The tumor necrosis factor (TNF) receptor associated factors
(TRAFs) constitute a family of genetically conserved adapter
proteins that has been found in mammals (TRAF1-6, see
review (Arch et al., 1998)), as well as in other multicellular
organisms such as Drosophila (Liu et al., 1999; Grech et al.,
2000; Medzhitov and Janeway, 2000; Zapata et al., 2000),
Dictyostelium discoideum (Regnier et al., 1995). Mammalian
TRAFs have emerged as the major signal transducers for the
TNF receptor superfamily and the interleukin-1 receptor/Toll-
like receptor (IL-1R/TLR) superfamily (Table 1). A wide range
of biological functions, such as adaptive and innate immunity,
embryonic development, stress response and bone metabolism,
are mediated by TRAFs through the induction of cell survival,
proliferation, differentiation and death. TRAFs are also
involved in the signal transduction of the Epstein-Barr virus
transforming protein LMP-1 (Mosialos et al., 1995). In
Drosophila, TRAFs are essential for dorsoventral polarization
and innate host defense by the signal transduction initiated
through the Toll receptor (Imler and Hoffmann, 2001; Preiss et
The TRAF proteins are characterized by the presence of a
novel TRAF domain at the C-terminus, which consists of a
coiled-coil domain followed by a conserved TRAF-C domain
(Rothe et al., 1994) (Fig. 1). The TRAF domain plays an
important role in TRAF function by mediating self-association
and upstream interactions with receptors and other signaling
proteins (Takeuchi et al., 1996). The N-terminal portion of
most TRAF proteins contains a RING finger and several zinc
finger motifs, which are important for downstream signaling
events (Rothe et al., 1995; Takeuchi et al., 1996).
Many of the biological effects of TRAF signaling appear to
be mediated through the activation of transcription factors of
et al., 1998) and
the NF-κB and AP-1 family. NF-κB promotes the expression
of genes involved in inflammatory and anti-apoptotic responses
(Baeuerle and Baltimore, 1996; Beg and Baltimore, 1996; Liu
et al., 1996). It is activated by the IκB kinase (IKK), which
consists of two kinase subunits, IKKα and IKKβ, and a
regulatory subunit, IKKγ/NEMO (DiDonato et al., 1997;
Regnier et al., 1997; Zandi et al., 1997; Krappmann et al.,
2000). Phosphorylation and degradation of IκB lead to the
release and translocation of NF-κB to the nucleus to activate
transcription (Stancovski and Baltimore, 1997). AP-1 activity
is stimulated by mitogen-activated protein (MAP) kinases
through either direct phosphorylation or transcription of AP-1
components (Karin, 1996). MAP kinases, which include
Ser/Thr kinases such as JNKs/SAPKs, ERKs and p38s, are at
the downstream end of a three-tiered system that also contains
MAP kinase kinase (MAP2K) and MAP kinase kinase kinase
(MAP3K). The stimulation of AP-1 activity by MAP kinases
may elicit stress responses and promote both cell survival and
cell death (Shaulian and Karin, 2001).
As adapter proteins, TRAFs elaborate receptor signal
transduction by serving as both a convergent and a divergent
platform. Therefore, different TRAFs are created with their
own specific biological roles. Their distinct upstream and
downstream signaling pathways may determine this specificity.
Recent structural and biochemical data have provided us with
a much better understanding of the upstream signaling
mechanism of TRAFs. Many of the current studies of TRAF
downstream signaling focus on the activation of NF-κB and
AP-1 transcription factors. However, accumulating evidence
points to the differential regulation of this apparently common
downstream pathway as well as to additional TRAF-specific
pathways for eliciting different biological functions. We further
suggest that signaling-dependent TRAF trafficking may be
another crucial regulatory factor. This commentary will focus
The tumor necrosis factor (TNF) receptor associated
factors (TRAFs) have emerged as the major signal
transducers for the TNF receptor superfamily and the
interleukin-1 receptor/Toll-like receptor (IL-1R/TLR)
superfamily. TRAFs collectively play important functions
in both adaptive and innate immunity. Recent functional
and structural studies have revealed the individuality of
each of the mammalian TRAFs and advanced our
understanding of the underlying molecular mechanisms.
Here, we examine this functional divergence among TRAFs
from a perspective of both upstream and downstream
TRAF signal transduction pathways and of signaling-
dependent regulation of TRAF trafficking. We raise
additional questions and propose hypotheses regarding the
molecular basis of TRAF signaling specificity.
Key words: TRAF, TNF, IL-1R/TLR, NF-κB, AP-1
All TRAFs are not created equal: common and distinct
molecular mechanisms of TRAF-mediated signal
Jee Y. Chung, Young Chul Park, Hong Ye and Hao Wu
Department of Biochemistry, Weill Medical College of Cornell University, New York, NY 10021, USA
Author for correspondence (e-mail: email@example.com)
Journal of Cell Science 115, 679-688 (2002) © The Company of Biologists Ltd
on the common and distinct molecular mechanisms of TRAF-
mediated signal transduction. For complementary information,
please refer to other recent reviews on TRAFs and TNF
receptors (Wallach et al., 1999; Inoue et al., 2000; Locksley et
al., 2001; Wajant et al., 2001).
Specific biological functions of mammalian TRAFs
Mammalian TRAF1 and TRAF2 were originally identified by
their association with TNFR2 (Rothe et al., 1994). The other
mammalian TRAFs were identified as follows: TRAF3 by its
interaction with CD40 and the Epstein-Barr virus transforming
protein LMP1 (Cheng et al., 1995; Mosialos et al., 1995; Sato
et al., 1995); TRAF4 by its overexpression in breast carcinoma
cells (Regnier et al., 1995); TRAF5 by its interaction with
CD40 and LTβR (Ishida et al., 1996; Nakano et al., 1996;
Mizushima et al., 1998) and TRAF6 by its participation in the
signal transduction of CD40 and interleukin-1, a cytokine that
is not related to TNF (Cao et al., 1996b; Ishida et al., 1996).
However, further extensive studies have shown that the specific
biological function of each TRAF protein is not necessarily
related to its origin of identification (Table 2, Fig. 2).
Since its discovery, TRAF2 has become the prototypical
member of the TRAF family. The paradigm of TRAF-mediated
NF-κB and MAP kinase activation was first demonstrated
using both TRAF2 overexpression and a dominant-negative
phenotype of a TRAF2 derivative lacking the RING domain
(Rothe et al., 1995; Hsu et al., 1996b; Takeuchi et al., 1996;
Duckett et al., 1997; Reinhard et al., 1997; Arch et al., 1998).
TRAF2 transcripts have been detected in almost every tissue
(Rothe et al., 1994), making TRAF2 the most widely expressed
TRAF family member.
TRAF2 plays a cytoprotective role, which was demonstrated
by the premature death of TRAF2-deficient mice owing to
severe runting. In addition, TRAF2-deficient cells are highly
sensitive to TNF-induced cell death (Yeh et al., 1997). The lack
of TRAF2 or the expression of a dominant-negative form of
TRAF2 only led to a modest defect in TNF-induced NF-κB
activation but resulted in a severe reduction of JNK/SAPK
activation (Lee et al., 1997; Yeh et al., 1997; Devin et al.,
2000). Recent data suggest that TRAF2 is important for NF-
κB activation, but this role may be partially compensated for
by the highly related TRAF5 (see below) (Nakano et al., 2000).
The sensitization to TNF-induced cell death in the absence of
TRAF2 must have been largely due to an NF-κB-independent
mechanism (Lee et al., 1997; Yeh et al., 1997; Lee et al., 1998).
One possibility may be related to the failure to recruit other
proteins such as cellular inhibitors of apoptosis proteins
(cIAPs) to the TNFR1 receptor signaling complex in the
absence of TRAF2 (Wang et al., 1998; Park et al., 2000). TNF
toxicity through TNFR1 appears to contribute significantly to
the survival defects in TRAF2-deficient mice because a double
Journal of Cell Science 115 (4)
Table 1. Current members of the TNF receptor and IL-1R/TLR superfamilies
TNF receptor superfamily
Receptors with intracellular death domains: TNFR1, Fas, DR3, DR4, DR5, DR6, NGFR
Receptors with no intracellular death domains: TNFR2, LTβR, CD40, CD30, OX40, CD27, 4-1BB, RANK/TRANCE-R, Troy, HveA, EDAR, XEDAR,
AITR, TACI, BCMA
IL-1 receptor family: IL-1R, IL-1RAcp, IL-18R, IL-18RAcp
Toll-like receptor family: TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10
Table 2. Summary of TRAF functions
Feedback regulation of receptor signaling
TRAF2 Anti-apoptotic signaling
TRAF3 T-cell-dependent antigen response
TRAF4 Tracheal formation
TRAF5 CD27 and CD40 signaling
TRAF2 and 5
Z Z Z Z ZR
R Z Z Z Z ZC-C
R Z Z Z Z Z Z Z C-C
Z Z Z Z Z
R Z Z Z Z Z ZC-CTRAF-C
Fig. 1. Sequence characteristics of TRAFs. (A) Domain organization
of the mammalian TRAFs. R, RING domain; Z, zinc-finger domain;
C-C, coiled-coil domain; TRAF-C, TRAF-C domain. The TRAF
domain comprises the coiled-coil domain and the TRAF-C domain.
(B) The proposed evolutionary tree for the mammalian, Drosophila
and C. elegans TRAFs. This figure is adapted from Grech et al.
(Grech et al., 2000).
deficiency in TRAF2 and TNFR1 resulted in increased survival
(Yeh et al., 1999).
TRAF1, unlike TRAF2 and other TRAFs, does not have the
N-terminal RING and zinc-finger domains (Rothe et al., 1994).
TRAF1 expression is fairly restricted (Rothe et al., 1994;
Mosialos et al., 1995) and can be upregulated in lymphoid
tumors and transformed lymphoid cells (Durkop et al., 1999;
Zapata et al., 2000). The current data are consistent with the
idea that TRAF1 is an NF-κB inducible protein that protects
cells from apoptosis and plays a role in the feedback regulation
of receptor signaling (Speiser et al., 1997; Wang et al., 1998;
Carpentier and Beyaert, 1999; Schwenzer et al., 1999; Nolan
et al., 2000). It appears that TRAF1 works in conjunction with
TRAF2 and cIAPs to fully suppress TNF-induced apoptosis.
This may be achieved through the direct suppression of caspase
activation in the TNFR1 signaling complex by cIAPs, which
are specifically recruited through
TRAF1 and TRAF2 (Wang et al.,
1998; Park et al., 2000).
Although TRAF3 possesses a
similar to TRAF2 and TRAF5,
overexpression of TRAF3 did not
activate NF-κB (Rothe et al., 1995).
In contrast, it was reported that
TRAF3 recruitment to LTβR led to
cell death (Force et al., 1997), and
that both N- and C-terminal domains
of TRAF3 negatively regulate NF-κB
activation induced by Ox40 (Takaori-
Kondo et al., 2000). However, it has
also been shown that there are a
variety of mRNA species of TRAF3
and that some splice variants do
Eyndhoven et al., 1999). Similar to
deficient mice have poor perinatal
and neonatal survival (Xu et al.,
1996). However, despite the runting
phenotype and the hypotrophy of the
spleen and thymus, which is similar
to the phenotype displayed by
TRAF2-deficient mice, the immune
system is fairly normal except in the
T-cell-dependent antigen responses
(Xu et al., 1996).
The biological importance of
TRAF4 was revealed by the gross
tracheal malformation displayed by
TRAF4-deficient mice (Shiels et al.,
2000), which suggested a parallel
function of TRAF4 with the Drosophila Toll pathway in
body organization. Analysis of TRAF4 expression has also
implicated TRAF4 in the function of neural multipotent cells
and epithelial stem cells in adult mammals (Krajewska et al.,
1998; Masson et al., 1998). Even though there is evidence that
TRAF4 may interact with several receptors in the TNF receptor
superfamily (Krajewska et al., 1998; Ye et al., 1999), further
studies are required to elucidate the molecular pathway of
TRAF5 is considered to be a close functional and structural
homologue of TRAF2, and overexpression of TRAF5 can also
activate NF-κB and AP-1 transcription factors (Ishida et al.,
1996; Nakano et al., 1996). However, deletion of TRAF5 did
not cause perinatal lethality, perhaps owing to the more
restricted expression pattern of TRAF5 compared with TRAF2
(Ishida et al., 1996; Nakano et al., 1996). TRAF5 deficiency
Fig. 2. TRAF signaling pathways. (A)
Membrane-proximal events in TRAF
signaling, showing direct receptor-TRAF
recruitment and indirect receptor-TRAF
interactions. (B) Downstream signaling
events for TRAFs, shown here for two
representative TRAF family members,
TRAF2 and TRAF6.
led to more specific defects in CD40- and CD27-mediated
lymphocyte activation, whereas TNF-mediated NF-κB
activation was not severely affected (Nakano et al., 1999).
Interestingly, TRAF2 and 5 double knockout animals did
exhibit a severe reduction in TNF-induced NF-κB activation,
which suggests that TRAF5 and TRAF2 are partially
functionally redundant (Nakano et al., 2000).
TRAF6 possesses a unique receptor-binding specificity that
results in its crucial role as the signaling mediator for both the
TNF receptor superfamily and the IL-1R/TLR superfamily. As
shown by targeted gene ablation, TRAF6 is functionally
important for both TRANCE-R-mediated osteoclast activation
and CD40 signaling (Lomaga et al., 1999; Naito et al., 1999;
Wong et al., 1999b), even though both CD40 and TRANCE-R
can also signal through TRAF2 (Pullen et al., 1998; Wong et
al., 1998). In the IL-1R/TLR superfamily, lack of TRAF6 leads
to defective signaling by IL-1 and IL-18 as well as hypo-
responsiveness to bacterial lipopolysaccharides (LPS), the cell
wall component of Gram-negative bacteria, which signals
through TLR4 (Lomaga et al., 1999; Naito et al., 1999). These
observations place TRAF6 as an important player in innate
immunity against pathogens.
The functional divergence of TRAFs appears to correlate
well with a proposed evolutionary relationship among TRAFs
in mammals and other organisms on the basis of sequence
conservation in the TRAF domain and gene structure analysis
(Grech et al., 2000) (Fig. 1). In this hypothesis, TRAF4 and
TRAF6 precursors appear to have arisen earlier in evolution.
We propose that TRAF4 and TRAF6 may be functional
descendents of dTRAF1 and dTRAF2, which have been
implicated in Toll signal transduction (Zapata et al., 2000; Shen
et al., 2001). This argument points to the existence of a yet to
be identified TRAF4-interacting receptor. On the other hand,
TRAF1, 2, 3 and 5 appear to be more recent siblings in the
TRAF family (Grech et al., 2000). This observation is
supported by the similar receptor-binding specificity of these
four TRAFs towards the TNF receptor superfamily (see below)
and the lack of known homologues of these receptors beyond
Common and distinct signal transduction
mechanisms up-stream of TRAFs
Each TRAF protein interacts with and mediates the signal
transduction of multiple receptors, and in turn each receptor
utilizes multiple TRAFs for specific functions (Arch et al.,
1998). There are at least three distinct ways that TRAF proteins
can be recruited to and activated by ligand-engaged receptors
(Fig. 2A). Members of the TNF receptor superfamily that do not
contain intracellular death domains, such as TNFR2 and CD40,
recruit TRAFs directly via short sequences in their intracellular
tails (Rothe et al., 1994; Cheng et al., 1995; Pullen et al., 1998).
Those that contain an intracellular death domain, such as
TNFR1, first recruit an adapter protein, TRADD, via a death-
domain–death-domain interaction (Hsu et al., 1995). TRADD
then serves as a central platform of the TNFR1 signaling
complex, which assembles TRAF2 (Hsu et al., 1996b) and RIP
(Stanger et al., 1995; Hsu et al., 1996a) for survival signaling,
and FADD and caspase-8 for the induction of apoptosis (Hsu et
al., 1996b). Members of the IL-1R/TLR superfamily contain a
protein interaction module known as the TIR domain (Xu et al.,
2000), which recruits, sequentially, MyD88 (Wesche et al.,
1997), a TIR domain and death domain containing protein, and
IRAKs (Cao et al., 1996a; Muzio et al., 1997; Wesche et al.,
1999), adapter Ser/Thr kinases with death domains. IRAKs in
turn associate with TRAF6 to elicit signaling by IL-1 and
pathogenic components such as LPS (Cao et al., 1996b; Zhang
et al., 1999; Hacker et al., 2000; Wang et al., 2001).
A common mechanism for the membrane-proximal event in
TRAF signaling has been revealed by the conserved trimeric
association in the crystal structure of the TRAF domain of
TRAF2 (Park et al., 1999; McWhirter et al., 1999). The
structure contains a stalk of a trimeric coiled-coil and a cap of
trimerized TRAF-C domain with a novel anti-parallel β-
sandwich fold, leading to a prominent mushroom shaped
structure (Fig. 3A). This trimeric stoichiometry of TRAFs
provides a structural basis for signal transduction across the
cellular membrane after receptor trimerization by trimeric
extracellular ligands in the TNF superfamily (Banner et al.,
1993). Interestingly, recent studies suggest that specific ligand-
induced receptor trimerization may be primed by non-signaling
receptor pre-association prior to ligand binding (Chan et al.,
2000; Siegel et al., 2000). Thermodynamic characterization
revealed the low affinity nature of monomeric TRAF2-
receptor interactions, which confirms the importance of
oligomerization-based affinity enhancement or avidity in
receptor-mediated TRAF recruitment (Ye and Wu, 2000).
Structural and biochemical studies have shown that a single
TRAF protein recognizes diverse receptor sequences via a
conserved mode of interaction but with a range of different
affinities. In several different TRAF2 complexes, receptor
sequences bind invariably to the surface groove on the TRAF-
C domain of TRAF2 in an extended conformation, making
main chain hydrogen bonding interactions with the edge of the
β-sandwich structure (Park et al., 1999; McWhirter et al., 1999;
Ye et al., 1999). The chain direction of the receptor peptides
allows the receptors to immediately latch on to the TRAF-C
domain after exiting from their transmembrane regions.
Although TRAF2-binding sequences from different receptors
bear limited sequence homology, their interactions with
TRAF2 are preserved by a few conserved structural contacts,
as shown in the consensus (P/S/T/A)x(Q/E)E (Ye et al., 1999)
(Fig. 3B). A deviation from this consensus, which bears the
sequence of PxQxxD, is present in the human Epstein-Barr
virus LMP-1 protein and binds to the same surface of TRAF2
via both similar and distinct features (Ye et al., 1999).
Thermodynamic characterization further showed variable
affinities of TRAF2 with different receptor sequences, which
are probably a consequence of affinity modulations by non-
conserved residues within and beyond the core binding motif
(Ye and Wu, 2000) (Table 3).
Further structural analyses have also revealed how several
different TRAFs can recognize a single receptor. The amino-
acid residues on the TRAF2 surface used for receptor
interactions are conserved among TRAF1, 2, 3 and 5,
explaining the overlapping specificity of these TRAFs for
different receptors (Park et al., 1999; Ye et al., 1999). However,
an identical sequence from CD40 exhibits alternative binding
modes to TRAF2 and TRAF3, suggesting that this conserved
interaction may vary to some extent in different TRAFs, which
modulates the strengths of the interactions (Fig. 3C). In the
TRAF3 complex, receptor residues distal to the central core
Journal of Cell Science 115 (4)
683 TRAF signaling
sequence also interact with TRAF3, leading to the formation
of a hairpin on the TRAF3 surface, which contributes strongly
to TRAF3 interaction (Ni et al., 2000).
The distinct mode of TRAF2 recruitment by TRADD was
revealed by the crystal structure of the TRAF2-TRADD
complex (Park et al., 2000) (Fig. 3D). The more extensive
TRAF2-TRADD interface overlaps spatially and therefore
potentially competes with TRAF2-receptor interactions.
resonance has shown that the TRAF2-TRADD interaction is
using surface plasmon
Fig. 3. Structural studies of upstream
interactions of TRAFs. (A) The
mushroom-shaped trimeric structure
of the TRAF domain of human
TRAF2 (left: three-fold axis into the
page; right: three-fold axis vertical) is
shown here in complex with TNFR2.
The coiled-coil region (stalk) is shown
as yellow helices. The β-sheet regions
of the three TRAF-C domains are
shown respectively in blue, green and
purple. Bound peptides from TNFR2
are shown as orange arrows, indicating
the direction of the peptide chains.
The proposed location of the cellular
membrane is shown. This figure is
modified from (Park et al., 1999).
(B) The structural superposition of
several TRAF2-interacting receptor
peptides is shown using stereo stick
models. Nitrogen atoms, blue; oxygen
atoms, red; sulfur atoms, green;
carbon atoms, yellow (CD40), gray
(CD30), green (Ox40), pink (4-1BB),
cyan (LMP1) and purple (TNFR2).
This figure is adapted from (Ye et al.,
1999). (C) The crystal structure of the
trimeric complex between the TRAF
domain of TRAF3 and a CD40
peptide bound in a hairpin
configuration (Ni et al., 2000). The
color-coding of the TRAF domain
follows that of (A) and the CD40
peptides are shown as orange arrows.
(D) A ribbon diagram of the complex
between TRADD and TRAF2 (left,
three-fold axis into the page; right,
three-fold axis vertical). TRAF2, blue,
green and purple; TRADD, magenta,
red and yellow. The TRAF2-TRADD
interface is more extensive and
exhibits higher affinity than TRAF2-
receptor-peptide interactions. This
figure was adapted from Park et al.
(Park et al., 2000).
unique in two distinct ways. First, TRAF2 has a significantly
higher affinity for TRADD than for peptide motifs in direct
receptor interactions (Table 3), which leads to more efficient
initiation of TRAF2 signaling by TRADD. Second, TRADD
has specificity for only TRAF1 and TRAF2, but not other
TRAF family members (Fig. 2A). It appears that TRAF1
and TRAF2 work in conjunction with associated caspase
inhibitors cIAPs to fully suppress TNF-induced apoptosis in
the TNFR1 signaling complex (Wang et al., 1998; Park et al.,
2000), leading to dominance of survival signaling for this
receptor under most circumstances.
TRAF6 directly interacts with CD40 and TRANCE-R,
which are members of the TNF receptor superfamily (Ishida et
al., 1996; Pullen et al., 1998; Darnay et al., 1999). For the
signal transduction of the IL-1R/TLR superfamily, TRAF6 is
indirectly coupled to receptor activation via IRAK and the
IRAK-TRAF6 pathway is evolutionarily analogous to the
Pelle-dTRAF pathway in Drosophila (Liu et al., 1999; Zapata
et al., 2000; Shen et al., 2001). Even though biochemical
characterizations suggest that TRAF6-receptor and TRAF6-
IRAK interactions differ from receptor recognition by other
TRAFs (Pullen et al., 1998; Darnay et al., 1999), elucidation
of the molecular mechanism of TRAF6 upstream interactions
awaits further structural information.
TRAF downstream signal transduction and
TRAF-mediated NF-κB and AP-1 activation has been
extensively studied for the representative TRAF family
members TRAF2 and TRAF6, which apparently utilize
different molecular pathways (Fig. 2B). Two models of
TRAF2 downstream signaling pathways have been proposed.
The TRAF2-mediated NF-κB activation may involve the
direct recruitment of the IKK complex in cooperation with
RIP (Yeh et al., 1997; Kelliher et al., 1998; Devin et al., 2000;
Nakano et al., 2000; Zhang et al., 2000). Furthermore,
artificial oligomerization of either TRAF2 or RIP was
sufficient for NF-κB activation (Baud et al., 1999; Poyet et al.,
2000). Alternatively, TRAF2 can associate with several
upstream MAP kinases to induce NF-κB and AP-1 activation.
These include NIK (Malinin et al., 1997; Song et al., 1997),
MEKK1 and MEKK3 (Baud et al., 1999; Yang et al., 2001)
for IKK activation and ASK1, MEKK1 and GCKR for
initiating MAP kinase pathways and AP-1 activation (Nishitoh
et al., 1998; Baud et al., 1999; Hoeflich et al., 1999; Shi et al.,
The activation of both NF-κB and AP-1 by TRAF6 in the
IL-1 signaling pathway appears to involve a MAP3K known
as TAK1 (Yamaguchi et al., 1995; Ninomiya-Tsuji et al., 1999)
and two adapter proteins TAB1 (Shibuya et al., 1996) and
TAB2 (Takaesu et al., 2000). Upon stimulation, TRAF6
associates with endogenous TAK1 and TAB1 (Ninomiya-Tsuji
et al., 1999) and interacts with TAB2 following the
translocation of TAB2 from the membrane to the cytosol
(Takaesu et al., 2001). Activated TAK1 appears to
phosphorylate NIK, which in turn activates IKK (Shirakabe et
al., 1997; Ninomiya-Tsuji et al., 1999) and initiates the MAP
kinase pathway. Surprisingly, it has been shown recently that
ubiquitination plays an important role in TAK1 activation
(Deng et al., 2000; Wang et al., 2001). It appears that as a
RING-domain-containing protein, TRAF6 operates together
with a ubiquitin-conjugating enzyme system to catalyze the
synthesis of unique polyubiquitin chains essential for TRAF6
The ability of multiple TRAFs to activate NF-κB and AP-
1 transcription factors raises the question of how are the
specific biological functions of different TRAFs realized. We
propose that the different signaling pathways, such as those
utilized by TRAF2 and TRAF6, may lead to preferential
activation of specific NF-κB and AP-1 components and
therefore the transcription of an overlapping but non-identical
set of genes. In addition, many TRAF-interacting proteins
have been identified and shown to regulate the activation of
NF-κB and AP-1 in a TRAF-specific manner. For example,
A20 is a TRAF1- and TRAF2-interacting protein (Song et al.,
1996) that inhibits NF-κB activation and regulates TNF-
induced cell death responses (Lee et al., 2000). A complete
review of these regulatory proteins is beyond the scope of this
commentary; however, their potential functions should not be
A different level of regulation was revealed by several recent
gene knockout studies in which certain proteins were shown to
regulate NF-κB transcriptional activity without affecting its
DNA-binding activity. For example, in mice deficient in the
MAP3K NIK, normal NF-κB DNA-binding activity was
observed upon treatment by a variety of cytokines, including
TNF, IL-1 and LTβ. However, gene transcription upon LTβR
activation was selectively affected by the absence of NIK (Yin
et al., 2001). Therefore, as different TRAFs may recruit a
different set of these regulatory proteins, their biological
functions may be modulated by them.
In addition to NF-κB and AP-1 activation, TRAF proteins
have been implicated in the crossover to additional signaling
pathways. One such example is TRAF6-mediated activation of
Src family kinases. In osteoclasts at least, TRAF6 plays an
indispensable role in the activation of c-Src and subsequently
the anti-apoptotic kinase PKB/Akt (Coffer et al., 1998; Wong
et al., 1999a). Similarly, TRAF6-dependent activation of
another protein tyrosine kinase Syk has been shown to mediate
IL-1-induced chemokine production (Yamada et al., 2001).
Therefore, the differential regulation of NF-κB and AP-1, as
well as the specific activation of other signaling pathways, may
collectively contribute to the specific functions of TRAFs.
Signaling-dependent TRAF trafficking
Accumulating evidence started to identify the intracellular
Journal of Cell Science 115 (4)
Table 3. Affinity characterization of the interactions of
TRAF2 with various receptor peptides and with TRADD
The data are from Park et al. (Park et al., 2000) and Ye and Wu (Ye and
Wu, 2000). The core sequences are shown in bold and aligned.
685 TRAF signaling
localization of TRAFs prior to, during and after receptor
activation as an important regulatory mechanism for TRAF-
mediated signal transduction. In resting cells, several TRAFs
have been shown to localize throughout the cytoplasm or to
intracellular punctate structures (Mosialos et al., 1995;
Hostager et al., 2000). Upon receptor stimulation, TRAFs are
redistributed to the cytoplasmic membrane or to plasma
membrane patches or caps (Mosialos et al., 1995; Kuhne et
al., 1997). More specifically, receptor recruitment of TRAFs
during CD40 signaling could lead to the partitioning of these
TRAFs into membrane rafts, which are specific regions of the
plasma membrane that are rich in sphingolipid and
cholesterol (Hostager et al., 2000; Vidalain et al., 2000). This
partitioning could be crucial for TRAF signaling as it
physically stabilizes the receptor signaling complexes and
places TRAFs in the vicinity of a number of signaling
proteins including the Src family kinases, which are
preferentially localized in these membrane rafts. In fact, it has
been found that among the known TRAFs, the ability to
redistribute to insoluble membrane fractions consistently
correlated with JNK activation. In addition, the forced
localization of TRAF3 to the cell membrane was sufficient to
convert this molecule into an activator of JNK (Dadgostar and
Although the redistribution of TRAFs into membrane
fractions may lead to a more sustained signaling of the
activated receptor, it could also lead to a depletion of
cytoplasmic TRAFs and therefore downregulate subsequent
TRAF-dependent signal transduction (Arch et al., 2000). Some
TRAFs can accumulate in perinuclear compartments after a
particular signaling event (Arch et al., 2000; Force et al., 2000)
but the eventual fate of these TRAFs is not clear. One
possibility is proteasome-dependent TRAF degradation
(Duckett and Thompson, 1997; Brown et al., 2001), which
would limit the recycling of TRAFs for further signal
transduction. Interestingly, several TRAFs have been shown to
interact with proteins of the cytoskeleton and/or of particular
membranes. These include the p62 nucleoporin, a component
of the nuclear pore central plug (Gamper et al., 2000), the
membrane-organizing protein caveolin-1 (Feng et al., 2001),
the microtubule-binding protein MIP-T3 (Ling and Goeddel,
2000) and filamin (Leonardi et al., 2000). Clearly, this is an
important field that requires further exploration and may hold
many of the clues to the specificity of TRAF-mediated signal
Since the identification of the first two TRAF family members
in 1994, it has become clear that different TRAFs exhibit
specific biological functions. The membrane-proximal events
for initiating differential TRAF signal transduction have been
relatively well established from the wealth of structural and
functional studies. The biggest challenge ahead is to further
elucidate the molecular mechanisms of specific TRAF
downstream signal transduction by differential TRAF
localization and interactions with various intracellular proteins.
We thank Joseph Aaron and Zheng-gang Liu for critical readings
of the manuscript and wish to apologize in advance for possible
Arch, R. H., Gedrich, R. W. and Thompson, C. B. (1998). Tumor necrosis
factor receptor-associated factors (TRAFs) – a family of adapter proteins
that regulates life and death. Genes. Dev. 12, 2821-2830.
Arch, R. H., Gedrich, R. W. and Thompson, C. B. (2000). Translocation of
TRAF proteins regulates apoptotic threshold of cells. Biochem. Biophys.
Res. Commun. 272, 936-945.
Baeuerle, P. A. and Baltimore, D. (1996). NF-kappa B: ten years after. Cell
Banner, D. W., D’Arcy, A., Janes, W., Gentz, R., Schoenfeld, H. J., Broger,
C., Loetscher, H. and Lesslauer, W. (1993). Crystal structure of the soluble
human 55 kd TNF receptor-human TNF beta complex: implications for TNF
receptor activation. Cell 73, 431-445.
Baud, V., Liu, Z. G., Bennett, B., Suzuki, N., Xia, Y. and Karin, M. (1999).
Signaling by proinflammatory cytokines: oligomerization of TRAF2 and
TRAF6 is sufficient for JNK and IKK activation and target gene induction
via an amino-terminal effector domain. Genes. Dev. 13, 1297-1308.
Beg, A. A. and Baltimore, D. (1996). An essential role for NF-kappaB in
preventing TNF-alpha-induced cell death. Science 274, 782-784.
Brown, K. D., Hostager, B. S. and Bishop, G. A. (2001). Differential
signaling and tumor necrosis factor receptor-associated factor (TRAF)
degradation mediated by CD40 and the Epstein-Barr virus oncoprotein
latent membrane protein 1 (LMP1). J. Exp. Med. 193, 943-954.
Cao, Z., Henzel, W. J. and Gao, X. (1996a). IRAK: A kinase associated with
the interleukin-1 receptor. Science 271, 1128-1131.
Cao, Z., Xiong, J., Takeuchi, M., Kurama, T. and Goeddel, D. V. (1996b).
TRAF6 is a signal transducer for interleukin-1. Nature 383, 443-446.
Carpentier, I. and Beyaert, R. (1999). TRAF1 is a TNF inducible regulator
of NF-kappaB activation. FEBS Lett. 460, 246-250.
Chan, F. K., Chun, H. J., Zheng, L., Siegel, R. M., Bui, K. L. and Lenardo,
M. J. (2000). A domain in TNF receptors that mediates ligand-independent
receptor assembly and signaling. Science 288, 2351-2354.
Cheng, G., Cleary, A. M., Ye, Z. S., Hong, D. I., Lederman, S. and
Baltimore, D. (1995). Involvement of CRAF1, a relative of TRAF, in CD40
signaling. Science 267, 1494-1498.
Coffer, P. J., Jin, J. and Woodgett, J. R. (1998). Protein kinase B (c-Akt): a
multifunctional mediator of phosphatidylinositol 3-kinase activation.
Biochem. J. 335, 1-13.
Dadgostar, H. and Cheng, G. (2000). Membrane localization of TRAF 3
enables JNK activation. J. Biol. Chem. 275, 2539-2544.
Darnay, B. G., Ni, J., Moore, P. A. and Aggarwal, B. B. (1999). Activation
of NF-kappaB by RANK requires tumor necrosis factor receptor- associated
factor (TRAF) 6 and NF-kappaB-inducing kinase. Identification of a novel
TRAF6 interaction motif. J. Biol. Chem. 274, 7724-7731.
Deng, L., Wang, C., Spencer, E., Yang, L., Braun, A., You, J., Slaughter,
C., Pickart, C. and Chen, Z. J. (2000). Activation of the IkappaB kinase
complex by TRAF6 requires a dimeric ubiquitin-conjugating enzyme
complex and a unique polyubiquitin chain. Cell 103, 351-361.
Devin, A., Cook, A., Lin, Y., Rodriguez, Y., Kelliher, M. and Liu, Z. (2000).
The distinct roles of TRAF2 and RIP in IKK activation by TNF-R1: TRAF2
recruits IKK to TNF-R1 while RIP mediates IKK activation. Immunity 12,
DiDonato, J. A., Hayakawa, M., Rothwarf, D. M., Zandi, E. and Karin,
M. (1997). A cytokine-responsive IkappaB kinase that activates the
transcription factor NF-kappaB. Nature 388, 548-554.
Duckett, C. S. and Thompson, C. B. (1997). CD30-dependent degradation
of TRAF2: implications for negative regulation of TRAF signaling and the
control of cell survival. Genes. Dev. 11, 2810-2821.
Duckett, C. S., Gedrich, R. W., Gilfillan, M. C. and Thompson, C. B.
(1997). Induction of nuclear factor kappaB by the CD30 receptor is
mediated by TRAF1 and TRAF2. Mol. Cell. Biol. 17, 1535-1542.
Durkop, H., Foss, H. D., Demel, G., Klotzbach, H., Hahn, C. and Stein, H.
(1999). Tumor necrosis factor receptor-associated factor 1 is overexpressed
in Reed-Sternberg cells of Hodgkin’s disease and Epstein-Barr virus-
transformed lymphoid cells. Blood 93, 617-623.
Feng, X., Gaeta, M. L., Madge, L. A., Yang, J. H., Bradley, J. R. and Pober,
J. S. (2001). Caveolin-1 associates with TRAF2 to form a complex that is
recruited to tumor necrosis factor receptors. J. Biol. Chem. 276, 8341-8349.
Force, W. R., Cheung, T. C. and Ware, C. F. (1997). Dominant negative
mutants of TRAF3 reveal an important role for the coiled coil domains in
cell death signaling by the lymphotoxin-beta receptor. J. Biol. Chem. 272,
Force, W. R., Glass, A. A., Benedict, C. A., Cheung, T. C., Lama, J. and
Ware, C. F. (2000). Discrete signaling regions in the lymphotoxin-beta
receptor for tumor necrosis factor receptor-associated factor binding,
subcellular localization, and activation of cell death and NF-kappaB
pathways. J. Biol. Chem. 275, 11121-11129.
Gamper, C., van Eyndhoven, W. G., Schweiger, E., Mossbacher, M., Koo,
B. and Lederman, S. (2000). TRAF-3 interacts with p62 nucleoporin, a
component of the nuclear pore central plug that binds classical NLS-
containing import complexes. Mol. Immunol. 37, 73-84.
Grech, A., Quinn, R., Srinivasan, D., Badoux, X. and Brink, R. (2000).
Complete structural characterisation of the mammalian and Drosophila
TRAF genes: implications for TRAF evolution and the role of RING finger
splice variants. Mol. Immunol. 37, 721-734.
Hacker, H., Vabulas, R. M., Takeuchi, O., Hoshino, K., Akira, S. and
Wagner, H. (2000). Immune cell activation by bacterial CpG-DNA through
myeloid differentiation marker 88 and tumor necrosis factor receptor-
associated factor (TRAF)6. J. Exp. Med. 192, 595-600.
Hoeflich, K. P., Yeh, W. C., Yao, Z., Mak, T. W. and Woodgett, J. R. (1999).
Mediation of TNF receptor-associated factor effector functions by apoptosis
signal-regulating kinase-1 (ASK1). Oncogene 18, 5814-5820.
Hostager, B. S., Catlett, I. M. and Bishop, G. A. (2000). Recruitment of
CD40 and tumor necrosis factor receptor-associated factors 2 and 3 to
membrane microdomains during CD40 signaling. J. Biol. Chem. 275,
Hsu, H., Huang, J., Shu, H. B., Baichwal, V. and Goeddel, D. V. (1996a).
TNF-dependent recruitment of the protein kinase RIP to the TNF receptor-
1 signaling complex. Immunity 4, 387-396.
Hsu, H., Shu, H.-B., Pan, M.-G. and Goeddel, D. V. (1996b). TRADD-
TRAF2 and TRADD-FADD interactions define two distinct TNF receptor
1 signal transduction pathways. Cell 84, 299-308.
Hsu, H., Xiong, J. and Goeddel, D. V. (1995). The TNF receptor 1-associated
protein TRADD signals cell death and NF-kB activation. Cell 81, 495-504.
Imler, J. L. and Hoffmann, J. A. (2001). Toll receptors in innate immunity.
Trends Cell Biol. 11, 304-311.
Inoue, J., Ishida, T., Tsukamoto, N., Kobayashi, N., Naito, A., Azuma, S.
and Yamamoto, T. (2000). Tumor necrosis factor receptor-associated factor
(TRAF) family: adapter proteins that mediate cytokine signaling. Exp. Cell
Res. 254, 14-24.
Ishida, T., Mizushima, S., Azuma, S., Kobayashi, N., Tojo, T., Suzuki, K.,
Aizawa, S., Watanabe, T., Mosialos, G., Kieff, E., Yamamoto, T. and
Inoue, J. (1996). Identification of TRAF6, a novel tumor necrosis factor
receptor-associated factor protein that mediates signaling from an amino-
terminal domain of the CD40 cytoplasmic region. J. Biol. Chem. 271,
Ishida, T. K., Tojo, T., Aoki, T., Kobayashi, N., Ohishi, T., Watanabe, T.,
Yamamoto, T. and Inoue, J. (1996). TRAF5, a novel tumor necrosis factor
receptor-associated factor family protein, mediates CD40 signaling. Proc.
Natl. Acad. Sci. USA 93, 9437-9442.
Karin, M. (1996). The regulation of AP-1 activity by mitogen-activated
protein kinases. Philos. Trans. R. Soc. Lond. B Biol. Sci. 351, 127-134.
Kelliher, M. A., Grimm, S., Ishida, Y., Kuo, F., Stanger, B. Z. and Leder,
P. (1998). The death-domain kinase RIP mediates the TNF-induced NF-kB
signal. Immunity 8, 297-303.
Krajewska, M., Krajewski, S., Zapata, J. M., Van Arsdale, T., Gascoyne,
R. D., Berern, K., McFadden, D., Shabaik, A., Hugh, J., Reynolds, A.,
Clevenger, C. V. and Reed, J. C. (1998). TRAF-4 expression in epithelial
progenitor cells. Analysis in normal adult, fetal, and tumor tissues. Am. J.
Pathol. 152, 1549-1561.
Krappmann, D., Hatada, E. N., Tegethoff, S., Li, J., Klippel, A., Giese, K.,
Baeuerle, P. A. and Scheidereit, C. (2000). The I kappa B kinase (IKK)
complex is tripartite and contains IKK gamma but not IKAP as a regular
component. J. Biol. Chem. 275, 29779-29787.
Kuhne, M. R., Robbins, M., Hambor, J. E., Mackey, M. F., Kosaka, Y.,
Nishimura, T., Gigley, J. P., Noelle, R. J. and Calderhead, D. M. (1997).
Assembly and regulation of the CD40 receptor complex in human B cells.
J. Exp. Med. 186, 337-342.
Lee, E. G., Boone, D. L., Chai, S., Libby, S. L., Chien, M., Lodolce, J. P.
and Ma, A. (2000). Failure to regulate TNF-induced NF-kappaB and cell
death responses in A20-deficient mice. Science 289, 2350-2354.
Lee, S. Y., Reichlin, A., Santana, A., Sokol, K. A., Nussenzweig, M. C. and
Choi, Y. (1997). TRAF2 is essential for JNK but not NF-kappaB activation
and regulates lymphocyte proliferation and survival. Immunity 7, 703-713.
Lee, S. Y., Kaufman, D. R., Mora, A. L., Santana, A., Boothby, M. and
Choi, Y. (1998). Stimulus-dependent synergism of the antiapoptotic tumor
necrosis factor receptor-associated factor 2 (TRAF2) and nuclear factor
kappaB pathways. J. Exp. Med. 188, 1381-1384.
Leonardi, A., Ellinger-Ziegelbauer, H., Franzoso, G., Brown, K. and
Siebenlist, U. (2000). Physical and functional interaction of filamin (actin-
binding protein- 280) and tumor necrosis factor receptor-associated factor
2. J. Biol. Chem. 275, 271-278.
Ling, L. and Goeddel, D. V. (2000). MIP-T3, a novel protein linking tumor
necrosis factor receptor- associated factor 3 to the microtubule network. J.
Biol. Chem. 275, 23852-23860.
Liu, Z. G., Hsu, H., Goeddel, D. V. and Karin, M. (1996). Dissection of
TNF receptor 1 effector functions: JNK activation is not linked to apoptosis
while NF-κb activation prevents cell death. Cell 87, 565-576.
Liu, H., Su, Y. C., Becker, E., Treisman, J. and Skolnik, E. Y. (1999). A
Drosophila TNF-receptor-associated factor (TRAF) binds the ste20 kinase
Misshapen and activates Jun kinase. Curr. Biol. 9, 101-104.
Locksley, R. M., Killeen, N. and Lenardo, M. J. (2001). The TNF and TNF
receptor superfamilies: integrating mammalian biology. Cell 104, 487-501.
Lomaga, M. A., Yeh, W., Sarosi, I., Duncan, G. S., Furlonger, C., Ho, A.,
Morony, S., Capparelli, C., Van, G., Kaufman, S. et al. (1999). TRAF6
deficiency results in osteopetrosis and defective interleukin-1, CD40, and
LPS signaling. Genes Dev. 13, 1015-1024.
Malinin, N. L., Boldin, M. P., Kovalenko, A. V. and Wallach, D. (1997).
MAP3K-related kinase involved in NF-κB induction by TNF, CD95 and IL-
1. Nature 385, 540-544.
Masson, R., Regnier, C. H., Chenard, M. P., Wendling, C., Mattei, M. G.,
Tomasetto, C. and Rio, M. C. (1998). Tumor necrosis factor receptor
associated factor 4 (TRAF4) expression pattern during mouse development.
Mech. Dev. 71, 187-191.
McWhirter, S. M., Pullen, S. S., Holton, J. M., Crute, J. J., Kehry, M. R.
and Alber, T. (1999). Crystallographic analysis of CD40 recognition and
signaling by human TRAF2. Proc. Natl. Acad. Sci. USA 96, 8408-8413.
Medzhitov, R. and Janeway, C., Jr (2000). Innate immune recognition:
mechanisms and pathways. Immunol. Rev. 173, 89-97.
Mizushima, S., Fujita, M., Ishida, T., Azuma, S., Kato, K., Hirai, M.,
Otsuka, M., Yamamoto, T. and Inoue, J. (1998). Cloning and
characterization of a cDNA encoding the human homolog of tumor necrosis
factor receptor-associated factor 5 (TRAF5). Gene 207, 135-140.
Mosialos, G., Birkenbach, M., Yalamanchili, R., VanArsdale, T., Ware, C.
and Kieff, E. (1995). The Epstein-Barr virus transforming protein LMP1
engages signaling proteins for the tumor necrosis factor receptor family. Cell
Muzio, M., Ni, J., Feng, P. and Dixit, V. M. (1997). IRAK (Pelle) family
member IRAK-2 and MyD88 as proximal mediators of IL-1 signaling.
Science 278, 1612-1615.
Naito, A., Azuma, S., Tanaka, S., Miyazaki, T., Takaki, S., Takatsu, K.,
Nakao, K., Nakamura, K., Katsuki, M., Yamamoto, T. and Inoue, J.
(1999). Severe osteopetrosis, defective interleukin-1 signalling and lymph
node organogenesis in TRAF6-deficient mice. Genes Cells 4, 353-362.
Nakano, H., Oshima, H., Chung, W., Williams-Abbott, L., Ware, C. F.,
Yagita, H. and Okumura, K. (1996). TRAF5, an activator of NF-kappaB
and putative signal transducer for the lymphotoxin-beta receptor. J. Biol.
Chem. 271, 14661-14664.
Nakano, H., Sakon, S., Koseki, H., Takemori, T., Tada, K., Matsumoto,
M., Munechika, E., Sakai, T., Shirasawa, T., Akiba, H. et al. (1999).
Targeted disruption of Traf5 gene causes defects in CD40- and CD27-
mediated lymphocyte activation. Proc. Natl. Acad. Sci. USA 96, 9803-
Nakano, H., Kurosawa, K., Sakon, S., Yagita, H., Yeh, W. C., Mak, T. W.
and Okumura, K. (2000). Impaired TNF-induced NF-kB activation and
high sensitivity to TNF-induced cell death in TRAF2- and TRAF5-double
deficient mice. Scand. J. Immuno. 51 (suppl 1), 71.
Ni, C. Z., Welsh, K., Leo, E., Chiou, C. K., Wu, H., Reed, J. C. and Ely,
K. R. (2000). Molecular basis for CD40 signaling mediated by TRAF3.
Proc. Natl. Acad. Sci. USA 97, 10395-10399.
Ninomiya-Tsuji, J., Kishimoto, K., Hiyama, A., Inoue, J., Cao, Z. and
Matsumoto, K. (1999). The kinase TAK1 can activate the NIK-I kappaB
as well as the MAP kinase cascade in the IL-1 signalling pathway. Nature
Nishitoh, H., Saitoh, M., Mochida, Y., Takeda, K., Nakano, H., Rothe, M.,
Miyazono, K. and Ichijo, H. (1998). ASK1 is essential for JNK/SAPK
activation by TRAF2. Mol. Cell 2, 389-395.
Nolan, B., Kim, R., Duffy, A., Sheth, K., De, M., Miller, C., Chari, R. and
Bankey, P. (2000). Inhibited neutrophil apoptosis: proteasome dependent
NF-kappaB translocation is required for TRAF-1 synthesis. Shock 14, 290-
Park, Y. C., Burkitt, V., Villa, A. R., Tong, L. and Wu, H. (1999). Structural
Journal of Cell Science 115 (4)
basis for self-association and receptor recognition of human TRAF2. Nature
Park, Y. C., Ye, H., Hsia, C., Segal, D., Rich, R. L., Liou, H. C., Myszka,
D. G. and Wu, H. (2000). A novel mechanism of TRAF signaling revealed
by structural and functional analyses of the TRADD-TRAF2 interaction.
Cell 101, 777-787.
Poyet, J. L., Srinivasula, S. M., Lin, J. H., Fernandes-Alnemri, T.,
Yamaoka, S., Tsichlis, P. N. and Alnemri, E. S. (2000). Activation of the
Ikappa B kinases by RIP via IKKgamma /NEMO-mediated oligomerization.
J. Biol. Chem. 275, 37966-37977.
Preiss, A., Johannes, B., Nagel, A. C., Maier, D., Peters, N. and Wajant,
(2001). Dynamic expression of Drosophila
embryogenesis and larval development. Mech. Dev. 100, 109-113.
Pullen, S. S., Miller, H. G., Everdeen, D. S., Dang, T. T., Crute, J. J. and
Kehry, M. R. (1998). CD40-tumor necrosis factor receptor-associated
factor (TRAF) interactions: regulation of CD40 signaling through multiple
TRAF binding sites and TRAF hetero-oligomerization. Biochemistry 37,
Regnier, C. H., Tomasetto, C., Moog-Lutz, C., Chenard, M. P., Wendling,
C., Basset, P. and Rio, M. C. (1995). Presence of a new conserved domain
in CART1, a novel member of the tumor necrosis factor receptor-associated
protein family, which is expressed in breast carcinoma. J. Biol. Chem. 270,
Regnier, C. H., Song, H. Y., Gao, X., Goeddel, D. V., Cao, Z. and Rothe,
M. (1997). Identification and characterization of an IkappaB kinase. Cell
Reinhard, C., Shamoon, B., Shyamala, V. and Williams, L. T. (1997).
Tumor necrosis factor alpha-induced activation of c-jun N-terminal kinase
is mediated by TRAF2. EMBO J. 16, 1080-1092.
Rothe, M., Wong, S. C., Henzel, W. J. and Goeddel, D. V. (1994). A novel
family of putative signal transducers associated with the cytoplasmic domain
of the 75 kDa tumor necrosis factor receptor. Cell 78, 681-692.
Rothe, M., Sarma, V., Dixit, V. M. and Goeddel, D. V. (1995). TRAF2-
mediated activation of NF-kappa B by TNF receptor 2 and CD40. Science
Sato, T., Irie, S. and Reed, J. C. (1995). A novel member of the TRAF family
of putative signal transducing proteins binds to the cytosolic domain of
CD40. FEBS Lett. 358, 113-118.
Schwenzer, R., Siemienski, K., Liptay, S., Schubert, G., Peters, N.,
Scheurich, P., Schmid, R. M. and Wajant, H. (1999). The human tumor
necrosis factor (TNF) receptor-associated factor 1 gene (TRAF1) is up-
regulated by cytokines of the TNF ligand family and modulates TNF-
induced activation of NF-kappaB and c-Jun N-terminal kinase. J. Biol.
Chem. 274, 19368-19374.
Shaulian, E. and Karin, M. (2001). AP-1 in cell proliferation and survival.
Oncogene 20, 2390-2400.
Shen, B., Liu, H., Skolnik, E. Y. and Manley, J. L. (2001). Physical and
functional interactions between Drosophila TRAF2 and Pelle kinase
contribute to Dorsal activation. Proc. Natl. Acad. Sci. USA 98, 8596-8601.
Shi, C. S., Leonardi, A., Kyriakis, J., Siebenlist, U. and Kehrl, J. H. (1999).
TNF-mediated activation of the stress-activated protein kinase pathway:
TNF receptor-associated factor 2 recruits and activates germinal center
kinase related. J. Immunol. 163, 3279-3285.
Shibuya, H., Yamaguchi, K., Shirakabe, K., Tonegawa, A., Gotoh, Y.,
Ueno, N., Irie, K., Nishida, E. and Matsumoto, K. (1996). TAB1: an
activator of the TAK1 MAPKKK in TGF-beta signal transduction. Science
Shiels, H., Li, X., Schumacker, P. T., Maltepe, E., Padrid, P. A., Sperling,
A., Thompson, C. B. and Lindsten, T. (2000). TRAF4 deficiency leads to
tracheal malformation with resulting alterations in air flow to the lungs. Am.
J. Pathol. 157, 679-688.
Shirakabe, K., Yamaguchi, K., Shibuya, H., Irie, K., Matsuda, S.,
Moriguchi, T., Gotoh, Y., Matsumoto, K. and Nishida, E. (1997). TAK1
mediates the ceramide signaling to stress-activated protein kinase/c-Jun N-
terminal kinase. J. Biol. Chem. 272, 8141-8144.
Siegel, R. M., Frederiksen, J. K., Zacharias, D. A., Chan, F. K., Johnson,
M., Lynch, D., Tsien, R. Y. and Lenardo, M. J. (2000). Fas preassociation
required for apoptosis signaling and dominant inhibition by pathogenic
mutations. Science 288, 2354-2357.
Song, H. Y., Rothe, M. and Goeddel, D. V. (1996). The tumor necrosis factor-
inducible zinc finger protein A20 interacts with TRAF1/TRAF2 and inhibits
NF-kappaB activation. Proc. Natl. Acad. Sci. USA 93, 6721-6725.
Song, H. Y., Regnier, C. H., Kirschning, C. J., Goeddel, D. V. and Rothe,
M. (1997). Tumor necrosis factor (TNF)-mediated kinase cascades:
bifurcation of nuclear factor-kappaB and c-jun N-terminal kinase
(JNK/SAPK) pathways at TNF receptor-associated factor 2. Proc. Natl.
Acad. Sci. USA 94, 9792-9796.
Speiser, D. E., Lee, S. Y., Wong, B., Arron, J., Santana, A., Kong, Y. Y.,
Ohashi, P. S. and Choi, Y. (1997). A regulatory role for TRAF1 in antigen-
induced apoptosis of T cells. J. Exp. Med. 185, 1777-1783.
Stancovski, I. and Baltimore, D. (1997). NF-kB activation: The IkB kinase
revealed? Cell 91, 299-302.
Stanger, B. Z., Leder, P., Lee, T., Kim, E. and Seed, B. (1995). RIP: a novel
protein containing a death domain that interacts with Fas/APO-1 (CD95) in
yeast and causes cell death. Cell 81, 513-523.
Takaesu, G., Kishida, S., Hiyama, A., Yamaguchi, K., Shibuya, H., Irie,
K., Ninomiya-Tsuji, J. and Matsumoto, K. (2000). TAB2, a novel adaptor
protein, mediates activation of TAK1 MAPKKK by linking TAK1 to TRAF6
in the IL-1 signal transduction pathway. Mol. Cell 5, 649-658.
Takaesu, G., Ninomiya-Tsuji, J., Kishida, S., Li, X., Stark, G. R. and
Matsumoto, K. (2001). Interleukin-1 (IL-1) receptor-associated kinase
leads to activation of TAK1 by inducing TAB2 translocation in the IL-1
signaling pathway. Mol. Cell Biol. 21, 2475-2484.
Takaori-Kondo, A., Hori, T., Fukunaga, K., Morita, R., Kawamata, S. and
Uchiyama, T. (2000). Both amino- and carboxyl-terminal domains of
TRAF3 negatively regulate NF-kappaB activation induced by OX40
signaling. Biochem. Biophys. Res. Commun. 272, 856-863.
Takeuchi, M., Rothe, M. and Goeddel, D. V. (1996). Anatomy of TRAF2.
Distinct domains for nuclear factor-kappaB activation and association with
tumor necrosis factor signaling proteins. J. Biol. Chem. 271, 19935-19942.
van Eyndhoven, W. G., Gamper, C. J., Cho, E., Mackus, W. J. and
Lederman, S. (1999). TRAF-3 mRNA splice-deletion variants encode
isoforms that induce NF- kappaB activation. Mol. Immunol. 36, 647-658.
Vidalain, P. O., Azocar, O., Servet-Delprat, C., Rabourdin-Combe, C.,
Gerlier, D. and Manie, S. (2000). CD40 signaling in human dendritic cells
is initiated within membrane rafts. EMBO J. 19, 3304-3313.
Wallach, D., Varfolomeev, E. E., Malinin, N. L., Goltsev, Y. V., Kovalenko,
A. V. and Boldin, M. P. (1999). Tumor necrosis factor receptor and Fas
signaling mechanisms. Annu. Rev. Immunol. 17, 331-367.
Wajant, H., Muhlenbeck, F. and Scheurich, P. (1998). Identification of a
TRAF (TNF receptor-associated factor) gene in Caenorhabditis elegans. J.
Mol. Evol. 47, 656-662.
Wajant, H., Henkler, F. and Scheurich, P. (2001). The TNF-receptor-
associated factor family: scaffold molecules for cytokine receptors, kinases
and their regulators. Cell Signal. 13, 389-400.
Wang, C., Deng, L., Hong, M., Akkaraju, G. R., Inoue, J. and Chen, Z. J.
(2001). TAK1 is a ubiquitin-dependent kinase of MKK and IKK. Nature
Wang, C. Y., Mayo, M. W., Korneluk, R. G., Goeddel, D. V. and Baldwin,
A. S., Jr (1998). NF-kappaB antiapoptosis: induction of TRAF1 and TRAF2
and c-IAP1 and c- IAP2 to suppress caspase-8 activation. Science 281, 1680-
Wang, Q., Dziarski, R., Kirschning, C. J., Muzio, M. and
transduction pathway that induces transcription of interleukin-8. Infect.
Immun. 69, 2270-2276.
Wesche, H., Henzel, W. J., Shillinglaw, W., Li, S. and Cao, Z. (1997).
MyD88: an adapter that recruits IRAK to the IL-1 receptor complex.
Immunity 7, 837-847.
Wesche, H., Gao, X., Li, X., Kirschning, C. J., Stark, G. R. and Cao, Z.
(1999). IRAK-M is a novel member of the Pelle/interleukin-1 receptor-
associated kinase (IRAK) family. J. Biol. Chem. 274, 19403-19410.
Wong, B. R., Josien, R., Lee, S. Y., Vologodskaia, M., Steinman, R. M. and
Choi, Y. (1998). The TRAF family of signal transducers mediates NF-
kappaB activation by the TRANCE receptor. J. Biol. Chem. 273, 28355-
Wong, B. R., Besser, D., Kim, N., Arron, J. R., Vologodskaia, M.,
Hanafusa, H. and Choi, Y. (1999). TRANCE, a TNF family member,
activates Akt/PKB through a signaling complex involving TRAF6 and c-
Src. Mol. Cell 4, 1041-1049.
Wong, B. R., Josien, R. and Choi, Y. (1999). TRANCE is a TNF family
member that regulates dendritic cell and osteoclast function. J. Leukoc. Biol.
Xu, Y., Cheng, G. and Baltimore, D. (1996). Targeted disruption of TRAF3
leads to postnatal lethality and defective T-dependent immune responses.
Immunity 5, 407-415.
Xu, Y., Tao, X., Shen, B., Horng, T., Medzhitov, R., Manley, J. L. and Tong,
and peptidoglycan activate
L. (2000). Structural basis for signal transduction by the Toll/interleukin-1
receptor domains. Nature 408, 111-115.
Yamada, T., Fujieda, S., Yanagi, S., Yamamura, H., Inatome, R.,
Yamamoto, H., Igawa, H. and Saito, H. (2001). Il-1 induced chemokine
production through the association of syk with tnf receptor-associated
factor-6 in nasal fibroblast lines. J Immunol 167, 283-288.
Yamaguchi, K., Shirakabe, K., Shibuya, H., Irie, K., Oishi, I., Ueno, N.,
Taniguchi, T., Nishida, E. and Matsumoto, K. (1995). Identification of a
member of the MAPKKK family as a potential mediator of TGF-beta signal
transduction. Science 270, 2008-2011.
Yang, J., Lin, Y., Guo, Z., Cheng, J., Huang, J., Deng, L., Liao, W., Chen,
Z., Liu, Z. and Su, B. (2001). The essential role of MEKK3 in TNF-induced
NF-kappaB activation. Nat. Immunol. 2, 620-624.
Ye, H., Park, Y. C., Kreishman, M., Kieff, E. and Wu, H. (1999). The
structural basis for the recognition of diverse receptor sequences by TRAF2.
Mol. Cell 4, 321-330.
Ye, H. and Wu, H. (2000). Thermodynamic characterization of the interaction
between TRAF2 and receptor peptides by isothermal titration calorimetry.
Proc. Natl. Acad. Sci. USA 97, 8961-8966.
Ye, X., Mehlen, P., Rabizadeh, S., VanArsdale, T., Zhang, H., Shin, H.,
Wang, J. J., Leo, E., Zapata, J., Hauser, C. A., Reed, J. C. and Bredesen,
D. E. (1999). TRAF family proteins interact with the common neurotrophin
receptor and modulate apoptosis induction. J. Biol. Chem. 274, 30202-
Yeh, W. C., Shahinian, A., Speiser, D., Kraunus, J., Billia, F., Wakeham,
A., de la Pompa, J. L., Ferrick, D., Hum, B., Iscove, N., Ohashi, P.,
Rothe, M., Goeddel, D. V. and Mak, T. W. (1997). Early lethality,
functional NF-kappaB activation, and increased sensitivity to TNF-induced
cell death in TRAF2-deficient mice. Immunity 7, 715-725.
Yeh, W. C., Hakem, R., Woo, M. and Mak, T. W. (1999). Gene targeting in
the analysis of mammalian apoptosis and TNF receptor superfamily
signaling. Immunol. Rev. 169, 283-302.
Yin, L., Wu, L., Wesche, H., Arthur, C. D., White, J. M., Goeddel, D. V.
and Schreiber, R. D. (2001). Defective lymphotoxin-beta receptor-induced
NF-kappaB transcriptional activity in NIK-deficient mice. Science 291,
Zandi, E., Rothwarf, D. M., Delhase, M., Hayakawa, M. and Karin, M.
(1997). The IkappaB kinase complex (IKK) contains two kinase subunits,
IKKalpha and IKKbeta, necessary for IkappaB phosphorylation and NF-
kappaB activation. Cell 91, 243-252.
Zapata, J. M., Krajewska, M., Krajewski, S., Kitada, S., Welsh, K.,
Monks, A., McCloskey, N., Gordon, J., Kipps, T. J., Gascoyne, R. D.,
Shabaik, A. and Reed, J. C. (2000). TNFR-associated factor family protein
expression in normal tissues and lymphoid malignancies. J. Immunol. 165,
Zapata, J. M., Matsuzawa, S., Godzik, A., Leo, E., Wasserman, S. A. and
Reed, J. C. (2000). The Drosophila tumor necrosis factor receptor-
associated factor-1 (DTRAF1) interacts with Pelle and regulates NFkappaB
activity. J. Biol. Chem. 275, 12102-12107.
Zhang, F. X., Kirschning, C. J., Mancinelli, R., Xu, X. P., Jin, Y., Faure,
E., Mantovani, A., Rothe, M., Muzio, M. and Arditi, M. (1999). Bacterial
lipopolysaccharide activates nuclear factor-kappaB through interleukin-1
signaling mediators in cultured human dermal endothelial cells and
mononuclear phagocytes. J. Biol. Chem. 274, 7611-7614.
Zhang, S. Q., Kovalenko, A., Cantarella, G. and Wallach, D. (2000).
Recruitment of the IKK signalosome to the p55 TNF receptor: RIP and A20
bind to NEMO (IKKgamma) upon receptor stimulation. Immunity 12, 301-
Journal of Cell Science 115 (4)