Telomere Biology: A New Player in the
Lorel Colgin and Roger Reddel
Yet another protein has been added to the crowd of
players found at the ends of chromosomes. Known
variously as PTOP, PIP1 or TINT1, this negative
regulator of telomere length connects some of the
key proteins already known to be present — TRF1,
TIN2, POT1, and TRF2 — and adds even more
complexity to telomere protein interactions.
The ends of linear chromosomes pose particular
problems for eukaryotic cells, which are solved with
the help of a team of specialised proteins. Core
members of this team include TRF1 and TRF2, which
bind directly to double-stranded telomeric DNA, and
POT1, which binds to the single-stranded DNA at the
telomere terminus. The team also includes many other
proteins that bind directly or indirectly to these three
(Figure 1A). POT1 does double duty because it is also
found in a protein complex with TRF1 and TIN2, which
binds directly to TRF1. How POT1 participates in this
complex was puzzling, however, because there was
no evidence that it binds either to TRF1 or to TIN2.
Three groups [1–3] have now identified a protein
that bridges this gap between TIN2 and POT1
(Figure 1B). It has been given three different names
which reflect the properties of the protein uncovered:
PTOP, for POT1 and TIN2 organizing protein ; PIP1,
for POT1 interacting protein ; and TINT1, for TIN2
interacting protein . PTOP/PIP1/TINT1 does not just
physically connect TIN2 and POT1: through binding to
TIN2, it may also be involved with interactions
between TRF1 and TRF2 [4,5].
With regard to the functions of PTOP/PIP1/TINT1,
most attention so far has concentrated on its role in
the TRF1 protein complex, which inhibits telomerase-
mediated lengthening of telomeres. As the telomere
increases in length, the amount of TRF1, TIN2 and
POT1 bound to it increases , which is thought to
result in an increasingly inhibitory environment for
telomerase and hence in the establishment of a
telomere length equilibrium or ‘set’ point. TIN2
modulates the activity of TRF1 by inhibiting the action
of tankyrase 1, which ADP ribosylates TRF1 and
causes it to be released from the telomere. Thus,
downregulation of either TRF1 or TIN2 results in
telomerase-dependent telomere lengthening  and
down-regulation of PTOP/PIP1/TINT1 has a similar
Although PTOP/PIP1/TINT1 recruits POT1 to the
TRF1 complex, the effects of modulating POT1 levels
are less predictable than for the other members of this
complex. Reducing the POT1 level by RNA interfer-
ence (RNAi) was found to cause telomere lengthening
. In other cellular contexts, however, elevated POT1
can cause telomerase-dependent telomere lengthen-
ing [1,8], as can a POT1–TERT fusion protein . Fur-
thermore, expression of the PTOP-binding domain of
POT1, or of a mutant version of POT1 that lacks the
DNA interacting OB fold, can also result in telomere
lengthening, perhaps by a dominant-negative effect
[1,10]. These seemingly contradictory results may
possibly be explained by as yet unknown interactions
of POT1 and/or by the dual role it plays as both a
member of the TRF1 complex and a single-stranded
telomeric DNA binding protein.
In addition to interacting with the TRF1 complex,
there is now evidence that PTOP/PIP1/TINT1 interacts
with TRF2 . This is perhaps not surprising, in view of
the accumulating evidence that TIN2 interacts directly
with both TRF1 and TRF2 [4,5]. TRF2 seems to be
important for the formation of telomere loops (t-loops)
, and overexpression of dominant-negative forms
of TRF2 results in dramatic telomere ‘uncapping’ .
Dominant-negative forms of TIN2 can extend telomere
length, like dominant-negative TRF1 , or they can
induce telomere uncapping, like dominant-negative
Houghtaling et al.  have suggested that the
TRF1 complex affects length regulation under normal
conditions, but when the ability of PTOP/PIP1/TINT1
to bind TRF1 is disrupted, the complex containing
TIN2 and PTOP/PIP1/TINT1 can bind TRF2 instead,
eventually stabilizing telomeres at a new length set
point. In this view, TRF2 complexed with TIN2 and
barrier to telomerase access. The interactions with
TRF1 and TRF2 may also occur simultaneously,
because a recent study  detected TRF1, TIN2,
PTOP/PIP1/TINT1 and POT1 all in association with
TRF2 and its binding partner hRAP1.
Why is there such a large crowd of players at the
telomere? The answer most likely lies in the complex-
ity of the functions that they perform. Telomeres need
to be protected from exonucleolytic attack and from
being recognised as double strand breaks, which may
result in repair by end-to-end fusion. This protection
may be achieved by formation of the t-loop structure,
which requires controlled processing of the terminus
to form a single-stranded DNA overhang. The area
where the terminus is inserted to form the loop resem-
bles a Holliday junction, resolution of which usually
needs to be suppressed.
The inserted overhang may be capable of priming
DNA replication, and this also needs to be sup-
pressed. The loop structure must open up to permit
DNA replication at the appropriate point of the cell
cycle, and in telomerase-positive cells telomerase
requires access in a controlled manner. Furthermore,
provide a secondary
Current Biology, Vol. 14, R901–R902, October 26, 2004, ©2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cub.2004.09.075
Cancer Research Unit, Children's Medical Research Institute,
214 Hawkesbury Road, Westmead, Sydney, NSW 2145,
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the telomere needs to interact with other nuclear Download full-text
structures. These include the telomere of its sister
chromatid after DNA replication, and recent evidence
indicates that there is a special mechanism for resolu-
tion of this interaction . Consideration of these
functions suggests that many more complexities
remain to be discovered regarding telomeric protein
team members and their interplay.
1. Liu, D., Safari, A., O'Connor, M. S., Chan, D.W., Laegeler, A., Qin, J.,
and Songyang, Z. (2004). PTOP interacts with POT1 and regulates
its localization to telomeres. Nat. Cell Biol. 6, 673-680.
2.Ye, J. Z., Hockemeyer, D., Krutchinsky, A.N., Loayza, D., Hooper,
S.M., Chait, B.T., and de Lange, T. (2004). POT1-interacting protein
PIP1: a telomere length regulator that recruits POT1 to the
TIN2/TRF1 complex. Genes Dev. 18, 1649-1654.
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TRF1 and TRF2 simultaneously and stabilizes the TRF2 complex on
telomeres. J. Biol. Chem. in press.
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(2003). Human POT1 facilitates telomere elongation by telomerase.
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9.Armbruster, B.N., Linardic, C. M., Veldman, T., Bansal, N.P.,
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Figure 1. The telomere team.
(A) Telomeres can form a protective t-
loop structure. Proteins that bind specifi-
cally to telomeric DNA include TRF1,
TRF2 and POT1. In addition, there is an
ever growing list of proteins that bind to
or modify these core components.
(B) One model is that PTOP/PIP1/TINT1
bridges the gap between TRF1 or TRF2,
TIN2 and POT1, possibly creating a
stable telomere structure that resists
lengthening by telomerase.