*Corresponding author. Keke Huo, Tel: 86-021-55664526; Fax:
86-021-55664526; E-mail: firstname.lastname@example.org, kkhuo2002@163.
com, Mingliang Ye, Tel: 86-411-84379620; Fax: 86-411-84379620;
#The first two authors contribute equally to this article.
Received 19 May 2009 , Accepted 26 August 2009
Keywords: CK2, Mass spectrometry, MSK1, Phosphorylation, Yeast
Casein Kinase 2 interacts with human mitogen- and
stress-activated protein kinase MSK1 and phosphorylates it at
Yan Shi1,#, Guanghui Han2,#, Huiling Wu1, Kan Ye1, Zhipeng Tian1, Jiaqi Wang1, Huili Shi1, Mingliang Ye2,*, Hanfa Zou2
& Keke Huo1,*
1State Key Laboratory of Genetic Engineering, Institute of Genetics, School of Life Sciences, Fudan University, 220 Handan Rd, Shanghai
200433, 2CAS Key Laboratory of Separation Sciences for Analytical Chemistry, National Chromatographic R&A Center, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Mitogen- and stress-activated protein kinase (MSK1) palys a
crucial role in the regulation of transcription downstream of
extracellular-signal-regulated kinase1/2 (ERK1/2) and mi-
togen-activated protein kinase p38. MSK1 can be phosphory-
lated and activated in cells by both ERK1/2 and p38α α. In this
study, Casein Kinase 2 (CK2) was identified as a binding and
regulatory partner for MSK1. Using the yeast two-hybrid sys-
tem, MSK1 was found to interact with the CK2β β regulatory
subunit of CK2. Interactions between MSK1 and the CK2α α cat-
alytic subunit and CK2β β subunit were demonstrated in vitro
and in vivo. We further found that CK2α α can only interact with
the C-terminal kinase domain of MSK1. Using site-directed
mutagenesis assay and mass spectrometry, we identified five
sites in the MSK1 C-terminus that could be phosphorylated by
CK2 in vitro: Ser757, Ser758, Ser759, Ser760 and Thr793. Of
these, Ser757, Ser759, Ser760 and Thr793 were previously
unknown. [BMB reports 2009; 42(12): 840-845]
Mitogen- and stress-activated protein kinase (MSK1) is a down-
stream kinase of mitogen-activated protein kinases (MAPKs).
MAPK pathways converge on extracellular signal-regulated
kinases (ERK, including ERK1 and ERK2 isoforms), c-Jun
NH2-terminal kinases (JNK, including JNK1, JNK2 and JNK3
isoforms), and p38 MAPKs (including p38α, p38β, p38γ and
p38δ isoforms) induce a variety of cellular functions such as
gene expression, mitosis, and apoptosis through the phosphor-
ylation of specific serine and/or threonine residues of target
proteins (1). MSK1 is most closely related to the p90 ribosomal
S6 kinase (RSK) family kinases, and like RSK, MSK1 contains
two distinct kinase domains within a single polypeptide (2-4).
The N-terminal domain is a member of the protein kinase
A/protein kinase G/protein kinase C/family kinases (AGC-type
kinases), while the C-terminal kinase domain is related to the
calmodulin-dependent protein kinase family (5). The molec-
ular mechanism of MSK1 activation is compelx. It requires the
phosphorylation of MSK1 at Ser360 and Thr581 by either
ERK1/2 or p38α (2-4, 6). These events activate the C-terminal
kinase domain of MSK1, which then autophosphorylates at
Ser212, Ser376, and Ser381, resulting in the activation of the
N-terminal kinase domain. Then Ser750, Ser752 and Ser758
were phosphorylated by the N-terminal kinase domain (7). In
addition to these eight phosphorylation sites, five novel sites,
Thr630, Ser647, Ser657, Ser695 and Thr700 have been re-
cently identified (6). Among these five sites, Thr700 was found
to be the third site phosphorylated by ERK1/2 or p38α α, per-
haps playing a key role in the activation of MSK1. Another
four sites can be phosphorylated by unknown kinases.
Although thirteen phosphorylation sites have been identified
in MSK1, the activation mechanism of it is unclear.
Activated MSK1 is required for the phosphorylation of sev-
eral factors in response to mitogenic and cellular stress, includ-
ing cAMP-response-element-binding protein (CREB), activating
transcription factor 1 (ATF1) (8, 9), the chromatin protein his-
tone H3, nuclear factor kappa B (NF-κB) (10) and high-mobi-
lity group protein 14 (HMG-14) (11, 12).
To further understand the phosphorylation regulation of
MSK1, it will be important to identify and characterize its in-
teracting kinases. We performed a yeast two-hybrid screen in
our current study to detect. These experiments identified multi-
ple isotypes of CK2β as direct MSK1 interacting partners.
Casein Kinase 2 (CK2) is one of the most conserved Ser/Thr
kinases, playing a key role in controlling gene expression, cell
growth and proliferation. It is a tetrameric enzyme composed
CK2 phosphorylates MSK1
Yan Shi, et al.
Fig. 1. MSK1 interacts with CK2. (A) Yeast two-hybrid interaction between MSK1 and CK2β. Yeast cells were co-transformed with pPC86-
CK2β/pDBLeu and pPC86-CK2β/pDBLeu-MSK1. β-galactosidase assays were used to detect interaction. pPC86-CK2β/pDBLeu showed no evident
interaction, pPC86-CK2β/pDBLeu-MSK1 indicated positive interaction. A, B, C, D, E are yeast controls with varying degrees of protein-protein
interaction, E indicates the strongest interaction. (B) In vitro GST pull-down assay. GST and GST-MSK1 were expressed in bacteria and im-
munoblotted with anti-GST antibody. Cell lysate from mammalian cells expressing Myc-CK2α or Myc-CK2β was incubated with gluta-
thione beads purified with GST or GST-MSK1 fusion proteins. The bound proteins were eluted, subjected to SDS-PAGE analysis and im-
munoblotted with anti-Myc antibody. (C) In vivo co-immunoprecipitation assay. Myc-CK2α or Myc-CK2β was co-transfected with Flag-MSK1
into HEK293T cells. Expression of proteins in co-transfected cells was analyzed by Western blotting with the relevant antibodies. The lysate
was immunoprecipitated with anti-Myc antibody and analyzed by immunoblotting with anti-Flag antibody.
of two catalytic (CK2α and/or CK2α') subunits and two regu-
latory (CK2β) subunits (13-16). CK2 can phosphorylate more
than 300 proteins, including a striking number of signaling
proteins. Here, we identified another five sites in the MSK1
C-terminus that could be phosphorylated by CK2 in vitro:
Ser757, Ser758, Ser759, Ser760 and Thr793. Of these, Ser757,
Ser759, Ser760 and Thr793 were previously unknown.
Identification of MSK1/CK2β β interaction in a yeast two-hy-
To identify proteins that interact with MSK1, we screened a
human liver cDNA library in a yeast two-hybrid system using
MSK1 as bait. Screening over 2 × 106 clones yielded 25
candidates. Sequencing indicated that 12 of the 25 encoded
the C-terminal portion of human CK2β (amino acids 74-216).
To confirm the interaction between MSK1 and CK2β,
full-length CK2β was cloned into the pPC86 vector, and co-
transfected into Mav203 with pDBLeu-MSK1 or pDBLeu.
Coexpression of MSK1 and CK2β showed activation of the re-
porter gene, but the negative control did not (Fig. 1A).
Identification of MSK1/CK2 interactions in vitro and in vivo
To confirm the physical interaction between MSK1 and CK2,
GST pull-down assays were employed. Bacterially expressed
GST-MSK1 efficiently pulled down Myc-CK2α and Myc-CK2β,
but the control GST did not (Fig. 1B). This indicated that MSK1
could bind with the two subunits of CK2 in vitro.
To determine whether MSK1 interacted with CK2 in mam-
malian cells, in vivo binding assays were performed. Myc-CK2α
or Myc-CK2β was transiently co-transfected with Flag-MSK1 in-
to HEK293T cells. As shown in Fig. 1C, Flag-MSK1 was de-
tected in Myc-CK2α and Myc-CK2β co-precipitated complex,
but not in the control. This indicated that the two subunits of
CK2 interacted with MSK1 directly in vivo.
Domains involved in the MSK1/CK2 interaction
To further understand the association between MSK1 and CK2,
we constructed two truncated mutants of MSK1, MSK1-NTK
(amino acids 1-395) and MSK1-CTK (amino acids 382-802) ac-
cording to its kinase domain (Fig. 2A). Co-immunoprecipit-
ation assay was used to validate the interaction between
MSK1-NTK and CK2. Flag-MSK1-NTK could only be detected
in the co-precipitated complex co-transfected with Myc-CK2β,
but not with Myc-CK2α (Fig. 2B). This indicates that CK2β
could interact with the N-terminal of MSK1 directly. Because
Flag-MSK1-CTK can not expressed in HEK293T cells, GST
pull-down assay was used to validate the interaction between
MSK1-CTK and the two subunits. pET32a-MSK1-CTK ex-
pressed in bacterial could easily pull Myc-CK2α and
Myc-CK2β down. The pET32a itself could not (Fig. 2C). This
result indicates that the two subunits of CK2 both can interact
with the C-terminal of MSK1. Taken together, CK2α only inter-
act with the C-terminal kinase domain of MSK1, but CK2β can
interact both with the N-terminal and C-terminal kinase do-
main of MSK1.
CK2 specifically phosphorylates MSK1-CTK in vitro
CK2 has been reported to phosphorylate a large variety of sub-
strates, so we investigated whether MSK1 could be a substrate.
MSK1 purified from bacterial systems autophosphorylates (6),
so it can not be used as to test phosphorylation by CK2.
GST-MSK1-T2 and pET32a-MSK1-CTK fusion proteins were
tested as CK2 substrates. GST-MSK1-T2 was unphosphory-
lated, had no autophosphorylation ability and was not phos-
phorylated in the presence of CK2 (Fig. 3A), while β-casein,
the positive control, was distinctly phosphorylated. In contrast,
MSK1-CTK was readily phosphorylated in the presence of CK2
(Fig. 3B), while the control showed little phosphorylation sig-
CK2 phosphorylates MSK1
Yan Shi, et al.
Fig. 3. MSK1 can be phosphorylated by CK2 in vitro, and the sites indentified on MSK1 can be phosphorylated by CK2. (A)
GST-MSK1-T2 recombinant protein was incubated with or without commercial CK2. Reactions were run on SDS-PAGE and visualized with
Pro-Q Diamond Phosphoprotein Gel Stain to see phosphorylated protein, and with coomassie blue stain to see total protein. CK2 hol-
oenzyme can be autophosphorylated (lanes 4, 5) and phosphorylates β-casein (2 mg/ml, last lane). But CK2 could not phosphorylate
GST-MSK1-T2. (B) Recombinant protein pET32a-MSK1-CTK was used as the substrate of CK2 purified as described in Materials and
Methods for in vitro kinase assays. (C) In vitro kinase assays were carried out in which recombinant protein pET32a-MSK1-Ser-757A and -
Ser758A were used as substrates for CK2 purified as described in Materials and Methods. (D) pET32a-Ser759A, -Ser760A and -Thr793A re-
combinant proteins were used as substrates. Equal quantities of the five proteins and MSK1-CTK were used in the assays.
Fig. 2. Identification of domains of MSK1 required for the interaction with CK2. (A) Schematic diagram of the structure of the truncated
mutant forms of MSK1. MSK1-T2, MSK1-NTK and MSK1-CTK are indicated. (B) CK2α and CK2β both can interact with MSK1-CTK. pET32a
and pET32a-MSK1-CTK recombinant proteins were expressed in E. coli BL21(DE)3 and purified with Ni2+-NTA agarose. Cell lysate ex-
pressed Myc-CK2α or myc-CK2β in mammalian cells were incubated with the agarose. The agarose were eluted and detected with an-
ti-myc antibody. (C) CK2β can interact with MSK1-NTK in vivo, but CK2α can not. Myc-CK2α and Myc-CK2β were co-transfected with
Flag-MSK1-NTK into HEK293T cells respectively. Lysates were immunoprecitated with anti-myc antibody and analyzed by immunoblotting
with anti-flag antibody.
nal in the absence of CK2. GST-MSK1 which had been auto-
phosphorylated was used as a positive control. These results
indicated that MSK1 could be phosphorylated by CK2 in vitro,
with the potential phosphorylation site in the MSK1 C-
CK2 phosphorylates MSK1
Yan Shi, et al.
Table 1. MSK1 phosphorylation sites identified for CK2 by mass
spectrometry. Three samples were prepared for MS/MS analysis.
Sample 1 was the purified recombinant proteins pET-32a-MSK1-CTK.
Sample 2 was autophosphorylation sample. Sample 3 was CK2 kinase
assay. The right column of the table was the corresponding phosphor-
ylation sites located by MS
Sample 1Sample 2Sample 3
T, digestion by trypsin; G, digestion by Glu-C; P, digestion by pepsin;
K, digestion by proteinase K; ✓, indicates the phosphoryltion site
identifed in this work.
Analysis of MSK1 phosphorylation sites by mass spectrometry
and in vitro kinase assays
To further analyze the CK2 phosphorylation sites in MSK1,
three samples described in methods were analyzed by MS.
The confidence of phosphopeptide identifications was set at a
false discovery rate (FDR) ＜2%. The identified phosphopep-
tides and locations of MSK1 phosphorylation sites are in Table
1. These results showed five additional phosphorylation sites
(Ser757, Ser758, Ser759, Ser760, Thr793) in sample 3, in addi-
tion to the sites in samples 1 and 2. These data suggested that
the five additional sites in MSK1 were phosphorylated by CK2.
The phosphorylated sites identified by MS were validated by
in vitro kinase assays. Five constructs were generated, in
which Ser757, Ser758, Ser759, Ser760 and Thr793 were mu-
tated to alanine. In vitro kinase assays were carried out using
these five mutant recombinant proteins, all adjusted to the
same concentration, as CK2 substrates. Ser757A, Ser758A,
Ser759A, and Ser760A all showed weak phosphorylation com-
pared to the wild type protein (Fig. 3C, D), which suggested
that these four sites were phosphorylated by CK2. The phos-
phorylation signal of Thr793A was unchanged, possibly be-
cause it is weakly phosphorylated, and could be detected by
the highly sensitive MS, but was under the detection limit of
the Pro-Q assay. Hence, use of both MS and in vitro kinase as-
says validated the identification of phosphorylation sites.
In this work, specific in vitro and in vivo interactions between
MSK1 and CK2 were identified. MSK1 is a downstream kinase
of p38 MAPK and can be activated by p38α under cellular
stress (2-4, 6). CK2 also contributes to p38 MAPK signaling
pathway regulation. In a phosphorylation-dependent manner,
p38α can directly interact with the α and β subunits of CK2 to
activate the holoenzyme (17). p38 and CK2 both co-im-
munoprecipitate with p53 (18, 19). Anisomycin and tumor ne-
crosis factor-α (TNF-α)-induced phosphorylation of p53 at
Ser392 requires p38 MAPK kinase and CK2 activities (17). In
additional, CK2 can phosphorylate IκBα at a cluster of C-termi-
nal sites through a mechanism that depends on the ultra-
violet-induced activation of p38 MAPK kinase (20).
CK2 is one of the most highly conserved Ser/Thr kinases,
and participates in many signal pathways, phosphorylating nu-
merous substrates. To determine if MSK1 is a substrate for
CK2, in vitro kinase assays were carried out using a series of
recombinant MSK1 proteins. MSK1 could be phosphorylated
by CK2 in its C-terminal kinase domain. CK2α interacted only
with the C-terminal kinase domain of MSK1, consistent with
the location of the phosphorylated sequences.
Five sites (Ser757, Ser758, Ser759, Ser760 and Thr793) in
the C-terminus of MSK1 were identified as CK2-phosphory-
lated sites by in vitro kinase assays and MS. Among these sites,
Ser758 has been reported as an autophosphorylated site in vi-
tro (7). Further research is necessary to determine if this site is
phosphorylated by CK2 or is autophosphorylated in vivo. All
five sites are located in the MSK1 C-terminus, close to Thr700,
which is conserved in MAPK-integrating kinase 1 and 2
(MNK1 and MNK2) (6). According to a recent activation model
of MSK1, phosphorylation of Thr700 relieves inhibition of
MSK1 by a C-terminal autoinhibitory helix, and helps induce a
conformational shift that protects Thr581 from dephosphor-
ylation (6). The five sites located C-terminal to Thr700 have no
equivalent sites in MSK2, RSK1, MAPKAPK2 and MNK, so
they are specific to MSK1. We hypothesize that they play a
similar role as Thr700. Under cellular stress, activated p38α
could phosphorylate MSK1 at Ser360, Thr581 and Thr700.
Simultaneously, the CK2 holoenzyme could be activated and
phosphorylates MSK1 at these five sites. Phosphorylation of
Thr700 relieve the inhibition of MSK1. These five phosphory-
lated sites phosphorylated by CK2 may help Thr700 induce
the shift of the C-terminal peptide of MSK1, contributing to ac-
tivation by providing a more stable MSK1 activated structure.
In conclusion, we have identified an interaction between
MSK1 and CK2, and located five potential CK2 phosphor-
ylation sites within the MSK1 C-terminal domain. Among
these, Ser757, Ser759, Ser760 and Thr793 are novel.
According to their spatial position, we hypothesize that these
five phosphorylation sites may help MSK1 maintain a stable
structure for subsequent activation. CK2 participates in many
signal pathways, phosphorylating numerous substrates result-
ing in regulating of gene activity, localization and protein-pro-
tein interaction. Whether this phosphorylation of MSK1 by
CK2 influence the processes such as MSK1 localization or ac-
tivity need further research. Nevertheless, our results found a
CK2 phosphorylates MSK1
Yan Shi, et al.
new regulative kinase CK2 for MSK1. And we proposed an ac-
tivation model for phosphorylation modification of MSK1.
MATERIALS AND METHODS
For protein interaction assays in yeast, MSK1 was inserted into
vector pDBLeu (GIBCO) and fused to Gal4 DNA binding domain
(BD). CK2β was cloned into pPC86 (GIBCO) vector and fused
to Gal4 activation domain (AD). For in vitro binding analysis,
MSK1 was cloned into a GST-tag vector pGEX-6p-1 (Amersham
Biosciences, Uppsala, Sweden). MSK1 deletion mutant MSK1-
CTK (amino acids 382-802) was inserted into a 6 × His-tag vector
pET32a (Novagen, Madison, WI, USA). For co-immunoprecipi-
tation assays, MSK1 and deletion mutant MSK1-NTK (amino
acids 1-395) were cloned into Flag-tag vectors. For binding as-
says, CK2α and CK2β were cloned into Myc-tag vectors. For
kinase assays, MSK1 transcript variant 2 (MSK1-T2, amino acids
1-505) was cloned into pGEX-6p-1. MSK-CTK and its muta-
genesis were cloned into pET32a. Mutagenesis of MSK-CTK was
performed using the Quik ChangeⓇ Site-Directed Mutagenesis
Kit (Stratagene, La Jolla, CA, USA).
Yeast two-hybrid assays
The yeast two-hybrid screen and co-transformation assay were
performed by GIBCO yeast two-hybrid screen manufacturer’s
protocol. Briefly, the autoactivation of the lacZ reporter gene
by pDBLeu-MSK1 was tested in Mav203 cells. Then the stable
BD-MSK1-transformed yeast cells were transformed with a hu-
man liver cDNA library (Invitrogen, Grand Island, NY, USA)
constructed in the pPC86 plasmid. More than 2 × 106 cDNA
colonies were screened on plates containing 25 mM 3AT
(Sigma, St Louis, MO, USA) and lacking histidine, leucine and
tryptophan. Positive clones were verified by β-galactosidase
assay. Prey plasmids were isolated from His+/Leu+/LacZ+
colonies and re-transformed into yeast along with either
pDBLeu-MSK1 or pDBLeu to verify specific interactions.
In vitro GST pull-down assays
GST fusion proteins were expressed in 0.1 mM IPTG-induced
E. coli strain BL21 and purified with glutathione-Sepharose 4B
beads (Invitrogen). The beads with bound proteins were in-
cubated with lysate from HEK293T cells expressing Myc-CK2α
or Myc-CK2β at 4oC for 4 h with gentle rotation. Beads were
washed three times with cell lysis buffer to disrupt non-specific
interactions, and bound proteins were separated by SDS-
PAGE. Mouse monoclonal antibody against GST was from
Sigma. Rabbit anti-mouse horseradish peroxidase second anti-
body was from Rockland (Philadelphia PA, USA).
Cell culture and co-immunoprecipitation assays
HEK293T cells were maintained in DMEM supplemented with
10% bovine calf serum, grown on 60 mm dishes at a concentration
of 6 × 105 cell/dish before the day of transfection. The relevant
plasmids were transfected with Lipofectamine (Invitrogen) into
HEK293T cells. After 48 h, cells were lysed in 400 μl lysis buffer
(Cell Signaling, Beverly, MA). Lysate was pretreated with protein
A/G agarose (Santa Cruz, CA, USA) and immunoprecipitated with
1-2 μg of relevant antibody and protein A/G agarose at 4oC
overnight. After washing three times with lysis buffer, the precip-
itates were analyzed by Western blot. Mouse monoclonal anti-
bodies against c-Myc, Flag M2, and 6 × His were from Sigma.
In vitro kinase assays
pET32a-CK2β, pET32a -MSK1-CTK and their mutated variants
were expressed in E. coli BL21 (DE3). Recombinant proteins
were purified with Ni2+-NTA agarose and eluted with elution
buffer (50 mM sodium phosphate, 300 mM NaCl, 250 mM
imidazole pH 8.0). GST-MSK1 and GST-CK2α were expressed
in E. coli BL21. Recombinant proteins were purified with
Sepharose 4B beads and eluted with glutathione solution (50
mM Tris-HCl, 20 mM reduced glutathione, pH 8.0). Approxi-
mately 1-2 μg recombinant protein was incubated at 30oC for
30 min in 20 μl kinase buffer with 1 mM ATP and either 1 μl
CK2 holoenzyme (New England Biolabs, Herts, United
Kingdom), or a mixture of 1 μg GST-CK2α and 2 μg pET32a-
CK2β purified as described above, that had CK2 holoenzyme
activity (Supporting Information 1). β-casein (Sigma, St. Louis,
MO, USA) was a special substrate for CK2, used as the positive
control. The reaction was run on SDS-PAGE followed by
PrO-Q Diamond Phosphoprotein Gel Stain (Invitrogen)
detection. For mass spectrometry, three samples were pre-
pared: 1) 1 mg purified recombinant protein MSK1-CTK; 2) 1
mg recombinant protein in 1 ml kinase buffer with 1 mM ATP;
3) 0.8 mg recombinant protein in 600 μl kinase buffer with 1
mM ATP and 15 μl CK2 holoenzyme (New England Biolabs).
The second and third reactions were carried out at 30oC for
30 min and all samples were denaturated with 8 M urea.
Identification of phosphorylation sites in MSK1 by mass
Mass spectrometry (MS) was used to localize phosphorylation
sites, using an MS2/MS3 target-decoy database search strategy
with multi-protease digestion (21). Briefly, the six steps in this ap-
proach were: 1) separate digestions of the phosphoprotein sam-
ple with multiple proteases; 2) enrichment of phosphopeptides
by Ti4+-IMAC (22, 23) from the individual peptide mixtures; 3)
analysis of enriched phosphopeptides with LC-MS2- MS3; 4) sub-
mission of acquired MS2 and MS3 spectra to an MS2/MS3 tar-
get-decoy search using a composite database that included
MSK1 and two subunits of CK2 as target sequences, and a re-
versed yeast database with 1,000 entries as the decoy database;
5) filtering the candidate phosphopeptides with parameters
(Rank’m, ΔCn’m and Xcorr’s) designed to achieve phosphopep-
tide identification at a specific false discovery rate (FDR); 6) deter-
mination of phosphorylation sites by Tscore as described by Jiang
et al. (24). Analysis was performed on a nano-RPLC-MS/MS sys-
tem using a LTQ linear ion trap mass spectrometer (Thermo
CK2 phosphorylates MSK1
Yan Shi, et al.
Finnigan). MS data were searched against database using
SEQUEST (version 0.27) in Bioworks (version 3.2).
We thank Dr. Jiahuai Han for pcDNA3-HA-RLPK plasmids,
and thank Dr. Jinsheng Dong and Dr. Guojun Hwang for
proofreading the manuscript and active discussion. This work
was supported by the China High Technology Research
Program Grant (2006AA02A310, 2006AA02A309, 2004BA
711A19); and the National Natural Sciences Foundation of
China (20735004, 90713017).
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