p38α- and DYRK1A-dependent phosphorylation of caspase-9 at an inhibitory site in response to hyperosmotic stress

The cysteine aspartyl protease caspase-9 is a critical component of the intrinsic apoptotic pathway. Activation of caspase-9 is inhibited by phosphorylation at Thr125, which is catalysed by the mitogen-activated protein kinases (MAPKs) ERK1/2 in response to growth factors, by the cyclin-dependent protein kinase CDK1-cyclin B1 during mitosis, and at a basal level by the dual-specificity tyrosine-phosphorylation regulated protein kinase DYRK1A. Here we show that inhibitory phosphorylation of caspase-9 at Thr125 is induced in mammalian cells by hyperosmotic stress. This response does not require ERK1/2 or ERK5, but it is diminished by ablation of DYRK1A expression by siRNA or chemical inhibition of DYRK1A by harmine. Phosphorylation of Thr125 in response to hyperosmotic stress is also reduced by chemical inhibition of p38 MAPK and is abolished in p38α^−/− mouse embryonic fibroblasts. These results show that both DYRK1A and p38α play roles in the inhibitory phosphorylation of caspase-9 following hyperosmotic stress and suggest a functional interaction between these protein kinases. Phosphorylation of caspase-9 at Thr125 may restrain apoptosis during the acute response to hyperosmotic stress.     

Phosphorylation of Mcl‐1 by CDK1–cyclin B1 initiates its Cdc20‐dependent destruction during mitotic arrest

The balance between cell cycle progression and apoptosis is important for both surveillance against genomic defects and responses to drugs that arrest the cell cycle. In this report, we show that the level of the human anti‐apoptotic protein Mcl‐1 is regulated during the cell cycle and peaks at mitosis. Mcl‐1 is phosphorylated at two sites in mitosis, Ser64 and Thr92. Phosphorylation of Thr92 by cyclin‐dependent kinase 1 (CDK1)–cyclin B1 initiates degradation of Mcl‐1 in cells arrested in mitosis by microtubule poisons. Mcl‐1 destruction during mitotic arrest requires proteasome activity and is dependent on Cdc20/Fizzy, which mediates recognition of mitotic substrates by the anaphase‐promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. Stabilisation of Mcl‐1 during mitotic arrest by mutation of either Thr92 or a D‐box destruction motif inhibits the induction of apoptosis by microtubule poisons. Thus, phosphorylation of Mcl‐1 by CDK1–cyclin B1 and its APC/C^Cdc20‐mediated destruction initiates apoptosis if a cell fails to resolve mitosis. Regulation of apoptosis, therefore, is linked intrinsically to progression through mitosis and is governed by a temporal mechanism that distinguishes between normal mitosis and prolonged mitotic arrest.     

Inhibition of caspase-9 through phosphorylation at Thr 125 by ERK MAPK

Many pro-apoptotic signals activate caspase-9, an initiator protease that activates caspase-3 and downstream caspases to initiate cellular destruction. However, survival signals can impinge on this pathway and suppress apoptosis. Activation of the Ras-Raf-MEK-ERK mitogen-activated protein kinase (MAPK) pathway is associated with protection of cells from apoptosis and inhibition of caspase-3 activation, although the targets are unknown. Here, we show that the ERK MAPK pathway inhibits caspase-9 activity by direct phosphorylation. In mammalian cell extracts, cytochrome c-induced activation of caspases-9 and -3 requires okadaic-acid-sensitive protein phosphatase activity. The opposing protein kinase activity is overcome by treatment with the broad-specificity kinase inhibitor staurosporine or with inhibitors of MEK1/2. Caspase-9 is phosphorylated at Thr 125, a conserved MAPK consensus site targeted by ERK2 in vitro, in a MEK-dependent manner in cells stimulated with epidermal growth factor (EGF) or 12-O-tetradecanoylphorbol-13-acetate (TPA). Phosphorylation at Thr 125 is sufficient to block caspase-9 processing and subsequent caspase-3 activation. We suggest that phosphorylation and inhibition of caspase-9 by ERK promotes cell survival during development and tissue homeostasis. This mechanism may also contribute to tumorigenesis when the ERK MAPK pathway is constitutively activated.     

Cleavage of claspin by caspase-7 during apoptosis inhibits the Chk1 pathway

Claspin is required for the phosphorylation and activation of the Chk1 protein kinase by ATR during DNA replication and in response to DNA damage. This checkpoint pathway plays a critical role in the resistance of cells to genotoxic stress. Here, we show that human Claspin is cleaved by caspase-7 during the initiation of apoptosis. In cells, induction of DNA damage by etoposide at first produced rapid phosphorylation of Chk1 at a site targeted by ATR. Subsequently, etoposide caused activation of caspase-7, cleavage of Claspin, and dephosphorylation of Chk1. In apoptotic cell extracts, Claspin was cleaved by caspase-7 at a single aspartate residue into a large N-terminal fragment and a smaller C-terminal fragment that contain different functional domains. The large N-terminal fragment was heavily phosphorylated in a human cell-free system in response to double-stranded DNA oligonucleotides, and this fragment retained Chk1 binding activity. In contrast, the smaller C-terminal fragment did not bind Chk1, but did associate with DNA and inhibited the DNA-dependent phosphorylation of Chk1 associated with its activation. These results indicate that cleavage of Claspin by caspase-7 inactivates the Chk1 signaling pathway. This mechanism may regulate the balance between cell cycle arrest and induction of apoptosis during the response to genotoxic stress.     

Identification of PP1alpha as a caspase-9 regulator in IL-2 deprivation-induced apoptosis

One of the mechanisms that regulate cell death is the reversible phosphorylation of proteins. ERK/MAPK phosphorylates caspase-9 at Thr(125), and this phosphorylation is crucial for caspase-9 inhibition. Until now, the phosphatase responsible for Thr(125) dephosphorylation has not been described. Here, we demonstrate that in IL-2-proliferating cells, phosphorylated serine/threonine phosphatase type 1alpha (PP1alpha) associates with phosphorylated caspase-9. IL-2 deprivation induces PP1alpha dephosphorylation, which leads to its activation and, as a consequence, dephosphorylation and activation of caspase-9 and subsequent dissociation of both molecules. In cell-free systems supplemented with ATP caspase-9 activation is induced by addition of cytochrome c and we show that in this process PP1alpha is indispensable for triggering caspase-9 as well as caspase-3 cleavage and activation. Moreover, PP1alpha associates with caspase-9 in vitro and in vivo, suggesting that it is the phosphatase responsible for caspase-9 dephosphorylation and activation. Finally, we describe two novel phosphatase-binding sites different from the previously described PP1alpha consensus motifs, and we demonstrate that these novel sites mediate the interaction of PP1alpha with caspase-9.     

A role for the phosphorylation of hRad9 in checkpoint signaling

The integrity of the human genome is preserved by signal transduction pathways called checkpoints, which delay progression through the cell cycle when DNA damage is present. Three checkpoint proteins, hRad9, hRad1, and hHus1, form a proliferating cell nuclear antigen-like, heterotrimeric complex that has been proposed to function in the initial detection of DNA structural abnormalities. hRad9 is highly modified by phosphorylation, in a constitutive manner and in response to both DNA damage and cell cycle position. Here we present evidence that Thr292 of hRad9 is subject to Cdc2-dependent phosphorylation in mitosis. Furthermore, our data are also consistent with four other hRad9 phosphorylation sites (Ser277, Ser328, Ser336, and Thr355) being regulated in part by Cdc2. We also identify Ser387 as a novel site of hRad9 constitutive phosphorylation and show that phosphorylation at Ser387 is a prerequisite for one form of DNA damage-induced hyperphosphorylation of hRad9. Characterization of nonphosphorylatable mutants has revealed that hRad9 phosphorylation plays a critical role in checkpoint signaling. Overexpression of these mutants blocks the interaction between hRad9 and the DNA damage-responsive protein TopBP1 and impairs the cellular response to DNA damage during S phase.     

Phosphorylation by Aurora B kinase regulates caspase-2 activity and function

Mitotic catastrophe (MC) is an important oncosuppressive mechanism that serves to eliminate cells that become polyploid or aneuploid due to aberrant mitosis. Previous studies have demonstrated that the activation and catalytic function of caspase-2 are key steps in MC to trigger apoptosis and/or cell cycle arrest of mitotically defective cells. However, the molecular mechanisms that regulate caspase-2 activation and its function are unclear. Here, we identify six new phosphorylation sites in caspase-2 and show that a key mitotic kinase, Aurora B kinase (AURKB), phosphorylates caspase-2 at the highly conserved residue S384. We demonstrate that phosphorylation at S384 blocks caspase-2 catalytic activity and apoptosis function in response to mitotic insults, without affecting caspase-2 dimerisation. Moreover, molecular modelling suggests that phosphorylation at S384 may affect substrate binding by caspase-2. We propose that caspase-2 S384 phosphorylation by AURKB is a key mechanism that controls caspase-2 activation during mitosis.Subject terms: Proteases, Proteolysis     

Mitotic reorganization of the intermediate filament protein nestin involves phosphorylation by cdc2 kinase

The intermediate filament protein nestin is expressed during early stages of development in the central nervous system and in muscle tissues. Nestin expression is associated with morphologically dynamic cells, such as dividing and migrating cells. However, little is known about regulation of nestin during these cellular processes. We have characterized the phosphorylation-based regulation of nestin during different stages of the cell cycle in a neuronal progenitor cell line, ST15A. Confocal microscopy of nestin organization and (32)P in vivo labeling studies show that the mitotic reorganization of nestin is accompanied by elevated phosphorylation of nestin. The phosphorylation-induced alterations in nestin organization during mitosis in ST15A cells are associated with partial disassembly of nestin filaments. Comparative in vitro and in vivo phosphorylation studies identified cdc2 as the primary mitotic kinase and Thr(316) as a cdc2-specific phosphorylation site on nestin. We generated a phosphospecific nestin antibody recognizing the phosphorylated form of this site. By using this antibody we observed that nestin shows constitutive phosphorylation at Thr(316), which is increased during mitosis. This study shows that nestin is reorganized during mitosis and that cdc2-mediated phosphorylation is an important regulator of nestin organization and dynamics during mitosis.     

Cell cycle-regulated phosphorylation of p21-activated kinase 1

Mammalian p21-activated kinase 1 (Pak1) is a highly conserved effector for the small GTPases Cdc42 and Rac1. In lower eukaryotes, Pak1 homologs are regulated during the cell cycle by phosphorylation. Here, we show that Pak1 is phosphorylated during mitosis in mammalian fibroblasts. This phosphorylation occurs at a single site, Thr 212, within a domain that is unique to Pak1. Cdc2 phosphorylates Pak1 at the identical site in vitro, and inhibition of Cdc2 abolishes Pak1 mitotic phosphorylation in vivo, indicating that Cdc2 is the kinase responsible for phosphorylating Pak1 in mitotic cells. Expression of a Pak1 mutant in which Thr 212 is replaced with a phosphomimic (aspartic acid) has marked effects on the rate and extent of postmitotic spreading of fibroblasts. The mitotic phosphorylation of Pak1 does not alter the basal or Rac-stimulated activity of this kinase, but it does affect the coimmunoprecipitation of at least three proteins with Pak1. These findings are the first to implicate a mammalian Pak in cell cycle regulation and suggest that Pakl, as a result of phosphorylation by Cdc2, alters its association with binding partners and/or substrates that are relevant to the morphologic changes associated with cell division.     

Phosphorylation of BAD at Ser-128 during mitosis and paclitaxel-induced apoptosis

Phosphorylation of BCL-2 family member BAD at different residues triggers different physiological effects, either inhibiting or promoting apoptosis. The recently identified phosphorylation site at Ser-128 enhances the apoptotic activity of BAD. We here show that BAD becomes phosphorylated at Ser-128 in the mitotic phase of the cell cycle in NIH3T3 cells. We also show that BAD-S128 is phosphorylated in taxol-treated mouse fibroblasts and MDA-MB-231 human breast cancer cells. However, expression of a phosphorylation-defective dominant negative BAD mutant did not block taxol-induced apoptosis. These data support the view that the phosphorylation of BAD Serine 128 exerts cell-specific effects on apoptosis. Whereas the BAD Serine 128 phosphorylation induces apoptosis in neuronal cells, it does not appear to promote apoptosis in proliferating non-neural cells during mitosis or upon exposure to the antineoplastic agent taxol.     

The docking interaction of caspase-9 with ERK2 provides a mechanism for the selective inhibitory phosphorylation of caspase-9 at threonine 125

Caspase-9 plays a critical role in the initiation of apoptosis by the mitochondrial pathway. Activation of caspase-9 is inhibited by phosphorylation at Thr(125) by ERK1/2 MAPKs in response to growth factors. Here, we show that phosphorylation of this site is specific for these classical MAPKs and is not strongly induced when JNK and p38alpha/beta MAPKs are activated by anisomycin. By deletion and mutagenic analysis, we identify domains in caspase-9 and ERK2 that mediate their interaction. Binding of ERK2 to caspase-9 and subsequent phosphorylation of caspase-9 requires a basic docking domain (D domain) in the N-terminal prodomain of the caspase. Mutational analysis of ERK2 reveals a (157)TTCD(160) motif required for recognition of caspase-9 that acts independently of the putative common docking domain. Molecular modeling supports the conclusion that Arg(10) in the D domain of caspase-9 interacts with Asp(160) in the TTCD motif of ERK2. Differences in the TTCD motif in other MAPK family members could account for the selective recognition of caspase-9 by ERK1/2. This selectivity may be important for the antiapoptotic role of classical MAPKs in contrast to the proapoptotic roles of stress-activated MAPKs.     

Myosin phosphatase is inactivated by caspase-3 cleavage and phosphorylation of myosin phosphatase targeting subunit 1 during apoptosis

In nonapoptotic cells, the phosphorylation level of myosin II is constantly maintained by myosin kinases and myosin phosphatase. During apoptosis, caspase-3–activated Rho-associated protein kinase I triggers hyperphosphorylation of myosin II, leading to membrane blebbing. Although inhibition of myosin phosphatase could also contribute to myosin II phosphorylation, little is known about the regulation of myosin phosphatase in apoptosis. In this study, we have demonstrated that, in apoptotic cells, the myosin-binding domain of myosin phosphatase targeting subunit 1 (MYPT1) is cleaved by caspase-3 at Asp-884, and the cleaved MYPT1 is strongly phosphorylated at Thr-696 and Thr-853, phosphorylation of which is known to inhibit myosin II binding. Expression of the caspase-3 cleaved form of MYPT1 that lacked the C-terminal end in HeLa cells caused the dissociation of MYPT1 from actin stress fibers. The dephosphorylation activity of myosin phosphatase immunoprecipitated from the apoptotic cells was lower than that from the nonapoptotic control cells. These results suggest that down-regulation of MYPT1 may play a role in promoting hyperphosphorylation of myosin II by inhibiting the dephosphorylation of myosin II during apoptosis.     

Proteasome inhibition-induced p38 MAPK/ERK signaling regulates autophagy and apoptosis through the dual phosphorylation of glycogen synthase kinase 3β

Proteasome inhibition is a promising approach for cancer treatment; however, the underlying mechanisms involved have not been fully elucidated. Here, we show that proteasome inhibition-induced p38 mitogen-activated protein kinase regulates autophagy and apoptosis by modulating the phosphorylation status of glycogen synthase kinase 3β (GSK3β) and 70kDa ribosomal S6 kinase (p70S6K). The treatment of MDA-MB-231 cells with MG132 induced endoplasmic reticulum stress through the induction of ATF6a, PERK phosphorylation, and CHOP, and apoptosis through the cleavage of Bax and procaspase-3. MG132 caused the phosphorylation of GSK3β at Ser(9) and, to a lesser extent, Thr(390), the dephosphorylation of p70S6K at Thr(389), and the phosphorylation of p70S6K at Thr(421) and Ser(424). The specific p38 inhibitor SB203080 reduced the p-GSK3β(Ser9) and autophagy through the phosphorylation of p70S6K(Thr389); however, it augmented the levels of p-ERK, p-GSK3β(Thr390), and p-70S6K(Thr421/Ser424) induced by MG132, and increased apoptotic cell death. The GSK inhibitor SB216763, but not lithium, inhibited the MG132-induced phosphorylation of p38, and the downstream signaling pathway was consistent with that in SB203580-treated cells. Taken together, our data show that proteasome inhibition regulates p38/GSK(Ser9)/p70S6K(Thr380) and ERK/GSK3β(Thr390)/p70S6K(Thr421/Ser424) kinase signaling, which is involved in cell survival and cell death.     

Cdk1/cyclin B1 controls Fas-mediated apoptosis by regulating caspase-8 activity

Caspase activation is a hallmark of apoptosis. However, the molecular mechanisms underlying the regulation of caspase-8 activation within the extrinsic death pathway are not well understood. In this study, we demonstrate that procaspase-8 is phosphorylated in mitotic cells by Cdk1/cyclin B1 on Ser-387, which is located at the N terminus of the catalytic subunit p10. This phosphorylation of procaspase-8 on Ser-387 occurs in cancer cell lines, as well as in primary breast tissues and lymphocytes. Furthermore, RNA interference-mediated silencing of cyclin B1 or treatment with the Cdk1 inhibitor RO-3306 enhances the Fas-mediated activation and processing of procaspase-8 in mitotic cells. A nonphosphorylatable procaspase-8 (S387A) facilitates Fas-induced apoptosis during mitosis. Our findings suggest that Cdk1/cyclin B1 activity shields human cells against extrinsic death stimuli and unravel the molecular details of the cross talk between cell cycle and extrinsic apoptotic pathways. Finally, this new mechanism may also contribute to tumorigenesis.     

Mapping of MST1 kinase sites of phosphorylation. Activation and autophosphorylation

MST1 is a member of the Sterile-20 family of cytoskeletal, stress, and apoptotic kinases. MST1 is activated by phosphorylation at previously unidentified sites. This study examines the role of phosphorylation at several sites and effects on kinase activation. We define Thr(183) in subdomain VIII as a primary site of phosphoactivation. Thr(187) is also critical for kinase activity. Phosphorylation of MST1 in subdomain VIII was catalyzed by active MST1 via intermolecular autophosphorylation, enhanced by homodimerization. Active MST1 (wild-type or T183E), but not inactive Thr(183)/Thr(187) mutants, was also highly autophosphorylated at the newly identified Thr(177) and Thr(387) residues. Cells expressing active MST1 were mostly detached, whereas with inactive MST1, adhesion was normal. Active MKK4, JNK, caspase-3, and caspase-9 were detected in the detached cells. These cells also contained all autophosphorylated and essentially all caspase-cleaved MST1. Similar phenotypes were elicited by a caspase-insensitive D326N mutant, suggesting that kinase activity, but not cleavage of MST1, is required. Interestingly, an S327E mutant mimicking Ser(327) autophosphorylation was also caspase-insensitive, but only when expressed in caspase-3-deficient cells. Together, these data suggest a model whereby MST1 activation is induced by existing, active MST kinase, which phosphorylates Thr(183) and possibly Thr(187). Dimerization promotes greater phosphorylation. This leads to induction of the JNK signaling pathway, caspase activation, and apoptosis. Further activation of MST1 by caspase cleavage is best promoted by caspase-3, although this appears to be unnecessary for signaling and morphological responses.     

Induction of mitotic arrest and apoptosis by evodiamine in human leukemic T-lymphocytes

Leukemias are a heterogenous group of diseases characterized by uncontrolled proliferation of abnormal blood cells of hematopoietic system. Evodiamine, a characteristic alkaloid extracted from Evodia fruits, has been reported to exhibit inhibitory effect on cell proliferation and migration in several types of cancer cells. However, there is no report elucidating the action target and anti-cancer mechanism of this potential natural compound. In this study, we have defined the anti-proliferative and apoptotic mechanisms of evodiamine in human acute leukemia CCRF-CEM cells. According to the MTT assay, the cell viability was inhibited by evodiamine in a concentration-dependent manner with an IC50 of 0.57 +/- 0.05 microM. Flow cytometry analysis showed that the apoptotic cell death proceeded by evodiamine was accompanied with a cell cycle arrest at the G2/M phase. Using Wright-Giemsa staining, we observed that evodiamine caused the cells to arrest in mitosis. It also profoundly caused an increase in polymerized tubulin levels and Bcl-2 phosphorylation on serine 70 in these cells. These data imply that the microtubular cytoskeleton appears to be one of the cellular targets in response to evodiamine. Moreover, treatment of CCRF-CEM cells with evodiamine was associated with increased levels of pro-apoptotic protein Bax, activation of caspase-3, and proteolytic cleavage of poly (ADP-ribose) polymerase, an endogenous caspase-3 substrate. Taken together, we demonstrate that evodiamine causes the mitotic arrest and a consequent apoptosis in CCRF-CEM cells through the enhancement of polymerized tubulin levels. Furthermore, several biological events including the Bcl-2 phosphorylation, Bax up-regulation and increase of caspase-3 activity could explain evodiamine-induced cell apoptosis.     

Phosphorylation of phosphatase inhibitor-2 at centrosomes during mitosis

Inhibitor-2 (I-2) is a regulator of protein phosphatase type-1 (PP1), known to be phosphorylated in vitro by multiple kinases. In particular Thr72 is a Thr-Pro phosphorylation site conserved from yeast to human, but there is no evidence that this phosphorylation responds to any physiological signals. Here, we used electrophoretic mobility shift and immunoblotting with a site-specific phospho-Thr72 antibody to establish Thr72 phosphorylation in HeLa cells and show a 25-fold increase in phosphorylation during mitosis. Mass spectrometry demonstrated I-2 in actively growing HeLa cells was also phosphorylated at three other sites, Ser120, Ser121, and an additional Ser located between residues 70 and 90. In vitro kinase assays using recombinant I-2 as a substrate showed that the Thr72 kinase(s) was activated during mitosis, and sensitivity to kinase inhibitors indicated that the principal I-2 Thr72 kinase was not GSK3 but instead a member of the cyclin-dependent protein kinase family. Immunocytochemistry confirmed Thr72 phosphorylation of I-2 during mitosis, with peak intensity at prophase, and revealed subcellular concentration of the phospho-Thr72 I-2 at centrosomes. Together, the data show dynamic changes in I-2 phosphorylation during mitosis and localization of phosphorylated I-2 at centrosomes, suggesting involvement in mammalian cell division.     

Protein kinase A regulates caspase-9 activation by Apaf-1 downstream of cytochrome c

The cyclic AMP signal transduction pathway modulates apoptosis in diverse cell types, although the mechanism is poorly understood. A critical component of the intrinsic apoptotic pathway is caspase-9, which is activated by Apaf-1 in the apoptosome, a large complex assembled in response to release of cytochrome c from mitochondria. Caspase-9 cleaves and activates effector caspases, predominantly caspase-3, resulting in the demise of the cell. Here we identified a distinct mechanism by which cyclic AMP regulates this apoptotic pathway through activation of protein kinase A. We show that protein kinase A inhibits activation of caspase-9 and caspase-3 downstream of cytochrome c in Xenopus egg extracts and in a human cell-free system. Protein kinase A directly phosphorylates human caspase-9 at serines 99, 183, and 195. However, mutational analysis demonstrated that phosphorylation at these sites is not required for the inhibitory effect of protein kinase A on caspase-9 activation. Importantly, protein kinase A inhibits cytochrome c-dependent recruitment of procaspase-9 to Apaf-1 but not activation of caspase-9 by a constitutively activated form of Apaf-1. These data indicate that extracellular signals that elevate cyclic AMP and activate protein kinase A may suppress apoptosis by inhibiting apoptosome formation downstream of cytochrome c release from mitochondria.     

Phospho-Bcl-xL(Ser62) plays a key role at DNA damage-induced G2 checkpoint

Accumulating evidence suggests that Bcl-xL, an anti-apoptotic member of the Bcl-2 family, also functions in cell cycle progression and cell cycle checkpoints. Analysis of a series of phosphorylation site mutants reveals that cells expressing Bcl-xL(Ser62Ala) mutant are less stable at the G₂ checkpoint and enter mitosis more rapidly than cells expressing wild-type Bcl-xL or Bcl-xL phosphorylation site mutants, including Thr41Ala, Ser43Ala, Thr47Ala, Ser56Ala and Thr115Ala. Analysis of the dynamic phosphorylation and location of phospho-Bcl-xL(Ser62) in unperturbed, synchronized cells and during DNA damage-induced G₂ arrest discloses that a pool of phospho-Bcl-xL(Ser62) accumulates into nucleolar structures in etoposide-exposed cells during G₂ arrest. In a series of in vitro kinase assays, pharmacological inhibitors and specific siRNAs experiments, we found that Polo kinase 1 and MAPK9/JNK2 are major protein kinases involved in Bcl-xL(Ser62) phosphorylation and accumulation into nucleolar structures during the G₂ checkpoint. In nucleoli, phospho-Bcl-xL(Ser62) binds to and co-localizes with Cdk1(cdc2), the key cyclin-dependent kinase required for entry into mitosis. These data indicate that during G₂ checkpoint, phospho-Bcl-xL(Ser62) stabilizes G₂ arrest by timely trapping of Cdk1(cdc2) in nucleolar structures to slow mitotic entry. It also highlights that DNA damage affects the dynamic composition of the nucleolus, which now emerges as a piece of the DNA damage response.