C-TAK1 regulates Ras signaling by phosphorylating the MAPK scaffold, KSR1

Kinase suppressor of Ras (KSR) is a conserved component of the Ras pathway that interacts directly with MEK and MAPK. Here we show that KSR1 translocates from the cytoplasm to the cell surface in response to growth factor treatment and that this process is regulated by Cdc25C-associated kinase 1 (C-TAK1). C-TAK1 constitutively associates with mammalian KSR1 and phosphorylates serine 392 to confer 14-3-3 binding and cytoplasmic sequestration of KSR1 in unstimulated cells. In response to signal activation, the phosphorylation state of S392 is reduced, allowing the KSR1 complex to colocalize with activated Ras and Raf-1 at the plasma membrane, thereby facilitating the phosphorylation reactions required for the activation of MEK and MAPK.

The human decatenation checkpoint

Chromatid catenation is actively monitored in human cells, with progression from G(2) to mitosis being inhibited when chromatids are insufficiently decatenated. Mitotic delay was quantified in normal and checkpoint-deficient human cells during treatment with ICRF-193, a topoisomerase II catalytic inhibitor that prevents chromatid decatenation without producing topoisomerase-associated DNA strand breaks. Ataxia telangiectasia (A-T) cells, defective in DNA damage checkpoints, showed normal mitotic delay when treated with ICRF-193. The mitotic delay in response to ICRF-193 was ablated in human fibroblasts expressing an ataxia telangiectasia mutated- and rad3-related (ATR) kinase-inactive ATR allele (ATR(ki)). BRCA1-mutant HCC1937 cells also displayed a defect in ICRF-193-induced mitotic delay, which was corrected by expression of wild-type BRCA1. Phosphorylations of hCds1 or Chk1 and inhibition of Cdk1 kinase activity, which are elements of checkpoints associated with DNA damage or replication, did not occur during ICRF-193-induced mitotic delay. Over-expression of cyclin B1 containing a dominant nuclear localization signal, and inhibition of Crm1-mediated nuclear export, reversed ICRF-193-induced mitotic delay. In combination, these results imply that ATR and BRCA1 enforce the decatenation G(2) checkpoint, which may act to exclude cyclin B1/Cdk1 complexes from the nucleus. Moreover, induction of ATR(ki) produced a 10-fold increase in chromosomal aberrations, further emphasizing the vital role for ATR in genetic stability.

The G(2) DNA damage checkpoint delays expression of genes encoding mitotic regulators

Transcriptional control of gene expression contributes to the regulation of diverse cellular processes including cell cycle progression and the cellular response to DNA damage. Global gene expression profiling was performed using p53-deficient human cells to identify genes with G(2)/M-specific and DNA damage-responsive expression. Numerous cell cycle-regulated genes were identified, but surprisingly the analysis failed to identify genes activated by ionizing radiation. Instead, significant delays in expression of G(2)/M-specific genes, including known mitotic regulators, were observed following DNA damage. Thus, in the absence of p53, gene induction does not contribute to the G(2) arrest following DNA damage. Rather, the DNA damage checkpoint elicits a G(2) cell cycle arrest, in part, by delaying accumulation of proteins required in mitosis.

ATR-mediated checkpoint pathways regulate phosphorylation and activation of human Chk1

Chk1 is an evolutionarily conserved protein kinase that regulates cell cycle progression in response to checkpoint activation. In this study, we demonstrated that agents that block DNA replication or cause certain forms of DNA damage induce the phosphorylation of human Chk1. The phosphorylated form of Chk1 possessed higher intrinsic protein kinase activity and eluted more quickly on gel filtration columns. Serines 317 and 345 were identified as sites of phosphorylation in vivo, and ATR (the ATM- and Rad3-related protein kinase) phosphorylated both of these sites in vitro. Furthermore, phosphorylation of Chk1 on serines 317 and 345 in vivo was ATR dependent. Mutants of Chk1 containing alanine in place of serines 317 and 345 were poorly activated in response to replication blocks or genotoxic stress in vivo, were poorly phosphorylated by ATR in vitro, and were not found in faster-eluting fractions by gel filtration. These findings demonstrate that the activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345. In addition, this study implicates ATR as a direct upstream activator of Chk1 in human cells.

Absence of apparent phenotype in mice lacking Cdc25C protein phosphatase

The Cdc25 family of protein phosphatases positively regulate the cell division cycle by activating cyclin-dependent protein kinases. In humans and rodents, three Cdc25 family members denoted Cdc25A, -B, and -C have been identified. The murine forms of Cdc25 exhibit distinct patterns of expression both during development and in adult mouse tissues. In order to determine unique contributions made by the Cdc25C protein phosphatase to embryonic and adult cell cycles, mice lacking Cdc25C were generated. We report that Cdc25C(-/-) mice are viable and do not display any obvious abnormalities. Among adult tissues in which Cdc25C is detected, its transcripts are most abundant in testis, followed by thymus, ovary, spleen, and intestine. Mice lacking Cdc25C were fertile, indicating that Cdc25C does not contribute an essential function during spermatogenesis or oogenesis in the mouse. T- and B-cell development was also found to be normal in Cdc25C(-/-) mice, and Cdc25C(-/-) mouse splenic T and B cells exhibited normal proliferative responses in vitro. Finally, the phosphorylation status of Cdc2, the timing of entry into mitosis, and the cellular response to DNA damage were unperturbed in mouse embryo fibroblasts lacking Cdc25C. These findings indicate that Cdc25A and/or Cdc25B may compensate for loss of Cdc25C in the mouse.

Immune system dysfunction and autoimmune disease in mice lacking Emk (Par-1) protein kinase

Emk is a serine/threonine protein kinase implicated in regulating polarity, cell cycle progression, and microtubule dynamics. To delineate the role of Emk in development and adult tissues, mice lacking Emk were generated by targeted gene disruption. Emk(-/-) mice displayed growth retardation and immune cell dysfunction. Although B- and T-cell development were normal, CD4(+)T cells lacking Emk exhibited a marked upregulation of the memory marker CD44/pgp-1 and produced more gamma interferon and interleukin-4 on stimulation through the T-cell receptor in vitro. In addition, B-cell responses to T-cell-dependent and -independent antigen challenge were altered in vivo. As Emk(-/-) animals aged, they developed splenomegaly, lymphadenopathy, membranoproliferative glomerulonephritis, and lymphocytic infiltrates in the lungs, parotid glands and kidneys. Taken together, these results demonstrate that the Emk protein kinase is essential for maintaining immune system homeostasis and that loss of Emk may contribute to autoimmune disease in mammals.

Localization of human Cdc25C is regulated both by nuclear export and 14-3-3 protein binding

Entry into mitosis requires activation of the Cdc2 protein kinase by the Cdc25C protein phosphatase. The interactions between Cdc2 and Cdc25C are negatively regulated throughout interphase and in response to G2 checkpoint activation. This is accomplished in part by maintaining the Cdc25 phosphatase in a phosphorylated form that binds 14-3-3 proteins. Here we report that 14-3-3 binding regulates the intracellular trafficking of Cdc25C. Although primarily cytoplasmic, Cdc25C accumulated in the nuclei of leptomycin B (LMB)-treated cells, indicating that Cdc25C is actively exported out of the nucleus. A mutant of Cdc25C that is unable to bind 14-3-3 was partially nuclear in the absence of LMB and its nuclear accumulation was greatly enhanced by LMB-treatment. A nuclear export signal (NES) was identified within the amino terminus of Cdc25C. Although mutation of the NES did not effect 14-3-3 binding, it did cause nuclear accumulation of Cdc25C. These results demonstrate that 14-3-3 binding is dispensable for the nuclear export of Cdc25C. However, complete nuclear accumulation of Cdc25C required loss of both NES function and 14-3-3 binding and this was accomplished both pharmacologically and by mutation. These findings suggest that the nuclear export of Cdc25C is mediated by an intrinsic NES and that 14-3-3 binding negatively regulates nuclear import.

Threonine 68 phosphorylation by ataxia telangiectasia mutated is required for efficient activation of Chk2 in response to ionizing radiation

Eukaryotic cells activate an evolutionarily conserved set of proteins that rapidly induce cell cycle arrest to prevent replication or segregation of damaged DNA before repair is completed. In response to ionizing radiation (IR), the cell cycle checkpoint kinase, Chk2 (hCds1), is phosphorylated and activated in an ataxia telangiectasia mutated (ATM)-dependent manner. Here we show that the ATM protein kinase directly phosphorylates T68 within the SQ/TQ-rich domain of Chk2 in vitro and that T68 is phosphorylated in vivo in response to IR in an ATM-dependent manner. Furthermore, phosphorylation of T68 was required for full activation of Chk2 after IR. Together, these data are consistent with the model that ATM directly phosphorylates Chk2 in vivo and that this event contributes to the activation of Chk2 in irradiated cells.

The Chk1 protein kinase and the Cdc25C regulatory pathways are targets of the anticancer agent UCN-01

A checkpoint operating in the G(2) phase of the cell cycle prevents entry into mitosis in the presence of DNA damage. UCN-01, a protein kinase inhibitor currently undergoing clinical trials for cancer treatment, abrogates G(2) checkpoint function and sensitizes p53-defective cancer cells to DNA-damaging agents. In most species, the G(2) checkpoint prevents the Cdc25 phosphatase from removing inhibitory phosphate groups from the mitosis-promoting kinase Cdc2. This is accomplished by maintaining Cdc25 in a phosphorylated form that binds 14-3-3 proteins. The checkpoint kinases, Chk1 and Cds1, are proposed to regulate the interactions between human Cdc25C and 14-3-3 proteins by phosphorylating Cdc25C on serine 216. 14-3-3 proteins, in turn, function to keep Cdc25C out of the nucleus. Here we report that UCN-01 caused loss of both serine 216 phosphorylation and 14-3-3 binding to Cdc25C in DNA-damaged cells. In addition, UCN-01 potently inhibited the ability of Chk1 to phosphorylate Cdc25C in vitro. In contrast, Cds1 was refractory to inhibition by UCN-01 in vitro, and Cds1 was still phosphorylated in irradiated cells treated with UCN-01. Thus, neither Cds1 nor kinases upstream of Cds1, such as ataxia telangiectasia-mutated, are targets of UCN-01 action in vivo. Taken together our results identify the Chk1 kinase and the Cdc25C pathway as potential targets of G(2) checkpoint abrogation by UCN-01.

DNA damage and replication checkpoints in fission yeast require nuclear exclusion of the Cdc25 phosphatase via 14-3-3 binding

In fission yeast as well as in higher eukaryotic organisms, entry into mitosis is delayed in cells containing damaged or unreplicated DNA. This is accomplished in part by maintaining the Cdc25 phosphatase in a phosphorylated form that binds 14-3-3 proteins. In this study, we generated a mutant of fission yeast Cdc25 that is severely impaired in its ability to bind 14-3-3 proteins. Loss of both the DNA damage and replication checkpoints was observed in fission yeast cells expressing the 14-3-3 binding mutant. These findings indicate that 14-3-3 binding to Cdc25 is required for fission yeast cells to arrest their cell cycle in response to DNA damage and replication blocks. Furthermore, the 14-3-3 binding mutant localized almost exclusively to the nucleus, unlike wild-type Cdc25, which localized to both the cytoplasm and the nucleus. Nuclear accumulation of wild-type Cdc25 was observed when fission yeast cells were treated with leptomycin B, indicating that Cdc25 is actively exported from the nucleus. Nuclear exclusion of wild-type Cdc25 was observed upon overproduction of Rad 24, one of the two fission yeast 14-3-3 proteins, indicating that one function of Rad 24 is to keep Cdc25 out of the nucleus. In support of this conclusion, Rad 24 overproduction did not alter the nuclear location of the 14-3-3 binding mutant. These results indicate that 14-3-3 binding contributes to the nuclear exclusion of Cdc25 and that the nuclear exclusion of Cdc25 is required for a normal checkpoint response to both damaged and unreplicated DNA.

Overproduction of human Myt1 kinase induces a G2 cell cycle delay by interfering with the intracellular trafficking of Cdc2-cyclin B1 complexes

The Myt1 protein kinase functions to negatively regulate Cdc2-cyclin B complexes by phosphorylating Cdc2 on threonine 14 and tyrosine 15. Throughout interphase, human Myt1 localizes to the endoplasmic reticulum and Golgi complex, whereas Cdc2-cyclin B1 complexes shuttle between the nucleus and the cytoplasm. Here we report that overproduction of either kinase-active or kinase-inactive forms of Myt1 blocked the nuclear-cytoplasmic shuttling of cyclin B1 and caused cells to delay in the G2 phase of the cell cycle. The COOH-terminal 63 amino acids of Myt1 were identified as a Cdc2-cyclin B1 interaction domain. Myt1 mutants lacking this domain no longer bound cyclin B1 and did not efficiently phosphorylate Cdc2-cyclin B1 complexes in vitro. In addition, cells overproducing mutant forms of Myt1 lacking the interaction domain exhibited normal trafficking of cyclin B1 and unperturbed cell cycle progression. These results suggest that the docking of Cdc2-cyclin B1 complexes to the COOH terminus of Myt1 facilitates the phosphorylation of Cdc2 by Myt1 and that overproduction of Myt1 perturbs cell cycle progression by sequestering Cdc2-cyclin B1 complexes in the cytoplasm.

A human Cds1-related kinase that functions downstream of ATM protein in the cellular response to DNA damage

Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. Much of our current understanding of checkpoints comes from genetic studies conducted in yeast. In the fission yeast Schizosaccharomyces pombe (Sp), SpRad3 is an essential component of both the DNA damage and DNA replication checkpoints. The SpChk1 and SpCds1 protein kinases function downstream of SpRad3. SpChk1 is an effector of the DNA damage checkpoint and, in the absence of SpCds1, serves an essential function in the DNA replication checkpoint. SpCds1 functions in the DNA replication checkpoint and in the S phase DNA damage checkpoint. Human homologs of both SpRad3 and SpChk1 but not SpCds1 have been identified. Here we report the identification of a human cDNA encoding a protein (designated HuCds1) that shares sequence, structural, and functional similarity to SpCds1. HuCds1 was modified by phosphorylation and activated in response to ionizing radiation. It was also modified in response to hydroxyurea treatment. Functional ATM protein was required for HuCds1 modification after ionizing radiation but not after hydroxyurea treatment. Like its fission yeast counterpart, human Cds1 phosphorylated Cdc25C to promote the binding of 14-3-3 proteins. These findings suggest that the checkpoint function of HuCds1 is conserved in yeast and mammals.

UCN-01 abrogates G2 arrest through a Cdc2-dependent pathway that is associated with inactivation of the Wee1Hu kinase and activation of the Cdc25C phosphatase

We have previously demonstrated that UCN-01, a potent protein kinase inhibitor currently in phase I clinical trials for cancer treatment, abrogates G2 arrest following DNA damage. Here we used murine FT210 cells, which contain temperature-sensitive Cdc2 mutations, to determine if UCN-01 abrogates G2 arrest through a Cdc2-dependent pathway. We report that UCN-01 cannot induce mitosis in DNA-damaged FT210 cells at the non-permissive temperature for Cdc2 function. Failure to abrogate G2 arrest was not due to UCN-01-inactivation at the elevated temperature because parental FM3A cells, which have wild-type Cdc2, were sensitive to UCN-01-induced G2 checkpoint abrogation. Having established that UCN-01 acted through Cdc2, we next assessed UCN-01's effect on the Cdc2-inhibitory kinase, Wee1Hu, and the Cdc2-activating phosphatase, Cdc25C. We found that Wee1Hu was indeed inactivated in UCN-01-treated cells, possibly just prior to Cdc2 activation and entry of DNA-damaged cells into mitosis. This inhibition appeared, however, to be a consequence of a further upstream action since in vitro studies revealed purified Wee1Hu was relatively resistant to UCN-01-inhibition. Consistent with such an upstream action, UCN-01 also promoted the hyperphosphorylation (activation) of Cdc25C in DNA-damaged cells. Our results suggest that UCN-01 abrogates G2 checkpoint function through inhibition of a kinase residing upstream of Cdc2, Wee1Hu, and Cdc25C, and that changes observed in these mitotic regulators are downstream consequences of UCN-01's actions.

Replication checkpoint requires phosphorylation of the phosphatase Cdc25 by Cds1 or Chk1

Checkpoints maintain the order and fidelity of events of the cell cycle by blocking mitosis in response to unreplicated or damaged DNA. In most species this is accomplished by preventing activation of the cell-division kinase Cdc2, which regulates entry into mitosis. The Chk1 kinase, an effector of the DNA-damage checkpoint, phosphorylates Cdc25, an activator of Cdc2. Phosphorylation of Cdc25 promotes its binding to 14-3-3 proteins, preventing it from activating Cdc2. Here we propose that a similar pathway is required for mitotic arrest in the presence of unreplicated DNA (that is, in the replication checkpoint) in fission yeast. We show by mutagenesis that Chk1 functions redundantly with the kinase Cds1 at the replication checkpoint and that both kinases phosphorylate Cdc25 on the same sites, which include serine residues at positions 99, 192 and 359. Mutation of these residues reduces binding of 14-3-3 proteins to Cdc25 in vitro and disrupts the replication checkpoint in vivo. We conclude that both Cds1 and Chk1 regulate the binding of Cdc25 to 14-3-3 proteins as part of the checkpoint response to unreplicated DNA.

14-3-3 proteins are required for maintenance of Raf-1 phosphorylation and kinase activity

By binding to serine-phosphorylated proteins, 14-3-3 proteins function as effectors of serine phosphorylation. The exact mechanism of their action is, however, still largely unknown. Here we demonstrate a requirement for 14-3-3 for Raf-1 kinase activity and phosphorylation. Expression of dominant negative forms of 14-3-3 resulted in the loss of a critical Raf-1 phosphorylation, while overexpression of 14-3-3 resulted in enhanced phosphorylation of this site. 14-3-3 levels, therefore, regulate the stoichiometry of Raf-1 phosphorylation and its potential activity in the cell. Phosphorylation of Raf-1, however, was insufficient by itself for kinase activity. Removal of 14-3-3 from phosphorylated Raf abrogated kinase activity, whereas addition of 14-3-3 restored it. This supports a paradigm in which the effects of phosphorylation on serine as well as tyrosine residues are mediated by inducible protein-protein interactions.

Crystal structure of the catalytic domain of the human cell cycle control phosphatase, Cdc25A

Cdc25 phosphatases activate the cell division kinases throughout the cell cycle. The 2.3 A structure of the human Cdc25A catalytic domain reveals a small alpha/beta domain with a fold unlike previously described phosphatase structures but identical to rhodanese, a sulfur-transfer protein. Only the active-site loop, containing the Cys-(X)5-Arg motif, shows similarity to the tyrosine phosphatases. In some crystals, the catalytic Cys-430 forms a disulfide bond with the invariant Cys-384, suggesting that Cdc25 may be self-inhibited during oxidative stress. Asp-383, previously proposed to be the general acid, instead serves a structural role, forming a conserved buried salt-bridge. We propose that Glu-431 may act as a general acid. Structure-based alignments suggest that the noncatalytic domain of the MAP kinase phosphatases will share this topology, as will ACR2, a eukaryotic arsenical resistance protein.

C-TAK1 protein kinase phosphorylates human Cdc25C on serine 216 and promotes 14-3-3 protein binding

Cdc25C is a dual-specificity protein kinase that controls entry into mitosis by dephosphorylating Cdc2 on both threonine 14 and tyrosine 15. Cdc25C is phosphorylated on serine 216 throughout interphase but not during mitosis. Serine 216 phosphorylation mediates the binding of 14-3-3 protein to Cdc25C, and Cdc25C/14-3-3 complexes are present throughout interphase but not during mitosis. Here we report the cloning of a human kinase denoted C-TAK1 (for Cdc twenty-five C associated protein kinase) that phosphorylates Cdc25C on serine 216 in vitro. C-TAK1 is ubiquitously expressed in human tissues and cell lines and is distinct from the DNA damage checkpoint kinase Chk1, shown previously to phosphorylate Cdc25C on serine 216. Cotransfection of Cdc25C with C-TAK1 resulted in enhanced phosphorylation of Cdc25C on serine 216. In addition, a physical interaction between C-TAK1 and Cdc25C was observed upon transient overexpression in COS-7 cells. Finally, coproduction of Cdc25C and C-TAK1 in bacteria resulted in the stoichiometric phosphorylation of Cdc25C on serine 216 and facilitated 14-3-3 protein binding in vitro. Taken together, these results suggest that one function of C-TAK1 may be to regulate the interactions between Cdc25C and 14-3-3 in vivo by phosphorylating Cdc25C on serine 216.

Serine phosphorylation-dependent association of the band 4.1-related protein-tyrosine phosphatase PTPH1 with 14-3-3beta protein

PTPH1 is a human protein-tyrosine phosphatase with homology to the band 4.1 superfamily of cytoskeletal-associated proteins. PTPH1 was found to associate with 14-3-3beta using a yeast two-hybrid screen, and its interaction could be reconstituted in vitro using recombinant proteins. Examination of the interaction between 14-3-3beta and various deletion mutants of PTPH1 by two-hybrid tests suggested that the integrity of the PTP is important for this binding. Although both PTPH1 and Raf-1 form complexes with 14-3-3beta, they appear to do so independently. Binding of 14-3-3beta to PTPH1 in vitro was abolished by pretreating PTPH1 with potato acid phosphatase and was greatly enhanced by pretreating with Cdc25C-associated protein kinase. Thus the association between PTPH1 and 14-3-3beta is phosphorylation-dependent. Two novel motifs RSLS359VE and RVDS853EP in PTPH1 were identified as major 14-3-3beta-binding sites, both of which are distinct from the consensus binding motif RSXSXP recently found in Raf-1. Mutation of Ser359 and Ser853 to alanine significantly reduced the association between 14-3-3beta and PTPH1. Furthermore, association of PTPH1 and 14-3-3beta was detected in several cell lines and was regulated in response to extracellular signals. These results raise the possibility that 14-3-3beta may function as an adaptor molecule in the regulation of PTPH1 and may provide a link between serine/threonine and tyrosine phosphorylation-dependent signaling pathways.

Conservation of the Chk1 checkpoint pathway in mammals: linkage of DNA damage to Cdk regulation through Cdc25

In response to DNA damage, mammalian cells prevent cell cycle progression through the control of critical cell cycle regulators. A human gene was identified that encodes the protein Chk1, a homolog of the Schizosaccharomyces pombe Chk1 protein kinase, which is required for the DNA damage checkpoint. Human Chk1 protein was modified in response to DNA damage. In vitro Chk1 bound to and phosphorylated the dual-specificity protein phosphatases Cdc25A, Cdc25B, and Cdc25C, which control cell cycle transitions by dephosphorylating cyclin-dependent kinases. Chk1 phosphorylates Cdc25C on serine-216. As shown in an accompanying paper by Peng et al. in this issue, serine-216 phosphorylation creates a binding site for 14-3-3 protein and inhibits function of the phosphatase. These results suggest a model whereby in response to DNA damage, Chk1 phosphorylates and inhibits Cdc25C, thus preventing activation of the Cdc2-cyclin B complex and mitotic entry.

Mitotic and G2 checkpoint control: regulation of 14-3-3 protein binding by phosphorylation of Cdc25C on serine-216

Human Cdc25C is a dual-specificity protein phosphatase that controls entry into mitosis by dephosphorylating the protein kinase Cdc2. Throughout interphase, but not in mitosis, Cdc25C was phosphorylated on serine-216 and bound to members of the highly conserved and ubiquitously expressed family of 14-3-3 proteins. A mutation preventing phosphorylation of serine-216 abrogated 14-3-3 binding. Conditional overexpression of this mutant perturbed mitotic timing and allowed cells to escape the G2 checkpoint arrest induced by either unreplicated DNA or radiation-induced damage. Chk1, a fission yeast kinase involved in the DNA damage checkpoint response, phosphorylated Cdc25C in vitro on serine-216. These results indicate that serine-216 phosphorylation and 14-3-3 binding negatively regulate Cdc25C and identify Cdc25C as a potential target of checkpoint control in human cells.

The human Myt1 kinase preferentially phosphorylates Cdc2 on threonine 14 and localizes to the endoplasmic reticulum and Golgi complex

Entry into mitosis requires the activity of the Cdc2 kinase. Cdc2 associates with the B-type cyclins, and the Cdc2-cyclin B heterodimer is in turn regulated by phosphorylation. Phosphorylation of threonine 161 is required for the Cdc2-cyclin B complex to be catalytically active, whereas phosphorylation of threonine 14 and tyrosine 15 is inhibitory. Human kinases that catalyze the phosphorylation of threonine 161 and tyrosine 15 have been identified. Here we report the isolation of a novel human cDNA encoding a dual-specificity protein kinase (designated Myt1Hu) that preferentially phosphorylates Cdc2 on threonine 14 in a cyclin-dependent manner. Myt1Hu is 46% identical to Myt1Xe, a kinase recently characterized from Xenopus laevis. Myt1Hu localizes to the endoplasmic reticulum and Golgi complex in HeLa cells. A stretch of hydrophobic and uncharged amino acids located outside the catalytic domain of Myt1Hu is the likely membrane-targeting domain, as its deletion results in the localization of Myt1Hu primarily to the nucleus.

Identification of a 95-kDa WEE1-like tyrosine kinase in HeLa cells

Human WEE1 (WEE1Hu) was cloned on the basis of its ability to rescue wee1+ mutants in fission yeast [Igarashi, M., Nagata, A., Jinno, S., Suto, K. & Okayama, H. (1991) Nature (London) 353, 80-83]. Biochemical studies carried out in vitro with recombinant protein demonstrated that WEE1Hu encodes a tyrosine kinase of approximately 49 kDa that phosphorylates p34cdc2 on Tyr-15 [Parker, L. L. & Piwnica-Worms, H. (1992) Science 257, 1955-1957]. To study the regulation of WEE1Hu in human cells, two polyclonal antibodies to bacterially produced p49WEE1Hu were generated. In addition, a peptide antibody generated against amino acids 361-388 of p49WEE1Hu was also used. Unexpectantly, these antibodies recognized a protein with an apparent molecular mass of 95 kDa in HeLa cells, rather than one of 49 kDa. Immunoprecipitates of p95 phosphorylated p34cdc2 on Tyr-15, indicating that p95 is functionally related to p49WEEIHu, and mapping studies demonstrated that p95 is structurally related to p49WEE1Hu. In addition, the substrate specificity of p95 was more similar to that of fission yeast p107wee1 than to that of human p49WEE1. Finally, the kinase activity of p95 toward p34cdc2/cyclin B was severely impaired during mitosis. Taken together, these results indicate that the original WEE1Hu clone isolated in genetic screens encodes only the catalytic domain of human WEE1 and that the authentic human WEE1 protein has an apparent molecular mass of approximately 95 kDa.

mik1+ encodes a tyrosine kinase that phosphorylates p34cdc2 on tyrosine 15

mik1+ and wee1+ function to regulate the tyrosine phosphorylation of p34cdc2 in Schizosaccharomyces pombe (Lundgren, K., Walworth, N., Booher, R., Dembski, M., Kirschner, M., and Beach, D. (1991) Cell 64, 1111-1122). wee1+ encodes a tyrosine kinase that directly phosphorylates p34cdc2 on tyrosine 15, resulting in the inactivation of the cyclin B/p34cdc2 complex. We have overproduced the mik1+ gene product in insect cells and in S. pombe in order to characterize it biochemically. Immunoprecipitates of Mik1 from both sources catalyzed the phosphorylation of p34cdc2 on tyrosine 15 whereas immunoprecipitates of a kinase-deficient mutant of Mik1 were negative in this assay. Mik1 overproduced in insect cells was partially purified by column chromatography, and column fractions were assayed for their ability to phosphorylate p34cdc2 on tyrosine 15. Two major peaks of Mik1 protein were detected by gel filtration chromatography. One peak eluted in the void volume, and a second peak eluted with an apparent molecular mass expected for monomeric Mik1 (approximately 68 kDa). The tyrosine 15 kinase activity co-eluted with the 68 kDa form of Mik1. These results indicate that mik1+ encodes a tyrosine kinase that directly phosphorylates p34cdc2 on tyrosine 15.