Characterization of synthetic peptide substrates for p34cdc2 protein kinase

Substrate specificity of CDK2-cyclin A. What is optimal?

The optimal amino acid sequence of substrates for recognition by the cyclin-dependent kinases is well established as -Ser/Thr0-Pro+1-Lys+2-Lys+3-. The catalytic efficiency of CDK2-cyclin A is impaired 2000-, 10-, and 150-fold, when Pro+1, Lys+2, or Lys+3, respectively, is substituted with Ala in a short synthetic peptide substrate. Yet, in physiological substrates of both CDK2-cyclin A and CDK2-cyclin E, it is found that Lys+2, and, occasionally, both Lys+2 and Lys+3 together are replaced with suboptimal determinants. Such suboptimal phosphorylation site motifs are invariably associated with a distinct cyclin-binding (Cy) motif, which has been shown to compensate for otherwise poor catalysis. Here we have investigated the kinetic basis for substrate recognition by CDK2-cyclin A. In the optimal motif, Pro+1 serves to dramatically enhance both substrate binding affinity as well as the rate of chemical phosphotransfer, whereas Lys+2 and Lys+3 both serve to enhance mainly substrate binding. When linked to a suboptimal phosphorylation site sequence (Lys+2 --> Pro) the Cy motif increases catalytic efficiency (kcat/Km) by increasing affinity without affecting turnover (kcat). When fused to the optimal sequence, however, catalytic efficiency is only minimally enhanced, because the resulting high substrate affinity impedes the rate of the phosphoryl transfer reaction. Our results provide kinetic insight into the basis for selecting suboptimal specificity determinants for the phosphorylation of cellular substrates.

Phosphorylation of Cdc28 and regulation of cell size by the protein kinase CKII in Saccharomyces cerevisiae

The CDK (cyclin-dependent kinase) family of enzymes is required for the G(1)-to-S-phase and G(2)-to-M-phase transitions during the cell-division cycle of eukaryotes. We have shown previously that the protein kinase CKII catalyses the phosphorylation of Ser-39 in Cdc2 during the G(1) phase of the HeLa cell-division cycle [Russo, Vandenberg, Yu, Bae, Franza and Marshak (1992) J. Biol. Chem. 267, 20317-20325]. To identify a functional role for this phosphorylation, we have studied the homologous enzymes in the budding yeast Saccharomyces cerevisiae. The S. cerevisiae homologue of Cdc2, Cdc28, contains a consensus CKII site (Ser-46), which is homologous with that of human Cdc2. Using in vitro kinase assays, metabolic labelling, peptide mapping and phosphoamino acid analysis, we demonstrate that this site is phosphorylated in Cdc28 in vivo as well in vitro. In addition, S. cerevisiae cells in which Ser-46 has been mutated to alanine show a decrease in both cell volume and protein content of 33%, and this effect is most pronounced in the stationary phase. Because cell size in S. cerevisiae is regulated primarily at the G(1) stage, we suggest that CKII contributes to the regulation of the cell cycle in budding yeast by phosphorylation of Cdc28 as a checkpoint for G(1) progression.

Mitosis-specific phosphorylation and subcellular redistribution of the RIIalpha regulatory subunit of cAMP-dependent protein kinase

Phosphorylation of the RII regulatory subunits of cyclic AMP-dependent protein kinases (PKAs) was examined during the HeLa cell cycle. Three RIIalpha isoforms of 51, 54, and 57 kDa were identified by RIIalpha immunodetection and labeling with 8-azido[32P]cAMP in different cell cycle phases. These isoforms were characterized as different phosphorylation states by the use of selective PKA and cyclin-directed kinase inhibitors. Whereas RIIalpha autophosphorylation by PKA caused RIIalpha to shift from 51 to 54 kDa, phosphorylation of RIIalpha by one other or a combination of several kinases activated during mitosis caused RIIalpha to shift from 51 to 57 kDa. In vivo incorporation of [32P]orthophosphate into mitotic cells and RIIalpha immunoprecipitation demonstrated that RIIalpha was hyperphosphorylated on a different site than the one phosphorylated by PKA. Deletion and mutation analysis demonstrated that the cyclin B-p34(cdc2) kinase (CDK1) phosphorylated human recombinant RIIalpha in vitro on Thr54. Whereas RIIalpha was associated with the Golgi-centrosomal region during interphase, it was dissociated from its centrosomal localization at metaphase-anaphase transition. Furthermore, particulate RIIalpha from HeLa cell extracts was solubilized following incubation with CDK1 in vitro. Our results suggest that at the onset of mitosis, CDK1 phosphorylates RIIalpha, and this may alter its subcellular localization.

Programmed cellular response to ionizing radiation damage

Three forms of radiation response were investigated to evaluate the hypothesis that cellular radiation response is the result of active molecular signaling and not simply a passive physicochemical process. The decision whether or not a cell should respond to radiation-induced damage either by induction of rescue systems, e.g. mobilization of repair proteins, or induction of suicide mechanisms, e.g. programmed cell death, appears to be the expression of intricate cellular biochemistry. A cell must recognize damage in its genetic material and then activate the appropriate responses. Cell type is important; the response of a fibroblast to radiation damage is both quantitatively and qualitatively different from that of a lymphocyte. The programmed component of radiation response is significant in radiation oncology and predicted to create unique opportunities for enhanced treatment success.

Determinants for substrate phosphorylation by p21-activated protein kinase (gamma-PAK)

gamma-PAK, originally designated PAK I and subsequently identified as a member of the p21-activated protein kinase family, has been shown to have cytostatic properties and to be involved in maintaining cells in a nondividing state [Rooney, R. D., et al., (1996) J. Biol. Chem. 271, 21498-21504]. The determinants for phosphorylation of substrates by gamma-PAK have been identified by examining the kinetics of phosphorylation of a series of synthetic peptides patterned after the sequence KKRKSGL, which is the site phosphorylated by gamma-PAK in the Rous sarcoma virus nucleocapsid protein NC in vivo and in vitro. With these peptides, the recognition sequence for gamma-PAK has been shown to contain two basic amino acids in the -2 and -3 positions, as represented by (K/R)RXS, in which the -2 position is an arginine, the -3 position is an arginine or a lysine, and X can be an acidic, basic, or neutral amino acid. A basic amino acid in the -1 or -4 position improves the rate of phosphorylation by increasing the Vmax and decreasing the Km. An acidic amino acid in the -1 position increases the rate (2.5-fold), as does an acidic residue in the -4 position, although to a lower extent (1.6-fold). Proline in the -1 or +1 position has a deleterious effect and inhibits phosphorylation by gamma-PAK. The substrate requirements of protein kinases that recognize basic amino acids on the N-terminal side of the phosphorylatable residue such as cAMP-dependent protein kinase (PKA) and Ca2+/phospholipid-dependent protein kinase (PKC) have been compared with gamma-PAK using the same peptides. An acidic residue in the -1 position negatively affects PKA and PKC; thus, peptides containing the sequence KRES can be used to identify gamma-PAK.

The highly basic ribosomal protein L41 interacts with the beta subunit of protein kinase CKII and stimulates phosphorylation of DNA topoisomerase IIalpha by CKII

Protein kinase CKII (CKII) is a heterotetramer composed of two catalytic (alpha or alpha') and two regulatory (beta) subunits. Using the yeast two-hybrid system, we have identified the highly basic, ribosomal protein L41 as a cellular protein capable of interacting with the beta subunit of CKII. We show, furthermore, using purified proteins, that L41 protein and CKIIbeta associate directly in vitro. L41 protein is not a substrate for CKII phosphorylation, and it does not stimulate CKII activity with either beta-casein or synthetic peptide substrate (RRREEETEEE). However, L41 protein stimulates the phosphorylation of DNA topoisomerase IIalpha by CKII by 2.5 times. Additionally, L41 protein enhances the autophosphorylation of CKIIalpha. The data indicate that L41 protein associates with CKII and can modulate its activity toward a specific substrate or substrates. The direct interaction of CKIIbeta with ribosomal proteins also suggests that CKIIbeta itself or CKII holoenzyme may be involved in ribosome assembly or translational control.

Renal tubule regeneration after ischemic injury is coupled to the up-regulation and activation of cyclins and cyclin dependent kinases

Proliferation of renal tubules after acute injury is a reactive process of renal regeneration for recovery of renal function. Molecular and cellular mechanisms of the re-entrance of renal cells into the cell cycle after injury remain largely unknown. We have measured the correlations among the extent of proliferative activity and expression of cyclins and CDKs, and activity of each CDK during the regeneration period in the outer medullae of kidneys after ischemic injury in rats. The ratio of proliferating cell nuclear antigen (PCNA) positively immuno-stained nuclei to total nuclei per each section of the outer medulla of kidney indicated the proliferative index (PI) for this study. PI in the control period was 0.1%. The PI was increased at day 1 (13.4%), remained at a plateau at days 3 and 5 (30.5 and 32.3%), and decreased at day 7 and day 14 (17.3 and 12.2%) after ischemic injury. Proliferative activity was readily detectable in renal tubules, but was hardly detectable in glomeruli or blood vessels. As the PI increased, the mRNA and protein levels of cyclins D1, D3 and B, the mRNA levels of cyclin A, the protein levels of CDK4 and CDK2, and the activities of CDKs (CDK4, CDK2 and cdc2) increased in the outer medullae of kidneys after ischemic injury. These findings suggest that the temporal induction of proliferative activity in outer medullary tubules was closely linked with the cyclin/CDK system for regeneration of kidney after ischemic injury.

Noninvasive, real-time method for the examination of thymidine uptake events--application of the method to V-79 cell synchrony studies

[14C]Thymidine uptake into V-79 hamster lung fibroblasts has been successfully demonstrated using a noninvasive, real-time method utilizing Cytostar-T scintillating microplates. These plates are standard format, tissue culture-treated, 96-well microplates with an integral scintillating base. The microplates permit the culture and observation of adherent cell monolayers. Biological activities of the cells can be studied by the provision of specific radiolabeled compounds. The biological activities of the adherent monolayer bring the specific radiolabel into proximity with the scintillating base and a scintillation signal is thereby generated. [14C]Thymidine incorporation on the microplates can be used to examine cell proliferation and cell cycle events. Using a combined mitotic shake-off/aphidicolin treatment to achieve synchronization, the thymidine incorporation activities of V-79 cells have been examined on Cytostar-T plates and correlated to traditional methods of determining incorporation. The method was further used to examine the effects of colcemid and olomoucine, both chemical inhibitors of cell proliferation, on synchronous populations of cells. The homogeneous detection format and the microplate nature of the method suggest a role for scintillating microplates in cell biology research and drug discovery.

Phosphorylation site specificity of the CDC2-related kinase PITALRE

PITALRE is a human protein kinase belonging to the cell division cycle 2 (CDC2) kinase family, and is the catalytic subunit of a multimeric complex that contains several cellular proteins. PITALRE complexes from several cell lines and tissues phosphorylate retinoblastoma protein and myelin basic protein (MBP). In the present work, we have found that MBP is phosphorylated by PITALRE complexes on both Ser and Thr residues. Two different antibodies raised to PITALRE purified virtually identical kinase activities, as analysed by MBP phosphopeptide mapping and phosphoamino acid analysis. We have identified the proline-directed residue Ser-162 of MBP as a major phosphorylation site for PITALRE. In addition, our results suggest that one of the two MBP proline-directed threonine residues, Thr-97, is also selectively phosphorylated by PITALRE. These data, together with analysis of different peptide substrates derived from sites on MBP that are phosphorylated by PITALRE, indicate that PITALRE is a Ser/Thr proline-directed kinase. In addition, our results show that PITALRE has a substrate site specificity distinguishable from those of the CDC2 and cyclin-dependent kinase 2 (CDK2).

Maturation promoting factor in ascidian oocytes is regulated by different intracellular signals at meiosis I and II

Using the fluorescent dye Calcium Green-dextran, we measured intracellular Ca2+ in oocytes of the ascidian Ciona intestinalis at fertilization and during progression through meiosis. The relative fluorescence intensity increased shortly after insemination in a single transient, the activation peak, and this was followed by several smaller oscillations that lasted for approximately 5 minutes (phase 1). The first polar body was extruded after the completion of the phase 1 transients, about 9 minutes after insemination, and then the intracellular calcium level remained at baseline for a period of 5 minutes (phase 2). At 14 minutes postinsemination a second series of oscillations was initiated that lasted 11 minutes (phase 3) and terminated at the time of second polar body extrusion. Phases 1 and 3 were inhibited by preloading oocytes with 5 mM heparin. Simultaneous measurements of membrane currents, in the whole-cell clamp configuration, showed that the 1–2 nA inward fertilization current correlated temporally with the activation peak, while a series of smaller oscillations of 0.1-0.3 nA amplitude were generated at the time of the phase 3 oscillations. Biochemical characterization of Maturation Promoting Factor (MPF) in ascidian oocytes led to the identification of a Cdc2-like kinase activity. Using p13suc1-sepharose as a reagent to precipitate the MPF complex, a 67 kDa (67 × 10(3) Mr) protein was identified as cyclin B. Histone H1 kinase activity was high at metaphase I and decreased within 5 minutes of insemination reaching a minimum level during phase 2, corresponding to telophase I. During phase 3, H1 kinase activity increased and then decayed again during telophase II. Oocytes preloaded with BAPTA and subsequently inseminated did not generate any calcium transients, nonetheless H1 kinase activity decreased 5 minutes after insemination, as in the controls, and remained low for at least 30 minutes. Injection of BAPTA during phase 2 suppressed the phase 3 calcium transients, and inhibited both the increase in H1 kinase activity normally encountered at metaphase II and second polar body extrusion.

A casein-kinase-2-related protein kinase is tightly associated with the large T antigen of simian virus 40

The simian virus 40 (SV40) large T antigen is a multifunctional protein involved in SV40 cell transformation and lytic virus infection. Some of its activities are regulated by interaction with cellular proteins and/or by phosphorylation of T antigen by various protein kinases. In this study, we show that immuno-purified T antigen from SV40-transformed cells and from baculovirus-infected insect cells is tightly associated with a protein kinase that phosphorylates T antigen in vitro. In the presence of heparin or a peptide resembling a protein kinase CK2 recognition site, the phosphorylation of T antigen by the associated kinase is reduced whereas a p34cdc2-kinase-specific peptide has no influence. In addition, the T-antigen-associated protein kinase can use GTP and ATP as phosphate donors. These properties together with the observation that immunopurified T antigen can be phosphorylated by the addition of protein kinase CK2 suggest that at least one of the T-antigen-associated protein kinases is CK2 or a protein-kinase-CK2-related enzyme. The association of recombinant CK2 with T antigen was strongly confirmed by in vitro binding studies. Experiments with temperature-sensitive SV40-transformed cells provide evidence for a close correlation between cell transformation and phosphorylation of T antigen by the associated protein kinase.

Cell cycle arrest induced by an inhibitor of glucosylceramide synthase. Correlation with cyclin-dependent kinases

In an attempt to define the basis for sphingolipid regulation of cell proliferation, we studied the effects of glucosylceramide (GlcCer) synthase inhibition by threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) on NIH 3T3 cells overexpressing insulin-like growth factor-1 (IGF-1) receptor. PDMP treatment resulted in a time-dependent decrease in GlcCer levels and an increase in cellular ceramide levels. PDMP abolished serum and IGF-1-stimulated cell proliferation, as measured by a reduction in [3H]thymidine incorporation, protein, and DNA levels. However it did not affect IGF-1-mediated early signaling events, including receptor tyrosine kinase, MAP kinase, and phosphatidylinositol 3-kinase activities. Two-color flow cytometry with propidium iodide and 5-bromo-2'-deoxyuridine monophosphate labeling revealed an arrest of the cell cycle at G1/S and G2/M transitions in an asynchronous population of cells. These changes were time dependent, with maximal effects seen by 12-24 h. Removal of PDMP from the cell medium resulted in reversal of the cell cycle changes, with cells re-entering the S phase. The cell cycle arrest at the G1/S and G2/M transitions was confirmed in cells synchronized by pretreatment with nocodazole, aphidicolin, or hydroxyurea, and released from blockade in the presence of PDMP. A decrease in the activities of two cyclin-dependent kinases, p34cdc2 kinase and cdk2 kinase, was observed with PDMP treatment. When cell ceramide levels were increased by N-acetylsphingosine, comparable changes in the cell cycle distribution were seen. However, sphingomyelinase treatment was without effect. Therefore, it appears that ceramide mediates in part the inhibitory effect of GlcCer synthase inhibition on IGF-1-induced cell proliferation in 3T3 cells. The rapid production of decreased cyclin-dependent kinase activities by PDMP suggests that one of the crucial sites of action of the inhibitor lies in this area.

Effects of ionizing radiation and caffeine treatment on cyclin dependent kinase complexes in V79 hamster cells

Exponentially growing V79 Chinese hamster lung fibroblasts irradiated with 7 Gy X-rays undergo cell cycle arrest in the S and G2 phases. These arrests are released, probably on completion of DNA repair. A premature release occurs after treatment of irradiated cells with caffeine. This release is accompanied by increased activity of the p34cdc2 serine/threonine protein kinase complex [Hain et al. (1993) Cancer Res. 53, 1507-1510]. We have investigated in V79 cells whether the association of p34cdc2 with its regulatory subunits cyclin A and B is affected by irradiation and subsequent caffeine treatment and found that this was not the case. The phosphorylation of p34cdc2 as assayed by mobility shift on SDS polyacrylamide gels was increased as early as 0.5 h after irradiation and decreased after subsequent caffeine treatment. A novel protein p40, detected with anti-PSTAIRE antibodies, appeared several fold more abundant than p34cdc2. Its phosphorylation state also changed after irradiation and after subsequent caffeine treatment.

Mitogen-activated protein kinase kinase 1 (MKK1) is negatively regulated by threonine phosphorylation

Mitogen-activated protein kinase kinase 1 (MKK1), a dual-specificity tyrosine/threonine protein kinase, has been shown to be phosphorylated and activated by the raf oncogene product as part of the mitogen-activated protein kinase cascade. Here we report the phosphorylation and inactivation of MKK1 by phosphorylation on threonine 286 and threonine 292. MKK1 contains a consensus phosphorylation site for p34cdc2, a serine/threonine protein kinase that regulates the cell division cycle, at Thr-286 and a related site at Thr-292. p34cdc2 catalyzes the in vitro phosphorylation of MKK1 on both of these threonine residues and inactivates MKK1 enzymatic activity. Both sites are phosphorylated in vivo as well. The data presented in this report provide evidence that MKK1 is negatively regulated by threonine phosphorylation.

Mitogen-activated protein kinase kinase 1 (MKK1) is negatively regulated by threonine phosphorylation

Mitogen-activated protein kinase kinase 1 (MKK1), a dual-specificity tyrosine/threonine protein kinase, has been shown to be phosphorylated and activated by the raf oncogene product as part of the mitogen-activated protein kinase cascade. Here we report the phosphorylation and inactivation of MKK1 by phosphorylation on threonine 286 and threonine 292. MKK1 contains a consensus phosphorylation site for p34cdc2, a serine/threonine protein kinase that regulates the cell division cycle, at Thr-286 and a related site at Thr-292. p34cdc2 catalyzes the in vitro phosphorylation of MKK1 on both of these threonine residues and inactivates MKK1 enzymatic activity. Both sites are phosphorylated in vivo as well. The data presented in this report provide evidence that MKK1 is negatively regulated by threonine phosphorylation.

Cell cycle-dependent regulation of the phosphorylation and metabolism of the Alzheimer amyloid precursor protein

Accumulation of the amyloid A beta peptide, which is derived from a larger precursor protein (APP), and the formation of plaques, are major events believed to be involved in the etiology of Alzheimer's disease. Abnormal regulation of the metabolism of APP may contribute to the deposition of plaques. APP is an integral membrane protein containing several putative phosphorylation sites within its cytoplasmic domain. We report here that APP is phosphorylated at Thr668 by p34cdc2 protein kinase (cdc2 kinase) in vitro, and in a cell cycle-dependent manner in vivo. At the G2/M phase of the cell cycle, when APP phosphorylation is maximal, the levels of mature APP (mAPP) and immature APP (imAPP) do not change significantly. However, imAPP is altered qualitatively. Furthermore, the level of the secreted extracellular N-terminal domain (APPS) is decreased and that of the truncated intracellular C-terminal fragment (APPCOOH) is increased. These findings suggest the possibility that phosphorylation-dependent events occurring during the cell cycle affect the metabolism of APP. Alterations in these events might play a role in the pathogenesis of Alzheimer's disease.

Dimerization and DNA binding alter phosphorylation of Fos and Jun

Fos and Jun form dimeric complexes that bind to activator protein 1 (AP-1) DNA sequences and regulate gene expression. The levels of expression and activities of these proteins are regulated by a variety of extracellular stimuli. They are thought to function in nuclear signal transduction processes in many different cell types. The role of Fos and Jun in gene transcription is complex and may be regulated in several ways including association with different dimerization partners, interactions with other transcriptional factors, effects on DNA topology, and reduction/oxidation of a conserved cysteine residue in the DNA-binding domain. In addition, phosphorylation has been suggested to control the activity of Fos and Jun. Here we show that phosphorylation of Fos and Jun by several protein kinases is affected by dimerization and binding to DNA. Jun homodimers are phosphorylated efficiently by casein kinase II, whereas Fos-Jun heterodimers are not. DNA binding also reduces phosphorylation of Jun by casein kinase II, p34cdc2 (cdc2) kinase, and protein kinase C. Phosphorylation of Fos by cAMP-dependent protein kinase and cdc2 is relatively insensitive to dimerization and DNA binding, whereas phosphorylation of Fos and Jun by DNA-dependent protein kinase is dramatically stimulated by binding to the AP-1 site. These results imply that different protein kinases can distinguish among Fos and Jun proteins in the form of monomers, homodimers, and heterodimers and between DNA-bound and non-DNA-bound proteins. Thus, potentially, these different states of Fos and Jun can be recognized and regulated independently by phosphorylation.

Phosphorylation at Ser-15 and Ser-392 in mutant p53 molecules from human tumors is altered compared to wild-type p53

The product of the p53 gene suppresses cell growth and plays a critical role in suppressing development of human tumors. p53 protein binds DNA, activates transcription, and can be phosphorylated at N- and C-terminal sites. Previously, wild-type p53 was shown to be hyperphosphorylated compared to mutant p53 during p53-mediated growth arrest in vivo. Here we show that Ser-15 and Ser-9 in the N-terminal transactivation domain of wild-type human p53 are phosphorylated in vivo in cells derived from the human glioblastoma line T98G. In [Ile237]p53 and [Ala143]p53, two natural p53 mutants from human tumors that are defective for activation of transcription, phosphorylation at Ser-15 was reduced and phosphorylation at Ser-392 was increased compared to wild-type p53. No change was observed at Ser-9. [His273]p53, a third mutant, had a phosphorylation state similar to that of wild-type p53. We suggest that phosphorylation of Ser-15 may depend on the ability of p53 to adopt a wild-type conformation and may contribute to p53's ability to block cell growth.

A conserved mitotic kinase active at late anaphase-telophase in syncytial Drosophila embryos

Mutations in the Drosophila gene polo cause abnormal mitotic and meiotic divisions. This gene encodes a 577-amino-acid protein that has an N-terminal putative kinase domain and a 300-residue C-terminal domain. In budding yeast, a homologous kinase is encoded by CDC5 (ref. 3), a gene required for nuclear division late in the mitotic cycle and during meiosis. Murine homologues have also been described. Here we show that the polo gene product immunoprecipitated from extracts of single Drosophila embryos can phosphorylate casein in vitro, and that the kinase activity peaks cyclically at late anaphase/telophase. This contrasts with the cyclical activity of cyclin B-associated p34cdc2 kinase, which is maximal upon entry into mitosis during the rapid cycles of mitosis in the syncytium.

Jun is phosphorylated by several protein kinases at the same sites that are modified in serum-stimulated fibroblasts

c-jun is a member of the family of immediate-early genes whose expression is induced by factors such as serum stimulation, phorbol ester, and differentiation signals. Here we show that increased Jun synthesis after serum stimulation is accompanied by a concomitant increase in phosphorylation. Several serine-threonine kinases were evaluated for their ability to phosphorylate Jun in vitro. p34cdc2, protein kinase C, casein kinase II, and pp44mapk phosphorylated Jun efficiently, whereas cyclic AMP-dependent protein kinase and glycogen synthase kinase III did not. The sites phosphorylated by p34cdc2 were similar to those phosphorylated in vivo after serum induction. The major sites of phosphorylation were mapped to serines 63, 73, and 246. Phosphorylation of full-length Jun with several kinases did not affect the DNA-binding activity of Jun homodimers or Fos-Jun heterodimers. Comparison of the DNA binding and in vitro transcription properties of wild-type and mutated proteins containing either alanine or aspartic acid residues in place of Ser-63, -73, and -246 revealed only minor differences among homodimeric complexes and no differences among Fos-Jun heterodimers. Thus, phosphorylation of Jun did not produce a significant change in dimerization, DNA-binding, or in vitro transcription activity. The regulatory role of phosphorylation in the modulation of Jun function is likely to be considerably more complex than previously suggested.

Jun is phosphorylated by several protein kinases at the same sites that are modified in serum-stimulated fibroblasts

c-jun is a member of the family of immediate-early genes whose expression is induced by factors such as serum stimulation, phorbol ester, and differentiation signals. Here we show that increased Jun synthesis after serum stimulation is accompanied by a concomitant increase in phosphorylation. Several serine-threonine kinases were evaluated for their ability to phosphorylate Jun in vitro. p34cdc2, protein kinase C, casein kinase II, and pp44mapk phosphorylated Jun efficiently, whereas cyclic AMP-dependent protein kinase and glycogen synthase kinase III did not. The sites phosphorylated by p34cdc2 were similar to those phosphorylated in vivo after serum induction. The major sites of phosphorylation were mapped to serines 63, 73, and 246. Phosphorylation of full-length Jun with several kinases did not affect the DNA-binding activity of Jun homodimers or Fos-Jun heterodimers. Comparison of the DNA binding and in vitro transcription properties of wild-type and mutated proteins containing either alanine or aspartic acid residues in place of Ser-63, -73, and -246 revealed only minor differences among homodimeric complexes and no differences among Fos-Jun heterodimers. Thus, phosphorylation of Jun did not produce a significant change in dimerization, DNA-binding, or in vitro transcription activity. The regulatory role of phosphorylation in the modulation of Jun function is likely to be considerably more complex than previously suggested.

A solid-phase screen for protein kinase substrate selectivity

Cassette mutagenesis was used to synthesize an Escherichia coli expression library of unique phosphorylation sites. The cassette encodes a central serine residue surrounded by every combination of Ala, Arg, Gln, Glu, Gly, and Pro residues over a 7-residue segment (a total of 6(7) approximately 2.8 x 10(5) sequences). The cassette was inserted into the gene of a suitable carrier protein and expressed in E. coli with the T7 expression system, and the resultant library was subjected to solid-phase protein phosphorylation assays on nitrocellulose filters. When the library was screened with TPK1 delta, the modified catalytic subunit of the Saccharomyces cerevisiae cAMP-dependent protein kinase, individual colonies that expressed substrates for this kinase were identified. By DNA sequencing through the cassette region of positive clones, the consensus recognition sequence for TPK1 delta was deduced and found to conform with the well-established substrate selectivity of its mammalian homolog (Arg-Arg-Xaa-Ser). Because a large number of clones can be sequenced rapidly, and the positions of invariant residues composing a recognition site identified, this approach may be useful as a general screen of protein kinase substrate selectivity.

Activation of the p34 CDC2 protein kinase at the start of S phase in the human cell cycle

Using a protocol for selecting cells on the basis of both size and age (with respect to the preceding mitosis), we isolated highly synchronous human G1 cells. With this procedure, we demonstrated that the p34 CDC2 kinase was activated at the start of S phase. Cyclin A synthesis began at the same time, and activation of the p34 CDC2 kinase at the start of S phase was, at least in part, due to its association with cyclin A. Furthermore, cells synchronized in late G1 by exposure to the drug mimosine contain active cyclin A/p34 CDC2 kinase, indicating that p34 CDC2 activation can occur before DNA synthesis begins. Thus, the cyclin A/CDC2 complex, which previously has been shown to be sufficient to start SV40 DNA synthesis in vitro, assembles and is activated at the start of S phase in vivo.

Human cyclin E, a new cyclin that interacts with two members of the CDC2 gene family

A new human cyclin, named cyclin E, was isolated by complementation of a triple cln deletion in S. cerevisiae. Cyclin E showed genetic interactions with the CDC28 gene, suggesting that it functioned at START by interacting with the CDC28 protein. Two human genes were identified that could interact with cyclin E to perform START in yeast containing a cdc28 mutation. One was CDC2-HS, and the second was the human homolog of Xenopus CDK2. Cyclin E produced in E. coli bound and activated the CDC2 protein in extracts from human G1 cells, and antibodies against cyclin E immunoprecipitated a histone H1 kinase from HeLa cells. The interactions between cyclin E and CDC2, or CDK2, may be important at the G1 to S transition in human cells.