Mitosis-specific phosphorylation of the nuclear oncoproteins Myc and Myb

Systematic identification of mitotic phosphoproteins

BACKGROUND

Cyclin-dependent kinases (CDKs) are thought to initiate and coordinate cell division processes by sequentially phosphorylating key targets; in most cases these substrates remain unidentified.

RESULTS

Using a screen that scores for phosphorylation of proteins, which were translated from pools of cDNA plasmids in vitro, by either phosphoepitope antibody recognition or electrophoretic mobility shifts, we have identified 20 mitotically phosphorylated proteins from Xenopus embryos, 15 of which have sequence similarity to other proteins. Of these proteins, five have previously been shown to be phosphorylated during mitosis (epithelial-microtubule associated protein-115, Oct91, Elongation factor 1gamma, BRG1 and Ribosomal protein L18A), five are related to proteins postulated to have roles in mitosis (epithelial-microtubule associated protein-115, Schizosaccharomyces pombe Cdc5, innercentrosome protein, BRG1 and the RNA helicase WM6), and nine are related to transcription factors (BRG1, negative co-factor 2alpha, Oct91, S. pombe Cdc5, HoxD1, Sox3, Vent2, and two isoforms of Xbr1b). Of 16 substrates tested, 14 can be directly phosphorylated in vitro by the mitotic CDK, cyclin B-Cdc2, although three of these may be physiological substrates of other kinases activated during mitosis.

CONCLUSIONS

Examination of this broad set of mitotic phosphoproteins has allowed us to draw three conclusions about how the activation of CDKs regulates cell-cycle events. First, Cdc2 itself appears to directly phosphorylate most of the mitotic phosphoproteins. Second, during mitosis most of the substrates are phosphorylated more than once and a number may be targets of multiple kinases, suggesting combinatorial regulation. Third, the large fraction of mitotic phosphoproteins that are presumptive transcription factors, two of which have been previously shown to dissociate from DNA during mitosis, suggests that an important function of mitotic phosphorylation is to strip the chromatin of proteins associated with gene expression.

Functional analysis of phosphorylation at serine 532 of human c-Myb by MAP kinase

The c-myb proto-oncogene encodes a transcription factor that is implicated in regulatory events during hematopoiesis. It contains negative regulatory domains at both the amino- and carboxy-termini. Here we describe that human c-Myb can be phosphorylated by mitogen-activated protein kinases (MAPK's) at serine 532 of the carboxy (C-) terminal regulatory domain in vitro. This serine residue can also be phosphorylated in vivo upon serum-stimulation of Jurkat cells. Expression of a constitutively active form of Ras together with c-Myb in transient transfection experiments had no effect on the transcriptional activity of c-Myb, while expression of a polypeptide containing the c-Myb C-terminal domain stimulated c-Myb activity. This effect is reduced upon MAPK-dependent phosphorylation of serine 532. Our data suggest that the MAPK-dependent state of phosphorylation modifies the cellular function of c-Myb by modulating its interaction with a putative inhibitory factor.

Cell cycle stage-specific phosphorylation of the Epstein-Barr virus immortalization protein EBNA-LP

EBNA-LP is a viral nuclear oncoprotein implicated in the immortalization of B lymphocytes by Epstein-Barr virus. An analysis of EBNA-LP migration on polyacrylamide gels was performed with protein derived from the X50-7 lymphoblastoid cell line blocked by hydroxyurea or aphidicolin at the G1/S phase of the cell cycle or by nocodazole at the G2/M phase. More slowly migrating species of EBNA-LP were detected in G2/M phase-arrested cell extracts. Release from nocodazole G2/M block or treatment with phosphatase caused the more slowly migrating species of EBNA-LP to disappear. Analyses of 32PO(4)(3-)-labeled EBNA-LP protein immunoprecipitated from the drug-synchronized cells showed that phosphorylated EBNA-LP was present throughout the cell cycle but that phosphorylation increased in G2 and was maximal at G2/M. Phosphoamino acid analysis revealed that all phosphorylation was on serine residues only. The ability of EBNA-LP to be phosphorylated by p34(cdc2) kinase and casein kinase II exclusively on serines implicates these enzymes as being potentially involved in EBNA-LP phosphorylation.

Mitotic regulation of TFIID: inhibition of activator-dependent transcription and changes in subcellular localization

Mitosis in higher eukaryotes is accompanied by a general inhibition of transcription. To begin to understand the mechanisms underlying this inhibition we have examined the behavior of the general transcription factor TFIID during mitosis. Immunocytochemistry and subcellular fractionation studies indicate that the majority of TFIID is displaced from the disassembling prophase nucleus to the mitotic cytoplasm around the time of nuclear envelope breakdown. However, a subpopulation of TFIID remains associated tightly with the condensed mitotic chromosomes. Metabolic labeling of mitotic cells revealed that several subunits of TFIID undergo mitosis-specific phosphorylation, but in spite of these changes, the TFIID complex remains intact. Functional analysis of purified TFIID from mitotic cells shows that phosphorylated forms are unable to direct activator-dependent transcription, but that this activity is restored upon dephosphorylation. These results demonstrate that TFIID regulation by phosphorylation is likely to have an important role in mitotic inhibition of RNA polymerase II transcription. In addition, they suggest a mechanism for regulating gene expression through the selective disruption of polymerase II promoter structures during mitosis.

Differential regulation of c-Myb-induced transcription activation by a phosphorylation site in the negative regulatory domain

The c-myb protooncogene encodes a highly conserved 75-89-kDa transcription factor that contains three functional domains, an amino-terminal DNA binding domain (DBD), a central acidic transactivation domain, and a carboxyl-terminal negative regulatory domain (NRD). Two acute transforming retroviruses, avian myeloblastosis virus and the E26 leukemia virus, transduced portions of c-myb and encode Myb proteins that are truncated in both the DBD and the NRD. Several conserved potential sites for phosphorylation by proline-directed serine/threonine protein kinases reside in or near the NRD, suggesting that phosphorylation might play a role in regulating c-Myb. We have previously demonstrated that serine 528, located in the NRD, is a target for p42(mapk) in vitro. Serine 528 is phosphorylated in vivo in several cell lines, and substitution of serine 528 to alanine (S528A) resulted in an increased ability of Myb to transactivate a synthetic promoter containing five copies of the mim-1A Myb-responsive element and a minimal herpes tk promoter. We have tested the ability of S528A Myb to transactivate a series of cellular target promoters and report that the serine to alanine substitution increased the ability of Myb to activate transcription from the CD34 promoter but not the c-myc or mim-1 promoters. This suggests that phosphorylation of serine 528 may differentially regulate c-Myb activity at different promoters. The DNA binding and multimerization activities of c-Myb appear to be unaffected by the S528A substitution, suggesting that phosphorylation of serine 528 may mediate its effect on the transcription transactivating activity of c-Myb by regulating interactions with other proteins.

Binding of A/T-rich DNA by three high mobility group-like domains in c-Abl tyrosine kinase

The c-Abl tyrosine kinase has been shown previously to bind DNA. Using polymerase chain reaction-based binding site-selection methods, no consensus high affinity binding site for c-Abl was found. Instead, oligonucleotides with runs of A/T sequences were isolated, and purified c-Abl was shown to bind A/T-containing oligonucleotides better than those without A/T sequences. DNA binding of c-Abl was dependent on three high mobility group 1-like boxes (HLBs), which bound cooperatively to the A/T-rich oligonucleotides. To distinguish binding to A/T sequences per se from binding to nonspecific DNA with a bend at the A/T-rich region, two oligonucleotides were compared for binding to c-Abl. Both oligonucleotides contained A/T sequences. In one, the A/T motif was part of an 80-mer duplex DNA. In another, the A/T motif was in the duplex arm of an 80-mer "bubble DNA" containing an internal unpaired 20-mer region to provide a flexible hinge. Interestingly, the HLBs of c-Abl bound better to the oligonucleotide containing the bubble, suggesting a higher affinity for bent DNA rather than A/T sequences per se. Taken together, these observations define a new class of DNA binding domains, the HLBs, which do not bind DNA with a high degree of sequence specificity, but may selectively bind to bent DNA or to sequences that are easier to distort.

The hbrm and BRG-1 proteins, components of the human SNF/SWI complex, are phosphorylated and excluded from the condensed chromosomes during mitosis

In yeast, the SNF/SWI complex is believed to regulate transcription by locally altering the chromatin structure. At the present time, three human homologues of yeast SNF/SWI proteins have been characterized: hbrm and BRG-1, homologues of SNF2/SWI2, and hSNF5, a homologue of SNF5. We show here that, during mitosis, hbrm and BRG-1 are phosphorylated and excluded from the condensed chromosomes. In this phase of the cell cycle, the level of hbrm protein is also strongly reduced, whereas the level of BRG-1 remains constant. The mitotic phosphorylation of hbrm and BRG-1 is found not to disrupt the association of these proteins with hSNF5 but correlates with a decreased affinity for the nuclear structure in early M phase. We suggest that chromosomal exclusion of the human SNF/SWI complex at the G2-M transition could be part of the mechanism leading to transcriptional arrest during mitosis.

Inhibition of cell proliferation by the Mad1 transcriptional repressor

Mad1 is a basic helix-loop-helix-leucine zipper protein that is induced upon differentiation of a number of distinct cell types. Mad1 dimerizes with Max and recognizes the same DNA sequences as do Myc:Max dimers. However, Mad1 and Myc appear to have opposing functions. Myc:Max heterodimers activate transcription while Mad:Max heterodimers repress transcription from the same promoter. In addition Mad1 has been shown to block the oncogenic activity of Myc. Here we show that ectopic expression of Mad1 inhibits the proliferative response of 3T3 cells to signaling through the colony-stimulating factor-1 (CSF-1) receptor. The ability of over-expressed Myc and cyclin D1 to complement the mutant CSF-1 receptor Y809F (containing a Y-to-F mutation at position 809) is also inhibited by Mad1. Cell cycle analysis of proliferating 3T3 cells transfected with Mad1 demonstrates a significant decrease in the fraction of cells in the S and G2/M phases and a concomitant increase in the fraction of G1 phase cells, indicating that Mad1 negatively influences cell cycle progression from the G1 to the S phase. Mutations in Mad1 which inhibit its activity as a transcription repressor also result in loss of Mad1 cell cycle inhibitory activity. Thus, the ability of Mad1 to inhibit cell cycle progression is tightly coupled to its function as a transcriptional repressor.

Winner of the Theodore E. Woodward Award: c-Myb and the coordinate regulation of thymic genes

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Casein kinase II phosphorylation site mutations in c-Myb affect DNA binding and transcriptional cooperativity with NF-M

Phosphorylation of c-Myb has been implicated in the regulation of the binding of c-Myb to DNA. We show that murine c-Myb is phosphorylated at Ser-11 and -12 in vivo and that these sites can be phosphorylated in vitro by casein kinase II (CKII), analogous to chicken c-Myb. An efficient method to study DNA binding properties of full-length c-Myb and Myb mutants under nondenaturing conditions was developed. It was found that a Myb mutant in which Ser-11 and -12 were replaced with Ala (Myb Ala-11/12), wild-type c-Myb, and Myb Asp-11/12 bound to the A site of the mim-1 promoter with decreasing affinities. In agreement with this finding, Myb Ala-11/12 transactivated better than wild-type c-Myb and Myb Asp-11/12 on the mim-1 promoter or a synthetic Myb-responsive promoter. Similar observations were made for the myeloid-specific neutrophil elastase promoter. The presence of NF-M or an NF-M-like activity abolished partially the differences seen with the Ser-11/12 mutants, suggesting that the reduced DNA binding due to negative charge at positions 11 and 12 can be compensated for by NF-M. Since no direct interaction of c-Myb and NF-M was observed, we propose that the cooperativity is mediated by a third factor. Our data offer two possibilities for how casein kinase II phosphorylation can influence c-Myb function: first, by reducing c-Myb DNA binding and thereby influencing transactivation, and second, by enhancing the apparent cooperativity between c-Myb and NF-M or an NF-M-like activity.

The regulation of protein transport to the nucleus by phosphorylation

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Modulation of c-Myb-induced transcription activation by a phosphorylation site near the negative regulatory domain

The c-myb protooncogene encodes a highly conserved transcription factor that functions as both an activator and a repressor of transcription. The v-myb oncogenes of E26 leukemia virus and avian myeloblastosis virus encode proteins that are truncated at both the amino and the carboxyl terminus, deleting portions of the c-Myb DNA-binding and negative regulatory domains. This has led to speculation that the deleted regions contain important regulatory sequences. We previously reported that the 42-kDa mitogen-activated protein kinase (p42mapk) phosphorylates chicken and murine c-Myb at multiple sites in the negative regulatory domain in vitro, suggesting that phosphorylation might provide a mechanism to regulate c-Myb function. We now report that three tryptic phosphopeptides derived from in vitro phosphorylated c-Myb comigrate with three tryptic phosphopeptides derived from metabolically labeled c-Myb immunoprecipitated from murine erythroleukemia cells. At least two of these peptides are phosphorylated on serine-528. Replacement of serine-528 with alanine results in a 2- to 7-fold increase in the ability of c-Myb to transactivate a Myb-responsive promoter/reporter gene construct. These findings suggest that phosphorylation serves to regulate c-Myb activity and that loss of this phosphorylation site from the v-Myb proteins may contribute to their transforming potential.

Cyclin-dependent protein kinases: key regulators of the eukaryotic cell cycle

Passage through the cell cycle requires the successive activation of different cyclin-dependent protein kinases (CDKs). These enzymes are controlled by transient associations with cyclin regulatory subunits, binding of inhibitory polypeptides and reversible phosphorylation reactions. To promote progression towards DNA replication, CDK/cyclin complexes phosphorylate proteins required for the activation of genes involved in DNA synthesis, as well as components of the DNA replication machinery. Subsequently, a different set of CDK/cyclin complexes triggers the phosphorylation of numerous proteins to promote the profound structural reorganizations that accompany the entry of cells into mitosis. At present, much research is focused on elucidating the links between CDK/cyclin complexes and signal transduction pathways controlling cell growth, differentiation and death. In future, a better understanding of the cell cycle machinery and its deregulation during oncogenesis may provide novel opportunities for the diagnostic and therapeutic management of cancer and other proliferation-related diseases.

H1TF2A, the large subunit of a heterodimeric, glutamine-rich CCAAT-binding transcription factor involved in histone H1 cell cycle regulation

H1TF2 is a CCAAT transcription factor that binds to the histone H1 subtype-specific consensus sequence, which has previously been shown to be necessary for temporal regulation of histone H1 transcription during the cell cycle (F. La Bella, P. Gallinari, J. McKinney, and N. Heintz, Genes Dev. 3:1982-1990, 1989). In this study, we report that H1TF2 is a heteromeric CCAAT-binding protein composed of two polypeptide doublets of 33 and 34 kDa and 43 and 44 kDa that are not antigenically related. The 33- and 34-kDa species were not detected in our previous studies (P. Gallinari, F. La Bella, and N. Heintz, Mol. Cell. Biol. 9:1566-1575, 1989) because of technical problems in detection of these heavily glycosylated subunits. The cloning of H1TF2A, the large subunit of this factor, reveals it to be a glutamine-rich protein with extremely limited similarity to previously cloned CCAAT-binding proteins. A monospecific antiserum produced against bacterially synthesized H1TF2A was used to establish that HeLa cell H1TF2A is phosphorylated in vivo and that, in contrast to the H2b transcription factor Oct1 (S. B. Roberts, N. Segil, and N. Heintz, Science 253:1022-1026, 1991; N. Segil, S. B. Roberts, and N. Heintz, Cold Spring Harbor Symp. Quant. Biol. 56:285-292, 1991), no gross change in H1TF2A phosphorylation is evident during the cell cycle. Further immunoprecipitation studies demonstrated that H1TF2 is heterodimeric in the absence of DNA in vivo and identified several H1TF2-interacting proteins that may play a role in H1TF2 function in vivo.

Production of a functional full-length Xenopus laevis c-Myc protein in insect cells

C-Myc is a nuclear phosphoprotein whose normal cellular function has not yet been clearly defined. Studies with this protein have always been constrained by the difficulty of obtaining full-length c-Myc in an active form, whatever the expression system used. We report here experimental conditions optimized to increase the solubility and the purification of c-Myc in a baculovirus expression system. Such conditions allow the production of both soluble and active full-length c-Myc. Interestingly, soluble c-Myc is found associated with a 500-kDa high-molecular-mass complex comparable to that found in human and Xenopus laevis embryos, and which may be required for its function in vivo.

H1TF2A, the large subunit of a heterodimeric, glutamine-rich CCAAT-binding transcription factor involved in histone H1 cell cycle regulation

H1TF2 is a CCAAT transcription factor that binds to the histone H1 subtype-specific consensus sequence, which has previously been shown to be necessary for temporal regulation of histone H1 transcription during the cell cycle (F. La Bella, P. Gallinari, J. McKinney, and N. Heintz, Genes Dev. 3:1982-1990, 1989). In this study, we report that H1TF2 is a heteromeric CCAAT-binding protein composed of two polypeptide doublets of 33 and 34 kDa and 43 and 44 kDa that are not antigenically related. The 33- and 34-kDa species were not detected in our previous studies (P. Gallinari, F. La Bella, and N. Heintz, Mol. Cell. Biol. 9:1566-1575, 1989) because of technical problems in detection of these heavily glycosylated subunits. The cloning of H1TF2A, the large subunit of this factor, reveals it to be a glutamine-rich protein with extremely limited similarity to previously cloned CCAAT-binding proteins. A monospecific antiserum produced against bacterially synthesized H1TF2A was used to establish that HeLa cell H1TF2A is phosphorylated in vivo and that, in contrast to the H2b transcription factor Oct1 (S. B. Roberts, N. Segil, and N. Heintz, Science 253:1022-1026, 1991; N. Segil, S. B. Roberts, and N. Heintz, Cold Spring Harbor Symp. Quant. Biol. 56:285-292, 1991), no gross change in H1TF2A phosphorylation is evident during the cell cycle. Further immunoprecipitation studies demonstrated that H1TF2 is heterodimeric in the absence of DNA in vivo and identified several H1TF2-interacting proteins that may play a role in H1TF2 function in vivo.

Hierarchical phosphorylation at N-terminal transformation-sensitive sites in c-Myc protein is regulated by mitogens and in mitosis

The N-terminal domain of the c-Myc protein has been reported to be critical for both the transactivation and biological functions of the c-Myc proteins. Through detailed phosphopeptide mapping analyses, we demonstrate that there is a cluster of four regulated and complex phosphorylation events on the N-terminal domain of Myc proteins, including Thr-58, Ser-62, and Ser-71. An apparent enhancement of Ser-62 phosphorylation occurs on v-Myc proteins having a mutation at Thr-58 which has previously been correlated with increased transforming ability. In contrast, phosphorylation of Thr-58 in cells is dependent on a prior phosphorylation of Ser-62. Hierarchical phosphorylation of c-Myc is also observed in vitro with a specific glycogen synthase kinase 3 alpha, unlike the promiscuous phosphorylation observed with other glycogen synthase kinase 3 alpha and 3 beta preparations. Although both p42 mitogen-activated protein kinase and cdc2 kinase specifically phosphorylate Ser-62 in vitro and cellular phosphorylation of Thr-58/Ser-62 is stimulated by mitogens, other in vivo experiments do not support a role for these kinases in the phosphorylation of Myc proteins. Unexpectedly, both the Thr-58 and Ser-62 phosphorylation events, but not other N-terminal phosphorylation events, can occur in the cytoplasm, suggesting that translocation of the c-Myc proteins to the nucleus is not required for phosphorylation at these sites. In addition, there appears to be an unusual block to the phosphorylation of Ser-62 during mitosis. Finally, although the enhanced transforming properties of Myc proteins correlates with the loss of phosphorylation at Thr-58 and an enhancement of Ser-62 phosphorylation, these phosphorylation events do not alter the ability of c-Myc to transactivate through the CACGTG Myc/Max binding site.

Hierarchical phosphorylation at N-terminal transformation-sensitive sites in c-Myc protein is regulated by mitogens and in mitosis

The N-terminal domain of the c-Myc protein has been reported to be critical for both the transactivation and biological functions of the c-Myc proteins. Through detailed phosphopeptide mapping analyses, we demonstrate that there is a cluster of four regulated and complex phosphorylation events on the N-terminal domain of Myc proteins, including Thr-58, Ser-62, and Ser-71. An apparent enhancement of Ser-62 phosphorylation occurs on v-Myc proteins having a mutation at Thr-58 which has previously been correlated with increased transforming ability. In contrast, phosphorylation of Thr-58 in cells is dependent on a prior phosphorylation of Ser-62. Hierarchical phosphorylation of c-Myc is also observed in vitro with a specific glycogen synthase kinase 3 alpha, unlike the promiscuous phosphorylation observed with other glycogen synthase kinase 3 alpha and 3 beta preparations. Although both p42 mitogen-activated protein kinase and cdc2 kinase specifically phosphorylate Ser-62 in vitro and cellular phosphorylation of Thr-58/Ser-62 is stimulated by mitogens, other in vivo experiments do not support a role for these kinases in the phosphorylation of Myc proteins. Unexpectedly, both the Thr-58 and Ser-62 phosphorylation events, but not other N-terminal phosphorylation events, can occur in the cytoplasm, suggesting that translocation of the c-Myc proteins to the nucleus is not required for phosphorylation at these sites. In addition, there appears to be an unusual block to the phosphorylation of Ser-62 during mitosis. Finally, although the enhanced transforming properties of Myc proteins correlates with the loss of phosphorylation at Thr-58 and an enhancement of Ser-62 phosphorylation, these phosphorylation events do not alter the ability of c-Myc to transactivate through the CACGTG Myc/Max binding site.

Does a synthetic peptide containing the leucine-zipper domain of c-myb form an alpha-helical structure in solution?

We have examined a synthetic peptide containing the putative leucine zipper domain of the chicken c-myb proto-oncogene using circular dichroism (CD) spectroscopy. The peptide adopts an alpha-helical structure only at low temperatures and in the presence 2,2,2-trifluoroethanol.

Cellular substrates of p34cdc2 and its companion cyclin-dependent kinases

Cyclin-dependent kinase (cdks) are key components of the engine that drives the cell proliferation cycle in all eukaryotes. These kinases are related to p34(cdc2) and associate with regulatory subunits belonging to the cyclin family. To understand how cdks promote cell cycle progression, it will be important to identify their physiological substrates and to determine how phosphorylation influences the functions of these substrates. This article discusses recent progress as well as some of the problems related to the quest for cdk substrates.

Cell cycle regulation of the c-Myc transcriptional activation domain

The product of the c-myc gene (c-Myc) is a sequence-specific DNA-binding protein that has previously been demonstrated to be required for cell cycle progression. Here we report that the c-Myc DNA binding site confers cell cycle regulation to a reporter gene in Chinese hamster ovary cells. The observed transactivation was biphasic with a small increase in G1 and a marked increase during the S-to-G2/M transition of the cell cycle. This cell cycle regulation of transactivation potential is accounted for, in part, by regulatory phosphorylation of the c-Myc transactivation domain. Together, these data demonstrate that c-Myc may have an important role in the progression of cells through both the G1 and G2 phases of the cell cycle.

Cell cycle regulation of the c-Myc transcriptional activation domain

The product of the c-myc gene (c-Myc) is a sequence-specific DNA-binding protein that has previously been demonstrated to be required for cell cycle progression. Here we report that the c-Myc DNA binding site confers cell cycle regulation to a reporter gene in Chinese hamster ovary cells. The observed transactivation was biphasic with a small increase in G1 and a marked increase during the S-to-G2/M transition of the cell cycle. This cell cycle regulation of transactivation potential is accounted for, in part, by regulatory phosphorylation of the c-Myc transactivation domain. Together, these data demonstrate that c-Myc may have an important role in the progression of cells through both the G1 and G2 phases of the cell cycle.

Targets of cyclin-dependent protein kinases

Our current understanding of the eukaryotic cell cycle attributes a key regulatory role to cyclin-dependent protein kinases. It is important, therefore, to identify the physiological substrates of these kinases, and to understand how the phosphorylation of such proteins promotes cell cycle progression.