Evidence of a functional interaction between serine 3 and serine 25 Mos phosphorylation sites. A dominant inhibitory role of serine 25 phosphorylation on Mos protein kinase

Cyclin B/cdc2 induces c-Mos stability by direct phosphorylation in Xenopus oocytes

The c-Mos proto-oncogene product plays an essential role during meiotic divisions in vertebrate eggs. In Xenopus, it is required for progression of oocyte maturation and meiotic arrest of unfertilized eggs. Its degradation after fertilization is essential to early embryogenesis. In this study we investigated the mechanisms involved in c-Mos degradation. We present in vivo evidence for ubiquitin-dependent degradation of c-Mos in activated eggs. We found that c-Mos degradation is not directly dependent on the anaphase-promoting factor activator Fizzy/cdc20 but requires cyclin degradation. We demonstrate that cyclin B/cdc2 controls in vivo c-Mos phosphorylation and stabilization. Moreover, we show that cyclin B/cdc2 is capable of directly phosphorylating c-Mos in vitro, inducing a similar mobility shift to the one observed in vivo. Tryptic phosphopeptide analysis revealed a practically identical in vivo and in vitro phosphopeptide map and allowed identification of serine-3 as the largely preferential phosphorylation site as previously described (Freeman et al., 1992). Altogether, these results demonstrate that, in vivo, stability of c-Mos is directly regulated by cyclin B/cdc2 kinase activity.

Hsp90 is required for c-Mos activation and biphasic MAP kinase activation in Xenopus oocytes

During Xenopus oocyte maturation, the Mos protein kinase is synthesized and activates the MAP kinase cascade. In this report, we demonstrate that the synthesis and activation of Mos are two separable processes. We find that Hsp90 function is required for activation and phosphorylation of Mos and full activation of the MAP kinase cascade. Once Mos is activated, Hsp90 function is no longer required. We show that Mos interacts with both Hsp90 and Hsp70, and that there is an inverse relationship between association of Mos with these two chaperones. We propose that Mos protein kinase is activated by a novel mechanism involving sequential association with Hsp70 and Hsp90 as well as phosphorylation. We also present evidence for a two-phase activation of MAP kinase in Xenopus oocytes.

Glutamic acid mutagenesis of retinoblastoma protein phosphorylation sites has diverse effects on function

The retinoblastoma tumor suppressor gene (Rb) has many functions within the cell including regulation of transcription, differentiation, apoptosis, and the cell cycle. Regulation of these functions is mediated by phosphorylation at as many as 16 cyclin-dependent kinase (CDK) phosphorylation sites in vivo. The contribution of these sites to the regulation of the various Rb functions is not well understood. To characterize the effect of phosphorylation at these sites, we systematically mutagenized the serines or threonines to glutamic acid. Thirty-five mutants with different combinations of modified phosphorylation sites were assayed for their ability to arrest the cell cycle and for their potential to induce differentiation. Only the most highly substituted mutants failed to arrest cell cycle progression. However, mutants with as few as four modified phosphorylation sites were unable to promote differentiation. Other mutants had increased activity in this assay. We conclude that modification of Rb phosphorylation sites can increase or decrease protein activity, that different Rb functions can be regulated independently by distinct combinations of sites, and that the effects of modification at any one site are context dependent.

Evidence for an important role of serine 16 and its phosphorylation in the stabilization of c-Mos

The c-Mos serine/threonine protein kinase is an essential component of cytostatic factor (CSF), which is required for metaphase II arrest of eggs in vertebrates. Previously, we showed that c-Mos residue Ser-16 is phosphorylated in the ts110 Mo-MuSV-encoded Gag-Mos fusion protein. Here we provide evidence that Mos is phosphorylated at Ser-16 in transfected COS-1 cells. To investigate the role of this phosphorylation, Ser-16 was substituted with alanine or glutamic acid in full-length v-Mos (an Env-Mos fusion protein that contains 31 additional amino acids at the amino terminus of c-Mos), its mouse c-Mos equivalent version (v-Mos residues 32-374, hereafter referred to as Mos), and mouse c-Mos. Constructs expressing mutant versions of Mos were transfected into COS-1 and NIH3T3 cells in a transient and stable manner, respectively. Synthesis and proteolysis of Mos were evaluated by pulse-chase analysis of 35S-methionine-labeled proteins. Our findings indicate that the S16A mutant of Mos was highly unstable. It accumulated to approximately 10% of the level of wild-type Mos or its S16E mutant. In addition, the S16A mutation but not the S16E mutation inhibited Mos interaction with a cellular protein, p35, suggesting that phosphorylation at Ser-16 may promote Mos interaction with p35. As expected from its destabilizing effect, the S16A mutation caused a dramatic decrease in the cellular transforming activity of Mos (determined by soft-agar colony-formation assay with the stably transfected NIH3T3 cells), which is known to correlate with its CSF function. Efficient ubiquitin-mediated proteolysis of c-Mos requires proline as the second residue from the amino-terminus. In contrast to Mos, neither the stability nor protein kinase activity of v-Mos (in which c-Mos residue Pro-2 becomes Pro-33) was affected by the S16A mutation. To provide further proof that, similar to c-Mos, the S16A mutant is recognized by the proteolysis system through Pro-2, we show that the effect of the S16A mutation is reversed by the Pro-2-Ala mutation. Thus, our results indicate that Ser-16 has an important role in the regulation of c-Mos and that phosphorylation at Ser-16 may inhibit proteolysis of c-Mos.

Evidence of an interaction between Mos and Hsp70: a role of the Mos residue serine 3 in mediating Hsp70 association

c-Mos is a germ cell-specific MAP kinase kinase kinase (MAPKKK) that plays an essential role during meiotic divisions of oocytes. c-Mos is a key component of an activity, cytostatic factor, required for metaphase II arrest of unfertilized eggs in vertebrates. To understand the regulation of c-Mos, we are investigating c-Mos-interacting proteins. We provide evidence that mouse c-Mos binds to Hsp70, a molecular chaperone. Hsp70 was found to associate with Mos ectopically expressed in COS-1 cells. Mos-Hsp70 complexes could be immunoprecipitated with both Mos and Hsp70 antibodies. Despite a low-abundance of Mos, the Hsp70 antibody immunoprecipitated Mos as the major protein. Of importance, the Mos protein present in anti-Hsp70 immunoprecipitates functioned as an active MAPKKK indicating that it is not grossly misfolded. It is known that c-Mos protein kinase activity in cell extracts of transfected COS-1 or NIH3T3 cells is labile. We found that the inclusion of adenosine triphosphate (ATP) in cell extracts protected against the loss of Mos kinase activity. In the absence of ATP from cell extracts, protein kinase activity of Mos was lost within 6 h on ice even though the Mos protein was not degraded and remained bound to Hsp70. Based on our identification of c-Mos-Hsp70 interaction, one of the roles of ATP may be to assist the regulation of c-Mos via ATP involvement in the protein-folding function of Hsp70 and possibly other molecular chaperones. We also detected by coimmunoprecipitation a physical association between endogenous c-Mos and Hsp70 in Xenopus eggs. To provide further evidence for the functional significance of Hsp70 interaction to Mos function, we show that the residue serine 3 in Mos, which is important for the regulation of protein kinase activity of Mos is also important for Hsp70 association.