title: Evidence for an important role of serine 16 and its phosphorylation in the stabilization of c-Mos.
abstract: 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.
authors.affiliation: Department of Molecular Pathology, University of Texas MD Anderson Cancer Center, Houston 77030, USA.
authors.forename: C D
authors.initials: CD
authors.lastname: Pham
authors.forename: V B
authors.initials: VB
authors.lastname: Vuyyuru
authors.forename: Y
authors.initials: Y
authors.lastname: Yang
authors.forename: W
authors.initials: W
authors.lastname: Bai
authors.forename: B
authors.initials: B
authors.lastname: Singh
mesh_terms: D016475:3T3 Cells; D000818:Animals; D019556:COS Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002845:Chromatography; D016521:Electrophoresis, Gel, Pulsed-Field; D015971:Gene Expression Regulation, Enzymologic; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D011233:Precipitin Tests; D017231:Proto-Oncogene Proteins c-mos / Q000378:metabolism / Q000502:physiology*; D012694:Serine / Q000502:physiology*; D013997:Time Factors; D014162:Transfection; D014410:Tumor Stem Cell Assay
chemical_list: D012694:Serine; D017231:Proto-Oncogene Proteins c-mos; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases