title: Modification of fos proteins: phosphorylation of c-fos, but not v-fos, is stimulated by 12-tetradecanoyl-phorbol-13-acetate and serum.
abstract: We have investigated the covalent modification of the proteins encoded by the murine fos proto-oncogene (c-fos) and that of the corresponding gene product of FBJ murine osteosarcoma virus (v-fos). Both proteins are posttranslationally processed in the cell, resulting in forms with lower electrophoretic mobilities than that of the initial translation product on sodium dodecyl sulfate-polyacrylamide gels. Treatment with alkaline phosphatase indicates that most, if not all, of this electrophoretic shift is due to phosphoesterification of both proteins. These phosphoryl groups stoichiometrically modify the v-fos and c-fos proteins on serine residues and turn over rapidly in vivo in the presence of protein kinase inhibitors (half-life, less than 15 min). Direct quantitative comparison of steady-state labeling studies with L-[35S]methionine and [32P]phosphate reveals that the c-fos protein is four- to fivefold more highly phosphorylated than the v-fos protein is. Comparison of tryptic fragments from [32P]phosphate-labeled proteins indicates that although the two proteins have several tryptic phosphopeptides in common, the c-fos protein contains unique major tryptic phosphopeptides that the v-fos protein lacks. These unique sites of c-fos phosphorylation have been tentatively localized to the carboxy-terminal 20 amino acid residues of the protein. Phosphorylation of the c-fos protein, but not the v-fos protein, can be stimulated at least fivefold in vivo by the addition of either 12-tetradecanoyl-phorbol-13-acetate or serum. This increase in the steady-state degree of phosphorylation of c-fos appears to be independent of protein kinase C since phosphorylation is Ca2+ and diacylglycerol independent. The possible role of phosphorylation of these proteins in cellular transformation is discussed.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001769:Blood; D002460:Cell Line; D003470:Culture Media; D015683:Gene Products, gag; D005796:Genes; D051379:Mice; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D016760:Proto-Oncogene Proteins c-fos; D011519:Proto-Oncogenes; D012190:Retroviridae / Q000235:genetics; D012191:Retroviridae Proteins / Q000235:genetics*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*
chemical_list: D003470:Culture Media; D015683:Gene Products, gag; D011518:Proto-Oncogene Proteins; D016760:Proto-Oncogene Proteins c-fos; D012191:Retroviridae Proteins; D013755:Tetradecanoylphorbol Acetate