Polyomavirus middle-sized tumor antigen modulates c-Jun phosphorylation and transcriptional activity.
Polyomavirus middle-sized tumor antigen (MT) increases the expression of c-jun through a phorbol 12-O-tetradecanoate 13-acetate response element in the c-jun promoter. To investigate the cellular signaling pathways affected by MT, we studied the role of the c-Ras and Raf-1 proteins in MT-induced transactivation of c-jun and cell transformation. There was an increase in GTP complexed to Ras in MT-expressing cells, indicating an increase in Ras activity. Coexpression of dominant inhibitory mutants of Ha-ras and raf-1 with MT inhibited MT-mediated transactivation and focus formation. Studies of the phosphorylation of c-Jun showed that MT expression increased the phosphorylation of Ser-63 and Ser-73 in the transactivation domain and decreased the phosphorylation of a peptide containing Ser-243, Ser-249, and Thr-231 in the DNA binding domain. MT increased the transcriptional activating ability of c-Jun but failed to increase the transcriptional activating ability of c-Jun mutants with Ser-63 and Ser-73 changed to nonphosphorylatable Ala, indicating that MT modulates c-Jun activity through phosphorylation. The dominant inhibitory mutants of Ha-ras and raf-1 interfered with the ability of MT to activate c-Jun. The results indicate that MT induces a phosphorylation cascade through the activation of c-Ras and Raf-1 and that c-Jun is one of the downstream targets that may cause changes in gene expression leading to cell transformation.
Srinivas
S
S
Molecular Biology and Virology Laboratory, Salk Institute, San Diego, CA 92186.
Schönthal
A
A
Eckhart
W
W
D016475:3T3 Cells; D000818:Animals; D000952:Antigens, Polyomavirus Transforming / Q000378:metabolism*; D015500:Chloramphenicol O-Acetyltransferase / Q000032:analysis / Q000096:biosynthesis; D015870:Gene Expression / Q000187:drug effects; D016758:Genes, jun; D011905:Genes, ras; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D051379:Mice; D010748:Phosphopeptides / Q000737:chemistry / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis; D016755:Proto-Oncogene Proteins c-jun / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf; D011519:Proto-Oncogenes; D012694:Serine / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate; D013912:Threonine / Q000378:metabolism; D014158:Transcription, Genetic; D014162:Transfection; D018631:ras Proteins / Q000378:metabolism
D000952:Antigens, Polyomavirus Transforming; D010748:Phosphopeptides; D011518:Proto-Oncogene Proteins; D016755:Proto-Oncogene Proteins c-jun; D006153:Guanosine Diphosphate; D013912:Threonine; D012694:Serine; D006160:Guanosine Triphosphate; D015500:Chloramphenicol O-Acetyltransferase; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D018631:ras Proteins; D013755:Tetradecanoylphorbol Acetate