Activation of the pp60c-src kinase by middle T antigen binding or by dephosphorylation.
The transforming protein of polyoma virus, middle T antigen, associates with the protein tyrosine kinase pp60c-src, and analysis of mutants of middle T suggests that this complex plays an important role in transformation by polyoma. It has recently been reported that pp60c-src from polyoma virus-transformed cells has enhanced tyrosine kinase activity in vitro. The data presented here confirm these findings and show that the enhanced kinase activity of pp60c-src is due to an increase in the Vmax of the enzyme. Sucrose density gradient analysis demonstrates that only the form of pp60c-src which is bound to middle T antigen is activated. The difference in enzyme activity between pp60c-src from normal and middle T-transformed cells is more marked when the enzyme is prepared from lysates containing the phosphotyrosine protein phosphatase inhibitor, sodium orthovanadate. pp60c-src from middle T transformed cells is unaffected, but pp60c-src from normal cells has reduced kinase activity if dephosphorylation is prevented. The kinase activity of pp60c-src thus appears to be regulated by its degree of phosphorylation at tyrosine, and data are presented which support this hypothesis. pp60c-src is the first example of a protein tyrosine kinase whose activity is inhibited by phosphorylation at tyrosine. Middle T antigen may increase the kinase activity of pp60c-src by preventing phosphorylation at this regulatory site.
Courtneidge
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D000818:Animals; D000952:Antigens, Polyomavirus Transforming; D000957:Antigens, Viral, Tumor / Q000494:pharmacology*; D002472:Cell Transformation, Viral; D004789:Enzyme Activation; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors; D010750:Phosphoproteins / Q000032:analysis*; D010766:Phosphorylation; D019000:Phosphotyrosine; D011494:Protein Kinases / Q000032:analysis*; D016392:Proto-Oncogene Proteins pp60(c-src); D051381:Rats; D014443:Tyrosine / Q000031:analogs & derivatives / Q000032:analysis; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology; D014764:Viral Proteins / Q000494:pharmacology*
D000952:Antigens, Polyomavirus Transforming; D000957:Antigens, Viral, Tumor; D010750:Phosphoproteins; D014764:Viral Proteins; D014639:Vanadium; D019000:Phosphotyrosine; D014638:Vanadates; D014443:Tyrosine; D011494:Protein Kinases; D016392:Proto-Oncogene Proteins pp60(c-src); D010749:Phosphoprotein Phosphatases