Activation of the pp60c‐src kinase by middle T antigen binding or by dephosphorylation.

SA Courtneidge - The EMBO journal, 1985 - embopress.org
SA Courtneidge
The EMBO journal, 1985embopress.org
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 …
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.
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