Purified polyoma virus medium T antigen has tyrosine-specific protein kinase activity but no significant phosphatidylinositol kinase activity.
Medium T antigen, the transforming protein of polyoma virus, is associated with pp60c-src and strongly activates its tyrosine-specific protein kinase activity. We investigated whether the medium T-pp60c-src complex is also associated with an activity that phosphorylates the membrane phospholipid phosphatidylinositol, as shown for pp60v-src and p68v-ros, the transforming proteins of Rous sarcoma virus and avian sarcoma virus UR2, respectively. Medium T was purified by affinity chromatography from extracts of polyoma virus-infected mouse fibroblasts. It was bound to antibodies against a peptide corresponding to the carboxy terminus of medium T and released from the immune complex with an excess of the same peptide. In a second step, the partially purified medium T was bound to antibodies against another peptide corresponding to an internal region of medium T and released with excess peptide. Further purification was carried out with a monoclonal antibody against pp60c-src. Samples from each purification step were examined for protein kinase and phosphatidylinositol kinase activity. The highly purified preparations of the medium T-pp60c-src complex showed very low levels of phosphatidylinositol kinase activity, and no difference between medium T from transforming viruses and nontransforming hr-t mutants was detected. In contrast, protein kinase activity was associated with medium T purified from transforming viruses but not from hr-t mutants.
D019870:1-Phosphatidylinositol 4-Kinase; D000957:Antigens, Viral, Tumor / Q000302:isolation & purification / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000378:metabolism; D010770:Phosphotransferases / Q000378:metabolism*; D011120:Polyomavirus / Q000201:enzymology*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D013379:Substrate Specificity
D000957:Antigens, Viral, Tumor; D010716:Phosphatidylinositols; D010770:Phosphotransferases; D019870:1-Phosphatidylinositol 4-Kinase; D011505:Protein-Tyrosine Kinases