An 81 kd protein complexed with middle T antigen and pp60^c-src: A possible phosphatidylinositol kinase

It has previously been shown that a proportion of middle T antigen molecules exist in a stable complex with pp60^c-src. Here we show that there appears to be a third component to the complex, a protein of molecular mass 81 kd (p81). p81 was phosphorylated exclusively on tyrosine residues in kinase assays performed using immunoprecipitates from polyoma virus-transformed cells and antibodies to both middle T and pp60^c-src, and was also detected when immunoprecipitates were made from lysates of ³²P-labeled cells. p81 was bound to middle T and pp60^c-src in cell lines containing transforming mutants of middle T, but not (in phosphorylated form) to all nontransforming mutants. A parallel investigation of phosphatidylinositol kinase activity in immune complexes containing these middle T mutants revealed a complete coincidence between the presence of p81 and phosphatidylinositol kinase activity. We therefore suggest that p81 is a phosphatidylinositol kinase.