The complex of polyoma virus middle-T antigen and pp60c-src

We have recently proposed that the transforming protein of polyoma virus, middle-T antigen, forms a complex with pp60c-src. Here we provide additional evidence for the existence of the complex using both monoclonal antibodies specific for middle-T and antibodies raised against synthetic peptides corresponding to sequences from both middle-T and pp60c-src. The complex was retained during partial purification of middle-T and was stable to incubation under various conditions. A survey of a number of mutants of middle-T antigen showed that there was a complete correlation between the ability of middle-T to accept phosphate in the in vitro kinase reaction and the presence of a middle-T: pp60c-src complex. This result is in accord with our hypothesis that middle-T itself is not a protein kinase but rather that pp60c-src phosphorylates middle-T. All mutant forms of middle-T antigen capable of transformation had associated pp60c-src. The middle-T of two non-transforming mutants (hr-t mutants) did not have associated pp60c-src, whereas other non-transforming middle-T species did associate with pp60c-src. We propose that the complex plays an essential role in transformation by polyoma virus, but that the existence of the complex per se may not be sufficient.