CELL FRACTIONATION IN NONIONIC DETERGENT DISTINGUISHES SUBPOPULATIONSOF POLYOMA-VIRUS MIDDLE T-ANTIGEN AND REVEALS A NOVEL FORM

The transforming function of polyoma virus, middle T antigen (MT), int eracts with several cellular enzymes, essential to its oncogenic activ ity. We have used cell fractionation to study the various MT/cellular protein complexes. We demonstrate that MT can be separated into three sub-species, dependent upon extraction in two buffers that we designat e A and B: Antigen extracted from whole cells by both buffers (called MT(1)) is associated with most of the phosphorylated phosphatidylinosi tol kinase 85 kD subunit, pp85, and protein phosphatase 2A. Antigen (M T(2)), associated with the greater portion of pp60c-src, is extracted by buffer B, but not buffer A. A third population (MT(3)), resistant t o extraction by either buffer, is not detectably associated with prote in phosphatase 2A or pp85. It is, however, associated with a low level kinase activity. The interaction between pp60c-src and MT appears to influence the formation of both MT(2) and MT(3). MT(2) fractionates wi th the cellular microtubule network, but does not appear to be directl y associated with it. MT(3), a previously undescribed population, comp rises about one third of MT in wild type antigen-containing cells. It is missing in mutants incapable of interacting with pp60c-src, but exi sts in the absence of an interaction with pp85. We suggest that MT(3) may be an intermediate in, or product of, one of the MT/pp60c-src sign alling pathways, distinct from that involving pp85.