The transforming gene of polyoma virus.

Polyoma virus appears to encode a transforming function. A variety of studies lead to the conclusion that this function resides mainly, and in some instances possibly exclusively, within the virally-coded middle T-antigen. Whereas this protein was originally isolated from membranes of cells infected with polyoma virus as a 55 kilodalton species, our recent studies, using monoclonal antibodies, suggest that middle T-antigen is a family of proteins, at least one member of which (but not all others) can be associated with a protein kinase activity. Investigation of the sub-cellular location of the middle T-antigen (s), using either immunofluorescence or immunoelectron-microscopy, and the monoclonal antibodies, show them to be associated with all cytoplasmic membranes. At early times post-infection, most of the middle T-antigen is found in association with the rough endoplasmic reticulum of the cell; only a few percent can be found located at the plasma membrane. At late times post-infection, this percentage increases. These data lead to the hypothesis that structurally similar viral proteins might have different functions, expressed at different locations within the cell. Which function(s) pertain to transformation remain to be defined.