PP60(C-SRC) BINDING TO POLYOMAVIRUS MIDDLE T-ANTIGEN (MT) REQUIRES RESIDUE-185 TO RESIDUE-210 OF THE MT SEQUENCE

Interaction with the src family of tyrosine kinases is crucial to the transforming action of polyomavirus middle T-antigen (MT). Association with MT activates the tyrosine kinase activity of pp60(c-src) and, th rough subsequent MT phosphorylation, creates binding sites for signall ing molecules whose stimulation culminates in cell transformation, Des pite this importance, and many studies, little is known of the mechani sms by which pp60(c-src) binds to MT, We report here isolation of the first. MT mutants that disrupt pp60(c-src) binding without affecting t he interaction between MT and protein phosphatase 2A (PP2A), Through d eletion analysis we established that interaction with pp60(c-src) requ ires the sequences between amino acids 185 and 210 of MT, but these re sidues have no effect on PP2A binding. Cells expressing these mutants showed few altered properties, indicating that the PP2A-MT interaction alone has little influence on cell phenotype. Subcellular location of these mutant MT molecules was indistinguishable by immunofluorescence analysis from that of wild-type MT but was altered markedly on loss o f PP2A binding, This suggests a possible role for PP2A in specifying s ubcellular distribution.