Structural and functional modification of pp60c-src associated with polyoma middle tumor antigen from infected or transformed cells.

The polyoma middle tumor antigen (MTAg) associates with the src proto-oncogene product pp60c-src in infected or transformed rodent cells. The tyrosine protein kinase activity of pp60c-src, as measured by in vitro phosphorylation of pp60c-src itself or the exogenous substrate enolase, was increased 10- to 20-fold in cells transformed or infected with transformation-competent polyoma virus compared with controls. pp60c-src associated with MTAg and precipitated with polyoma antitumor serum had a novel site(s) of in vitro tyrosine phosphorylation within its amino-terminal domain. These observations suggest that association of MTAg with pp60c-src alters the accessibility of pp60c-src tyrosine residues for phosphorylation in vitro and increases pp60c-src protein kinase activity. Several transformation-defective mutants of MTAg did not cause amino-terminal tyrosine phosphorylation of pp60c-src in vitro or enhance its protein kinase activity, suggesting that these properties correlate with the transforming ability of MTAg. However, one transformation-defective MTAg mutant, dl1015, did cause amino-terminal tyrosine phosphorylation of pp60c-src in vitro and did enhance its protein kinase activity. This suggests that properties of MTAg, in addition to modifying the structure and function of pp60c-src, may be important for transformation.