Phosphorylation of middle T by pp60 c- src: A switch for binding of phosphatidylinositol 3-kinase and optimal tumorigenesis

Abstract

Substitution of phenylalanine for tyrosine 315 of the polyoma virus middle T (mT) protein lowers the incidence and limits the spectrum of tumors induced following inoculation of the virus into newborn mice. This substitution removes the major site of phosphorylation by pp60 c- src without altering the ability of mT to associate with or to activate pp60 c- src . The mutant mT fails to show binding of a phosphatidylinositol 3-kinase (PtdIns 3-kinase) activity that is normally present in wild-type mT complexes. Furthermore, an anti-peptide antiserum that specifically recognizes mT lacking phosphate at tyrosine 315 precipitates binary (mT-pp60 c- src ) but not ternary (mT-pp60 c- src -PtdIns 3-kinase) complexes from wild-type infected cell extracts. Reprecipitation with either anti-pp60 c- src or anti-mT serum brings down ternary complexes containing mT phosphorylated on tyrosine 315. Phosphorylation of mT by pp60 c- src in vivo is therefore a critical event for binding of PtdIns 3-kinase and for expression of the full tumorigenic potential of the virus.