Local mutagenesis of Rous sarcoma virus: the major sites of tyrosine and serine phosphorylation of pp60src are dispensable for transformation.

We have constructed mutants of Rous sarcoma virus expressing p60srcs that are underphosphorylated on serine or tyrosine, by linker insertion or insertion/deletion into cloned Rous sarcoma virus DNA, and recovery of mutant virus by transfection of chicken embryo fibroblasts. Cells infected with mutants whose p60srcs lack the major site of either serine or tyrosine phosphorylation were morphologically transformed and formed colonies in soft agar. The tyrosine kinase activities of the mutant p60srcs measured in vivo and in vitro were close to the wild type activity. Peptide mapping showed that phosphorylation on tyrosine and serine of p60src is independent: the major phosphorylated tyrosine and the major phosphorylated serine can each be phosphorylated in the absence of phosphorylation of the other.