Site-directed mutagenesis of polyomavirus middle-T antigen sequences encoding tyrosine 315 and tyrosine 250

Tyrosine residues of middle-T and tyrosine phosphorylation are thought to be important in the transformation of cultured rodent cells by polyomavirus. Of the potential tyrosine sites in the carboxyl-terminal half of middle-T, tyrosines 297, 315, and 322 have been studied previously, whereas tyrosine 250 has not. Two mutant plasmids, XD121 and pT250, encode polyomavirus middle-T species in which the tyrosine 250 residue is affected. XD121 is a deletion mutant in which the region encoding tyrosine 250, together with three adjacent amino acids, is deleted, whereas pT250 is a point mutant in which the tyrosine 250 codon has been converted to a phenylalanine codon. The plasmids were handicapped in transforming ability, as judged by focus formation on a monolayer of Rat-1 cells. Both demonstrated a reduction in the number of foci produced and a lag in the time of appearance of foci when compared with wild-type plasmid. The importance of residue 250 in this phenotype was indicated by the observation that plasmids containing multiple mutations proximal to the tyrosine 250 codon were wild type in their transforming ability. Furthermore, a revertant of pT250 (pT250-w.t.), which utilized the alternative tyrosine codon of TAC, was shown to regain full transforming activity. A combined-mutant plasmid, pTH, encodes a middle-T species in which both tyrosines 250 and 315 are converted to phenylalanine. This plasmid was totally defective in the transformation of rodent cells in a focus formation assay; however, it did impart a small measure of anchorage-independent growth when the encoded protein was expressed in NIH 3T3 cells. The in vitro kinase activity and pp60c-src association of the mutant middle-T antigens were examined. These assays demonstrated a reduction in phosphate acceptor activity for the middle-T species encoded by pT250 and pTH. Quantitative kinase assays showed that all of the tyrosine-mutant middle-T species, encoded by pAS131 (containing the tyrosine 315 codon-to-phenylalanine codon mutation), pT250, and pTH, were able to enhance pp60c-src kinase activity but only at levels which were intermediate and which reflected their transforming abilities relative to wild type.