THE ROLE OF THE SHC-PHOSPHOTYROSINE INTERACTION PHOSPHOTYROSINE BINDING-DOMAIN AND TYROSINE PHOSPHORYLATION SITES IN POLYOMA MIDDLE T-ANTIGEN-MEDIATED CELL-TRANSFORMATION

The phosphotyrosine Interaction (PI)/phosphotyrosine binding (PTB) dom ain of She binds specific tyrosine-phosphorylated motifs found on acti vated growth factor receptors and proteins such as polyoma virus middl e T antigen (MT), Phenylalanine 198 (Phe(198)) has been identified as a crucial residue involved in the interaction of the She PI/PTB with p hosphopeptides. In MH3T3 cells expressing MT, p52 She carrying the F19 8V mutation is weakly phosphorylated and does not bind MT or Grb2. Ove rexpression of the PI/PTB domain alone as She amino acids 1-238 acted in a dominant interfering fashion blocking MT-induced transformation, However, expression of a slightly longer construct, She 1-260, which e ncompasses Tyr(239)/Tyr(240), a novel She tyrosine phosphorylation sit e, did not block transformation, This was found to be due to the abili ty of She 1-260 to become tyrosine-phosphorylated and bind Grb2. Furth ermore, full-length She in which Tyr(239)/Tyr(240) had been mutated to phenylalanine did not become tyrosine-phosphorylated or bind Grb2 but did inhibit colony formation in soft agar, Conversely, p52 She carryi ng a mutation in the other tyrosine phosphorylation site, Tyr(317), be came heavily tyrosine-phosphorylated, bound Grb2, and gave rise to col onies in soft agar.