Reduced phosphotyrosine binding by the v-Src SH2 domain is compatible with wild-type transformation

The SH2 domain of v-Src binds phosphotyrosyl-proteins in vivo and in vitro. The function of this domain is necessary for transformation of Rat-2 cells and for morphologically wild-type transformation of chicken embryo fibroblasts (CEF). The phosphate group of phosphotyrosine interacts directly with a conserved arginine residue in the FLVRES motif of the SH2 domain, R175 in v-Src. To examine the role of phosphotyrosine binding in transformation by v-Src, we have characterized the effects of R175 mutations on the transforming ability of v-Src and on the interaction of the v-Src SH2 domain with phosphotyrosyl-proteins. The R175H mutation, and to a lesser extent the R175K mutation, reduced but did not eliminate the binding of phosphotyrosyl-proteins to the v-Src SH2 domain. However neither mutation affected transformation of CEF or Rat-2 cells by v-Src and neither mutation resulted in major changes in the level or pattern of protein-tyrosine phosphorylation in transformed CEF. In contrast, the R175E mutant of v-Src induced fusiform transformation of CEF and failed to transform Rat-2 cells; the mutant SH2 domain was insoluble when expressed in bacteria, suggesting that the R175E mutation disrupts the structure of the v-Src SH2 domain. We conclude that, although the Arg residue in the FLVRES motif is invariant in most if not all SH2 domains, at position 175 in the v-Src SH2 domain residues other than arginine can support the binding of phosphotyrosyl-proteins, albeit at reduced levels. Furthermore under the expression conditions normally used, that is when v-Src is expressed under the control of a retroviral LTR, the reduced binding of phosphotyrosyl-proteins is compatible with wild-type transformation.