THE SRC SH3 DOMAIN IS REQUIRED FOR DNA-SYNTHESIS INDUCED BY PLATELET-DERIVED GROWTH-FACTOR AND EPIDERMAL GROWTH-FACTOR

The Src family of protein tyrosine kinases has been implicated in the response of cells to platelet-derived growth factor (PDGF) or epiderma l growth factor (EGF). We recently described a microinjection approach that we used to demonstrate that kinase activity of Src family member s is required for PDGF- and EGF-induced S-phase entry of fibroblasts. We have now used this approach to ask whether a functional SH3 domain of Src is required to transduce the mitogenic signal upon PDGF or EGF stimulation. Microinjection of plasmids encoding Src mutants lacking t he SH3 domain (Src Delta SH3) or point-mutated within the ligand bindi ng surface of the SH3 domain, but with intact kinase domains, inhibite d the mitogenic effect of PDGF and EGF in fibroblasts. Src Delta SH3 c ould still associate with the PDGF receptor, suggesting that the inhib itory effect of the Src SH3 mutants was brought about by a failure of the PDGF receptor Src Delta SH3 complex to relay the mitogenic signal further downstream. Chimeric molecules in which the Src SH3 domain was replaced with that of spectrin or Lck also blocked PDGF-induced DNA s ynthesis, whereas a chimera containing the Fyn SH3 domain did not. The se data suggest that the Src or Fyn SH3 domain is required either for correct substrate selection or to recruit other proteins to the PDGF r eceptor.