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 epidermal growth factor (EGF). We recently described a microinjection approach that we used to demonstrate that kinase activity of Src family members 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 the SH3 domain (SrcDeltaSH3) or point-mutated within the ligand binding surface of the SH3 domain, but with intact kinase domains, inhibited the mitogenic effect of PDGF and EGF in fibroblasts. SrcDeltaSH3 could still associate with the PDGF receptor, suggesting that the inhibitory effect of the Src SH3 mutants was brought about by a failure of the PDGF receptor.SrcDeltaSH3 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 synthesis, whereas a chimera containing the Fyn SH3 domain did not. These data suggest that the Src or Fyn SH3 domain is required either for correct substrate selection or to recruit other proteins to the PDGF receptor.