AN SH3 DOMAIN IS REQUIRED FOR THE MITOGENIC ACTIVITY OF MICROINJECTEDPHOSPHOLIPASE C-GAMMA-1

Phospholipase activity is elevated in dividing cells. In response to g rowth factor stimulation, phospholipase C-gamma (PLC-gamma) binds to a ctivated tyrosine kinase receptors via SH2 binding domains, resulting in phosphorylation of PLC-gamma and activation of its enzyme activity. These observations suggest that PLC-gamma participates in the signal transduction pathway employed by growth factors to promote mitogenesis . Consistent with this hypothesis, microinjection of purified bovine P LC-gamma into quiescent fibroblasts has been previously reported to in itiate a mitogenic response [Smith et al. (1989) Proc. Natl. Acad. Sci . 86, 3659]. We have reproduced this result using recombinant rat PLC- gamma protein. Surprisingly however, a catalytically inactive mutant o f PLC-gamma, H335Q, also elicited a full mitogenic response. The capac ity to induce mitogenesis by microinjection of PLC-gamma was mapped to the 'Z' domain of the protein, which contains PLC-gamma's SH2 and SH3 motifs. Inactivation of the phosphorylated tyrosine binding propertie s of both SH2 domains had no effect on the mitogenic activity of the Z -domain peptide. However, deletion of the SH3 domain resulted in a com plete loss of activity. These results suggest that PLC-gamma's mitogen ic properties do not require the enzyme's phospholipase activity, but are instead mediated by a novel pathway for mitogenic stimulation whic h is dependent upon an intact SH3 domain.