Engagement of the T lymphocyte antigen receptor regulates association of son-of-sevenless homologues with the SH3 domain of phospholipase C gamma 1

One mechanism for transducing signals downstream of lymphocyte receptor activation involves the stable association between signaling proteins. To identify protein ligands of the signal activator phospholipase C gamma 1 (PLC gamma 1), we screened T cell cDNA libraries with the PLC gamma 1-SH3 domain by the yeast two-hybrid assay. We observed association between the PLC gamma 1-SH3 domain and the human Ras guanine nucleotide exchange factor son-of-sevenless-2 (hSos2) through a proline-rich domain interaction. Stable and abundant hSos2/PLC gamma 1 and hSos1/PLC gamma 1 complexes were observed in unstimulated T cells. The interaction between these enzymes was augmented following engagement of the T cell antigen receptor (TCR/CD3). The kinetics of protein complex enhancement correlated with TCR/CD3-induced tyrosine phosphorylation of PLC gamma 1; however, those PLC gamma 1 molecules in complex with hSos2 were non-phosphorylated after TCR/CD3 stimulation, in contrast to the phosphorylated PLC gamma 1 associated with the linker for activation of T cells, LAT. The Grb2 adapter protein was detected in complex with hSos/PLC gamma 1, suggesting a regulatory role for Grb2. SH3 domains from both Grb2 and PLC gamma 1, but not RasGAP, bound directly to hSos homologues. The SH2 domain from Grb2 formed an association with the hSos/PLC gamma 1 complex, which was enhanced following TCR/CD3 ligation. Together, the data suggest a mechanism for the son-of-sevenless and PLC gamma 1 signal transducing enzymes in recruitment to protein complexes with potentially differential signaling consequences in T lymphocytes.