PLC-γ directly binds activated c-Src, which is necessary for carbachol-mediated inhibition of NHE3 activity in Caco-2/BBe cells

Elevated levels of intracellular Ca2+ ([Ca2+](i)) inhibit Na+/H+ exchanger 3 (NHE3) activity in the intact intestine. We previously demonstrated that PLC-gamma directly binds NHE3, an interaction that is necessary for [Ca2+](i) inhibition of NHE3 activity, and that PLC-gamma Src homology 2 (SH2) domains may scaffold Ca2+ signaling proteins necessary for regulation of NHE3 activity. [Ca2+](i) regulation of NHE3 activity is also c-Src dependent; however, the mechanism by which c-Src is involved is undetermined. We hypothesized that the SH2 domains of PLC-gamma might link c-Src to NHE3-containing complexes to mediate [Ca2+](i) inhibition of NHE3 activity. In Caco-2/BBe cells, carbachol (CCh) decreased NHE3 activity by similar to 40%, an effect abolished with the c-Src inhibitor PP2. CCh treatment increased the amount of active c-Src as early as 1 min through increased Y-416 phosphorylation. Coimmunoprecipitation demonstrated that c-Src associated with PLC-gamma, but not NHE3, under basal conditions, an interaction that increased rapidly after CCh treatment and occurred before the dissociation of PLC-gamma and NHE3 that occurred 10 min after CCh treatment. Finally, direct binding to c-Src only occurred through the PLC-gamma SH2 domains, an interaction that was prevented by blocking the PLC-gamma SH2 domain. This study demonstrated that c-Src 1) activity is necessary for [Ca2+](i) inhibition of NHE3 activity, 2) activation occurs rapidly (similar to 1 min) after CCh treatment, 3) directly binds PLC-gamma SH2 domains and associates dynamically with PLC-gamma under elevated [Ca2+](i) conditions, and 4) does not directly bind NHE3. Under elevated [Ca2+](i) conditions, PLC-gamma scaffolds c-Src into NHE3-containing multiprotein complexes before dissociation of PLC-gamma from NHE3 and subsequent endocytosis of NHE3.