Homer 2 tunes G protein–coupled receptors stimulus intensity by regulating RGS proteins and PLCβ GAP activities

Homers are scaffolding proteins that bind G protein–coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP(3)) receptors (IP(3)Rs), ryanodine receptors, and TRP channels. However, their role in Ca(2+) signaling in vivo is not known. Characterization of Ca(2+) signaling in pancreatic acinar cells from Homer2(−/−) and Homer3(−/−) mice showed that Homer 3 has no discernible role in Ca(2+) signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca(2+) signaling by GPCRs to regulate the frequency of [Ca(2+)](i) oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCβ and evoke Ca(2+) release and oscillations. This was not due to aberrant localization of IP(3)Rs in cellular microdomains or IP(3)R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca(2+) signaling in vivo. Moreover, Homer 2 preferentially bound to PLCβ in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCβ in an in vitro reconstitution system, with minimal effect on PLCβ-mediated PIP(2) hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCβ GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca(2+)](i) oscillations.