The heterotrimeric G protein Gq regulates neuromuscular control as well as platelet activation, blood pressure and cardiac function, and plays a central role in the development of hypertension and cardiac hypertrophy. Activated G?q directly interacts with proteins that have been shown to be important for these processes, including the effector enzymes phospholipase C2 (PLC2) and p63RhoGEF, and the GTPase activating protein regulator of G protein signaling protein 2 (RGS2). We recently determined structures of the autoinhibited catalytic core of invertebrate PLC2 and the G?q -p63RhoGEF-RhoA complex, which led to dramatic new insights into how G?q regulates effector activity, and of the RGS2-G1q complex, the first structure of an RGS protein in complex with a member of the G1q subfamily. In Aim 1, we will confirm our model for how G?q allosterically regulates PLC2 in living cells and in the model organism C. elegans, and investigate how the C-terminal regulatory region of PLC23 enhances catalysis and affinity for G1q using functional studies and X-ray crystallographic and electron microscopic analyses of full-length PLC2 in complex with G1q. In Aim 2, we determine the autoinhibited basal structure of p63RhoGEF by solution NMR and assess how its structure is perturbed upon complex formation with G?q which will help fully describe its mechanism of activation. In Aim 3, we will determine the role of unique interactions formed between RGS2 and the helical domain of G?q and test their contribution to the selective regulation of G1q by RGS2. Collectively, our experiments are expected to reveal the molecular basis for how G?q activates its effectors, and how RGS2 appears to have uniquely evolved to regulate G?q . This knowledge is essential for understanding fundamental processes that underlie cardiac and neuromuscular function, and for developing new approaches to treat cardiovascular disease.