Phospholipase C-β (PLCβ) is a key regulator of intracellular calcium levels whose activity is controlled by heptahelical receptors that couple to Gq. We have determined atomic structures of two invertebrate homologs of PLCβ (PLC21) from cephalopod retina and identified a helix from the C-terminal regulatory region that interacts with a conserved surface of the catalytic core of the enzyme. Mutations designed to disrupt the analogous interaction in human PLCβ3 dramatically increase basal activity and diminish stimulation by Gαq. Gαq binding requires displacement of the autoinhibitory helix from the catalytic core, thus providing an allosteric mechanism for activation of PLCβ.

Phospholipase Cβ (PLCβ) is activated in response to specific extracellular stimuli by direct interactions with the heterotrimeric G protein Gαq. However, the molecular mechanisms underlying this regulation are poorly understood. In this work we present four new structures of PLCβ that capture the enzyme in its basal state and in complex with Gαq. These structures not only provide the first high‐resolution models of the C‐terminal regulatory domain in the context of full‐length PLCβ, but also identify regions important for PLCβ regulation and Gαq binding and activation. Deletion or mutation of these regions has profound consequences in the basal activity of PLCβ, the magnitude of Gαq‐stimulated activation, and the affinity for activated Gαq. Taken together, these structures reveal an allosteric mechanism of PLCβ activation by Gαq and define the role of the C‐terminal regulatory domain in this process. Support for this work was provided by NIH grants HL086865 and HL071818 to J.J.G.T. and an American Heart Association postdoctoral fellowship to A.M.L.

Phospholipase Cβ (PLCβ), which catalyzes the hydrolysis of phosphatidylinositol‐4,5‐bisphosphate (PIP2) to the second messengers diacylglycerol (DAG) and inositol‐1,4,5‐triphosphate (IP3), is activated in response to specific extracellular stimuli through interactions with the heterotrimeric G protein Gαq. However, the structural and functional mechanisms underlying the regulation of PLCβ activity is poorly understood. In this work we present two new structures of PLCβ homologs purified from endogenous sources that include a highly conserved element from the C‐terminal regulatory domain. This conserved element forms a microdomain that interacts with a conserved surface of the catalytic core. Deletion of or mutation of residues within this microdomain in PLCβ3 results in increased PLCβ activity. Based on the recent structure of the Gαq‐PLCβ3 complex, this microdomain is likely displaced from the catalytic core upon Gαq binding. Thus, the C‐terminal microdomain appears to represent an autoinhibitory element that modulates the basal activity of PLCβ and is involved in allosteric changes mediated by Gαq.Support was provided by NIH grants HL086865 and HL071818.