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Abstract
Heterotrimeric G proteins and G protein–coupled receptors represent conserved protein families with origins in the prokaryotes, but the various G protein–regulated effectors are heterogeneous in structure and function. The effectors apparently evolved ways to listen to G proteins late in their evolutionary histories. The structure of a complex between the effector protein phospholipase C-β3 (PLC-β3) and its activator, Gαq, suggests that several effectors independently evolved a structurally similar helix-turn-helix segment for G protein recognition. PLC-βs are also guanosine triphosphatase (GTPase)–activating proteins (GAPs) for the Gq that activates them. In a second example of convergent evolution, the GAP activity of these proteins depends on a flexible asparagine-containing loop that resembles the GAP site on RGS proteins, another family of G protein GAPs. Together, these two sites are proposed to cooperate to enable fast binding to activated Gαq, followed by fast deactivation. This cycle allows rapid sampling of the activation state of Gq-coupled receptors while providing efficient signal transduction.
