Gαq and Phospholipase C-β : Turn On, Turn Off, and Do It Fast

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-beta 3 (PLC-beta 3) and its activator, G alpha(q), suggests that several effectors independently evolved a structurally similar helix-turn-helix segment for G protein recognition. PLC-beta s are also guanosine triphosphatase (GTPase)-activating proteins (GAPs) for the G(q) 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 G alpha(q), followed by fast deactivation. This cycle allows rapid sampling of the activation state of G(q)-coupled receptors while providing efficient signal transduction.