A regulator of G protein signaling interaction surface linked to effector specificity

Proteins of the regulator of G protein signaling (RGS) family accelerate GTP hydrolysis by the α subunits (G(α)) of G proteins, leading to rapid recovery of signaling cascades. Many different RGS proteins can accelerate GTP hydrolysis by an individual G(α), and GTP hydrolysis rates of different G(α)s can be enhanced by the same RGS protein. Consequently, the mechanisms for specificity in RGS regulation and the residues involved remain unclear. Using the evolutionary trace (ET) method, we have identified a cluster of residues in the RGS domain that includes the RGS-G(α) binding interface and extends to include additional functionally important residues on the surface. One of these is within helix α3, two are in α5, and three are in the loop connecting α5 and α6. A cluster of surface residues on G(α) previously identified by ET, and composed predominantly of residues from the switch III region and helix α3, is spatially contiguous with the ET-identified residues in the RGS domain. This cluster includes residues proposed to interact with the γ subunit of G(tα)'s effector, cGMP phosphodiesterase (PDEγ). The proximity of these clusters suggests that they form part of an interface between the effector and the RGS-G(α) complex. Sequence variations in these residues correlate with PDEγ effects on GTPase acceleration. Because ET identifies residues important for all members of a protein family, these residues likely form a general site for regulation of G protein-coupled signaling cascades, possibly by means of effector interactions.