Proteins of the regulator of G protein signaling (RCS) family accelerate GT
P hydrolysis by the or subunits (G(alpha)) of G proteins. leading to rapid
recovery of signaling cascades. Many different RCS proteins can accelerate
GTP hydrolysis by an individual G(alpha), and CTP hydrolysis rates of diffe
rent G alpha(s) can be enhanced by the same RCS protein. Consequently, the
mechanisms for specificity in RGS regulation and the residues involved rema
in unclear. Using the evolutionary trace (ET) method, we have identified a
cluster of residues in the RGS domain that includes the RCS-C, binding inte
rface and extends to include additional functionally important residues on
the surface. One of these is within helix alpha 3, two are in alpha 5, and
three are in the loop connecting alpha 5 and alpha 6. A cluster of surface
residues on G(alpha) previously identified by ET, and composed predominantl
y of residues from the switch III region and helix alpha 3, is spatially co
ntiguous with the PT-identified residues in the RGS domain. This cluster in
cludes residues proposed to interact with the gamma subunit of G(t alpha)'s
effector, cGMP phosphodiesterase (PDE gamma), The proximity of these clust
ers suggests that they form part of an interface between the effector and t
he RGS-G(alpha) complex. Sequence variations in these residues correlate wi
th PDE gamma 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, poss
ibly by means of effector interactions.