The GTPase-activating protein RGS4 stabilizes the transition state for nucleotide hydrolysis

RGS proteins constitute a newly appreciated group of negative regulators of G protein signaling. Discovered by genetic screens in yeast, worms, and other organisms, two mammalian RGS proteins, RGS4 and GAIP, act as GTPase- activating proteins for members of the G(i) family of G protein α subunits. We have purified recombinant RGS4 to homogeneity and demonstrate that it acts catalytically to stimulate GTP hydrolysis by G(i) proteins. Furthermore, RGS4 stabilizes the transition state for GTP hydrolysis, as evidenced by its high affinity for the GDP-AlF ₄/ ^--bound forms of G(oα) and G(iα) and its relatively low affinity for the GTPγS- and GDP-bound forms of these proteins. Consequently, RGS4 is most likely not a downstream effector for activated G(α) subunits. All members of the G(i) subfamily of proteins tested are substrates for RGS4 (including G(tα) and G(zα)); the protein has lower affinity for G(qα), and it does not stimulate the GTPase activity of G(8α) or G(12α).