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 Gi family of G protein alpha subunits. We have purified recombinant RGS4 to homogeneity and demonstrate that it acts catalytically to stimulate GTP hydrolysis by Gi proteins. Furthermore, RGS4 stabilizes the transition state for GTP hydrolysis, as evidenced by its high affinity for the GDP-AlF4--bound forms of Goalpha and Gialpha and its relatively low affinity for the GTPgammaS- and GDP-bound forms of these proteins. Consequently, RGS4 is most likely not a downstream effector for activated Galpha subunits. All members of the Gi subfamily of proteins tested are substrates for RGS4 (including Gtalpha and Gzalpha); the protein has lower affinity for Gqalpha, and it does not stimulate the GTPase activity of Gsalpha or G12alpha.