Modulation of the Affinity and Selectivity of RGS Protein Interaction with Gα Subunits by a Conserved Asparagine/Serine Residue

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Abstract
The crystal structure of the complex between a G protein α subunit (Giα1) and its GTPase-activating protein (RGS4) demonstrated that RGS4 acts predominantly by stabilization of the transition state for GTP hydrolysis [Tesmer, J. J., et al. (1997) Cell89, 251]. However, attention was called to a conserved Asn residue (Asn128) that could play a catalytic role by interacting, directly or indirectly, with the hydrolytic water molecule. We have analyzed the effects of several disparate substitutions for Asn128 on the GAP activity of RGS4 toward four Gα substrates (Go, Gi, Gq, and Gz) using two assay formats. The results substantiate the importance of this residue but indicate that it is largely involved in substrate binding and that its function may vary with different Gα targets. Various mutations decreased the apparent affinity of RGS4 for substrate Gα proteins by several orders of magnitude, but had variable and modest effects on maximal rates of GTP hydrolysis when tested with different Gα subunits. One mutation, N128F, that differentially decreased the GAP activity toward Gαi compared with that toward Gαq could be partially suppressed by mutation of the nearby residue in Gαi to that found in Gαq (K180P). Detection of GAP activities of the mutants was enhanced in sensitivity up to 100-fold by assay at steady state in proteoliposomes that contain heterotrimeric G protein and receptor.