Mechanism of RGS4, a GTPase-activating Protein for G Protein α Subunits*
- From the Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
Abstract
GTP hydrolysis by guanine nucleotide-binding proteins, an essential step in many biological processes, is stimulated by GTPase-activating proteins (GAPs). The mechanisms whereby GAPs stimulate GTP hydrolysis are unknown. We have used mutational, biochemical, and structural data to investigate how RGS4, a GAP for heterotrimeric G protein α subunits, stimulates GTP hydrolysis. Many of the residues of RGS4 that interact with Giα1 are important for GAP activity. Furthermore, optimal GAP activity appears to require the additive effects of interactions along the RGS4-Gα interface. GAP-defective RGS4 mutants invariably were defective in binding Gα subunits in their transition state; furthermore, the apparent strengths of GAP and binding defects were correlated. Thus, none of these residues of RGS4, including asparagine 128, the only residue positioned at the active site of Giα1, is required exclusively for catalyzing GTP hydrolysis. These results and structural data (Tesmer, J. G. G., Berman, D. M., Gilman, A. G., and Sprang, S. R. (1997) Cell 89, 251–261) indicate that RGS4 stimulates GTP hydrolysis primarily by stabilizing the transition state conformation of the switch regions of the G protein, favoring the transition state of the reactants. Therefore, although monomeric and heterotrimeric G proteins are related, their GAPs have evolved distinct mechanisms of action.
Footnotes
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↵* This work was supported by United States Public Health Service Training Grant AR07279-18 (to N. W.) and by National Institutes of Health Grant GM44592 and a grant from Monsanto (to K. J. B).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ Contributed equally to this work.
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↵§ Established Investigator of the American Heart Association. To whom correspondence should be addressed: Dept. of Cell Biology and Physiology, Washington University School of Medicine, 660 S. Euclid Ave., St. Louis, MO 63110. Tel.: 314-362-1668; Fax: 314-362-7463; E-mail: kblumer{at}cellbio.wustl.edu.
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↵1 The abbreviations used are: GAPs, GTPase-activating proteins; GTPγS, guanosine 5′-O-(3-thiotriphosphate).
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- Received June 27, 1997.
- Revision received November 6, 1997.
- The American Society for Biochemistry and Molecular Biology, Inc.
