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J. Biol. Chem., Vol. 283, Issue 20, 14153-14164, May 16, 2008

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Effector Proteins Exert an Important Influence on the Signaling-active State of the Small GTPase Cdc42^*

Matthew J. Phillips, Guillermo Calero, Britton Chan, Sekar Ramachandran, and Richard A. Cerione^¶1

From the Department of Chemistry and Chemical Biology, Baker Laboratory, ^¶Department of Molecular Medicine, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853 and the Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305

GTP-binding (G) proteins regulate the flow of information in cellular signaling pathways by alternating between a GTP-bound "active" state and a GDP-bound "inactive" state. Cdc42, a member of the Rho family of Ras-related small G-proteins, plays key roles in the regulation of cell shape, motility, and growth. Here we describe the high resolution x-ray crystal structure for Cdc42 bound to the GTP analog guanylyl β,-methylene-diphosphonate (GMP-PCP) (i.e. the presumed signaling-active state) and show that it is virtually identical to the structures for the signaling-inactive, GDP-bound form of the protein, contrary to what has been reported for Ras and other G-proteins. Especially surprising was that the GMP-PCP- and GDP-bound forms of Cdc42 did not show detectable differences in their Switch I and Switch II loops. Fluorescence studies using a Cdc42 mutant in which a tryptophan residue was introduced at position 32 of Switch I also showed that there was little difference in the Switch I conformation between the GDP- and GMP-PCP-bound states (i.e. <10%), which again differed from Ras where much larger changes in Trp-32 fluorescence were observed when comparing these two nucleotide-bound states (>30%). However, the binding of an effector protein induced significant changes in the Trp-32 emission specifically from GMP-PCP-bound Cdc42, as well as in the phosphate resonances for GTP bound to this G-protein as indicated in NMR studies. An examination of the available structures for Cdc42 complexed to different effector proteins, versus the x-ray crystal structure for GMP-PCP-bound Cdc42, provides a possible explanation for how effectors can distinguish between the GTP- and GDP-bound forms of this G-protein and ensure that the necessary conformational changes for signal propagation occur.

Received for publication, July 30, 2007 , and in revised form, February 25, 2008.

The atomic coordinates and structure factors (code 2QRZ) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported, in whole or in part, by National Institutes of Health Grants GM40654 and GM47458 (to R. A. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 607-253-3888; Fax: 607-253-3659; E-mail: rac1{at}cornell.edu.

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