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© The Rockefeller University Press, 0021-9525/1997//1555 $5.00
The Journal of Cell Biology, Volume 137, Number 7, , 1997 1555-1566

Article

IQGAP1, a Rac- and Cdc42-binding Protein, Directly Binds and Cross-links Microfilaments



Anne-Marie Bashour*, Aaron T. Fullerton*, Matthew J. Hart{ddagger}, and George S. Bloom*

* Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas 75235; and {ddagger} Onyx Pharmaceuticals, Richmond, California 94806

Activated forms of the GTPases, Rac and Cdc42, are known to stimulate formation of microfilament-rich lamellipodia and filopodia, respectively, but the underlying mechanisms have remained obscure. We now report the purification and characterization of a protein, IQGAP1, which is likely to mediate effects of these GTPases on microfilaments. Native IQGAP1 purified from bovine adrenal comprises two ~190-kD subunits per molecule plus substoichiometric calmodulin. Purified IQGAP1 bound directly to F-actin and cross-linked the actin filaments into irregular, interconnected bundles that exhibited gel-like properties. Exogenous calmodulin partially inhibited binding of IQGAP1 to F-actin, and was more effective in the absence, than in the presence of calcium. Immunofluorescence microscopy demonstrated cytochalasin D–sensitive colocalization of IQGAP1 with cortical microfilaments. These results, in conjunction with prior evidence that IQGAP1 binds directly to activated Rac and Cdc42, suggest that IQGAP1 serves as a direct molecular link between these GTPases and the actin cytoskeleton, and that the actin-binding activity of IQGAP1 is regulated by calmodulin.

1. Abbreviations used in this paper: FPLC, fast protein liquid chromatography; GAP, GTPase activating protein; MF, microfilament; MT, microtubule.

This work partially fulfills the Ph.D. requirements for A.-M. Bashour, and was supported by grants to G.S. Bloom from the National Institutes of Health (NS30485), the Robert A. Welch Foundation (I-1236), and the American Cancer Society (CB-58E).

Please address all correspondence to Dr. George S. Bloom, University of Texas, Southwestern Medical Center, Department of Cell Biology and Neuroscience, 5323 Harry Hines Boulevard, Dallas, TX 75235. Tel.: (214) 648-7680. Fax: (214) 648-8694. e-mail: bloom{at}utsw.swmed.edu


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