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