IQGAP1 Integrates Ca2+/Calmodulin and Cdc42 Signaling

doi:10.1074/jbc.274.1.464

IQGAP1 Integrates Ca²⁺/Calmodulin and Cdc42 Signaling*

From the Department of Pathology, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts 02115

Abstract

Calmodulin regulates diverse Ca²⁺-dependent cellular processes, including cell cycle progression and cytoskeletal rearrangement. A recently identified calmodulin-binding protein, IQGAP1, interacts with both actin and Cdc42. In this study, evidence is presented that, in the absence of Ca²⁺, IQGAP1 bound to Cdc42, which maintained Cdc42 in the active GTP-bound state. Addition of Ca²⁺ both directly abrogated the effect of IQGAP1 on the intrinsic GTPase activity of Cdc42 and, in the presence of calmodulin, dissociated Cdc42 from IQGAP1. In addition, in vitro binding assays revealed that calmodulin associated with both the calponin homology domain and the IQ motifs of IQGAP1. Moreover, F-actin competed with Ca²⁺/calmodulin for binding to the calponin homology domain, but not the IQ motifs, of IQGAP1. Analysis of cell lysates revealed that calmodulin bound to IQGAP1 in a ternary complex with Cdc42. Increasing the Ca²⁺ concentration enhanced the interaction between calmodulin and IQGAP1, with a concomitant decrease in the association of IQGAP1 with Cdc42. Our data suggest that IQGAP1 functions as a scaffolding protein, providing a molecular link between Ca²⁺/calmodulin and Cdc42 signaling.

Footnotes

↵* This work was supported in part by grants from the U.S. Army DAMD17-98-1-8040 (to D. B. S.), and the Massachusetts Department of Public Health (to J. L. J.).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.
↵‡ Contributed equally to the results of this work.
↵§ To whom correspondence should be addressed: Brigham and Women’s Hospital, Thorn 530, 75 Francis St., Boston, MA 02115. Tel.: 617-732-6627; Fax: 617-278-6921.
Abbreviations:

GAP

GTPase-activating protein

GST

glutathione S-transferase

GTPγS

guanosine 5′-O-(3-thiotriphosphate)

PAGE

polyacrylamide gel electrophoresis

PVDF

polyvinylidene difluoride

ECL

enhanced chemiluminescence

PBS

phosphate-buffered saline

CHD

calponin homology domain.
- Received October 6, 1998.
The American Society for Biochemistry and Molecular Biology, Inc.