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J. Biol. Chem., Vol. 281, Issue 28, 18961-18972, July 14, 2006

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Caveolin-1 Functions as a Novel Cdc42 Guanine Nucleotide Dissociation Inhibitor in Pancreatic -Cells^*

From the Department of Biochemistry and Molecular Biology and Center for Diabetes Research, Indiana University School of Medicine, Indianapolis, Indiana 46202

The cycling of the small Rho family GTPase Cdc42 is required for insulin granule exocytosis, although the regulatory proteins involved in Cdc42 cycling in pancreatic -cells are unknown. Here we demonstrate that the caveolar protein caveolin-1 (Cav-1) is a Cdc42-binding protein in -cells. Cav-1 associated with Cdc42-VAMP2-bound granules present near the plasma membrane under basal conditions. However, stimulation with glucose induced the dissociation of Cav-1 from Cdc42-VAMP2 complexes, coordinate with the timing of Cdc42 activation. Analyses of the Cav-1 scaffolding domain revealed a motif conserved in guanine nucleotide dissociation inhibitors (GDIs), which suggested a novel role for Cav-1 as a Cdc42 GDI in -cells. The novel role was further supported by: 1) in vitro binding analyses that demonstrated a direct interaction between Cav-1 and Cdc42; 2) GST-Cdc42 interaction assays showing preferential Cav-1 binding to GDP-Cdc42 over that of GTP-Cdc42; 3) Cav-1 depletion studies resulting in an inappropriate 40% induction of activated Cdc42 in the absence of stimuli and also a 40% increase in basal insulin release from both MIN6 cells and islets. Expression of wild-type Cav-1 in Cav-1-depleted cells restored basal level secretion to normal, whereas expression of a scaffolding domain mutant of Cav-1 failed to normalize secretion. Taken together, these data suggest that Cav-1 functions as a Cdc42 GDI in -cells, maintaining Cdc42 in an inactive state and regulating basal secretion in the absence of stimuli. Through its interaction with the Cdc42-VAMP2-bound insulin granule complex, Cav-1 may contribute to the specific targeting of granules to "active sites" of exocytosis organized by caveolae.

Received for publication, April 13, 2006 , and in revised form, May 16, 2006.

^* This work was supported by a predoctoral fellowship from the National Institutes of Health, Diabetes Training Grant T32 DK064466 (to A. K. N.), and American Diabetes Association Career Development Award 1-03-CD-10 (to D. C. T.). 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: Dept. of Biochemistry and Molecular Biology, Center for Diabetes Research, Indiana University School of Medicine, Indianapolis, IN 46202. Tel.: 317-274-1551; Fax: 317-274-4686; E-mail: dthurmon{at}iupui.edu.

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