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J. Biol. Chem., Vol. 281, Issue 1, 484-490, January 6, 2006

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GLUT4 Distribution between the Plasma Membrane and the Intracellular Compartments Is Maintained by an Insulin-modulated Bipartite Dynamic Mechanism^*

Ola J. Martin, Adrian Lee, and Timothy E. McGraw1

From the Department of Biochemistry and Training Program in Chemical Biology, Weill Cornell Medical College, New York, New York 10013

The GLUT4 glucose transporter is predominantly retained inside basal fat and muscle cells, and it is rapidly recruited to the plasma membrane with insulin stimulation. There is controversy regarding the mechanism of basal GLUT4 retention. One model is that GLUT4 retention is dynamic, based on slow exocytosis and rapid internalization of the entire pool of GLUT4 (Karylowski, O., Zeigerer, A., Cohen, A., and McGraw, T. E. (2004) Mol. Biol. Cell 15, 870–882). In this model, insulin increases GLUT4 in the plasma membrane by modulating GLUT4 exocytosis and endocytosis. The second model is that GLUT4 retention is static, with 90% of GLUT4 stored in compartments that are not in equilibrium with the cell surface in basal conditions (Govers, R., Coster, A. C., and James, D. E. (2004) Mol. Cell Biol. 24, 6456–6466). In this model, insulin increases GLUT4 in the plasma membrane by releasing it from the static storage compartment. Here we show that under all experimental conditions examined, basal GLUT4 retention is by a bipartite dynamic mechanism involving slow efflux and rapid internalization. To establish that the dynamic model developed in studies of the extreme conditions of >100 nM insulin and no insulin also describes GLUT4 behavior at more physiological insulin concentrations, we characterized GLUT4 trafficking in 0.5 nM insulin. This submaximal insulin concentration promotes an intermediate effect on both GLUT4 exocytosis and endocytosis, resulting in an intermediate degree of redistribution to the plasma membrane. These data establish that changes in the steady-state surface/total distributions of GLUT4 are the result of gradated, insulin-induced changes in GLUT4 exocytosis and endocytosis rates.

Received for publication, June 1, 2005 , and in revised form, September 29, 2005.

^* This work was supported by National Institutes of Health Grant DK52582 (to T. E. M.) and a predoctoral fellowship from the W. M. Keck Foundation (to O. J. M.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1 and 2.

¹ To whom correspondence should be addressed: Dept. of Biochemistry and Training Program in Chemical Biology, Weill Cornell Medical College, 1300 York Ave., New York, NY 10013. Tel.: 212-746-4982; Fax: 212-746-8875; E-mail: temcgraw{at}med.cornell.edu.

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