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Molecular and Cellular Biology, December 2005, p. 11102-11112, Vol. 25, No. 24
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.24.11102-11112.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Evidence for High-Capacity Bidirectional Glucose Transport across the Endoplasmic Reticulum Membrane by Genetically Encoded Fluorescence Resonance Energy Transfer Nanosensors

Marcus Fehr, Hitomi Takanaga, David W. Ehrhardt, and Wolf B. Frommer^*

Carnegie Institution, 260 Panama St., Stanford, California 94305

Received 14 June 2005/ Returned for modification 20 July 2005/ Accepted 5 October 2005

Glucose release from hepatocytes is important for maintenance of blood glucose levels. Glucose-6-phosphate phosphatase, catalyzing the final metabolic step of gluconeogenesis, faces the endoplasmic reticulum (ER) lumen. Thus, glucose produced in the ER has to be either exported from the ER into the cytosol before release into circulation or exported directly by a vesicular pathway. To measure ER transport of glucose, fluorescence resonance energy transfer-based nanosensors were targeted to the cytosol or the ER lumen of HepG2 cells. During perfusion with 5 mM glucose, cytosolic levels were maintained at 80% of the external supply, indicating that plasma membrane transport exceeded the rate of glucose phosphorylation. Glucose levels and kinetics inside the ER were indistinguishable from cytosolic levels, suggesting rapid bidirectional glucose transport across the ER membrane. A dynamic model incorporating rapid bidirectional ER transport yields a very good fit with the observed kinetics. Plasma membrane and ER membrane glucose transport differed regarding sensitivity to cytochalasin B and showed different relative kinetics for galactose uptake and release, suggesting catalysis by distinct activities at the two membranes. The presence of a high-capacity glucose transport system on the ER membrane is consistent with the hypothesis that glucose export from hepatocytes occurs via the cytosol by a yet-to-be-identified set of proteins.

Authors contributed equally.

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