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J. Biol. Chem., Vol. 275, Issue 34, 25959-25964, August 25, 2000

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Characterization of the Vibrio parahaemolyticus Na⁺/Glucose Cotransporter
A BACTERIAL MEMBER OF THE SODIUM/GLUCOSE TRANSPORTER (SGLT) FAMILY^*

Zhiyi Xie, Eric Turk, and Ernest M. Wright

From the Department of Physiology, UCLA School of Medicine, Los Angeles, California 90095-1751

The Vibrio parahaemolyticus sodium/glucose transporter (vSGLT) is a bacterial member of the SGLT gene family. Wild-type and mutant vSGLT proteins were expressed in Escherichia coli, and transport activity was measured in intact cells and plasma membrane vesicles. Two cysteine-less vSGLT proteins exhibited sugar transport rates comparable with that of the wild-type protein. Six residues in two regions of vSGLT known to be of functional importance in SGLT1 were replaced individually with cysteine in the cysteine-less protein. Characterization of these single cysteine-substituted vSGLTs showed that two residues (Gly-151 and Gln-428) are essential for transport function, whereas the other four residues (Leu-147, Leu-149, Ala-423, and Gln-425) are not. 2-Aminoethylmethanethiosulfonate (MTSEA) blocked Na⁺/glucose transport by only the transporter bearing a cysteine at position 425 (Q425C). MTSEA inhibition was reversed by dithiothreitol and blocked by the presence of both Na⁺ and D-glucose, indicating that conformational changes of the vSGLT protein are involved in Na⁺/glucose transport. A split version of vSGLT was generated by co-expression of the N-terminal (N₇) and C-terminal (C₇) halves of the transporter. The split vSGLT maintained Na⁺-dependent glucose transport activity. Chemical cross-linking of split vSGLT, with a cysteine in each N₇ and C₇ fragment, suggested that hydrophilic loops between helices 4 and 5 and between helices 10 and 11 are within 8 Å of each other. We conclude that the mechanism of Na⁺/glucose transport by vSGLT is similar to mammalian SGLTs and that further studies on vSGLT will provide novel insight to the structure and function of this class of cotransporters.

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