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Volume 271, Number 41, Issue of October 11, 1996 pp. 25139-25144
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Transport Mechanism of the Cloned Potato H⁺/Sucrose Cotransporter StSUT1

(Received for publication, May 30, 1996, and in revised form, July 25, 1996)

Kathryn J. Boorer , Donald D. F. Loo , Wolf B. Frommer ^¶ and Ernest M. Wright

From the  Department of Physiology, UCLA School of Medicine, Los Angeles, California 90095-1751 and the ^¶ Institut für Botanik, Eberhard-Karls-Universität, Auf der Morgenstelle 1, D-72076 Tübingen, Germany

The transport mechanism of the potato StSUT1 H⁺/sucrose cotransporter expressed in Xenopus oocytes was investigated using the 2-electrode voltage clamp and radiotracer flux methods. Sucrose induced inward currents through the transporter that were dependent on the extracellular sucrose and H⁺ concentrations and the membrane voltage. The activation of StSUT1 by H⁺ and sucrose displayed Michaelis-Menten-type kinetics suggestive of a 1:1 H⁺:sucrose stoichiometry. This was confirmed by simultaneously measuring inward currents and sucrose flux in voltage-clamped oocytes. The apparent affinities K_0.5 for H⁺ and sucrose were voltage-dependent. At 150 mV K^suc_0.5 was 0.5 ± 0.07 mM at 10 µM H⁺_o, and K^H_0.5 was 0.1 ± 0.05 µM at 20 mM sucrose_o. StSUT1 exhibited presteady-state transient currents, which relaxed with time constants between <1 and 4 ms and fitted to the Boltzmann equation: maximum charge transfer Q_max 1.8 nanocoulombs; apparent valence z  1; potential for 50% charge transfer V_0.5  15 mV at 0.032 µM H⁺_o and 45 mV at 10 µM H⁺_o. The steady-state data were used to formulate a kinetic model for sucrose transport, and computer simulations were performed to obtain rate constants for the partial reaction steps. Our model is consistent with protons binding to StSUT1 before sucrose with both ligands transported simultaneously across the membrane.

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