Substrate Specificity of the Arabidopsis thaliana Sucrose Transporter AtSUC2

The Arabidopsis sucrose transporter AtSUC2 is expressed in the companion cells of the phloem (specialized vascular tissue) and is essential for the long distance transport of carbohydrates within the plant. A variety of glucosides are known to inhibit sucrose uptake into yeast expressing AtSUC2; however, it remains unknown whether glucosides other than sucrose could serve as transported substrates. By expression of AtSUC2 in Xenopus oocytes and two-electrode voltage clamping, we have tested the ability of AtSUC2 to transport a range of physiological and synthetic glucosides. Sucrose induced inward currents with a K_0.5 of 1.44 mM at pH 5 and a membrane potential of -137 mV. Of the 24 additional sugars tested, 8 glucosides induced large inward currents allowing kinetic analysis. These glucosides were maltose, arbutin (hydroquinone-β-D-glucoside), salicin (2-(hydroxymethyl)phenyl-β-D-glucoside), α-phenylglucoside, β-phenylglucoside, α-paranitrophenylglucoside, β -paranitrophenylglucoside, and paranitrophenyl-β-thioglucoside. In addition, turanose and α-methylglucoside induced small but significant inward currents indicating that they were transported by AtSUC2. The results indicate that AtSUC2 is not highly selective for α- over β-glucosides and may function in transporting glucosides besides sucrose into the phloem, and the results provide insight into the structural requirements for transport by AtSUC2.