Analysis of the transport activity of barley sucrose transporter HvSUT1

Localization studies indicate that barley (Hordeum vulgare) sucrose transporter HvSUT1 functions in sucrose uptake into seeds during grain filling. To further understand the physiological function of HvSUT1, we have expressed the HvSUT1 cDNA in Xenopus laevis oocytes and analyzed the transport activity by two-electrode voltage clamping. Consistent with a H+-coupled transport mechanism, sucrose induced large inward currents in HvSUT1-expressing oocytes with a K-0.5 of 3.8 mM at pH 5.0 and a membrane potential of -157 mV. Of 21 other sugars tested, four glucosides were also transported by HvSUT1. These glucosides were maltose, salicin (2-(hydroxymethyl) phenyl beta-D-glucoside), alpha-phenylglucoside and alpha-paranitrophenylglucoside. Kinetic analysis of transport of these substrates by HvSUT1 was performed and K-0.5 values were measured. The apparent affinity for all substrates was dependent on membrane potential and pH with lower K,., values at lower external pH and more negative membrane potentials. HvSUT1 was more selective for alpha-glucosides over beta-glucosides than the Arabidopsis sucrose transporter AtSUC2. Several substrates transported by AtSUC2 (beta-phenylglucoside, beta-paranitrophenylglucoside, a-methylglucoside, turanose, and arbutin (hydroquinone beta-D-glucoside)) showed low or undetectable transport by HvSUT1. Of these, beta-paranitrophenylglucoside inhibited sucrose transport by HvSUT1 indicating that it interacts with the transporter while arbutin and a-methyl glucoside did not inhibit. The results demonstrate significant differences in substrate specificity between HvSUT1 and AtSUC2.