Comparison of the transport properties of three plant sucrose carriers expressed in Xenopus oocytes

Abstract

Previous work has shown different patterns of expression for the three plant sucrose transporters PmSUC1, PmSUC2 and AtSUC1. Each carrier may therefore have different properties appropriate for the transport function at each location in the plant. To test this hypothesis, a detailed characterisation of each carrier was performed by expressing the protein in Xenopus oocytes. All three carriers were functionally active in oocytes and transported both sucrose and maltose with the sugars eliciting inward currents, which increased at more negative membrane voltages. This effect was greater for AtSUC1 and PmSUC1 than for PmSUC2. The affinities of the carriers for each sugar were different, with PmSUC2 having a 10-fold higher K_m for sucrose compared to AtSUC1 and PmSUC1. However, these values for all three carriers were dependent on the external pH and membrane voltage. For both PmSUC1 and PmSUC2 the maximal sugar transport was achieved at external pH values of 6.5–7.5, values which do not appear to be consistent with a proton-coupled mechanism of transport. However, the normal saline used for oocyte experiments contains relatively high concentrations of NaCl that could interfere with the function of the carrier protein. To test this idea the sucrose-elicited currents were measured in saline containing lower concentrations of these ions. Sodium, but not chloride, ions modified the sugar transport activities of the carriers and these effects were different for each carrier, inhibiting AtSUC1 and PmSUC1, but stimulating PmSUC2. The properties of each carrier are discussed in relation to their expression in oocytes and what is known about their pattern of expression in planta.