The heterologous expression of H(+)-coupled transporters in Xenopus oocytes.

The Xenopus oocyte has become a convenient and robust system for the expression of many different animal transport proteins and it has now been demonstrated that oocytes can also translate, process and target plant membrane proteins. Here we review the background to this expression system and discuss how it can be used to express plant transporters. A H+/hexose cotransporter (STP1) from Arabidopsis thaliana has been successfully expressed in oocytes and further characterization has shown the transporter to have properties similar to the same transporter expressed in yeast. However, the expression of H(+)-coupled transporters may present problems for oocyte intracellular pH regulation, because the influx of protons during their operation may acidify the cell. To investigate this possibility, proton-selective microelectrodes have been used to measure intracellular pH during symport of H+ and hexose in oocytes expressing STP1. These measurements showed that oocyte cytosolic pH is unaltered during H+/glucose cotransport, but is sensitive to changes in external pH.