HETEROLOGOUS EXPRESSION OF THE H+/HEXOSE COTRANSPORTER FROM CHLORELLA IN XENOPUS OOCYTES AND ITS CHARACTERIZATION WITH RESPECT TO SUGAR SPECIFICITY, PH AND MEMBRANE-POTENTIAL

The cRNA transcribed from a full length clone that codes for the inducible H+/hexose cotransporter from Chlorella kessleri was injected into Xenopus oocytes. The functional expression was documented by the occurrence of inward current when D-glucose was offered to the oocytes. The fact that substitution of sodium ions by choline did not change the sugar-dependent charge flow supports the idea that the current was carried by protons. Thus, direct proof was obtained for the electrogenicity of sugar transport by the Chlorella carrier. The sugar specificity of proton flow was found to be in good agreement with the well characterized sugar transport by Chlorella cells. The K(m) values determined for proton flux were similar to the values found previously for transport of D-glucose and other sugars. An exception was mannose, which elicited current although it was poorly transported by Chlorella cells. Both proton gradient and membrane potential affected the proton flux. It is concluded that the heterologous expression of cloned transporters in Xenopus oocytes is not only an excellent way to identify transporter clones but also to study interactions of substrate and charge flow.