Functional expression of the Vibrio parahaemolyticus Na⁺/galactose (vSGLT) cotransporter in Xenopus laevis oocytes

We have successfully expressed a bacterial cotransporter in a functional form in the Xenopus laevis oocyte expression system. The goals were to compare the kinetics and selectivity of the cotransporter expressed in oocytes with those obtained in bacteria and in proteoliposomes, and to determine if it is possible to measure the electrical properties of the bacterial cotransporter expressed in oocytes. The Vibrio parahaemolyticus Na⁺/galactose cotransporter (vSGLT) expressed in oocytes has functional properties that are similar to those expressed in bacteria and those of the purified cotransporter reconstituted into liposomes. vSGLT is a Na⁺ -dependent transporter that is saturable with Na⁺ (K_0.5 = 17 mM) and D-galactose (K_0.5 = 237 μM) and is sensitive to both D-fucose and phlorizin. In addition, vSGLT in oocytes shows a sugar specificity in the order of D-galactose > D-fucose > D-glucose, distinguishing it from the animal members of the Na⁺/glucose cotransporter family. The level of transport by vSGLT in oocytes is lower overall (V_max ∼ 10 pmol/oocyte/hour) compared to other plant and animal cotransporters (V_max ∼ 1000 pmol/oocyte/hour). The low level of expression does not permit us to carry out electrophysiological studies of the bacterial cotransporter. This study shows the potential and unique advantages of utilizing a eukaryotic oocyte expression system to study bacterial cotransporters.