Molecular characterization of Vibrio parahaemolyticus vSGLT. A model for sodium-coupled sugar cotransporters

The Na⁺/galactose cotransporter (vSGLT) of Vibrio parahaemolyticus, tagged with C-terminal hexahistidine, has been purified to apparent homogeneity by Ni²⁺ affinity chromatography and gel filtration. Resequencing the vSGLT gene identified an important correction: The N terminus constitutes an additional 13 functionally essential residues. The mass of His-tagged vSGLT expressed under its native promoter, as determined by electrospray ionization-mass spectrometry (ESI-MS), verifies these 13 residues in wild-type vSGLT. A fusion protein of vSGLT and green fluorescent protein, comprising a mass of over 90 kDa, was also successfully analyzed by ESI-MS. Reconstitution of purified vSGLT yields proteoliposomes active in Na⁺-dependent galactose uptake, with sugar preferences (galactose > glucose > fucose) reflecting those of wild-type vSGLT in vivo. Substrates are transported with apparent 1:1 stoichiometry and apparent K(m) values of 129 mM (Na⁺) and 158 μM (galactose). Freeze-fracture electron microscopy of functional proteoliposomes shows intramembrane particles of a size consistent with vSGLT existing as a monomer. We conclude that vSGLT is a suitable model for the study of sugar cotransporter mechanisms and structure, with potential applicability to the larger SGLT family of important sodium:solute cotransporters. It is further demonstrated that ESI-MS is a powerful tool for the study of proteomics of membrane transporters.