RESIDUES IN THE PATHWAY THROUGH A MEMBRANE TRANSPORTER

The structure of solute transporters is understood largely from analys is of their amino acid sequences, and more direct information is great ly needed. Here we report work that applies cysteine scanning mutagene sis to describe structure-function relations in UhpT, a bacterial memb rane transporter. By using an impermeant SH-reactive agent to probe si ngle-cysteine variants, we show that UhpT transmembrane segment 7 span s the membrane as an alpha-helix and that the central portion of this helix is exposed to both membrane surfaces, forming part of the transl ocation pathway through this transporter.