Structure of a glycerol-conducting channel and the basis for its selectivity

Membrane channel proteins of the aquaporin family are highly selective for permeation of specific small molecules, with absolute exclusion of ions and charged solutes and without dissipation of the electrochemical potential a cross the cell membrane. We report the crystal structure of the Escherichia coli glycerol facilitator (GlpF) with its primary permeant substrate glyce rol at 2.2 angstrom resolution. Glycerol molecules line up in an amphipathi c channel in single file. In the narrow selectivity filter of the channel t he glycerol alkyl backbone is wedged against a hydrophobic corner, and succ essive hydroxyl groups form hydrogen bonds with a pair of acceptor, and don or atoms. Two conserved aspartic acid-proline-alanine motifs form a key int erface between two gene-duplicated segments that each encode three-and-one- half membrane-spanning helices around the channel. This structure elucidate s the mechanism of selective permeability for linear carbohydrates and sugg ests how ions and water are excluded.