Structure of a Glycerol-Conducting Channel and the Basis for Its Selectivity

Abstract

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 across the cell membrane. We report the crystal structure of the Escherichia coli glycerol facilitator (GlpF) with its primary permeant substrate glycerol at 2.2 angstrom resolution. Glycerol molecules line up in an amphipathic channel in single file. In the narrow selectivity filter of the channel the glycerol alkyl backbone is wedged against a hydrophobic corner, and successive hydroxyl groups form hydrogen bonds with a pair of acceptor, and donor atoms. Two conserved aspartic acid-proline-alanine motifs form a key interface between two gene-duplicated segments that each encode three-and-one-half membrane-spanning helices around the channel. This structure elucidates the mechanism of selective permeability for linear carbohydrates and suggests how ions and water are excluded.

A refined structure of human aquaporin‐1

“…Superposition of the backbone traces of the three AQP1 models, coloured in yellow (1FQY), magenta (1IH5) and green (our re¢ned model). 1FX8 denotes the PDB code of the X-ray structure of GlpF [3], 1FQY and 1IH5 the two AQP1 models [1,2], and re¢ denotes our re¢ned AQP1 structure. 1FX8 denotes the PDB code of the X-ray structure of GlpF [3], 1FQY and 1IH5 the two AQP1 models [1,2], and re¢ denotes our re¢ned AQP1 structure.…”

Members of the aquaporin family in the developing pea seed coat include representatives of the PIP, TIP, and NIP subfamilies

“…In GlpF glycerol is transported through an amphipathic channel. The selectivity filter in this channel is lined by four residues, , which are crucial for the selective transport of glycerol (Fu et al, 2000;Sui et al, 2001). Alignment of several glycerol-transporting MIPs showed that Trp-48 in GlpF is conserved in Fsp1, the glycerol facilitator from Saccharomyces cerevisiae, whereas it is replaced by phenylalanine in mammalian aquaglyceroporins such as AQP3.…”

“…High-resolution atomic structures of prototypes of each category, aquaporin-1 (AQP1) from mammals and the glycerol facilitator (GlpF) from Escherichia coli, have elucidated aspects of the selectivity of these transporters (Fu et al, 2000;Sui et al, 2001). To some extent the glycerol-transporting MIPs, including GlpF, also allow the passage of water (Borgnia and Agre, 2001), hence their designation as aquaglyceroporins.…”

Functions of aquaporins in the eye

“…There is a considerable body of information on AQP protein structure, because, compared to other membrane proteins, the AQPs are relatively easy to isolate and crystallize for structural analysis by x-ray or electron crystallography. High-resolution x-ray crystal structures exist for AQP1 (Sui et al, 2001), the bacterial glycerol-transporter Glpf (Fu et al, 2000), and the major intrinsic protein of lens fiber AQP0 (Harries et al, 2004). AQP1 monomers contain six tilted alpha-helical domains forming a barrel-like structure in which the first and last 3 helices exhibit inverted symmetry (reviewed in Fujiyoshi et al, 2002;Stroud et al, 2003).…”