title: The 6.9-A structure of GlpF: a basis for homology modeling of the glycerol channel from Escherichia coli.
abstract: The three-dimensional structure of GlpF, the glycerol facilitator of Escherichia coli, was determined by cryo-electron microscopy. The 6.9-A density map calculated from images of two-dimensional crystals shows the GlpF helices to be similar to those of AQP1, the erythrocyte water channel. While the helix arrangement of GlpF does not reflect the larger pore diameter as seen in the projection map, additional peripheral densities observed in GlpF are compatible with the 31 additional residues in loops C and E, which accordingly do not interfere with the inner channel construction. Therefore, the atomic structure of AQP1 was used as a basis for homology modeling of the GlpF channel, which is predicted to be free of bends, wider, and more vertically oriented than the AQP1 channel. Furthermore, the residues facing the GlpF channel exhibit an amphiphilic nature, being hydrophobic on one side and hydrophilic on the other side. This property may partially explain the contradiction of glycerol diffusion but limited water permeation capacity.
authors.affiliation: M. E. Müller Institute for Microscopy, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland.
authors.forename: H
authors.initials: H
authors.lastname: Stahlberg
authors.forename: T
authors.initials: T
authors.lastname: Braun
authors.forename: B
authors.initials: B
authors.lastname: de Groot
authors.forename: A
authors.initials: A
authors.lastname: Philippsen
authors.forename: M J
authors.initials: MJ
authors.lastname: Borgnia
authors.forename: P
authors.initials: P
authors.lastname: Agre
authors.forename: W
authors.initials: W
authors.lastname: Kühlbrandt
authors.forename: A
authors.initials: A
authors.lastname: Engel
mesh_terms: D051398:Aquaporin 1; D020346:Aquaporins / Q000737:chemistry; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry*; D004926:Escherichia coli / Q000737:chemistry*; D029968:Escherichia coli Proteins; D005990:Glycerol; D008958:Models, Molecular; D015394:Molecular Structure; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D020346:Aquaporins; D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; C078043:GlpF protein, E coli; D051398:Aquaporin 1; D005990:Glycerol