Doctor Peter Nollert
Macromolecular Structure Group, Department of Biochemistry & Biophysics
University of California
San Francisco, California

Monday, October 29, 2001
3:00 pm (CT)
3269 Beckman Institute

Abstract

Membrane channel proteins of the aquaporin family are highly selective for the permeation of specific small molecules, with absolute exclusion of ions including protons or OH- anions and charged solutes, and without dissipating the electrochemical potential across the cell membrane. The family includes channels that conduct water called aquaporins, and channels that conduct glycerol or other linear polyalcohols (aquaglyceroporins), and ion conducting channels. The E. coli glycerol facilitator (GlpF) was cloned, expressed, purified, and crystallized with its primary permeant substrate glycerol. The crystal structure of GlpF was determined by multiple isomorphous replacement at 2.2 ? resolution and shows aspects of the channel that illustrate the basis for its selectivity. Three 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 OH groups form hydrogen bonds with a pair of acceptor, and donor atoms. Two conserved -Asn Pro Ala- motifs meet in the center of the membrane and form a conserved 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 functional assay was developed, and shows that the channel is both stereo, and enantio selective. The conservation of sequence shows how the water channels of the same family conduct water and exclude glycerol. A comparison of features of the channel itself, and of the space through the center of the tetrameric arrangement of channels leads to conclusions about what is required for ion conducting channels.

Tea and coffee will be served in R3151 Beckman Institute at 2:15pm.