title: Crystal structure of AqpZ tetramer reveals two distinct Arg-189 conformations associated with water permeation through the narrowest constriction of the water-conducting channel.
abstract: AqpZ is a homotetramer of four water-conducting channels that facilitate rapid water movements across the plasma membrane of Escherichia coli. Here we report a 3.2 angstroms crystal structure of the tetrameric AqpZ (tAqpZ). All channel-lining residues in the four monomeric channels are found orientated in nearly identical positions with one marked exception at the narrowest channel constriction, where the side chain of a highly conserved Arg-189 adopts two distinct conformational orientations. In one of the four monomers, the guanidino group of Arg-189 points toward the periplasmic vestibule, opening up the constriction to accommodate the binding of a water molecule through a tridentate H-bond. In the other three monomers, the Arg-189 guanidino group bends over to form an H-bond with carbonyl oxygen of the Thr-183, thus occluding the channel. Therefore, the tAqpZ structure reveals two distinct Arg-189 confirmations associated with water permeation through the channel constrictions. Alternation between the two Arg-189 conformations disrupts continuous flow of water, thus regulating the open probability of the water pore. Further, the difference in Arg-189 displacements is correlated with a strong electron density found between the first transmembrane helices of two open channels, suggesting that the observed Arg-189 conformations are stabilized by asymmetrical subunit interactions in tAqpZ.
authors.affiliation: Department of Biology, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: Jiansheng
authors.initials: J
authors.lastname: Jiang
authors.forename: Brenda V
authors.initials: BV
authors.lastname: Daniels
authors.forename: Dax
authors.initials: D
authors.lastname: Fu
mesh_terms: D020346:Aquaporins / Q000737:chemistry* / Q000378:metabolism*; D001692:Biological Transport; D003461:Crystallography; D004926:Escherichia coli; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D014867:Water / Q000378:metabolism*
chemical_list: D020346:Aquaporins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; C106425:aqpZ protein, E coli; D014867:Water