title: High resolution AFM topographs of the Escherichia coli water channel aquaporin Z.
abstract: Aquaporins form a large family of membrane channels involved in osmoregulation. Electron crystallography has shown monomers to consist of six membrane spanning alpha-helices confirming sequence based predictions. Surface exposed loops are the least conserved regions, allowing differentiation of aquaporins. Atomic force microscopy was used to image the surface of aquaporin Z, the water channel of Escherichia coli. Recombinant protein with an N-terminal fragment including 10 histidines was isolated as a tetramer by Ni-affinity chromatography, and reconstituted into two-dimensional crystals with p42(1)2 symmetry. Small crystalline areas with p4 symmetry were found as well. Imaging both crystal types before and after cleavage of the N-termini allowed the cytoplasmic surface to be identified; a drastic change of the cytoplasmic surface accompanied proteolytic cleavage, while the extracellular surface morphology did not change. Flexibility mapping and volume calculations identified the longest loop at the extracellular surface. This loop exhibited a reversible force-induced conformational change.
authors.affiliation: M.E.Müller Institute for Structural Biology at the Biozentrum, University of Basel, Basel CH-4056, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Scheuring
authors.forename: P
authors.initials: P
authors.lastname: Ringler
authors.forename: M
authors.initials: M
authors.lastname: Borgnia
authors.forename: H
authors.initials: H
authors.lastname: Stahlberg
authors.forename: D J
authors.initials: DJ
authors.lastname: Müller
authors.forename: P
authors.initials: P
authors.lastname: Agre
authors.forename: A
authors.initials: A
authors.lastname: Engel
mesh_terms: D000595:Amino Acid Sequence; D020346:Aquaporins / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure*; D003460:Crystallization; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D018625:Microscopy, Atomic Force; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000648:ultrastructure; D013499:Surface Properties
chemical_list: D020346:Aquaporins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D011994:Recombinant Proteins; C106425:aqpZ protein, E coli