title: Functional reconstitution and characterization of AqpZ, the E. coli water channel protein.
abstract: Understanding the selectivity of aquaporin water channels will require structural and functional studies of wild-type and modified proteins; however, expression systems have not previously yielded aquaporins in the necessary milligram quantities. Here we report expression of a histidine-tagged form of Escherichia coli aquaporin-Z (AqpZ) in its homologous expression system. 10-His-AqpZ is solubilized and purified to near homogeneity in a single step with a final yield of approximately 2.5 mg/l of culture. The histidine tag is removed by trypsin, yielding the native protein with the addition of three N-terminal residues, as confirmed by microsequencing. Sucrose gradient sedimentation analysis showed that the native, solubilized AqpZ protein is a trypsin-resistant tetramer. Unlike other known aquaporins, AqpZ tetramers are not readily dissociated by 1% SDS at neutral pH. Hydrophilic reducing agents have a limited effect on the stability of the tetramer in 1% SDS, whereas incubations for more than 24 hours, pH values below 5.6, or exposure to the hydrophobic reducing agent ethanedithiol cause dissociation into monomers. Cys20, but not Cys9, is necessary for the stability of the AqpZ tetramer in SDS. Upon reconstitution into proteoliposomes, AqpZ displays very high osmotic water permeability (pf > or = 10 x 10(-14) cm3 s-1 subunit-1) and low Arrhenius activation energy (Ea = 3.7 kcal/mol), similar to mammalian aquaporin-1 (AQP1). No permeation by glycerol, urea or sorbitol was detected. Expression of native and modified AqpZ in milligram quantities has permitted biophysical characterization of this remarkably stable aquaporin tetramer, which is being utilized for high-resolution structural studies.
authors.affiliation: Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205-2185, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Borgnia
authors.forename: D
authors.initials: D
authors.lastname: Kozono
authors.forename: G
authors.initials: G
authors.lastname: Calamita
authors.forename: P C
authors.initials: PC
authors.lastname: Maloney
authors.forename: P
authors.initials: P
authors.lastname: Agre
mesh_terms: D000595:Amino Acid Sequence; D020346:Aquaporins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism; D003545:Cysteine / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics; D029968:Escherichia coli Proteins; D006863:Hydrogen-Ion Concentration; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009154:Mutation; D009994:Osmolar Concentration; D010539:Permeability; D011487:Protein Conformation / Q000187:drug effects; D011510:Proteolipids / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D019163:Reducing Agents / Q000494:pharmacology; D012967:Sodium Dodecyl Sulfate / Q000494:pharmacology; D012995:Solubility; D013329:Structure-Activity Relationship; D014357:Trypsin / Q000378:metabolism; D014867:Water / Q000378:metabolism
chemical_list: D020346:Aquaporins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D011510:Proteolipids; D011993:Recombinant Fusion Proteins; D019163:Reducing Agents; C106425:aqpZ protein, E coli; C020485:proteoliposomes; D014867:Water; D012967:Sodium Dodecyl Sulfate; D014357:Trypsin; D003545:Cysteine