title: Structural and functional roles of the surface-exposed loops of the beta-barrel membrane protein OmpA from Escherichia coli.
abstract: The N-terminal domain of the OmpA protein from Escherichia coli, consisting of 170 amino acid residues, is embedded in the outer membrane, in the form of an antiparallel beta-barrel whose eight transmembrane beta-strands are connected by three short periplasmic turns and four relatively large surface-exposed hydrophilic loops. This protein domain serves as a paradigm for the study of membrane assembly of integral beta-structured membrane proteins. In order to dissect the structural and functional roles of the surface-exposed loops, they were shortened separately and in all possible combinations. All 16 loop deletion mutants assembled into the outer membrane with high efficiency and adopted the wild-type membrane topology. This systematic approach proves the absence of topogenic signals (e.g., in the form of loop sizes or charge distributions) in these loops. The shortening of surface-exposed loops did not reduce the thermal stability of the protein. However, none of the mutant proteins, with the exception of the variant with the fourth loop shortened, served as a receptor for the OmpA-specific bacteriophage K3. Furthermore, all loops were necessary for the OmpA protein to function in the stabilization of mating aggregates during F conjugation. An OmpA deletion variant with all four loops shortened, consisting of only 135 amino acid residues, constitutes the smallest beta-structured integral membrane protein known to date. These results represent a further step toward the development of artificial outer membrane proteins.
authors.affiliation: Max-Planck-Institut für Biologie, Abteilung Mikrobiologie, D-72076 Tübingen, Germany. koebnik@iname.com
authors.forename: R
authors.initials: R
authors.lastname: Koebnik
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D002462:Cell Membrane / Q000378:metabolism; D003227:Conjugation, Genetic; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D016297:Mutagenesis, Site-Directed; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017384:Sequence Deletion
chemical_list: D001425:Bacterial Outer Membrane Proteins; D011994:Recombinant Proteins; C078167:OMPA outer membrane proteins