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Journal of Bacteriology, June 1999, p. 3688-3694, Vol. 181, No. 12
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Structural and Functional Roles of the Surface-Exposed Loops of the beta -Barrel Membrane Protein OmpA from Escherichia coli

Ralf Koebnik*

Max-Planck-Institut für Biologie, Abteilung Mikrobiologie, D-72076 Tübingen, Germany

Received 25 January 1999/Accepted 21 April 1999

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.

* Present address: Martin-Luther-University, Institute of Genetics, Weinbergweg 22, D-06120 Halle (Saale), Germany. Phone: 49-345-5526293. Fax: 49-345-5527259. E-mail: koebnik{at}iname.com.

Journal of Bacteriology, June 1999, p. 3688-3694, Vol. 181, No. 12
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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Copyright © 1999 by the American Society for Microbiology. All rights reserved.