The two-domain, 325 residue outer membrane protein OmpA is one of the most abundant proteins of Escherichia coli playing a role in the maintenance of the integrity of the cell surface. The N-terminal domain of about 170 amino acid residues is embedded in the membrane, presumably in form of a beta-barrel consisting of eight amphipathic transmembrane strands. Pairs of these proposed transmembrane strands were permuted at the DNA level in order to dissect the process of membrane assembly. All three possible circular permutations led to variants, which were, in comparison with the wild-type protein, less efficiently assembled. In contrast, with none of 18 non-circularly permuted variants could any membrane assembly be detected. We take this as an indication that the "right" (wild-type) order of beta-strands is a necessary and sufficient prerequisite for an at least partially successful membrane assembly. This may be the consequence of packing constraints and/or a failure to adopt the wild-type arrangement of beta-strands, which would ask for crossing of the periplasmic turns.
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