This family includes homologs of the well characterized OmpA protein of Escherichia coli. Members of this family (similar to members of the OprF family) show a modular architecture which is composed of two distinctstructural domains: an N-terminal ß-barrel domain formed by 8 ß strands with short turns at the periplasmic ends and long flexible loops at the external ends that anchors the protein to the outer membrane; and a C-terminal domain that protrudes into the periplasmic space interacting with peptidoglycans. The C-terminal domain (named OmpA-like domain), is also found in a number of outer membrane lipoproteins, therefore it is not characteristic of ß-barrels. OmpA is largely expressed in the outer membrane of Escherichia coli and is required for bacterial conjugation and for maintenance of outer membrane stability. OmpA of E. coli K1 is a potential vaccine candidate because of its predominant contribution to bacterial pathogenesis and sub-cellular localization.There is also evidence that OmpA exhibits a small channel activity. There is a distant homology to Neisserial opacity (Opa) adhesins and members of the Ail/Lom/OmpX family but the barrels, though they possess the same number of strands (8), show different characteristics (shear numbers, inclination with respect to the membrane). A molecular-dynamics simulations of the full-length model of the OmpA dimer proposed by Robinson and co-workers revealed a large dimerization interface within the membrane environment, ensuring that the dimer is stable over the course of the simulations. The linker is flexible, expanding and contracting to pull the globular C-terminal domain up toward the membrane or push it down toward the periplasm, suggesting a possible mechanism for providing mechanical stability to the cell.

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