With TolR probably plays a role in maintaining the position of the peptidoglycan cell wall in the periplasm (Probable). Plays a role in resistance to environmental stress, and a role in outer membrane functionality and cell shape (PubMed:11906175, PubMed:361695). Non-covalently binds peptidoglycan (PubMed:25135663) (Probable). Acts as a porin with low permeability that allows slow penetration of small solutes (PubMed:1370823, PubMed:20004640, PubMed:21069910). A very abundant protein, there can be up to 210,000 OmpA molecules per cell (PubMed:24766808). Reconstitution in unilamellar lipid vesicles shows only about 3% of the protein is in an open conformation, which allows diffusion of L-arabinose at a rate comparable to that of OmpF porin; the pores interconvert very rarely (PubMed:7517935). Native and reconstituted protein forms ion channels with 2 conductance states of (50-80 pS) and (260-320 pS); the states are interconvertible in this study. Interconversion requires refolding of the periplasmic domain (PubMed:10636850). Small pores are converted into large pores by increasing temperature; in this model the C-terminal periplasmic domain forms 8 more beta sheets to form a larger pore (PubMed:15850404). The center of the isolated beta-barrel is polar and has a central gate (involving Glu-73, Lys-103, Glu-149 and Arg-159, sandwiched between Tyr-29, Phe-40 and Tyr-94), with no obvious passage for water or ions (PubMed:9808047) (Probable). Gating involves the Glu-73-Arg-159 salt bridge; gate opening probably involves formation of alternate salt bridges Glu-149-Arg-159 and Glu-73-Lys-103 (PubMed:17041590). Modeling suggests that non-covalent binding of OmpA (from the outer membrane) and TolR (from the inner membrane) to peptidoglycan maintains the position of the cell wall in the periplasm, holding it approximately equidistant from both the inner and outer membranes. Trimeric Lpp controls the width of the periplasm, adjusts its tilt angle to accommodate to the available space, and can compensate in part for an absence of OmpA (Probable). Required for F plasmid cell conjugation; purified protein plus lipopolysaccharide (LPS) inhibits conjugation in a concentration-dependent manner. OmpA probably acts as the receptor on recipient cells (PubMed:321438) (Probable). Required for the stabilization of mating aggregates during F plasmid conjugative transfer, may interact with F plasmid-encoded TraN, but not with TraN from plasmid R100-1 (PubMed:9696748, PubMed:16272376). All 4 external, surface-exposed loops are required for F plasmid conjugation (PubMed:10368142). (Microbial infection) Mutants with decreased or altered protein are resistant to bacteriophage TuII* (PubMed:1107069, PubMed:791936). Mutants which have no or greatly reduced protein levels are resistant to a number of bacteriophages, including K3, K4, K5, Ox2, Ox3, Ox4, Ox5, Ml, and Ac3 (Probable) (PubMed:783129, PubMed:3902787). Mutations in this protein render the bacteria partially or completely susceptible to a number of bacteriophages for which is it probably the receptor (PubMed:6086577, PubMed:3902787). All but the last external, surface-exposed loops are required for phage K3 infection (PubMed:10368142). (Microbial infection) A mutation in this locus (called tolG) renders the cell tolerant to bacteriocin JF246 but does not affect its sensitivity to colicins A, C, El, E2, E3, K, Ia, or Ib (PubMed:4591955). Mutations in this protein render the bacteria partially or completely susceptible to colicin K or colicin L, for which is it probably the receptor (PubMed:6086577). Monomer (PubMed:1370823, PubMed:10764596, PubMed:9808047). Homodimer (PubMed:16079137, PubMed:21697552, PubMed:24746938). Interacts with Lpp (PubMed:3013869). About 10% of the C-terminal periplasmic domain dimerizes when expressed without the N-terminal domain (PubMed:25135663, PubMed:24746938). Interacts with F plasmid-encoded TraN during conjugation (Probable) (PubMed:16272376). The 8 beta strands are tilted by about 45 degrees relative to the membrane normal (PubMed:9808047, PubMed:10764596, PubMed:11276254, PubMed:16719475). Evenly distributed on the outer membrane (PubMed:10947984). The N-terminus transmembrane region forms low conductance pores (50-80 pS); larger conductance pores (260-320 pS) are formed by the whole protein. Refolding of the periplasmic domain may be required to make the large conductance pores (PubMed:10636850). The structure of the whole protein is controversial; the periplasmic domain may or may not be inserted into the cell outer membrane to form a larger pore. The protein with a narrow 8 sheet beta-barrel and a periplasmic domain may be an intermediate in formation of the larger pore (Probable). The disulfide bond is necessary for formation of the larger pore; disulfide-bonded protein inserts into the membrane without reduction (PubMed:2211627, PubMed:21069910). The isolated C-terminal periplasmic domain binds peptidoglycan (PubMed:25135663, PubMed:27866852). A region in the isolated periplasmic domain bulges from the rest of the protein (resides 302-328) and is stabilized by the disulfide bond (PubMed:25135663). Hydroxybutyrylation on Ser-184 and/or Ser-188 can add up to 10 R-3-hydroxybutyrate residues; it is not clear if one or both residues are modified in vivo. Hydrophobic residues adjacent to the modified Ser residues (i.e. Leu-183, Leu-185 and Val-187) are important for hydroxylation. Hydroxybutyrylation occurs in the cytoplasm (PubMed:17659252). Hydroxybutyrylation is required for stable pore formation, unmodified protein may not be able to insert into the outer membrane (PubMed:20004640). Further hydroxybutyrylation occurs in the C-terminal fragment (residues 285-346); this modification probably occurs in the periplasm (PubMed:21069910). OmpA is degraded by human neutrophil elastase (ELANE) in vitro; this coincides with bacterial death. Mice with a homozygous knockout of ELANE are more easily killed by wild-type E.coli but not by E.coli with an ompA deletion; this experiment was certainly not performed with E.coli strain K12 which is no longer virulent. Double lpp-ompA mutants are spherical and only grow in the presence of electrolytes such as 30 mM Mg(2+), and are sensitive to hydrophobic antibiotics and detergents. The peptidoglycan layer is no longer attached to the cell outer membrane which undergoes abundant blebbing (PubMed:361695). Decreased efficiency of bacterial conjugation of the F plasmid (PubMed:9696748). Deletions grow poorly on SDS, cholate, at pH 3.8 or at 5 M NaCl (PubMed:11906175). Grows very slowly in the absence of NaCl, wild-type growth in 1% NaCl (PubMed:17041590). E.coli is no longer killed by human neutrophil elastase (ELANE) in vitro (PubMed:10947984). Belongs to the outer membrane OOP (TC 1.B.6) superfamily. OmpA family.