With TolR probably plays a role in maintaining the position of the peptidoglycan cell wall in the periplasm. Acts as a porin with low permeability that allows slow penetration of small solutes; an internal gate slows down solute passage. Reconstitution in planar bilayers with lithium dodecyl sulfate-solublized P5 yields narrow pores (58 pS conductance) with a low probability of opening, whereas n-octyl-bD-glucopyranoside-solubilized P5 forms large pores (1.1 nS conductance) with high open probability. The large pore easily converts to the smaller pore at room temperature; at 42 degrees Celsius the smaller pore converts to the larger one. Monomer and homodimer. The extracellular loops are most variable in sequence, and in some bacteria confer sensitivity to phage and/or colicins. Cross reacts with antisera againt E.coli OmpA. Belongs to the outer membrane OOP (TC 1.B.6) superfamily. OmpA family.