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. Required for conjugation with F-type plasmids; probably serves as the mating receptor on recipient cells. Monomer and homodimer. The extracellular loops are most variable in sequence, and in some bacteria confer sensitivity to phage and/or colicins. Belongs to the outer membrane OOP (TC 1.B.6) superfamily. OmpA family.