May play a role in ammonia secretion that neutralizes the medium at pH 5.5, although it does not play a direct role in ammonia transport. Binds peptidoglycan (By similarity). Expression depends on arfB and arfC. Does not have a cleavable signal sequence. Expressed equally at pH 5.5 and 7.2 (at protein level). Part of the arfA-arfB-arfC operon. No effect of a single gene deletion on growth at pH 7.0 nor of whole-operon deletion on growth at pH 4.6, 5.5 or 7.2. No effect of whole-operon deletion on growth in macrophages. Belongs to the outer membrane OOP (TC 1.B.6) superfamily. ArfA family.