Part of the tripartite efflux system MacAB-TolC. MacA stimulates the ATPase activity of MacB by promoting the closed ATP-bound state of MacB, increases the capacity of MacB to bind macrolides such as erythromycin, and provides a physical link between MacB and TolC. When overexpressed, the system confers resistance against macrolides composed of 14- and 15-membered lactones but no or weak resistance against 16-membered ones. In addition, MacA binds tightly rough-core lipopolysaccharide (R-LPS), suggesting that the system could also transport R-LPS or a similar glycolipid. Homohexamer. Part of the tripartite efflux system MacAB-TolC, which is composed of an inner membrane transporter, MacB, a periplasmic membrane fusion protein, MacA, and an outer membrane component, TolC. The complex forms a large protein conduit and can translocate molecules across both the inner and outer membranes. MacA interacts with MacB and TolC, and this interaction changes its conformation. Expressed at very low levels under standard laboratory conditions. The periplasmic domain is sufficient to bind MacB. The membrane proximal (MP) region plays a critical role in the stimulation of MacB ATPase activity. Belongs to the membrane fusion protein (MFP) (TC 8.A.1) family. Extended N-terminus.