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. Confers resistance against macrolides (By similarity). 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 (By similarity). Belongs to the membrane fusion protein (MFP) (TC 8.A.1) family.