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FUNCTION Part of the tripartite efflux system MacAB-TolC (PubMed:11544226, PubMed:17214741, PubMed:18955484, PubMed:21696464, PubMed:28504659, PubMed:40083904, PubMed:40461577). 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 (PubMed:17214741, PubMed:18955484, PubMed:21696464). When overexpressed, the system confers resistance against macrolides composed of 14- and 15-membered lactones but no or weak resistance against 16-membered ones; also confers resistance against bacitracin and colistin (PubMed:11544226, PubMed:18955484). In addition, MacA binds tightly rough-core lipopolysaccharide (R-LPS), suggesting that the system could also transport R-LPS or a similar glycolipid (PubMed:23974027). As part of the system, involved in the efflux of the immediate heme precursor, protoporphyrin IX (PPIX), which is probably an endogenous substrate (PubMed:25257218).SUBUNIT Homohexamer (PubMed:19254725). 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 (PubMed:17214741, PubMed:18955484, PubMed:21325274, PubMed:21696464, PubMed:28504659, PubMed:40083904, PubMed:40461577). The complex forms a large protein conduit and can translocate molecules across both the inner and outer membranes (PubMed:17214741, PubMed:18955484, PubMed:21325274, PubMed:21696464, PubMed:28504659, PubMed:40083904, PubMed:40461577). MacA interacts with MacB and TolC, and this interaction changes its conformation (PubMed:17214741, PubMed:18955484, PubMed:21325274, PubMed:21696464, PubMed:28504659, PubMed:40461577). Upon the binding of substrate, MacB ATPase may become active, resulting in the hydrolysis of ATP and the expulsion of the substrate, followed by the subsequent dissociation of MacB from the full pump (PubMed:40083904, PubMed:40461577). In the absence of substrate, MacA and TolC may form a bipartite complex (PubMed:40083904, PubMed:40461577).INTERACTION Expressed at very low levels under standard laboratory conditions.DOMAIN The periplasmic domain is sufficient to bind MacB (PubMed:17214741, PubMed:21696464). The membrane proximal (MP) region plays a critical role in the stimulation of MacB ATPase activity (PubMed:17214741, PubMed:21696464).DISRUPTION PHENOTYPE In a macB mutant background, grown in the presence of 5-aminolevulinic acid (5-ALA), does not accumulate porphyrins (PubMed:25257218). In a combined macB and entF mutant background, grown in the presence of iron chelator 2-2 dipyridyl, shows stronger fluorescence than wild-type (PubMed:25257218). Total intracellular porphyrins increase about 20-fold in the presence of iron chelators, in a combined macB and entF mutant background (PubMed:25257218).SIMILARITY Belongs to the membrane fusion protein (MFP) (TC 8.A.1) family.SEQUENCE CAUTION Extended N-terminus.
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