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FUNCTION Part of the tripartite efflux system MacAB-TolC (PubMed:11544226, PubMed:28504659, PubMed:29109272, PubMed:40083904, PubMed:40461577). MacB is a non-canonical ABC transporter that contains transmembrane domains (TMD), which form a pore in the inner membrane, and an ATP-binding domain (NBD), which is responsible for energy generation (PubMed:17214741, PubMed:18955484, PubMed:29109272). 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:28504659, PubMed:29109272). Involved in secretion of enterotoxin STII (PubMed:29109272). In addition, 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).ACTIVITY REGULATION ATPase activity is stimulated by interaction with MacA and inhibited by vanadate.BIOPHYSICOCHEMICAL PROPERTIES kcat is 0.78 sec(-1) for ATP in the presence of MacA. kcat is 0.10 sec(-1) for ATP in the absence of MacA.SUBUNIT Homodimer (PubMed:18955484). 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:21696464, PubMed:28504659, PubMed:29109272, 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:21696464, PubMed:28504659, PubMed:40083904). Interacts with MacA (PubMed:17214741, PubMed:18955484, PubMed:21696464). 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:29109272, PubMed:40083904, PubMed:40461577). In the absence of substrate, MacA and TolC may form a bipartite complex (PubMed:40083904, PubMed:40461577).INTERACTION In a macA mutant background, grown in the presence of 5-aminolevulinic acid (5-ALA), does not accumulate porphyrins (PubMed:25257218). In a combined macA 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 macA and entF mutant background (PubMed:25257218).SIMILARITY Belongs to the ABC transporter superfamily. Macrolide exporter (TC 3.A.1.122) family.
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