Part of an export system, which is required for biosynthesis and secretion of siderophores. Essential for virulence. Overexpression of the system confers non-target based resistance to azoles, clofazimine and bedaquiline, via an efflux mechanism. Interacts with MmpL5. Repressed by MmpR5 (PubMed:24737322). Repressed by iron (PubMed:12065475). Regulation is IdeR-independent (PubMed:12065475). Deletion mutant does not exhibit a low iron growth phenotype, but has attenuated virulence compared to the wild-type strain. Deletion of both mmpS4 and mmpS5 drastically decreases synthesis and secretion of siderophores, and greatly reduces virulence in mice. Belongs to the MmpS family.