MmpL12 transports lipooligosaccharides and impacts virulence in Mycobacterium marinum

Lipooligosaccharides (LOSs) are polar glycolipids found in the cell envelope of many pathogenic mycobacteria. Here, we show that LOS transport in Mycobacterium marinum requires mmpL12, a member of the resistance-nodulation-division family of membrane proteins. Deletion of mmpL12 resulted in a rough colony morphology and increased hydrophobicity. The △mmpL12 mutant accumulated three of the biosynthesis intermediates of LOSs (LOS-I, LOS-II and LOS-III) intracellularly and failed to produce the final product, LOS-IV, suggesting that final glycosylation of LOS-III to yield LOS-IV occurs extracellularly after LOS-III export. In silico structural analysis of the MmpL12 suggests that it is a proton-driven transporter that shares very similar organization with other subclass 1 MmpLs (MmpL1, 2, 4–8 and 9–10), featuring a large periplasmic loop (PD3 domain) which is predicted to form a large coiled coil that may be involved in the trimerization of this subset of MmpL transporters. Furthermore, the long C-terminal extension domain, which is unique to MmpL12, may provide additional trimerization support and scaffold for assembly of additional LOS biosynthetic enzymes. The absence of any extracellular LOS intermediates and of LOS-IV had an impact on virulence, with the mutant strain exhibiting a larger bacterial burden in infected zebrafish embryos.