Structure, assembly, and mechanism of the MmpL family of transporters

SUMMARY The mycobacterial cell envelope, one of the most complex membranes found in bacteria, plays a major role in bacterial pathogenesis, virulence, and antimicrobial resistance. Biogenesis and modeling of this cell envelope are heavily influenced by the mycobacterial membrane protein large (MmpL) family of transporters due to their ability to export fatty acids and lipid components. Select MmpL transporters can also function as siderophore exporters to help regulate the acquisition of iron, which is critical for mycobacterial survival. Additionally, certain MmpLs can participate in active efflux of antimycobacterial drugs, directly contributing to antimicrobial resistance. Given the physiological significance of these MmpL membrane proteins and their potential to serve as important antimycobacterial targets, questions regarding their functional roles, cellular assemblies, interactions, and regulation need to be fully addressed. In this review, we summarize our current knowledge on the structures and functions of these MmpL transporters. It is our hope that researchers in the field will continue to build upon these efforts and apply various structural, biophysical, and biochemical methodologies to fully elucidate how MmpL transporters coordinate to participate in cell envelope biogenesis, cell elongation and division, and antimicrobial resistance.