Crystal Structures of Membrane Transporter MmpL3, an Anti-TB Drug Target

Two Accessory Proteins Govern MmpL3 Mycolic Acid Transport in Mycobacteria

Mycolic acids are the signature lipid of mycobacteria and constitute an important physical component of the cell wall, a target of mycobacterium-specific antibiotics and a mediator of Mycobacterium tuberculosis pathogenesis. Mycolic acids are synthesized in the cytoplasm and are thought to be transported to the cell wall as a trehalose ester by the MmpL3 transporter, an antibiotic target for M. tuberculosis However, the mechanism by which mycolate synthesis is coupled to transport, and the full MmpL3 transport machinery, is unknown. Here, we identify two new components of the MmpL3 transport machinery in mycobacteria. The protein encoded by MSMEG_0736/Rv0383c is essential for growth of Mycobacterium smegmatis and M. tuberculosis and is anchored to the cytoplasmic membrane, physically interacts with and colocalizes with MmpL3 in growing cells, and is required for trehalose monomycolate (TMM) transport to the cell wall. In light of these findings, we propose MSMEG_0736/Rv0383c be named "TMM transport factor A", TtfA. The protein encoded by MSMEG_5308 also interacts with the MmpL3 complex but is nonessential for growth or TMM transport. However, MSMEG_5308 accumulates with inhibition of MmpL3-mediated TMM transport and stabilizes the MmpL3/TtfA complex, indicating that it may stabilize the transport system during stress. These studies identify two new components of the mycobacterial mycolate transport machinery, an emerging antibiotic target in M. tuberculosis IMPORTANCE The cell envelope of Mycobacterium tuberculosis, the bacterium that causes the disease tuberculosis, is a complex structure composed of abundant lipids and glycolipids, including the signature lipid of these bacteria, mycolic acids. In this study, we identified two new components of the transport machinery that constructs this complex cell wall. These two accessory proteins are in a complex with the MmpL3 transporter. One of these proteins, TtfA, is required for mycolic acid transport and cell viability, whereas the other stabilizes the MmpL3 complex. These studies identify two new components of the essential cell envelope biosynthetic machinery in mycobacteria.

Direct Inhibition of MmpL3 by Novel Antitubercular Compounds

MmpL3, an essential transporter involved in the export of mycolic acids, is the proposed target of a number of antimycobacterial inhibitors under development. Whether MmpL3 serves as the direct target of these compounds, however, has been called into question after the discovery that some of them dissipated the proton motive force from which MmpL transporters derive their energy. Using a combination of in vitro and whole-cell-based approaches, we here provide evidence that five structurally distinct MmpL3 inhibitor series, three of which impact proton motive force in Mycobacterium tuberculosis, directly interact with MmpL3. Medium- to high-throughput assays based on these approaches were developed to facilitate the future screening and optimization of MmpL3 inhibitors. The promiscuity of MmpL3 as a drug target and the mechanisms through which missense mutations located in a transmembrane region of this transporter may confer cross-resistance to a variety of chemical scaffolds are discussed in light of the exquisite vulnerability of MmpL3, its apparent mechanisms of interaction with inhibitors, and evidence of conformational changes induced both by the inhibitors and one of the most commonly identified resistance mutations in MmpL3.

MmpL3 is a lipid transporter that binds trehalose monomycolate and phosphatidylethanolamine

The cell envelope of Mycobacterium tuberculosis is notable for the abundance of mycolic acids (MAs), essential to mycobacterial viability, and of other species-specific lipids. The mycobacterial cell envelope is extremely hydrophobic, which contributes to virulence and antibiotic resistance. However, exactly how fatty acids and lipidic elements are transported across the cell envelope for cell-wall biosynthesis is unclear. Mycobacterial membrane protein Large 3 (MmpL3) is essential and required for transport of trehalose monomycolates (TMMs), precursors of MA-containing trehalose dimycolates (TDM) and mycolyl arabinogalactan peptidoglycan, but the exact function of MmpL3 remains elusive. Here, we report a crystal structure of Mycobacterium smegmatis MmpL3 at a resolution of 2.59 Å, revealing a monomeric molecule that is structurally distinct from all known bacterial membrane proteins. A previously unknown MmpL3 ligand, phosphatidylethanolamine (PE), was discovered inside this transporter. We also show, via native mass spectrometry, that MmpL3 specifically binds both TMM and PE, but not TDM, in the micromolar range. These observations provide insight into the function of MmpL3 and suggest a possible role for this protein in shuttling a variety of lipids to strengthen the mycobacterial cell wall.

MmpL Proteins in Physiology and Pathogenesis of M. tuberculosis

Mycobacterium tuberculosis (Mtb) remains an important human pathogen. The Mtb cell envelope is a critical bacterial structure that contributes to virulence and pathogenicity. Mycobacterial membrane protein large (MmpL) proteins export bulky, hydrophobic substrates that are essential for the unique structure of the cell envelope and directly support the ability of Mtb to infect and persist in the host. This review summarizes recent investigations that have enabled insight into the molecular mechanisms underlying MmpL substrate export and the role that these substrates play during Mtb infection.