Druggable redox pathways against Mycobacterium abscessus in cystic fibrosis patient-derived airway organoids.

Mycobacterium abscessus (Mabs) drives life-shortening mortality in cystic fibrosis (CF) patients, primarily because of its resistance to chemotherapeutic agents. To date, our knowledge on the host and bacterial determinants driving Mabs pathology in CF patient lung remains rudimentary. Here, we used human airway organoids (AOs) microinjected with smooth (S) or rough (R-)Mabs to evaluate bacteria fitness, host responses to infection, and new treatment efficacy. We show that S Mabs formed biofilm, and R Mabs formed cord serpentines and displayed a higher virulence. While Mabs infection triggers enhanced oxidative stress, pharmacological activation of antioxidant pathways resulted in better control of Mabs growth and reduced virulence. Genetic and pharmacological inhibition of the CFTR is associated with better growth and higher virulence of S and R Mabs. Finally, pharmacological activation of antioxidant pathways inhibited Mabs growth, at least in part through the quinone oxidoreductase NQO1, and improved efficacy in combination with cefoxitin, a first line antibiotic. In conclusion, we have established AOs as a suitable human system to decipher mechanisms of CF-driven respiratory infection by Mabs and propose boosting of the NRF2-NQO1 axis as a potential host-directed strategy to improve Mabs infection control.

Altered serine metabolism promotes drug tolerance in Mycobacterium abscessus via a WhiB7-mediated adaptive stress response.

Mycobacterium abscessus is an emerging opportunistic pathogen responsible for chronic lung diseases, especially in patients with cystic fibrosis. Treatment failure of M. abscessus infections is primarily associated with intrinsic or acquired antibiotic resistance. However, there is growing evidence that antibiotic tolerance, i.e., the ability of bacteria to transiently survive exposure to bactericidal antibiotics through physiological adaptations, contributes to the relapse of chronic infections and the emergence of acquired drug resistance. Yet, our understanding of the molecular mechanisms that underlie antibiotic tolerance in M. abscessus remains limited. In the present work, a mutant with increased cross-tolerance to the first- and second-line antibiotics cefoxitin and moxifloxacin, respectively, has been isolated by experimental evolution. This mutant harbors a mutation in serB2, a gene involved in L-serine biosynthesis. Metabolic changes caused by this mutation alter the intracellular redox balance to a more reduced state that induces overexpression of the transcriptional regulator WhiB7 during the stationary phase, promoting tolerance through activation of a WhiB7-dependant adaptive stress response. These findings suggest that alteration of amino acid metabolism and, more generally, conditions that trigger whiB7 overexpression, makes M. abscessus more tolerant to antibiotic treatment.

Mycobacteria-host interactions in human bronchiolar airway organoids.

Respiratory infections remain a major global health concern. Tuberculosis is one of the top 10 causes of death worldwide, while infections with Non-Tuberculous Mycobacteria are rising globally. Recent advances in human tissue modeling offer a unique opportunity to grow different human "organs" in vitro, including the human airway, that faithfully recapitulates lung architecture and function. Here, we have explored the potential of human airway organoids (AOs) as a novel system in which to assess the very early steps of mycobacterial infection. We reveal that Mycobacterium tuberculosis (Mtb) and Mycobacterium abscessus (Mabs) mainly reside as extracellular bacteria and infect epithelial cells with very low efficiency. While the AO microenvironment was able to control, but not eliminate Mtb, Mabs thrives. We demonstrate that AOs responded to infection by modulating cytokine, antimicrobial peptide, and mucin gene expression. Given the importance of myeloid cells in mycobacterial infection, we co-cultured infected AOs with human monocyte-derived macrophages and found that these cells interact with the organoid epithelium. We conclude that adult stem cell (ASC)-derived AOs can be used to decipher very early events of mycobacteria infection in human settings thus offering new avenues for fundamental and therapeutic research.

Solution structure of the type I polyketide synthase Pks13 from Mycobacterium tuberculosis.

BACKGROUND: Type I polyketide synthases (PKSs) are multifunctional enzymes responsible for the biosynthesis of a group of diverse natural compounds with biotechnological and pharmaceutical interest called polyketides. The diversity of polyketides is impressive despite the limited set of catalytic domains used by PKSs for biosynthesis, leading to considerable interest in deciphering their structure-function relationships, which is challenging due to high intrinsic flexibility. Among nineteen polyketide synthases encoded by the genome of Mycobacterium tuberculosis, Pks13 is the condensase required for the final condensation step of two long acyl chains in the biosynthetic pathway of mycolic acids, essential components of the cell envelope of Corynebacterineae species. It has been validated as a promising druggable target and knowledge of its structure is essential to speed up drug discovery to fight against tuberculosis. RESULTS: We report here a quasi-atomic model of Pks13 obtained using small-angle X-ray scattering of the entire protein and various molecular subspecies combined with known high-resolution structures of Pks13 domains or structural homologues. As a comparison, the low-resolution structures of two other mycobacterial polyketide synthases, Mas and PpsA from Mycobacterium bovis BCG, are also presented. This study highlights a monomeric and elongated state of the enzyme with the apo- and holo-forms being identical at the resolution probed. Catalytic domains are segregated into two parts, which correspond to the condensation reaction per se and to the release of the product, a pivot for the enzyme flexibility being at the interface. The two acyl carrier protein domains are found at opposite sides of the ketosynthase domain and display distinct characteristics in terms of flexibility. CONCLUSIONS: The Pks13 model reported here provides the first structural information on the molecular mechanism of this complex enzyme and opens up new perspectives to develop inhibitors that target the interactions with its enzymatic partners or between catalytic domains within Pks13 itself.

The final assembly of trehalose polyphleates takes place within the outer layer of the mycobacterial cell envelope.

Trehalose polyphleates (TPP) are high-molecular-weight, surface-exposed glycolipids present in a broad range of nontuberculous mycobacteria. These compounds consist of a trehalose core bearing polyunsaturated fatty acyl substituents (called phleic acids) and a straight-chain fatty acid residue and share a common basic structure with trehalose-based glycolipids produced by Mycobacterium tuberculosis TPP production starts in the cytosol with the formation of a diacyltrehalose intermediate. An acyltransferase, called PE, subsequently catalyzes the transfer of phleic acids onto diacyltrehalose to form TPP, and an MmpL transporter promotes the export of TPP or its precursor across the plasma membrane. PE is predicted to be an anchored membrane protein, but its topological organization is unknown, raising questions about the subcellular localization of the final stage of TPP biosynthesis and the chemical nature of the substrates that are translocated by the MmpL transporter. Here, using genetic, biochemical, and proteomic approaches, we established that PE of Mycobacterium smegmatis is exported to the cell envelope following cleavage of its signal peptide and that this process is required for TPP biosynthesis, indicating that the last step of TPP formation occurs in the outer layers of the mycobacterial cell envelope. These results provide detailed insights into the molecular mechanisms controlling TPP formation and transport to the cell surface, enabling us to propose an updated model of the TPP biosynthetic pathway. Because the molecular mechanisms of glycolipid production are conserved among mycobacteria, these findings obtained with PE from M. smegmatis may offer clues to glycolipid formation in M. tuberculosis.

ESX-1 and phthiocerol dimycocerosates of Mycobacterium tuberculosis act in concert to cause phagosomal rupture and host cell apoptosis.

Although phthiocerol dimycocerosates (DIM) are major virulence factors of Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis, little is known about their mechanism of action. Localized in the outer membrane of mycobacterial pathogens, DIM are predicted to interact with host cell membranes. Interaction with eukaryotic membranes is a property shared with another virulence factor of Mtb, the early secretory antigenic target EsxA (also known as ESAT-6). This small protein, which is secreted by the type VII secretion system ESX-1 (T7SS/ESX-1), is involved in phagosomal rupture and cell death induced by virulent mycobacteria inside host phagocytes. In this work, by the use of several knock-out or knock-in mutants of Mtb or Mycobacterium bovis BCG strains and different cell biological assays, we present conclusive evidence that ESX-1 and DIM act in concert to induce phagosomal membrane damage and rupture in infected macrophages, ultimately leading to host cell apoptosis. These results identify an as yet unknown function for DIM in the infection process and open up a new research field for the study of the interaction of lipid and protein virulence factors of Mtb.

Interplay between siderophores and colibactin genotoxin biosynthetic pathways in Escherichia coli.

In Escherichia coli, the biosynthetic pathways of several small iron-scavenging molecules known as siderophores (enterobactin, salmochelins and yersiniabactin) and of a genotoxin (colibactin) are known to require a 4'-phosphopantetheinyl transferase (PPTase). Only two PPTases have been clearly identified: EntD and ClbA. The gene coding for EntD is part of the core genome of E. coli, whereas ClbA is encoded on the pks pathogenicity island which codes for colibactin. Interestingly, the pks island is physically associated with the high pathogenicity island (HPI) in a subset of highly virulent E. coli strains. The HPI carries the gene cluster required for yersiniabactin synthesis except for a gene coding its cognate PPTase. Here we investigated a potential interplay between the synthesis pathways leading to the production of siderophores and colibactin, through a functional interchangeability between EntD and ClbA. We demonstrated that ClbA could contribute to siderophores synthesis. Inactivation of both entD and clbA abolished the virulence of extra-intestinal pathogenic E. coli (ExPEC) in a mouse sepsis model, and the presence of either functional EntD or ClbA was required for the survival of ExPEC in vivo. This is the first report demonstrating a connection between multiple phosphopantetheinyl-requiring pathways leading to the biosynthesis of functionally distinct secondary metabolites in a given microorganism. Therefore, we hypothesize that the strict association of the pks island with HPI has been selected in highly virulent E. coli because ClbA is a promiscuous PPTase that can contribute to the synthesis of both the genotoxin and siderophores. The data highlight the complex regulatory interaction of various virulence features with different functions. The identification of key points of these networks is not only essential to the understanding of ExPEC virulence but also an attractive and promising target for the development of anti-virulence therapy strategies.

4'-Phosphopantetheinyl transferase PptT, a new drug target required for Mycobacterium tuberculosis growth and persistence in vivo.

The cell envelope of Mycobacterium tuberculosis, the causative agent of tuberculosis in humans, contains lipids with unusual structures. These lipids play a key role in both virulence and resistance to the various hostile environments encountered by the bacteria during infection. They are synthesized by complex enzymatic systems, including type-I polyketide synthases and type-I and -II fatty acid synthases, which require a post-translational modification to become active. This modification consists of the covalent attachment of the 4'-phosphopantetheine moiety of Coenzyme A catalyzed by phosphopantetheinyl transferases (PPTases). PptT, one of the two PPTases produced by mycobacteria, is involved in post-translational modification of various type-I polyketide synthases required for the formation of both mycolic acids and lipid virulence factors in mycobacteria. Here we identify PptT as a new target for anti-tuberculosis drugs; we address all the critical issues of target validation to demonstrate that PptT can be used to search for new drugs. We confirm that PptT is essential for the growth of M. bovis BCG in vitro and show that it is required for persistence of M. bovis BCG in both infected macrophages and immunodeficient mice. We generated a conditional expression mutant of M. tuberculosis, in which the expression of the pptT gene is tightly regulated by tetracycline derivatives. We used this construct to demonstrate that PptT is required for the replication and survival of the tubercle bacillus during the acute and chronic phases of infection in mice. Finally, we developed a robust and miniaturized assay based on scintillation proximity assay technology to search for inhibitors of PPTases, and especially of PptT, by high-throughput screening. Our various findings indicate that PptT meets the key criteria for being a therapeutic target for the treatment of mycobacterial infections.

Detection of soluble co-factor dependent protein expression in vivo: application to the 4'-phosphopantetheinyl transferase PptT from Mycobacterium tuberculosis.

The need for early-on diagnostic tools to assess the folding and solubility of expressed protein constructs in vivo is of great interest when dealing with recalcitrant proteins. In this paper, we took advantage of the picomolar sensitivity of the bipartite GFP1-10/GFP11 system to investigate the solubility of the Mycobacterium tuberculosis 4'-phosphopantetheinyl transferase PptT, an enzyme essential for the viability of the tubercle bacillus. In vivo and in vitro complementation assays clearly showed the improved solubility of the full-length PptT compared to its N- and C-terminally truncated counterparts. However, initial attempts to purify the full-length enzyme overexpressed in Escherichia coli cells were hampered by aggregation issues overtime that caused the protein to precipitate within hours. The fact that the naturally occurring Coenzyme A and Mg(2+), essentials for PptT to carry out its function, could play a role in stabilizing the enzyme was confirmed using DSF experiments. In vitro activity assays were performed using the ACP substrate from the type I polyketide synthase PpsC from M. tuberculosis, a 2188 amino-acid enzyme that plays a major role in the virulence and pathogenicity of this microbial pathogen. We selected the most soluble and compact ACP fragment (2042-2188), identified by genetic selection of in-frame fragments from random library experiments, to monitor the transfer of the P-pant moiety from Coenzyme A onto a conserved serine residue of this ACP domain.

Biochemical and structural study of the atypical acyltransferase domain from the mycobacterial polyketide synthase Pks13.

Pks13 is a type I polyketide synthase involved in the final biosynthesis step of mycolic acids, virulence factors, and essential components of the Mycobacterium tuberculosis envelope. Here, we report the biochemical and structural characterization of a 52-kDa fragment containing the acyltransferase domain of Pks13. This fragment retains the ability to load atypical extender units, unusually long chain acyl-CoA with a predilection for carboxylated substrates. High resolution crystal structures were determined for the apo, palmitoylated, and carboxypalmitoylated forms. Structural conservation with type I polyketide synthases and related fatty-acid synthases also extends to the interdomain connections. Subtle changes could be identified both in the active site and in the upstream and downstream linkers in line with the organization displayed by this singular polyketide synthase. More importantly, the crystallographic analysis illustrated for the first time how a long saturated chain can fit in the core structure of an acyltransferase domain through a dedicated channel. The structures also revealed the unexpected binding of a 12-mer peptide that might provide insight into domain-domain interaction.

In vivo imaging of MmpL transporters reveals distinct subcellular locations for export of mycolic acids and non-essential trehalose polyphleates in the mycobacterial outer membrane.

The mycobacterial cell envelope consists of a typical plasma membrane, surrounded by a complex cell wall and a lipid-rich outer membrane. The biogenesis of this multilayer structure is a tightly regulated process requiring the coordinated synthesis and assembly of all its constituents. Mycobacteria grow by polar extension and recent studies showed that cell envelope incorporation of mycolic acids, the major constituent of the cell wall and outer membrane, is coordinated with peptidoglycan biosynthesis at the cell poles. However, there is no information regarding the dynamics of incorporation of other families of outer membrane lipids during cell elongation and division. Here, we establish that the translocation of non-essential trehalose polyphleates (TPP) occurs at different subcellular locations than that of the essential mycolic acids. Using fluorescence microscopy approaches, we investigated the subcellular localization of MmpL3 and MmpL10, respectively involved in the export of mycolic acids and TPP, in growing cells and their colocalization with Wag31, a protein playing a critical role in regulating peptidoglycan biosynthesis in mycobacteria. We found that MmpL3, like Wag31, displays polar localization and preferential accumulation at the old pole whereas MmpL10 is more homogenously distributed in the plasma membrane and slightly accumulates at the new pole. These results led us to propose a model in which insertion of TPP and mycolic acids into the mycomembrane is spatially uncoupled.

Mycobacterium trehalose polyphleates are required for infection by therapeutically useful mycobacteriophages BPs and Muddy.

Mycobacteriophages are good model systems for understanding their bacterial hosts and show promise as therapeutic agents for nontuberculous mycobacterium infections. However, little is known about phage recognition of Mycobacterium cell surfaces, or mechanisms of phage resistance. We show here that surface-exposed trehalose polyphleates (TPPs) are required for infection of Mycobacterium abscessus and Mycobacterium smegmatis by clinically useful phages BPs and Muddy, and that TPP loss leads to defects in adsorption, infection, and confers resistance. Transposon mutagenesis indicates that TPP loss is the primary mechanism for phage resistance. Spontaneous phage resistance occurs through TPP loss, and some M. abscessus clinical isolates are phage-insensitive due to TPP absence. Both BPs and Muddy become TPP-independent through single amino acid substitutions in their tail spike proteins, and M. abscessus mutants resistant to TPP-independent phages reveal additional resistance mechanisms. Clinical use of BPs and Muddy TPP-independent mutants should preempt phage resistance caused by TPP loss.

Therapeutically useful mycobacteriophages BPs and Muddy require trehalose polyphleates.

Mycobacteriophages show promise as therapeutic agents for non-tuberculous mycobacterium infections. However, little is known about phage recognition of Mycobacterium cell surfaces or mechanisms of phage resistance. We show here that trehalose polyphleates (TPPs)-high-molecular-weight, surface-exposed glycolipids found in some mycobacterial species-are required for infection of Mycobacterium abscessus and Mycobacterium smegmatis by clinically useful phages BPs and Muddy. TPP loss leads to defects in adsorption and infection and confers resistance. Transposon mutagenesis shows that TPP disruption is the primary mechanism for phage resistance. Spontaneous phage resistance occurs through TPP loss by mutation, and some M. abscessus clinical isolates are naturally phage-insensitive due to TPP synthesis gene mutations. Both BPs and Muddy become TPP-independent through single amino acid substitutions in their tail spike proteins, and M. abscessus mutants resistant to TPP-independent phages reveal additional resistance mechanisms. Clinical use of BPs and Muddy TPP-independent mutants should preempt phage resistance caused by TPP loss.

Phthiocerol dimycocerosates of M. tuberculosis participate in macrophage invasion by inducing changes in the organization of plasma membrane lipids.

Phthiocerol dimycocerosates (DIM) are major virulence factors of Mycobacterium tuberculosis (Mtb), in particular during the early step of infection when bacilli encounter their host macrophages. However, their cellular and molecular mechanisms of action remain unknown. Using Mtb mutants deleted for genes involved in DIM biosynthesis, we demonstrated that DIM participate both in the receptor-dependent phagocytosis of Mtb and the prevention of phagosomal acidification. The effects of DIM required a state of the membrane fluidity as demonstrated by experiments conducted with cholesterol-depleting drugs that abolished the differences in phagocytosis efficiency and phagosome acidification observed between wild-type and mutant strains. The insertion of a new cholesterol-pyrene probe in living cells demonstrated that the polarity of the membrane hydrophobic core changed upon contact with Mtb whereas the lateral diffusion of cholesterol was unaffected. This effect was dependent on DIM and was consistent with the effect observed following DIM insertion in model membrane. Therefore, we propose that DIM control the invasion of macrophages by Mtb by targeting lipid organisation in the host membrane, thereby modifying its biophysical properties. The DIM-induced changes in lipid ordering favour the efficiency of receptor-mediated phagocytosis of Mtb and contribute to the control of phagosomal pH driving bacilli in a protective niche.

Phosphopantetheinyl transferase binding and inhibition by amidino-urea and hydroxypyrimidinethione compounds.

Owing to their role in activating enzymes essential for bacterial viability and pathogenicity, phosphopantetheinyl transferases represent novel and attractive drug targets. In this work, we examined the inhibitory effect of the aminido-urea 8918 compound against the phosphopantetheinyl transferases PptAb from Mycobacterium abscessus and PcpS from Pseudomonas aeruginosa, two pathogenic bacteria associated with cystic fibrosis and bronchiectasis, respectively. Compound 8918 exhibits inhibitory activity against PptAb but displays no activity against PcpS in vitro, while no antimicrobial activity against Mycobacterium abscessus or Pseudomonas aeruginosa could be detected. X-ray crystallographic analysis of 8918 bound to PptAb-CoA alone and in complex with an acyl carrier protein domain in addition to the crystal structure of PcpS in complex with CoA revealed the structural basis for the inhibition mechanism of PptAb by 8918 and its ineffectiveness against PcpS. Finally, in crystallo screening of potent inhibitors from the National Cancer Institute library identified a hydroxypyrimidinethione derivative that binds PptAb. Both compounds could serve as scaffolds for the future development of phosphopantetheinyl transferases inhibitors.

Phthiocerol Dimycocerosates From Mycobacterium tuberculosis Increase the Membrane Activity of Bacterial Effectors and Host Receptors.

Mycobacterium tuberculosis (Mtb) synthesizes a variety of atypical lipids that are exposed at the cell surface and help the bacterium infect macrophages and escape elimination by the cell's immune responses. In the present study, we investigate the mechanism of action of one family of hydrophobic lipids, the phthiocerol dimycocerosates (DIM/PDIM), major lipid virulence factors. DIM are transferred from the envelope of Mtb to host membranes during infection. Using the polarity-sensitive fluorophore C-Laurdan, we visualized that DIM decrease the membrane polarity of a supported lipid bilayer put in contact with mycobacteria, even beyond the site of contact. We observed that DIM activate the complement receptor 3, a predominant receptor for phagocytosis of Mtb by macrophages. DIM also increased the activity of membrane-permeabilizing effectors of Mtb, among which the virulence factor EsxA. This is consistent with previous observations that DIM help Mtb disrupt host cell membranes. Taken together, our data show that transferred DIM spread within the target membrane, modify its physical properties and increase the activity of host cell receptors and bacterial effectors, diverting in a non-specific manner host cell functions. We therefore bring new insight into the molecular mechanisms by which DIM increase Mtb's capability to escape the cell's immune responses.

Molecular Basis for Extender Unit Specificity of Mycobacterial Polyketide Synthases.

Mycobacterium tuberculosis is the causative agent of the tuberculosis disease, which claims more human lives each year than any other bacterial pathogen. M. tuberculosis and other mycobacterial pathogens have developed a range of unique features that enhance their virulence and promote their survival in the human host. Among these features lies the particular cell envelope with high lipid content, which plays a substantial role in mycobacterial pathogenicity. Several envelope components of M. tuberculosis and other mycobacteria, e.g., mycolic acids, phthiocerol dimycocerosates, and phenolic glycolipids, belong to the "family" of polyketides, secondary metabolites synthesized by fascinating versatile enzymes-polyketide synthases. These megasynthases consist of multiple catalytic domains, among which the acyltransferase domain plays a key role in selecting and transferring the substrates required for polyketide extension. Here, we present three new crystal structures of acyltransferase domains of mycobacterial polyketide synthases and, for one of them, provide evidence for the identification of residues determining extender unit specificity. Unravelling the molecular basis for such specificity is of high importance considering the role played by extender units for the final structure of key mycobacterial components. This work provides major advances for the use of mycobacterial polyketide synthases as potential therapeutic targets and, more generally, contributes to the prediction and bioengineering of polyketide synthases with desired specificity.

Conformational flexibility of coenzyme A and its impact on the post-translational modification of acyl carrier proteins by 4'-phosphopantetheinyl transferases.

One central question surrounding the biosynthesis of fatty acids and polyketide-derived natural products is how the 4'-phosphopantetheinyl transferase (PPTase) interrogates the essential acyl carrier protein (ACP) domain to fulfill the initial activation step. The triggering factor of this study was the lack of structural information on PPTases at physiological pH, which could bias our comprehension of the mechanism of action of these important enzymes. Structural and functional studies on the family II PPTase PptAb of Mycobacterium abscessus show that pH has a profound effect on the coordination of metal ions and on the conformation of endogenously bound coenzyme A (CoA). The observed conformational flexibility of CoA at physiological pH is accompanied by a disordered 4'-phosphopantetheine (Ppant) moiety. Finally, structural and dynamical information on an isolated mycobacterial ACP domain, in its apo form and in complex with the activator PptAb, suggests an alternate mechanism for the post-translational modification of modular megasynthases.

Molecular dissection of the biosynthetic relationship between phthiocerol and phthiodiolone dimycocerosates and their critical role in the virulence and permeability of Mycobacterium tuberculosis.

Phthiocerol dimycocerosates and related compounds are important molecules in the biology of Mycobacterium tuberculosis, playing a key role in the permeability barrier and in pathogenicity. Both phthiocerol dimycocerosates, the major compounds, and phthiodiolone dimycocerosates, the minor constituents, are found in the cell envelope of M. tuberculosis, but their specific roles in the biology of the tubercle bacillus have not been established yet. According to the current model of their biosynthesis, phthiocerol is produced from phthiodiolone through a two-step process in which the keto group is first reduced and then methylated. We have previously identified the methyltransferase enzyme that is involved in this process, encoded by the gene Rv2952 in M. tuberculosis. In this study, we report the construction and biochemical analyses of an M. tuberculosis strain mutated in gene Rv2951c. This mutation prevents the formation of phthiocerol and phenolphthiocerol derivatives, but leads to the accumulation of phthiodiolone dimycocerosates and glycosylated phenolphthiodiolone dimycocerosates. These results provide the formal evidence that Rv2951c encodes the ketoreductase catalyzing the reduction of phthiodiolone and phenolphthiodiolone to yield phthiotriol and phenolphthiotriol, which are the substrates of the methyltransferase encoded by gene Rv2952. We also compared the resistance to SDS and replication in mice of the Rv2951c mutant, deficient in synthesis of phthiocerol dimycocerosates but producing phthiodiolone dimycocerosates, with those of a wild-type strain and a mutant without phthiocerol and phthiodiolone dimycocerosates. The results established the functional redundancy between phthiocerol and phthiodiolone dimycocerosates in both the protection of the mycobacterial cell and the pathogenicity of M. tuberculosis in mice.

MmpL transporter-mediated export of cell-wall associated lipids and siderophores in mycobacteria.

Mycobacteria produce a large variety of surface-exposed lipids with unusual structures. Some of these compounds are ubiquitously present in mycobacteria and play an important role in the structural organization of the cell envelope, while others are species-specific. The biosynthesis of most of these lipids requires modular polyketide synthases (PKS) or non-ribosomal peptide synthetases (NRPS) that are intracellular, suggesting that the assembly of these compounds takes place in the cytosolic compartment or near the inner leaflet of the plasma membrane. The molecular mechanisms that mediate the export of these lipid components across the cell envelope remain poorly understood. Mycobacterial membrane protein Large (MmpL) transporters, a subclass of Resistance-Nodulation-Cell Division (RND) transporters, appear to play a major role in this process, acting as scaffold proteins that couple lipid synthesis and transport. Recent studies have shown that this family of transporters also contributes to siderophore secretion in Mycobacterium tuberculosis. The goal of this review is to provide the most recent advances in our understanding of the molecular mechanisms involved in lipid and siderophore transport mediated by MmpL transporters.