Literature highlights the effects of mycolic acid (MA) fine-structure on biological activity, pathogenicity, virulence and cell wall structure and permeability. Knowledge on MA-structure and how their conformations are dependent on their precise molecular composition becomes essential in exploiting these properties in drug-design and in advancing our understanding of the disease. In our group evidence for a structural or functional relationship between cholesterol and MAs have been discovered. The aim of the experimental part of this work was to study this relationship further by attempting to quantify the interaction between cholesterol and MAs in liposomes on an evanescent field biosensor. The binding profiles that were obtained could not be evaluated with kinetic software and the interaction between cholesterol and MAs was not linearly dependant on the concentration of cholesterol. However, novel insight into the interaction was gained when it was observed that cholesterol only accumulated on MA liposomes when cholesterol liposomes containing concentrations of cholesterol resulting in a suspected liquid ordered phase, were used. This is significant since it implies that cholesterol in membrane rafts of the host cell that exist in a liquid ordered phase would be able to interact with MAs under physiological conditions. The theoretical part of this work represents the first molecular modeling study in which MAs are allowed to fold with no conformational restrictions. It is proposed that MAs fold as a function of their functional groups, stereochemistry, and various chain lengths. It was also investigated whether methylation of the acid group changes conformational preferences. The effect of chain length on cyclopropane structure and the viability of systematic conformational searching in MAs were shown using quantum mechanics. Replicate molecular dynamics simulations were done for 4 ns in vacuo on alpha-; methoxy-; methoxy methyl ester- and keto-MAs. MAs had an open starting conformation without conformational restrictions. Results were analysed using eight distances characteristic of the conformational fold. Using these distances, W-, U- and Z-shaped folds were identified. Principal component analysis (PCA) and self-organising maps (SOMs) were used to evaluate differences and trends in MA-conformations. Quantum chemical results showed that chain length did not affect cyclopropane structure and that the systematic plotting of potential energy surfaces is an effective tool to analyse effects of changes in geometry on the energy of the molecule and to predict favoured conformations. Remarkably, single MAs assumed W-, U- and Z-folds in vacuo during molecular dynamics simulations that have previously been observed in monolayers. PCA and SOM plots showed that keto-MA folded faster than other MAs. Alpha-MA showed the highest frequency of W-, U- and Z-folds. Methoxy-MA did not readily fold at its cis-cyclopropane group. Methylation of the acid group of methoxy-MA did not show remarkable differences in the conformations assumed, but almost doubled the frequency of WUZ-structures obtained as compared to non-esterified methoxy-MA. The inherent structural differences between MA-subclasses clearly affect the trends in structural folds that they assume. Molecular modeling of MAs proved to be a versatile tool for resolving structure-function relationships at the molecular level. Copyright 2008, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Prinsloo, W 2008, Computational models for conformations of cell wall mycolates from Mycobacterium tuberculosis, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-06122009-114802 / > E1400/gm / Dissertation (MSc)--University of Pretoria, 2009. / Biochemistry / unrestricted

Tuberculosis (TB) is a collective name for the bacterial infection, which is caused by members of the Mycobacterium tuberculosis (M. tb) complex and can infect the lungs (pulmonary) as well as the kidneys, lymph nodes, bones and joints (extra-pulmonary). The re-emergence of drug-resistant strains and the HIV epidemic are among the main reasons for the resurgence of TB and there is a need for new drugs and diagnostic assays which are rapid and sensitive. Serodiagnostic assays have the potential of being rapid, inexpensive and relatively non-invasive. The most abundant antigen in the cell wall of M. tb, which has been analysed with ELISA and resonant mirror biosensor assays for use in serodiagnosis, is mycolic acid (MA). The sensitivity previously obtained in the ELISA assay was however inadequate for serodiagnostic purposes. It was believed that MA mimicked the structure of cholesterol, thereby causing anti-cholesterol human antibodies from TB negative sera to bind to MA and result in a large number of false positives. Within this work the apparent molecular mimicry between MA and cholesterol was investigated using a competitive enzyme linked inhibition assay (CELIA) assay. The results suggested that MA in liposomes resembled the liquid ordered arrangement of cholesterol in liposomes, rather than a direct mimicry of individual molecules. The nature of the antibody from TB negative patient sera binding to MA coated onto ELISA plates was also investigated. The results obtained from this study have not disproved the hypothesis of a cross-reactive anti-cholesterol antibody, but it would appear that the MA signal from TB negative serum was partially due to the binding of anti-MA antibodies. The presence of anti-MA antibodies in TB negative serum could have been the result of prior BCG vaccination, latent infection or due to constant immune stimulation from saprophytic mycobacteria. This creates the potential of using antibodies to MA to distinguish between latent TB infection and active disease. Furthermore, in order to overcome the low sensitivity of the ELISA assay due to high background signals from TB negative serum, members of our group previously developed a resonant mirror biosensor inhibition assay based on MA contained in liposomes. The biosensor measured mass accumulation and the identity of the binding molecules were unknown. It was shown here that one of the serum components binding to the immobilised MA liposomes in the biosensor inhibition assay was immunoglobulin G antibodies. The specificity of both the ELISA and biosensor assays previously analysed using a natural mixture of MA however, remained poor, and in the search for a more specific antigen, this study investigated the potential of MA subclasses for TB serodiagnosis using ELISA. It was observed that the antibody binding signal to the MA subclasses depended on the polarity of the coating solution, for which hexane was the preferred solvent. Both the alpha- and keto-MA subclasses could better distinguish between a range of TB positive patient and TB negative sera compared with the natural mixture of MA. These results suggested that a particular subclass applied in the biosensor inhibition assay could enhance the test to reach the required sensitivity and specificity required for the serodiagnosis TB. / Dissertation (MSc)--University of Pretoria, 2011. / Biochemistry / unrestricted

The mycolic acid bacteria are a distinct suprageneric group of asporogenous Grampositive, high GC-content bacteria, distinguished by the presence of mycolic acids in their cell envelope. They exhibit great diversity in their cell and morphology; although primarily non-pathogens, this group contains three major pathogens Mycobacterium leprae, Mycobacterium tuberculosis complex, and Corynebacterium diphtheria. Although the mycolic acid bacteria are a clearly defined group of bacteria, the taxonomic relationships between its constituent genera and species are less well defined. Two approaches were tested for their suitability in describing the taxonomy of the group. First, a Multilocus Sequence Typing (MLST) experiment was assessed and found to be superior to monophyletic (16S small ribosomal subunit) in delineating a total of 52 mycolic acid bacterial species. Phylogenetic inference was performed using the neighbor-joining method. To further refine phylogenetic analysis and to take advantage of the widespread availability of bacterial genome data, a computational framework that simulates DNA-DNA hybridisation was developed and validated using multiscale bootstrap resampling. The tool classifies microbial genomes based on whole genome DNA, and was deployed as a web-application using PHP and Javascript. It is accessible online at http://cbrc.kaust.edu.sa/dna_hybridization/ A third study was a computational and statistical methods in the identification and analysis of a putative minimal mycolic acid bacterial genome so as to better understand (1) the genomic requirements to encode a mycolic acid bacterial cell and (2) the role and type of genes and genetic elements that lead to the massive increase in genome size in environmental mycolic acid bacteria. Using a reciprocal comparison approach, a total of 690 orthologous gene clusters forming a putative minimal genome were identified across 24 mycolic acid bacterial species. In order to identify new potential drug candidates against the pathogenic mycolic acid bacteria, a drug target analysis of the protein-coding genes, that are conserved across 15 pathogens, identified a total of 175 potential broad spectrum drug candidates. The activity of the predicted targets can be modified by a total of 306 chemical compounds, 34 of which have been approved by the Food and Drug Administration (FDA), for human treatment.

Les Corynebacteriales sont un groupe de bactéries qui comprend des pathogènes de l’Homme, comme les agents de la tuberculose, de la lèpre et de la diphtérie. Ces bactéries se distinguent pas une enveloppe atypique formée de peptidoglycane, lié covalemment à un polymére sacharidique, l’arabinogalactane, lui-même estérifié par des acides mycoliques. Les acides mycoliques sont une classe d’acide gras α-hydroxylés β-branchés qui peuvent être retrouvés soit sous forme “libre”, où ils estérifient du tréhalose, soit sous forme “liés”, où ils estérifient l’arabinogalactane. Les acides mycoliques sont transférés sur leurs accepteurs par une classe d’enzymes spécifiques, les mycoloyltransférases. Les acides mycoliques sont les composants majeurs de la membrane externe des Corynebacteriales (“mycomembrane”). Récemment, il a été démontré que les acides mycoliques peuvent aussi être transférés sur des protéines.Jusqu’à récemment, l’utilisation de la spectrométrie de masse n’avait permis d’identifier que 3 protéines mycoloylées. PorA et PorH sont de petites protéines (45 et 57 résidus) qui forment un pore hétéro-oligomérique dans la membrane externe, alors que la fonction de ProtX (38 résidus) n’est pas connue. La mycoloyltransférase MytC est essentielle pour la mycoloylation des protéines.Dans le but de mieux comprendre la généralité et le rôle de la mycoloylation des protéines, une de nos objectifs était de déterminer si d’autres protéines sont mycoloylées chez C. glutamicum et, si c’est le cas, de les identifier. Pour ce faire, nous avons développé, chez cet organisme, une méthode de marquage métabolique dans laquelle un analogue d’acide gras est utilisé pour marquer les acides mycoliques. Si incorporé, cette sonde pourra réagir avec un conjugué azido-biotine, permettant alors la détection et/ou l’enrichissement des protéines lipidés. Nos résultats montrent que PorA, PorH et ProtX peuvent être marqués spécifiquement par cet analogue et que le marquage est dépendant de la présence de MytC et de Pks13, une enzyme clé dans la synthèse des acides mycoliques. Ces résultats indiquent donc qu’un analogue d’acide gras peut être utilisé pour caractériser des protéines mycoloylées potentielles. En effet, deux autres candidats, PorB et PorC ont été testés par cette approche et répondent positivement dans le test. Dans le cas de PorB et PorC, la mycoloylation semble favoriser l’association à l’enveloppe. / Corynebacteriales are a group of bacteria that comprise human pathogens, including the agents of tuberculosis, leprosy, and diphtheria. These bacteria are distinguished by an atypical envelope formed of peptidoglycan bonded to the arabinogalactan saccharide polymer, itself esterified with mycolic acids. Mycolic acids are a specific class of α-hydroxylated β-linked fatty acids, they are found either in a so-called "free" form where they esterify trehalose, or in a "linked" form, where they are covalently bound to arabinogalactan. Mycolic acids are transferred to their acceptors through a specific class of enzymes, mycoloyltransferases. Mycolic acids are the major components of the outer membrane of Corynebacteriales called mycomembrane. Recently, mycolic acids were also found covalently attached to proteins in C. glutamicum.So far, using mass spectrometry, only 3 proteins have been identified as mycoloylated. PorA and PorH are small proteins (45 and 57 residues) forming a hetero-oligomeric pore in the outer membrane whereas the function of ProtX (38 residues) is still unknown. The mycoloyltransferase MytC is essential for protein mycoloylation.In order to gain insights in the extent and function of protein mycoloylation, one of our objectives was to determine whether other proteins are mycoloylated in C. glutamicum and if so, identify them. To do so, we developed in this organism a metabolic labeling approach in which a fatty acid chemical reporter is used to label mycolic acids. If incorporated, this reporter could react with an azido-biotin conjugate, allowing the detection and/or enrichment of lipidated proteins. Our results show that PorA, PorH and ProtX can be specifically labeled with such a reporter, and that this labeling is strictly dependent on the presence of MytC and Pks13, the key enzyme allowing mycolic acids biosynthesis. These results hence indicate that fatty acid chemical reporter can be used to characterize putative mycoloylated proteins. Indeed, two other candidates, PorB and PorC, were tested for mycoloylation using this approach and found to positively respond in the click-chemistry assay. In the case of PorB and PorB, mycoloylation appears to favor association to the envelope.

Chaque année, Mycobacterium tuberculosis (Mtu) est responsable d‘environ 2 million de morts dans le monde. L’apparition de souches multi-résistantes aux antibiotiques rend nécessaire le développement de nouveaux antituberculeux. Dans ce contexte, les voies de biosynthèse des composés essentiels de l’enveloppe, tels que les acides mycoliques (AM), représentent des cibles thérapeutiques de choix. Les AM, cruciaux pour l’architecture et la perméabilité de l’enveloppe, jouent également un rôle important dans la virulence et dans la persistance bactérienne. La biosynthèse des AM fait appel à une Fatty Acid Synthase de type II (FAS-II), système multienzymatique qui produit les chaînes méromycoliques des AM et qui contient au moins deux déshydratases, HadAB et HadBC. Lors d’une analyse approfondie du génome de Mtu dans l’équipe, onze hydratases/déshydratases (R)-spécifiques potentielles ont été identifiées, dont la protéine Rv0504c, nommée "HadDMtu". L’objectif de mes travaux a été de caractériser la fonction enzymatique et le rôle physiologique de la protéine HadDMtu chez Mtu et de son orthologue potentiel chez M. smegmatis (Msm), un modèle d’étude mycobactérien, non pathogène et à croissance rapide. Les tests d’activité enzymatique in vitro sur la protéine HadDMsm ont montré qu'elle possède une activité hydratase/deshydratase. Lorsque le gène hadD est délété, chez Msm et chez Mtu, les profils et les structures fines des AM sont modifiés. Chez Msm, le mutant n’est plus capable de produire d’AM à longues chaînes α et epoxy. Par contre, chez Mtu, c’est la structure fine et la proportion des AM keto qui sont affectées. En parallèle de mes travaux, il a été montré que la protéine HadDMsm de Msm interagit spécifiquement avec HadAB. L'ensemble de nos données a apporté la preuve que HadD fait partie du système FAS-II. Il suggère fortement que dans les deux espèces, l’enzyme intervient dans les derniers cycles d’élongations de la chaîne méromycolique. En accord avec ces résultats, des expériences de complémentation croisée des souches mutantes de Mtu ΔhadDMtu et de Msm ΔhadDMsm, ont montré que les deux protéines HadD ont une fonction proche in vivo mais elles ne sont pas orthologues. Des tests phénotypiques ont montré que les propriétés de surface et l’intégrité de l’enveloppe des mutants ΔhadD sont profondément altérées. De plus, chez la souris, la souche mutante de Mtu apparait beaucoup moins virulente avec une baisse des charges bactériennes dans les poumons et dans la rate. / Mycobacterium tuberculosis (Mtu) kills 2 million people every year in the world. The development of new antituberculous drugs is urgently needed due to the emergence of drug-resistant strains. In this context, the biosynthesis pathways of essential compounds from the mycobacterial envelope, such as mycolic acids (MAs), constitute relevant therapeutic targets. The MAs are crucial for both the architecture and the permeability of the envelope and play important roles in the bacterial virulence and persistence. The MA biosynthesis involves a Fatty Acid Synthase type II (FAS-II), a multi-enzyme system that produces the main meromycolic chains of MA and contains at least two dehydratases, HadAB and HadBC. A thorough analysis of the Mtu genome led to the identification of eleven potential (R)-specific hydratases/dehydratases, including the protein Rv0504c, named “HadDMtu”. The objective of my work was to characterize the enzymatic function and the physiological role of HadDMtu protein in Mtu and of its putative orthologue in M. smegmatis (Msm), a non-pathogenic and fast growing mycobacteria model. The in vitro enzymatic activity tests with HadDMsm protein showed that it has a hydratase/dehydratase activity. When hadD gene is deleted, in Msm and in Mtu, the MA distribution and fine structures were changed. In Msm, the mutant could not produce long chain α- and epoxy-MAs while in Mtu, it is the fine structure and the proportion of the keto-MAs that were affected. In parallel of my work, it was that HadDMsm protein specifically interacts with HadAB. All these data show that HadD belongs to the FAS-II system. They strongly suggest that, in the both two strains, the enzyme occurs in the late steps of the meromycolic chain elongation. According to these results, cross-complementation assays of Mtu and Msm ΔhadD mutants showed that both HadD proteins have a similar function in vivo but they are not orthologs. Phenotypic tests showed that the surface properties and the envelope integrity of the ΔhadD mutants were profoundly altered. Also, in the mouse model of infection, Mtu mutant exhibited a significant loss of virulence with a decrease of bacterial loads in the lungs and in the spleen.

Chemotherapy of tuberculosis (TB), a disease caused by Mycobacterium tuberculosis (M. tuberculosis), is successful against actively-growing bacilli but ineffective against dormant/persistent organisms, found mainly in a protective lipid-laden granuloma, possibly necessitating the use of lipophilic antibiotics. In vitro, these bacilli are encased in lipid-rich biofilms. In this study, the antimycobacterial activity of one such agent, clofazimine, and its nanoparticle formulation, have been investigated against Mycobacterium smegmatis (M. smegmatis), as a surrogate for M. tuberculosis, by determining the bacteriostatic and bactericidal activities of the native (NC) and spray-dried (SDC) preparations of this agent on planktonic and biofilm populations, as well as their effects on biofilm formation and its lipid compositions, specifically free mycolic acid (FM) content. Both preparations were comparable, being bacteriostatic for rapidly-proliferating bacilli, bactericidal for slow-growing, biofilm-producing sessile bacteria, but ineffective against non-replicating, biofilm-encased M. smegmatis organisms. However, similar studies in M. tuberculosis are required. / Dissertation (MSc)--University of Pretoria, 2013. / Immunology / Unrestricted

Systems biology adopts an integrated approach to study and understand the function of biological systems, particularly, the response of such systems to perturbations, such as the inhibition of a reaction in a pathway, or the administration of a drug. The complexity and large scale of biological systems make modelling and simulation an essential and critical part of systems-level studies. Systems-level modelling of pathogenic organisms has the potential to significantly enhance drug discovery programmes. In this thesis, we show how systems--level models can positively impact anti-tubercular drug target identification. *Mycobacterium tuberculosis*, the principal aetiological agent of tuberculosis in humans, is estimated to cause two million deaths every year. The existing drugs, although of immense value in controlling the disease to some extent, have several shortcomings, the most important of them being the emergence of drug resistance rendering even the front-line drugs inactive. As drug discovery efforts are increasingly becoming rational, focussing at a molecular level, the identification of appropriate targets becomes a fundamental pre-requisite. We have constructed many system-level models, to identify drug targets for tuberculosis. We construct a constraint-based stoichiometric model of mycolic acid biosynthesis, and simulate it using flux balance analysis, to identify critical points in mycobacterial metabolism for targeting drugs. We then analyse protein--protein functional linkage networks to identify influential hubs, which can be targeted to disrupt bacterial metabolism. An important aspect of tuberculosis is the emergence of drug resistance. A network analysis of potential information pathways in the cell helps to identify important proteins as co-targets, targeting which could counter the emergence of resistance. We integrate analyses of metabolism, protein--protein interactions and protein structures to develop a generic drug target identification pipeline, for identifying most suitable drug targets. Finally, we model the interplay between the pathogen and the human immune system, using Boolean networks, to elucidate critical factors influencing the outcome of infection. The strategies described can be applied to understand various pathogens and can impact many drug discovery programmes.

Etwa ein Drittel der Weltbevölkerung ist mit Mycobacterium tuberculosis (Mtb), der Erreger der Tuberkulose (TB), infiziert. Daher ist es unbedingt notwendig vorhandenen Behandlungsstrategien weiter zu verbessern. Diese Study beschäftigt sich mit dem Mtb Protein Ag85C, einer Mycolyltransferase, als ein neues Ziel für die medikamentöse Behandlung der TB. Ag85C ist eines von drei verwandten Proteinen, Ag85A, B und C, welche zusammen an der Biogenese der Zellwand von Mtb beteiligt sind. Eine Gruppe von chemischen Molekülen mit den Namen Ag85C-1 bis -4 wurde als Inhibitoren von Ag85C getestet. Alle Verbindungen waren in der Lage das Wachstum von Mtb in Flüssigkulturen zu inhibieren, aber nur Ag85C-3 hatte ebenfalls einen Effekt auf intrazelluläre Bakterien, welches in einem Makrophagen-Infektions-System getestet wurde. Hervorzuheben ist, dass Ag85C-3 darüber hinaus auch das in vitro Überleben eines MDR Stammes inhibierte. Dies macht dieses Molekül zu einem interessanten Kandidaten für neue anti-mycobakterielle Therapieansätze. Desweiteren wurden detaillierte, funktionelle Charakterisierungen der Effekte von Ag85C-3 auf Mtb durchgeführt. Die Verbindung modifiziert die Lipide der mykolischen Säuren in der Zellwand durch die Blockierung der Ag85 Funktionen. Dieser Effekt führt dann zu einer Veränderung in der Durchlässigkeit der Außenhülle von Mtb. Mit Hilfe der microarray Analyse wurden die Regulierungen der Signalwege durch Ag85C-3 umfassend untersucht. Es konnte gezeigt werden, dass lebensnotwendige Siderophore durch das Molekül modifiziert werden, was auf mehrere Wirkungsmechanismen schließen lässt. Diese Erkenntnisse machen Ag85C, als Ziel, und Ag85C-3, als Inhibitor, zu vielversprechende Kandidaten für zukünftige Medikamentenforschung auf dem Gebiet der TB-Therapien. Diese Studie hebt zudem die zielbasierte Identifizierung von chemischen Inhibitoren als wichtigen und wertvollen Ansatz für die Medikamentenentwicklung hervor. / Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB) infects about one-third of the world’s population. Therefore there is an urgent need to improve existing intervention strategies. This study evaluated the Mtb Ag85C protein, a mycolyl transferase, as a novel target for drug mediated intervention. Ag85C belongs to a family of three cognate proteins, Ag85A, B and C. They are involved in the final steps of Mtb cell envelope biogenesis. A panel of chemical molecules, Ag85C-1-4, which bind to Ag85C were utilized as inhibitors of Ag85C. All compounds inhibited growth of Mtb in vitro in liquid medium cultures but only Ag85C-3 had an effect on intracellular bacteria in macrophage infection system. Importantly, Ag85C-3 can inhibit in vitro survival of a MDR strain of Mtb making it a relevant molecule in the search for novel classes of anti-mycobacterial compounds. Furthermore a detailed functional characterization of Ag85C-3 effect on Mtb was performed. It modified the cell wall mycolic acid containing lipid amounts by blocking Ag85 function that led to changes in permeability of Mtb envelope. A comprehensive analysis of Mtb signalling pathways regulated by Ag85C-3 was investigated through microarray analysis. It showed modification of vital siderophore biosynthesis indicating multiple mechanisms of action. Thus the target, Ag85C and the inhibitor, Ag85C-3 are promising candidates for future TB drug research aimed at combating broad spectrum resistance development. This study also reinforces target based identification of chemical inhibitors as a valid and valuable approach in drug development.

L’émergence de la résistance à une multitude d’agents antimicrobiens chez des bactéries pathogènes est considérée comme une menace majeure pour la santé publique (2). Ces souches sont reconnues comme des organismes multirésistants aux médicaments ou MDR (multidrug-resistant) (4). Les recherches progressent chez les bactéries, à Gram positif, à Gram négatif et acido-alcoolo-résistantes au vu de l’ampleur de la menace pour la santé publique, ces bactéries multirésistantes sont devenues les cibles potentielles à cette fin de recherche. De ce fait, les objectifs de la présente étude ont consisté en la caractérisation structurale et fonctionnelle de différents transporteurs transmembranaires de la famille des RND (Resistance-Nodulation-Division) encore énigmatiques, à savoir: le MmpL3 chez Mycobacterium tuberculosis (Mtb) via l’étude de son orthologue CmpL1 chez Corynebacterium glutamicum (Cgl) et le TriAxBC chez Pseudomonas aeruginosa (P. aeruginosa). Ainsi, comme première démarche présentée dans le chapitre 2, la structure du transporteur MmpL3 Mtb (un transporteur d'acides mycoliques – sous forme de tréhalose de monomycolates (ou TMM) - essentiel pour la viabilité de Mtb) (5) et celle de son orthologue CmpL1 Cgl ont été prédites via le serveur I-TASSER (6-8). Ces structures ont été validées par la suite en comparant à la carte électronique générée pour CmpL1 (18 Å) par des analyses de microscopie électronique en transmission à coloration négative (TEM). La caractérisation du transporteur CmpL1 purifié par chromatographie à exclusion stérique a confirmé le complexe trimérique de taille avoisinant les 315 KDa (incluant la couronne du détergent) en accord avec des analyses par gel SDS-PAGE. Des études génétiques et biochimiques en collaboration ont d’autre part identifié des résidus engagés dans le transport du TMM chez MmpL3 ainsi que d’autres impliqués dans la résistance à des inhibiteurs ciblant ce transporteur. L’ensemble de ces données a mis en évidence la localisation des résidus essentiels au transport et à la résistance au niveau du canal central du modèle trimérique de MmpL3. La région de MmpL3 activant le transport par force protomotrice a été localisée au niveau d’une cavité centrale qui est une caractéristique intrinsèque de la famille des RND. Les cartes électroniques de faible résolution déjà obtenues pour la protéine CmpL1 font de ce projet une des directions futures du laboratoire. Dans le chapitre 3, nous illustrons le deuxième aspect du présent projet qui repose sur l’extension de l’étude du potentiel thérapeutique du ciblage du transporteur transmembranaire MmpL3 chez les différentes souches de Mycobacterium. Nos collaborateurs ont effectué une analyse biochimique de l’effet thérapeutique des inhibiteurs les plus prometteurs du transporteur MmpL3 Mtb sur certaines souches mycobactériennes non-tuberculeuses (NTB) multi-résistantes. Basés sur nos modélisations structurales comparatives obtenues par I-TASSER (6-8), nous avons pu complémenter les informations biochimiques en soulignant les similitudes et les différences de structure entre les souches TB et NTB ainsi que leurs impacts fonctionnels. Ce chapitre met en évidence l’intérêt du ciblage thérapeutique de MmpL3 chez les espèces NTB. En effet, l’efficacité de certains inhibiteurs de MmpL3 Mtb sélectionnés sur le traitement des infections pulmonaires NTB promet de pouvoir généraliser cette nouvelle voie de traitement pour d’autres souches multi-résistantes NTB voire à contribuer à remédier à la problématique de la résistance aux antibiotiques et décomplexifier le traitement actuel. D’autres études en collaboration entreprenant les mêmes approches d’études structurales ont été réalisées pour les transporteurs tripartites TriAxBC (P. aeruginosa), des pompes à efflux appartenant à la famille des RND. Le but du chapitre 4 était de générer une structure du complexe et de déchiffrer son mode d’assemblage et d’expulsion des antibiotiques vers le milieu externe. Un modèle à structure quaternaire de TriAxBC a été prédit par I-TASSER (6-8) et validé contre sa carte électronique à 4.3 Å générée en Cryo-EM. Le complexe TriAxBC a été également caractérisé par filtration sur gel confirmant une taille approximative de 620 KDa et sa composition en trimère par visualisation sur gel SDS-PAGE. En conclusion, nous avons pu à travers cette étude combiner différentes approches biochimiques, génétiques et structurales soutenant la nécessité d’une approche multidisciplinaire pour l’approfondissement de la compréhension de la structure et du mode de fonctionnement des transporteurs RND. Ces derniers demeurent toujours énigmatiques; toutefois, nos avancées et d’autres à venir permettront la génération de nouveaux médicaments spécifiques traitant les bactéries multirésistantes. / The emergence of resistance to a multitude of antimicrobial agents in pathogenic bacteria is considered a major threat to public health (2). These strains are recognized as multidrug resistant organisms (MDR) (4). Research is progressing in Gram positive, Gram positive high GC and Gram negative bacteria, and given the scale of the public health threat, these MDR have become potential targets for this research. The objectives of the present study consist of the structural and functional characterization of various transmembrane transporters of the still enigmatic RND (Resistance-Nodulation-Division) family, namely: MmpL3 in Mycobacterium tuberculosis (Mtb) via the study of its ortholog CmpL1 in Corynebacterium glutamicum (Cgl) and TriAxBC in Pseudomonas aeruginosa (P. aeruginosa). The first component of this project, presented in Chapter 2, studies the structure of the transporter MmpL3 Mtb (a TMM mycolic acid transporter essential for the viability of Mtb (5) and that of its CmpL1 Cgl orthologue, which have been predicted via the I- Tasser Pack (6-8). These structures were subsequently validated by comparing to the electronic map generated for CmpL1 (18 Å) by negative staining transmission electron microscopy (TEM). Characterization of the purified CmpL1 transporter by size exclusion chromatography confirmed the trimeric complex size around 315 KDa (including the detergent crown) corroborated by SDS-PAGE gel analyses. Collaborative genetic and biochemical studies have also identified residues involved in the transport of TMM in MmpL3 as well as those residues conferring antibiotic resistance. This data highlighted the location of the essential residues of transport and resistance in the central channel of the trimeric Mmpl3 model. The MmpL3 region activating proto-motor transport has been located at a central cavity, which is an intrinsic feature of the RND family. The low-resolution electronic maps obtained for the protein CmpL1 may serve as the foundation of future studies. In Chapter 3 we explore the therapeutic potential of the targeting of the transmembrane transporter MmpL3 in different Mycobacterium strains. Our collaborators studied the therapeutic effect of the most promising inhibitors of the MmpL3 Mtb transporter on certain multi-resistant mycobacterial non-tuberculous (NTB) strains. Based on our comparative structural modeling obtained by I-TASSER (6-8), we supplemented the biochemical data by highlighting the structural similarities and differences between the TB and NTB strains as well as their functional impacts. This chapter highlights the interest of direct or indirect targeting of MmpL3 in NTB species. Indeed, the efficacy of certain selected MmpL3 Mtb inhibitors on the treatment of NTB pulmonary infection have potential as generalizable treatment options for other NTB multi-resistant strains, or even to help address the problem of resistance to antibiotics and simplify current combination approaches. Other collaborative studies undertaking the same structural approaches were carried out for TriAxBC tripartite carriers (P. aeruginosa), efflux pumps belonging to the RND family. The purpose of Chapter 4 was to generate a structure of the complex and decipher its mode of assembly and expulsion of antibiotics from the intracellular environment. A quaternary structure model of TriAxBC was predicted by I-TASSER (6-8) and validated against its 4.3 Å electronic map generated by Cryo-EM. The TriAxBC complex was also characterized by gel filtration confirming an approximate size of 620 KDa and its trimer composition by SDS-PAGE. In conclusion, this study is combining different biochemical, genetic and structural approaches to highlight the need for a multidisciplinary approach to characterizing the structure function of RND transporters. The latter remain enigmatic; however, our contribution and the progress of others will allow the generation of new specific drugs targeting multiresistant strains.