An attenuated mutant of the Rv1747 ATP-binding cassette transporter of Mycobacterium tuberculosis and a mutant of its cognate kinase, PknF, show increased expression of the efflux pump-related iniBAC operon

Compartmentalization of galactan biosynthesis in mycobacteria

Galactan polymer is a prominent component of the mycobacterial cell wall core. Its biogenesis starts at the cytoplasmic side of the plasma membrane by a build-up of the linker disaccharide [rhamnosyl (Rha) - N-acetyl-glucosaminyl (GlcNAc) phosphate] on the decaprenyl-phosphate carrier. This decaprenyl-P-P-GlcNAc-Rha intermediate is extended by two bifunctional galactosyl transferases, GlfT1 and GlfT2, and then it is translocated to the periplasmic space by an ABC transporter Wzm-Wzt. The cell wall core synthesis is finalized by the action of an array of arabinosyl transferases, mycolyl transferases, and ligases that catalyze an attachment of the arabinogalactan polymer to peptidoglycan through the linker region. Based on visualization of the GlfT2 enzyme fused with fluorescent tags it was proposed that galactan polymerization takes place in a specific compartment of the mycobacterial cell envelope, the intracellular membrane domain, representing pure plasma membrane free of cell wall components (previously denoted as the "PMf" domain), which localizes to the polar region of mycobacteria. In this work, we examined the activity of the galactan-producing cellular machine in the cell-wall containing cell envelope fraction and in the cell wall-free plasma membrane fraction prepared from Mycobacterium smegmatis by the enzyme assays using radioactively labeled substrate UDP-[14C]-galactose as a tracer. We found that despite a high abundance of GlfT2 in both of these fractions as confirmed by their thorough proteomic analyses, galactan is produced only in the reaction mixtures containing the cell wall components. Our findings open the discussion about the distribution of GlfT2 and the regulation of its activity in mycobacteria.

Structure of the Mycobacterium tuberculosis cPknF and conformational changes induced in forkhead-associated regulatory domains

Mycobacterium tuberculosis (Mtb) has 11 Serine-Threonine Protein Kinases (STPK) that control numerous physiological processes, including cell growth, cell division, metabolic flow, and transcription. PknF is one of the 11 Mtb STPKs that has, among other substrates, two FHA domains (FHA-1 and FHA-2) of the ATP-Binding Cassette (ABC) transporter Rv1747. Phosphorylation in T152 and T210 located in a non-structured linker that connects Rv1747 FHA domains is considerate to be the regulatory mechanism of the transporter. In this work, we resolved the three-dimensional structure of the PknF catalytic domain (cPknF) in complex with the human kinase inhibitor IKK16. cPknF is conserved when compared to other STPKs but shows specific residues in the binding site where the inhibitor is positioned. In addition, using Small Angle X-Ray Scattering analysis we monitored the behavior of the wild type and three FHA-phosphomimetic mutants in solution, and measured the cPknF affinity for these domains. The kinase showed higher affinity for the non-phosphorylated wild type domain and preference for phosphorylation of T152 inducing the rapprochement of the domains and significant structural changes. The results shed some light on the process of regulating the transporter's activity by phosphorylation and arises important questions about evolution and importance of this mechanism for the bacillus.

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Rv1747 is an ABC transporter which activity is regulated by PknF.

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cPknF is a typical Serine/Threonine Protein Kinase that can be explored as drug target.

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The higher affinity of cPknF for FHA-2 is important for further conformational changes.

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Rv1747 activation model reveals a concatenated activity essential for the system.

The ATP-Binding Cassette (ABC) Transport Systems in Mycobacterium tuberculosis: Structure, Function, and Possible Targets for Therapeutics

Simple Summary

Mycobacterium tuberculosis is a bacterium of great medical importance because it causes tuberculosis, a disease that affects millions of people worldwide. Two important features are related to this bacterium: its ability to infect and survive inside the host, minimizing the immune response, and the burden of clinical isolates that are highly resistant to antibiotics treatment. These two phenomena are directly affected by cell envelope proteins, such as proteins from the ATP-Binding Cassette (ABC transporters) superfamily. In this review, we have compiled information on all the M. tuberculosis ABC transporters described so far, both from a functional and structural point of view, and show their relevance for the bacillus and the potential targets for studies aiming to control the microorganism and structural features.

Abstract

Mycobacterium tuberculosis is the etiological agent of tuberculosis (TB), a disease that affects millions of people in the world and that is associated with several human diseases. The bacillus is highly adapted to infect and survive inside the host, mainly because of its cellular envelope plasticity, which can be modulated to adapt to an unfriendly host environment; to manipulate the host immune response; and to resist therapeutic treatment, increasing in this way the drug resistance of TB. The superfamily of ATP-Binding Cassette (ABC) transporters are integral membrane proteins that include both importers and exporters. Both types share a similar structural organization, yet only importers have a periplasmic substrate-binding domain, which is essential for substrate uptake and transport. ABC transporter-type importers play an important role in the bacillus physiology through the transport of several substrates that will interfere with nutrition, pathogenesis, and virulence. Equally relevant, exporters have been involved in cell detoxification, nutrient recycling, and antibiotics and drug efflux, largely affecting the survival and development of multiple drug-resistant strains. Here, we review known ABC transporters from M. tuberculosis, with particular focus on the diversity of their structural features and relevance in infection and drug resistance.

The thick waxy coat of mycobacteria, a protective layer against antibiotics and the host's immune system

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

Chemical Genetic Interaction Profiling Reveals Determinants of Intrinsic Antibiotic Resistance in Mycobacterium tuberculosis

Chemotherapy for tuberculosis (TB) is lengthy and could benefit from synergistic adjuvant therapeutics that enhance current and novel drug regimens. To identify genetic determinants of intrinsic antibiotic susceptibility in Mycobacterium tuberculosis, we applied a chemical genetic interaction (CGI) profiling approach. We screened a saturated transposon mutant library and identified mutants that exhibit altered fitness in the presence of partially inhibitory concentrations of rifampin, ethambutol, isoniazid, vancomycin, and meropenem, antibiotics with diverse mechanisms of action. This screen identified the M. tuberculosis cell envelope to be a major determinant of antibiotic susceptibility but did not yield mutants whose increase in susceptibility was due to transposon insertions in genes encoding efflux pumps. Intrinsic antibiotic resistance determinants affecting resistance to multiple antibiotics included the peptidoglycan-arabinogalactan ligase Lcp1, the mycolic acid synthase MmaA4, the protein translocase SecA2, the mannosyltransferase PimE, the cell envelope-associated protease CaeA/Hip1, and FecB, a putative iron dicitrate-binding protein. Characterization of a deletion mutant confirmed FecB to be involved in the intrinsic resistance to every antibiotic analyzed. In contrast to its predicted function, FecB was dispensable for growth in low-iron medium and instead functioned as a critical mediator of envelope integrity.

Cmr is a redox-responsive regulator of DosR that contributes to M. tuberculosis virulence

Mycobacterium tuberculosis (MTb) is the causative agent of pulmonary tuberculosis (TB). MTb colonizes the human lung, often entering a non-replicating state before progressing to life-threatening active infections. Transcriptional reprogramming is essential for TB pathogenesis. In vitro, Cmr (a member of the CRP/FNR super-family of transcription regulators) bound at a single DNA site to act as a dual regulator of cmr transcription and an activator of the divergent rv1676 gene. Transcriptional profiling and DNA-binding assays suggested that Cmr directly represses dosR expression. The DosR regulon is thought to be involved in establishing latent tuberculosis infections in response to hypoxia and nitric oxide. Accordingly, DNA-binding by Cmr was severely impaired by nitrosation. A cmr mutant was better able to survive a nitrosative stress challenge but was attenuated in a mouse aerosol infection model. The complemented mutant exhibited a ∼2-fold increase in cmr expression, which led to increased sensitivity to nitrosative stress. This, and the inability to restore wild-type behaviour in the infection model, suggests that precise regulation of the cmr locus, which is associated with Region of Difference 150 in hypervirulent Beijing strains of Mtb, is important for TB pathogenesis.

Mycobacterium tuberculosis universal stress protein Rv2623 interacts with the putative ATP binding cassette (ABC) transporter Rv1747 to regulate mycobacterial growth

We have previously shown that the Mycobacterium tuberculosis universal stress protein Rv2623 regulates mycobacterial growth and may be required for the establishment of tuberculous persistence. Here, yeast two-hybrid and affinity chromatography experiments have demonstrated that Rv2623 interacts with one of the two forkhead-associated domains (FHA I) of Rv1747, a putative ATP-binding cassette transporter annotated to export lipooligosaccharides. FHA domains are signaling protein modules that mediate protein-protein interactions to modulate a wide variety of biological processes via binding to conserved phosphorylated threonine (pT)-containing oligopeptides of the interactors. Biochemical, immunochemical and mass spectrometric studies have shown that Rv2623 harbors pT and specifically identified threonine 237 as a phosphorylated residue. Relative to wild-type Rv2623 (Rv2623WT), a mutant protein in which T237 has been replaced with a non-phosphorylatable alanine (Rv2623T237A) exhibits decreased interaction with the Rv1747 FHA I domain and diminished growth-regulatory capacity. Interestingly, compared to WT bacilli, an M. tuberculosis Rv2623 null mutant (ΔRv2623) displays enhanced expression of phosphatidyl-myo-inositol mannosides (PIMs), while the ΔRv1747 mutant expresses decreased levels of PIMs. Animal studies have previously shown that ΔRv2623 is hypervirulent, while ΔRv1747 is growth-attenuated. Collectively, these data have provided evidence that Rv2623 interacts with Rv1747 to regulate mycobacterial growth; and this interaction is mediated via the recognition of the conserved Rv2623 pT237-containing FHA-binding motif by the Rv1747 FHA I domain. The divergent aberrant PIM profiles and the opposing in vivo growth phenotypes of ΔRv2623 and ΔRv1747, together with the annotated lipooligosaccharide exporter function of Rv1747, suggest that Rv2623 interacts with Rv1747 to modulate mycobacterial growth by negatively regulating the activity of Rv1747; and that Rv1747 might function as a transporter of PIMs. Because these glycolipids are major mycobacterial cell envelope components that can impact on the immune response, our findings raise the possibility that Rv2623 may regulate bacterial growth, virulence, and entry into persistence, at least in part, by modulating the levels of bacillary PIM expression, perhaps through negatively regulating the Rv1747-dependent export of the immunomodulatory PIMs to alter host-pathogen interaction, thereby influencing the fate of M. tuberculosis in vivo.