Vitamin B12 uptake across the mycobacterial outer membrane is influenced by membrane permeability in Mycobacterium marinum

Identification of Rv1133c (MetE) as a marker of Mycobacterium tuberculosis replication and as a highly immunogenic antigen with potential immunodiagnostic power

The immunization of mice with the sterile culture medium supernatants of Mycobacterium tuberculosis (Mtb) H37Rv permitted the production of several monoclonal antibodies (mAbs) specific for secreted and/or released antigens. Two mAbs bound and immunoprecipitated an 80-kDa protein that was identified by mass spectrometry as Rv1133c, the methionine synthase MetE. The protein MetE is ubiquitous among prokaryota and shows a significant sequence homology in many bacteria. We produced both the full-length recombinant MetE and its N-terminal fragment, whose sequence is more conserved among mycobacteria, to select mAbs recognizing an Mtb-specific region of MetE. Finally, we produced and selected eight mAbs that specifically detect the MetE protein in the supernatant and cell lysate of Mtb and BCG, but not other bacteria such as non-tuberculous mycobacteria (NTM), Streptococcus pneumoniae, Staphylococcus aureus, Acinetobacter baumanii, or Escherichia coli. Taking advantage of our mAbs, we studied (i) the vitamin B12 dependence for the synthesis of MetE in Mtb and NTM and (ii) the kinetics of MetE production and secretion in supernatants during the in vitro reproduced replicative, dormant, and resuscitation cycle of Mtb. Our data demonstrate that dormant Mtb, which are assumed to be prevalent in latent infections, as well as NTM do not produce and secrete MetE. Results indicate an unexpected specificity for Mtb of our anti-MetE mAbs and encourage the use of rMetE and our mAbs as tools for the immunodiagnosis of TB and its stages.

A structure-based mechanism of adenosylcobinamide kinase/adenosylcobinamide phosphate guanylyltransferase (MpaCobU) from Methylocapsa palsarum

Adenosylcobinamide kinase/adenosylcobinamide phosphate guanylyltransferase (CobU) is one of the key enzymes that participate in the biosynthesis of cobalamin, specifically lining the lower ligand 5,6-dimethylbenzimidazole in the α-position of cyclic tetrapyrrolidine. During this process, CobU exhibits two distinct activities: kinase and nucleotidyl transferase, using two nucleoside triphosphates. A structural study of CobU from Salmonella typhimurium showed that guanosine triphosphate binding induces a conformational rearrangement of helix 2. This rearrangement decreases the distance between the phosphate binding loop (P-loop) and helix 2, which is important for the subsequent guanylylation step of the reaction. However, these findings provide only partial insights into the mechanism of CobU at the structural level, and the precise molecular details of this mechanism have not yet been studied. As a first step towards elucidating the molecular mechanisms and sequence of events involved in the phosphorylation and guanylylation steps, we report the high-resolution crystal structures of phosphorylated -MpaCobU (1.8 Å), the C91S mutant (1.5 Å), the guanosine diphosphate complex (1.9 Å), and the adenosylcobinamide-phosphate complex (2.6 Å) from Methylocapsa palsarum for the first time. High-resolution structures revealed the crucial elements governing the catalytic steps of MpaCobU, thereby contributing to understanding the catalytic mechanism of CobU at the molecular level.

Modulating mycobacterial envelope integrity for antibiotic synergy with benzothiazoles

Developing effective tuberculosis drugs is hindered by mycobacteria's intrinsic antibiotic resistance because of their impermeable cell envelope. Using benzothiazole compounds, we aimed to increase mycobacterial cell envelope permeability and weaken the defenses of Mycobacterium marinum, serving as a model for Mycobacterium tuberculosis Initial hit, BT-08, significantly boosted ethidium bromide uptake, indicating enhanced membrane permeability. It also demonstrated efficacy in the M. marinum-zebrafish embryo infection model and M. tuberculosis-infected macrophages. Notably, BT-08 synergized with established antibiotics, including vancomycin and rifampicin. Subsequent medicinal chemistry optimization led to BT-37, a non-toxic and more potent derivative, also enhancing ethidium bromide uptake and maintaining synergy with rifampicin in infected zebrafish embryos. Mutants of M. marinum resistant to BT-37 revealed that MMAR_0407 (Rv0164) is the molecular target and that this target plays a role in the observed synergy and permeability. This study introduces novel compounds targeting a new mycobacterial vulnerability and highlights their cooperative and synergistic interactions with existing antibiotics.