Mycobacterial outer membranes: in search of proteins

106

A major step in the biogenesis of newly synthesized precursor proteins in bacteria is their targeting to the Sec translocon at the inner membrane. In Gram-negative bacteria, the chaperone SecB binds nonnative forms of precursors and specifically transfers them to the SecA motor component of the translocase, thus facilitating their export. The major human pathogen Mycobacterium tuberculosis is an unusual Gram-positive bacterium with a well-defined outer membrane and outer membrane proteins. Assistance to precursor proteins by chaperones in this bacterium remains largely unexplored. Here we show that the product of the previously uncharacterized Rv1957 gene of M. tuberculosis can substitute for SecB functions in Escherichia coli and prevent preprotein aggregation in vitro. Interestingly, in M. tuberculosis, Rv1957 is clustered with a functional stress-responsive higB-higA toxin-antitoxin (TA) locus of unknown function. Further in vivo experiments in E. coli and in Mycobacterium marinum strains that do not possess the TA-chaperone locus show that the severe toxicity of the toxin was entirely inhibited when the antitoxin and the chaperone were jointly expressed. We found that Rv1957 acts directly on the antitoxin by preventing its aggregation and protecting it from degradation. Taken together, our results show that the SecB-like chaperone Rv1957 specifically controls a stress-responsive TA system relevant for M. tuberculosis adaptive response.DnaK | molecular chaperones | trigger factor | protease

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

SecB-like chaperone controls a toxin–antitoxin stress-responsive system in Mycobacterium tuberculosis

Bordes

Cirinesi

Ummels

et al. 2011

106

“…Such an approach may lead to a new class of anti-infectives that could sensitize pathogens to host immune responses by inhibiting bacterial defense mechanisms as recently described by Carl Nathan and colleagues (27). Targeting MctB might be a promising strategy because the surface accessibility of MctB (28) may enable putative inhibitors to reach its target directly without the need to cross the notoriously impermeable outer membrane of Mtb, a major determinant of intrinsic drug resistance (29).…”

Section: Discussionmentioning

confidence: 99%

Copper resistance is essential for virulence of Mycobacterium tuberculosis

Wolschendorf

Ackart

Shrestha

et al. 2011

Self Cite

288

333

Copper (Cu) is essential for many biological processes, but is toxic when present in excessive amounts. In this study, we provide evidence that Cu plays a crucial role in controlling tuberculosis. A Mycobacterium tuberculosis ( Mtb ) mutant lacking the outer membrane channel protein Rv1698 accumulated 100-fold more Cu and was more susceptible to Cu toxicity than WT Mtb . Similar phenotypes were observed for a M. smegmatis mutant lacking the homolog Ms3747, demonstrating that these mycobacterial copper transport proteins B (MctB) are essential for Cu resistance and maintenance of low intracellular Cu levels. Guinea pigs responded to infection with Mtb by increasing the Cu concentration in lung lesions. Loss of MctB resulted in a 1,000- and 100-fold reduced bacterial burden in lungs and lymph nodes, respectively, in guinea pigs infected with Mtb . In mice, the persistence defect of the Mtb mctB mutant was exacerbated by the addition of Cu to the diet. These experiments provide evidence that Cu is used by the mammalian host to control Mtb infection and that Cu resistance mechanisms are crucial for Mtb virulence. Importantly, Mtb is much more susceptible to Cu than other bacteria and is killed in vitro by Cu concentrations lower than those found in phagosomes of macrophages. Hence, this study reveals an Achilles heel of Mtb that might be a promising target for tuberculosis chemotherapy.

“…To date, only a handful of proteins have been suggested to be outer membrane proteins (15), but their functions have not been confirmed by phenotypes of the corresponding mutants of M. tuberculosis. This study, to our knowledge, represents the first example in which the phenotype of a gene deletion mutant-that is, an uptake defect for a small molecule-corresponds with the channel-forming activity of a purified M. tuberculosis protein.…”

Section: Discussionmentioning

confidence: 99%

“…Similarly, proteins secreted by M. tuberculosis need to cross both an inner and an outer membrane (14), a barrier of notoriously low permeability in M. tuberculosis (15). However, the only known secretion systems capable of translocating proteins across both M. tuberculosis membranes are the type VII secretion systems encoded by esx operons.…”

mentioning

confidence: 99%

An outer membrane channel protein of Mycobacterium tuberculosis with exotoxin activity

Danilchanka¹,

Sun²,

Pavlenok³

et al. 2014

Self Cite

114

136

The ability to control the timing and mode of host cell death plays a pivotal role in microbial infections. Many bacteria use toxins to kill host cells and evade immune responses. Such toxins are unknown in Mycobacterium tuberculosis. Virulent M. tuberculosis strains induce necrotic cell death in macrophages by an obscure molecular mechanism. Here we show that the M. tuberculosis protein Rv3903c (channel protein with necrosis-inducing toxin, CpnT) consists of an N-terminal channel domain that is used for uptake of nutrients across the outer membrane and a secreted toxic C-terminal domain. Infection experiments revealed that CpnT is required for survival and cytotoxicity of M. tuberculosis in macrophages. Furthermore, we demonstrate that the C-terminal domain of CpnT causes necrotic cell death in eukaryotic cells. Thus, CpnT has a dual function in uptake of nutrients and induction of host cell death by M. tuberculosis.transport | pore | secretion