Type VII secretion systems: structure, functions and transport models

ABSTRACT Recently, a novel type of secretory pathway, type VII secretion systems (T7SSs), has been characterized in mycobacteria. The chromosomes of Mycobacterium tuberculosis and Mycobacterium bovis encode five T7SSs (ESX-1 to ESX-5). The best characterized of them, ESX-1, is involved in host-pathogen interactions, and its deletion is one of the main causes of M. bovis BCG attenuation. Another T7SS, ESX-3, has been previously shown to be transcriptionally controlled by the zinc uptake repressor (Zur) and by the iron-dependent transcriptional repressor (IdeR), suggesting that it might be involved in zinc and iron homeostasis. In this study, we characterized an M. tuberculosis conditional mutant in which transcription of the ESX-3 gene cluster can be downregulated by anhydrotetracycline. We showed that this T7SS is essential for growth and that this phenotype can be complemented by zinc, iron, or supernatant from a wild-type parental strain culture, demonstrating that the ESX-3 secretion system is responsible for the secretion of some soluble factor(s) required for growth that is probably involved in optimal iron and zinc uptake.

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Mycobacterial Esx-3 Requires Multiple Components for Iron Acquisition

mBio ◽

10.1128/mbio.01073-14 ◽

2014 ◽

Vol 5 (3) ◽

Cited By ~ 50

Author(s):

M. Sloan Siegrist ◽

Magnus Steigedal ◽

Rushdy Ahmad ◽

Alka Mehra ◽

Marte S. Dragset ◽

...

Keyword(s):

Mass Spectrometry ◽

Iron Acquisition ◽

Multiple Reaction Monitoring ◽

Protein Export ◽

Secretion Systems ◽

Experimental Conditions ◽

Content Type ◽

Type Vii Secretion ◽

Wide Range

ABSTRACT The type VII secretion systems are conserved across mycobacterial species and in many Gram-positive bacteria. While the well-characterized Esx-1 pathway is required for the virulence of pathogenic mycobacteria and conjugation in the model organism Mycobacterium smegmatis, Esx-3 contributes to mycobactin-mediated iron acquisition in these bacteria. Here we show that several Esx-3 components are individually required for function under low-iron conditions but that at least one, the membrane-bound protease MycP3 of M. smegmatis, is partially expendable. All of the esx-3 mutants tested, including the ΔmycP 3ms mutant, failed to export the native Esx-3 substrates EsxH ms and EsxG ms to quantifiable levels, as determined by targeted mass spectrometry. Although we were able to restore low-iron growth to the esx-3 mutants by genetic complementation, we found a wide range of complementation levels for protein export. Indeed, minute quantities of extracellular EsxH ms and EsxG ms were sufficient for iron acquisition under our experimental conditions. The apparent separation of Esx-3 function in iron acquisition from robust EsxG ms and EsxH ms secretion in the ΔmycP 3ms mutant and in some of the complemented esx-3 mutants compels reexamination of the structure-function relationships for type VII secretion systems. IMPORTANCE Mycobacteria have several paralogous type VII secretion systems, Esx-1 through Esx-5. Whereas Esx-1 is required for pathogenic mycobacteria to grow within an infected host, Esx-3 is essential for growth in vitro. We and others have shown that Esx-3 is required for siderophore-mediated iron acquisition. In this work, we identify individual Esx-3 components that contribute to this process. As in the Esx-1 system, most mutations that abolish Esx-3 protein export also disrupt its function. Unexpectedly, however, ultrasensitive quantitation of Esx-3 secretion by multiple-reaction-monitoring mass spectrometry (MRM-MS) revealed that very low levels of export were sufficient for iron acquisition under similar conditions. Although protein export clearly contributes to type VII function, the relationship is not absolute.

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Evolutionary thrift: mycobacteria repurpose plasmid diversity during adaptation of type VII secretion systems

10.1101/067207 ◽

2016 ◽

Author(s):

Tatum D. Mortimer ◽

Alexandra M. Weber ◽

Caitlin S. Pepperell

Keyword(s):

Model Organism ◽

Gene Families ◽

Type Iv Secretion ◽

Paralogous Gene ◽

Secretion Systems ◽

Type Vii Secretion ◽

Type Iv ◽

Selection For ◽

Plasmid Diversity ◽

Chromosomal Loci

AbstractMycobacteria have a distinct secretion system, termed type VII (T7SS), which is encoded by paralogous chromosomal loci (ESX) and associated with pathogenesis, conjugation, and metal homeostasis. Evolution of paralogous gene families is of interest because duplication is an important mechanism by which novel genes evolve, but there are potential conflicts between adaptive forces that stabilize duplications and those that enable evolution of new functions. Our objective was to delineate the adaptive forces underlying diversification of T7SS. Plasmid-borne ESX were described recently, and we found evidence that the initial duplication and divergence of ESX systems occurred on plasmids and was driven by selection for advantageous mutations. Plasmid conjugation has been linked to T7SS and type IV secretion systems (T4SS) in mycobacteria, and we discovered that T7SS and T4SS genes evolved in concert on the plasmids. We hypothesize that differentiation of plasmid ESX helps to prevent conjugation among cells harboring incompatible plasmids. Plasmid ESX appear to have been repurposed following migration to the chromosome, and there is evidence of positive selection driving further differentiation of chromosomal ESX. We hypothesize that ESX loci were initially stabilized on the chromosome by mediating their own transfer. These results emphasize the diverse adaptive paths underlying evolution of novelty, which in this case involved plasmid duplications, selection for advantageous mutations in the mobile and core genomes, migration of the loci between plasmids and chromosomes, and lateral transfer among chromosomes. We discuss further implications for the choice of model organism to study ESX functions in Mycobacterium tuberculosis.

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A type VII secretion system in Group B Streptococcus mediates cytotoxicity and virulence

PLoS Pathogens ◽

10.1371/journal.ppat.1010121 ◽

2021 ◽

Vol 17 (12) ◽

pp. e1010121

Author(s):

Brady L. Spencer ◽

Uday Tak ◽

Jéssica C. Mendonça ◽

Prescilla E. Nagao ◽

Michael Niederweis ◽

...

Keyword(s):

Group B Streptococcus ◽

Bioinformatic Analysis ◽

Effector Proteins ◽

Dependent Manner ◽

Secretion Systems ◽

Gram Positive Bacteria ◽

Type Vii Secretion ◽

Type Vii Secretion System ◽

Group B

Type VII secretion systems (T7SS) have been identified in Actinobacteria and Firmicutes and have been shown to secrete effector proteins with functions in virulence, host toxicity, and/or interbacterial killing in a few genera. Bioinformatic analysis indicates that isolates of Group B Streptococcus (GBS) encode at least four distinct subtypes of T7SS machinery, three of which encode adjacent putative T7SS effectors with WXG and LXG motifs. However, the function of T7SS in GBS pathogenesis is unknown. Here we assessed the role of the most abundant GBS T7SS subtype during GBS pathogenesis. In a murine model of hematogenous meningitis, mice infected with GBS lacking a functional T7SS or lacking the secreted WXG100 effector EsxA exhibited less mortality, lower bacterial burdens in tissues, and decreased inflammation in the brain compared to mice infected with the parental GBS strain. We further showed that this T7SS induces cytotoxicity in brain endothelium and that EsxA contributes to these cytotoxicity phenotypes in a WXG motif-dependent manner. Finally, we determined that EsxA is a pore-forming protein, thus demonstrating the first role for a non-mycobacterial EsxA homolog in pore formation. This work reveals the importance of a T7SS in host–GBS interactions and has implications for T7SS effector function in other Gram-positive bacteria.

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Genome Sequence of Mycobacterium abscessus Phage phiT46-1

Microbiology Resource Announcements ◽

10.1128/mra.01421-20 ◽

2021 ◽

Vol 10 (2) ◽

Author(s):

Elizabeth D. Amarh ◽

Rebekah M. Dedrick ◽

Rebecca A. Garlena ◽

Daniel A. Russell ◽

Deborah Jacobs-Sera ◽

...

Keyword(s):

Genome Sequence ◽

Mycobacterium Abscessus ◽

Spontaneous Release ◽

Secretion Systems ◽

Content Type ◽

Type Vii Secretion

ABSTRACT Mycobacteriophage phiT46-1 is a newly isolated Mycobacterium phage that was isolated by spontaneous release from Mycobacterium abscessus strain Taiwan-46 and infects M. abscessus strain BWH-C. Phage phiT46-1 is unrelated to previously described mycobacteriophages, has a 52,849-bp genome, and includes a polymorphic toxin-immunity cassette associated with type VII secretion systems.

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The structure of the endogenous ESX-3 secretion system

eLife ◽

10.7554/elife.52983 ◽

2019 ◽

Vol 8 ◽

Cited By ~ 15

Author(s):

Nicole Poweleit ◽

Nadine Czudnochowski ◽

Rachel Nakagawa ◽

Donovan D Trinidad ◽

Kenan C Murphy ◽

...

Keyword(s):

Metabolic Regulation ◽

Model Organism ◽

Single Copy ◽

Secretion Systems ◽

Epitope Tag ◽

Protein Substrates ◽

Gram Positive Bacteria ◽

Type Vii Secretion ◽

Cryo Electron Microscopy ◽

Range Of Functions

The ESX (or Type VII) secretion systems are protein export systems in mycobacteria and many Gram-positive bacteria that mediate a broad range of functions including virulence, conjugation, and metabolic regulation. These systems translocate folded dimers of WXG100-superfamily protein substrates across the cytoplasmic membrane. We report the cryo-electron microscopy structure of an ESX-3 system, purified using an epitope tag inserted with recombineering into the chromosome of the model organism Mycobacterium smegmatis. The structure reveals a stacked architecture that extends above and below the inner membrane of the bacterium. The ESX-3 protomer complex is assembled from a single copy of the EccB3, EccC3, and EccE3 and two copies of the EccD3 protein. In the structure, the protomers form a stable dimer that is consistent with assembly into a larger oligomer. The ESX-3 structure provides a framework for further study of these important bacterial transporters.

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A type VII secretion system of Streptococcus gallolyticus subsp. gallolyticus contributes to gut colonization and the development of colon tumors

PLoS Pathogens ◽

10.1371/journal.ppat.1009182 ◽

2021 ◽

Vol 17 (1) ◽

pp. e1009182

Author(s):

John Culver Taylor ◽

Xinsheng Gao ◽

Juan Xu ◽

Michael Holder ◽

Joseph Petrosino ◽

...

Keyword(s):

Bacterial Culture ◽

Secretion System ◽

Host Cells ◽

Secretion Systems ◽

Colon Tumors ◽

Type Vii Secretion ◽

Streptococcus Gallolyticus ◽

Gut Colonization ◽

Type Vii Secretion System

Streptococcus gallolyticus subspecies gallolyticus (Sgg) has a strong clinical association with colorectal cancer (CRC) and actively promotes the development of colon tumors. However, the molecular determinants involved in Sgg pathogenicity in the gut are unknown. Bacterial type VII secretion systems (T7SS) mediate pathogen interactions with their host and are important for virulence in pathogenic mycobacteria and Staphylococcus aureus. Through genome analysis, we identified a locus in Sgg strain TX20005 that encodes a putative type VII secretion system (designated as SggT7SST05). We showed that core genes within the SggT7SST05 locus are expressed in vitro and in the colon of mice. Western blot analysis showed that SggEsxA, a protein predicted to be a T7SS secretion substrate, is detected in the bacterial culture supernatant, indicating that this SggT7SST05 is functional. Deletion of SggT7SST05 (TX20005Δesx) resulted in impaired bacterial adherence to HT29 cells and abolished the ability of Sgg to stimulate HT29 cell proliferation. Analysis of bacterial culture supernatants suggest that SggT7SST05-secreted factors are responsible for the pro-proliferative activity of Sgg, whereas Sgg adherence to host cells requires both SggT7SST05-secreted and bacterial surface-associated factors. In a murine gut colonization model, TX20005Δesx showed significantly reduced colonization compared to the parent strain. Furthermore, in a mouse model of CRC, mice exposed to TX20005 had a significantly higher tumor burden compared to saline-treated mice, whereas those exposed to TX20005Δesx did not. Examination of the Sgg load in the colon in the CRC model suggests that SggT7SST05-mediated activities are directly involved in the promotion of colon tumors. Taken together, these results reveal SggT7SST05 as a previously unrecognized pathogenicity determinant for Sgg colonization of the colon and promotion of colon tumors.

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