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Bunduc CM, Ding Y, Kuijl C, Marlovits TC, Bitter W, Houben ENG. Reconstitution of a minimal ESX-5 type VII secretion system suggests a role for PPE proteins in the outer membrane transport of proteins. mSphere 2023; 8:e0040223. [PMID: 37747201 PMCID: PMC10597459 DOI: 10.1128/msphere.00402-23] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2023] [Accepted: 08/03/2023] [Indexed: 09/26/2023] Open
Abstract
Mycobacteria utilize type VII secretion systems (T7SSs) to secrete proteins across their highly hydrophobic and diderm cell envelope. Pathogenic mycobacteria have up to five different T7SSs, called ESX-1 to ESX-5, which are crucial for growth and virulence. Here, we use a functionally reconstituted ESX-5 system in the avirulent species Mycobacterium smegmatis that lacks ESX-5, to define the role of each esx-5 gene in system functionality. By creating an array of gene deletions and assessing protein levels of components and membrane complex assembly, we observed that only the five components of the inner membrane complex are required for its assembly. However, in addition to these five core components, active secretion also depends on both the Esx and PE/PPE substrates. Tagging the PPE substrates followed by subcellular fractionation, surface labeling and membrane extraction showed that these proteins localize to the mycobacterial outer membrane. This indicates that they could play a role in secretion across this enigmatic outer barrier. These results provide the first full overview of the role of each esx-5 gene in T7SS functionality. IMPORTANCE Pathogenic mycobacteria, such as the notorious Mycobacterium tuberculosis, are highly successful as pathogens, in part due to their specific and diderm cell envelope, with a mycolic acid-containing outer membrane. The architecture of this highly impermeable membrane is little understood and the proteins that populate it even less so. To transport proteins across their cell envelope, mycobacteria employ a specialized transport pathway called type VII secretion. While recent studies have elucidated the type VII secretion membrane channel that mediates transport across the inner membrane, the identity of the outer membrane channel remains a black box. Here, we show evidence that specific substrates of the type VII pathway could form these channels. Elucidating the pathway and mechanism of protein secretion through the mycobacterial outer membrane will allow its exploitation for the development of novel mycobacterial therapeutics.
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Affiliation(s)
- C. M. Bunduc
- Molecular Microbiology Section, Amsterdam Institute for Life and Environment (A-Life), Vrije Universiteit, Amsterdam, The Netherlands
- Centre for Structural Systems Biology, Notkestraße, Hamburg, Germany
- Institute of Structural and Systems Biology, University Medical Center Hamburg-Eppendorf, Notkestraße, Hamburg, Germany
- German Electron Synchrotron Centre, Notkestraße, Hamburg, Germany
| | - Y. Ding
- Molecular Microbiology Section, Amsterdam Institute for Life and Environment (A-Life), Vrije Universiteit, Amsterdam, The Netherlands
| | - C. Kuijl
- Department of Medical Microbiology and Infection Control, Amsterdam UMC, Amsterdam, The Netherlands
| | - T. C. Marlovits
- Centre for Structural Systems Biology, Notkestraße, Hamburg, Germany
- Institute of Structural and Systems Biology, University Medical Center Hamburg-Eppendorf, Notkestraße, Hamburg, Germany
- German Electron Synchrotron Centre, Notkestraße, Hamburg, Germany
| | - W. Bitter
- Molecular Microbiology Section, Amsterdam Institute for Life and Environment (A-Life), Vrije Universiteit, Amsterdam, The Netherlands
- Department of Medical Microbiology and Infection Control, Amsterdam UMC, Amsterdam, The Netherlands
| | - E. N. G. Houben
- Molecular Microbiology Section, Amsterdam Institute for Life and Environment (A-Life), Vrije Universiteit, Amsterdam, The Netherlands
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Berger TM, Michaelis C, Probst I, Sagmeister T, Petrowitsch L, Puchner S, Pavkov-Keller T, Gesslbauer B, Grohmann E, Keller W. Small Things Matter: The 11.6-kDa TraB Protein is Crucial for Antibiotic Resistance Transfer Among Enterococci. Front Mol Biosci 2022; 9:867136. [PMID: 35547396 PMCID: PMC9083827 DOI: 10.3389/fmolb.2022.867136] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2022] [Accepted: 03/14/2022] [Indexed: 11/18/2022] Open
Abstract
Conjugative transfer is the most important means for spreading antibiotic resistance genes. It is used by Gram-positive and Gram-negative bacteria, and archaea as well. Conjugative transfer is mediated by molecular membrane-spanning nanomachines, so called Type 4 Secretion Systems (T4SS). The T4SS of the broad-host-range inc18-plasmid pIP501 is organized in a single operon encoding 15 putative transfer proteins. pIP501 was originally isolated from a clinical Streptococcus agalactiae strain but is mainly found in Enterococci. In this study, we demonstrate that the small transmembrane protein TraB is essential for pIP501 transfer. Complementation of a markerless pIP501∆traB knockout by traB lacking its secretion signal sequence did not fully restore conjugative transfer. Pull-downs with Strep-tagged TraB demonstrated interactions of TraB with the putative mating pair formation proteins, TraF, TraH, TraK, TraM, and with the lytic transglycosylase TraG. As TraB is the only putative mating pair formation complex protein containing a secretion signal sequence, we speculate on its role as T4SS recruitment factor. Moreover, structural features of TraB and TraB orthologs are presented, making an essential role of TraB-like proteins in antibiotic resistance transfer among Firmicutes likely.
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Affiliation(s)
- Tamara M.I. Berger
- Institute of Molecular Biosciences, Department of Structural Biology, University of Graz, Graz, Austria
| | - Claudia Michaelis
- Faculty of Life Sciences and Technology, Department of Microbiology, Berliner Hochschule für Technik, Berlin, Germany
| | - Ines Probst
- Division of Infectious Diseases, University Medical Center Freiburg, Freiburg, Germany
| | - Theo Sagmeister
- Institute of Molecular Biosciences, Department of Structural Biology, University of Graz, Graz, Austria
| | - Lukas Petrowitsch
- Institute of Molecular Biosciences, Department of Structural Biology, University of Graz, Graz, Austria
| | - Sandra Puchner
- Faculty of Life Sciences and Technology, Department of Microbiology, Berliner Hochschule für Technik, Berlin, Germany
| | - Tea Pavkov-Keller
- Institute of Molecular Biosciences, Department of Structural Biology, University of Graz, Graz, Austria
- Field of Excellence BioHealth, University of Graz, Graz, Austria
- BioTechMed-Graz, Graz, Austria
| | - Bernd Gesslbauer
- Institute of Pharmaceutical Sciences, Division of Pharmaceutical Chemistry, University of Graz, Graz, Austria
| | - Elisabeth Grohmann
- Faculty of Life Sciences and Technology, Department of Microbiology, Berliner Hochschule für Technik, Berlin, Germany
| | - Walter Keller
- Institute of Molecular Biosciences, Department of Structural Biology, University of Graz, Graz, Austria
- Field of Excellence BioHealth, University of Graz, Graz, Austria
- BioTechMed-Graz, Graz, Austria
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