Structural and functional studies on the mechanisms of effector recruitment by a Xanthomonadaceae type VI secretion system

• Grant number: 26/01352-8
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: March 01, 2026
• End date: February 28, 2029
• Field of knowledge: Biological Sciences - Genetics - Molecular Genetics and Genetics of Microorganisms
• Principal Investigator: Cristina Elisa Alvarez Martinez
• Grantee: Bharat Ramasubramanian Iyer
• Host Institution: Instituto de Biologia (IB). Universidade Estadual de Campinas (UNICAMP). Campinas , SP, Brazil
• Associated research grant: 21/10577-0 - Biology of Bacteria and Bacteriophages Research Center, AP.CEPID

Abstract

The type VI secretion system (T6SS) is a specialized apparatus comprised of a minimum set of 13 distinct proteins, which traverses the bacterial envelope and injects protein substrates directly into target cells. Assembly of the T6SS in the bacterial inner membrane relies on a complex formed by TssJ-TssL-TssM, which recruits the baseplate components TssE-TssF-TssG-TssK. The baseplate serves as a platform for polymerization of a tail tube formed by Hcp protein, which is surrounded by a TssB-TssC contractile sheath; and contains a VgrG/PAAR puncturing device at its tip (spike complex). Effector recruitment by the T6SS is mediated by interactions with the tube components Hcp, VgrG and PAAR and may involve a variety of protein adaptors and chaperones. The T6SS has been mainly characterized for its role in interbacterial competition, secreting toxins directly into bacterial cells. Anti-eukaryotic T6SS have also been identified in several bacterial species, but much less is known on their mode of assembly, activation and protein delivery. The X. citri T6SS targets eukaryotic cells and is required for resistance to predation by the bacterivorous amoeba Dictyostelium discoideum, providing a fitness advantage in the soil and plant environment. Interestingly, a similar T6SS gene cluster is found in the genome of a clinical isolate of Stenotrophomonas maltophilia, another member of the Xanthomonadaceae family that is found in the rhizophere and as an opportunistic pathogen. S. maltophilia encodes two T6SS gene clusters and their role in resistance against amoeba predation and interbacterial competition is currently under investigation in the research group. The T6SS cluster found in both X. citri and S. maltophilia harbors a gene that encodes a putative adaptor protein of the VgrG/PAAR spike. In addition, structural prediction of the VgrG protein from X. citri revealed some features distinct from those described for homologs present in antibacterial systems, suggesting a different effector recruitment mechanism. This works aims to structurally and functionally characterize the puncturing device formed by VgrG and PAAR, also testing for the presence of the putative adaptor protein in this complex. In addition, the project aims to identify the effectors secreted by the T6SS of S. maltophilia and perform biochemical and structural studies to describe their function and activity. (AU)