Subcellular localization of Type VI secretion system assembly in response to cell–cell contact

Abstract

Bacteria require a number of systems, including the type VI secretion system (T6SS), for interbacterial competition and pathogenesis. The T6SS is a large nanomachine that can deliver toxins directly across membranes of proximal target cells. Since major reassembly of T6SS is necessary after each secretion event, accurate timing and localization of T6SS assembly can lower the cost of protein translocation. Although critically important, mechanisms underlying spatiotemporal regulation of T6SS assembly remain poorly understood. Here, we used super‐resolution live‐cell imaging to show that while Acinetobacter and Burkholderia thailandensis can assemble T6SS at any site, a significant subset of T6SS assemblies localizes precisely to the site of contact between neighboring bacteria. We identified a class of diverse, previously uncharacterized, periplasmic proteins required for this dynamic localization of T6SS to cell–cell contact (TslA). This precise localization is also dependent on the outer membrane porin OmpA. Our analysis links transmembrane communication to accurate timing and localization of T6SS assembly as well as uncovers a pathway allowing bacterial cells to respond to cell–cell contact during interbacterial competition. image The assembly of the Type VI secretion system in Acinetobacter and Burkholderia can be precisely positioned to the site of a cell–cell contact allowing for efficient toxin translocation during interbacterial competition. Super‐resolution microscopy shows that a significant subset of T6SS assemblies initiate at the site of a contact between neighboring bacteria. T6SS baseplate and membrane complex is required for the formation of small sheath foci followed by delayed sheath polymerization. A previously uncharacterized periplasmic protein TslA and the outer membrane porin OmpA are required for localization of T6SS assembly at the cell–cell contact sites.