Assembly and release of a Tc toxin

Tripartite Tc toxins are virulence factors of bacterial pathogens. Although their structure and mechanism of action are well understood, it remains elusive where this large macromolecular complex is assembled and how it is released. Here we show by an integrative multiscale imaging approach that Yersinia entomophaga Tc (YenTc) toxin components are expressed only in a subpopulation of cells that are “primed” with several other potential virulence factors, including filaments of the protease M66/StcE. A phage-like lysis cassette (LC) is required for YenTc release; however, before resulting in complete cell lysis, the LC generates intermediate “ghost” cells, which may serve as assembly compartments and become densely packed with assembled YenTc holotoxins. We hypothesize that this stepwise mechanism evolved to minimize the number of cells that need to be sacrificed. The occurrence of similar lysis cassettes in diverse organisms indicates a conserved mechanism for Tc toxin release that may apply to other extracellular macromolecular machines.