Mounting, structure and autocleavage of a type VI secretion-associated Rhs polymorphic toxin

Bacteria have evolved effectors or toxins to outcompete other bacteria or to hijack host cell pathways. One broad family of bacterial polymorphic toxins regroups multidomain proteins with a modular organization, that comprise a C-terminal toxin domain fused to a N-terminal domain that adapt to the delivery apparatus. Polymorphic toxins include bacteriocins, contact-dependent growth inhibition systems, and specialized Hcp, VgrG, PAAR or Rhs Type VI secretion (T6SS) components. We recently described and characterized Tre23, a toxin domain fused to a T6SS-associated Rhs protein in Photorhabdus laumondii. Here, we show that the Rhs polymorphic toxin forms a complex with the T6SS spike protein VgrG and a cognate chaperone EagR. Using truncation derivatives and cross-linking mass spectrometry, we demonstrate that VgrG-EagR-Rhs complex formation requires the VgrG C-terminal β-helix and the Rhs Nterminal prePAAR and PAAR domains. We then report the cryo-electron-microscopy structure of the Rhs-EagR complex, demonstrating that the Rhs central region forms a β-barrel cage-like structure that encapsulates the C-terminal toxin domain, and provide evidence for processing of the Rhs protein through aspartyl autoproteolysis. We propose a model for Rhs loading on the T6SS, transport and delivery into the target cell.