Salmonella antibacterial Rhs polymorphic toxin inhibits translation through ADP-ribosylation of EF-Tu P-loop

Rearrangement hot spots (Rhs) proteins are members of the broad family of polymorphic toxins. Polymorphic toxins are modular proteins composed of an N-terminal region that specifies their mode of secretion into the medium or into the target cell, a central delivery module and a C-terminal domain that has toxic activity. Here, we structurally and functionally characterize the C-terminal toxic domain of the antibacterial Rhsmain protein, which is delivered by the Type VI secretion system of Salmonella enterica Typhimurium. We show that this domain adopts an ADP-ribosyltransferase fold and inhibits protein synthesis by transferring an ADP-ribose group from NAD+ to the elongation factor EFTu. This modification is specifically placed on the sidechain of the conserved D21 residue located on the P-loop of the EF-Tu G-domain. Finally, we demonstrate that its cognate immunity protein neutralizes Rhsmain C-terminal toxin activity by acting like a lid that closes the catalytic site and traps the NAD+.