Mechanism of threonine ADP-ribosylation of F-actin by a Tc toxin

Tc toxins deliver toxic enzymes into host cells by a unique injection mechanism. One of these enzymes is TccC3, an ADP-ribosyltransferase from Photorhabdus luminescens. Once TccC3 is translocated into the target cell, the enzyme ADP-ribosylates actin, resulting in clustering of the actin cytoskeleton and ultimately cell death. Here, we combine biochemistry, solution and solid-state NMR spectroscopy and cryo-EM to show in atomic detail how TccC3 modifies actin. We find that the ADP-ribosyltransferase does not bind to G-actin but interacts with two consecutive actin subunits of F-actin. The binding of TccC3 to F-actin occurs via an induced-fit mechanism that facilitates access of NAD+ to the nucleotide binding pocket. The following nucleophilic substitution reaction results in the transfer of ADP-ribose to threonine-148 of F-actin. We demonstrate that this site-specific modification of F-actin prevents its interaction with depolymerization factors, such as cofilin, which impairs actin network turnover and leads to steady actin polymerization. Our findings reveal in atomic detail a new mechanism of action of a bacterial toxin through specific targeting and modification of F-actin.