Photorhabdus luminescens Tc toxin is inhibited by the protease inhibitor MG132 and activated by protease cleavage resulting in increased binding to target cells.

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Photorhabdus luminescens Tc toxins consist of the cell-binding component TcA, the linker component TcB, and the enzyme component TcC. TccC3, a specific isoform of TcC, ADP-ribosylates actin and causes redistribution of the actin cytoskeleton. TccC5, another isoform of TcC, ADP-ribosylates and activates Rho proteins. Here, we report that the proteasome inhibitor MG132 blocks the intoxication of cells by Tc toxin. The inhibitory effect of MG132 was not observed, when the ADP-ribosyltransferase domain of the TcC component was introduced into target cells by protective antigen, which is the binding and delivery component of anthrax toxin. Additionally, MG132 affected neither pore formation by TcA in artificial membranes nor binding of the toxin to cells. Furthermore, the in vitro ADP-ribosylation of actin by the enzyme domain of TccC3 was not affected by MG132. Similar to MG132, several calpain inhibitors blocked the action of the Tc toxin. Proteolytic cleavage of the binding component TcA induced by P.luminescens protease PrtA1 or by collagenase largely increased the toxicity of the Tc toxin. MG132 exhibited no inhibitory effect on the cleaved TcA component. Moreover, binding of TcA to target cells was largely increased after cleavage. The data indicate that Tc toxin is activated by proteolytic processing of the TcA component, resulting in increased receptor binding. Toxin processing is probably inhibited by MG132.