A cysteine protease-like domain enhances the cytotoxic effects of the toxin PaTox.

The nematode mutualistic bacterium produces a large virulence-associated multifunctional protein toxin named PaTox. A glycosyltransferase domain and a deamidase domain of this large toxin function as effectors that specifically target host Rho GTPases and heterotrimeric G proteins, respectively. Modification of these intracellular regulators results in toxicity toward insects and mammalian cells. In this study, we identified a cysteine protease-like domain spanning PaTox residues 1844-2114 (PaTox), upstream of these two effector domains and characterized by three conserved amino acid residues (Cys-1865, His-1955, and Asp-1975). We determined the crystal structure of the PaTox C1865A variant by native single-wavelength anomalous diffraction of sulfur atoms (sulfur-SAD). At 2.0 Å resolution, this structure revealed a catalytic site typical for papain-like cysteine proteases, comprising a catalytic triad, oxyanion hole, and typical secondary structural elements. The PaTox structure had highest similarity to that of the AvrPphB protease from classified as a C58-protease. Furthermore, we observed that PaTox shares structural homology also with non-C58-cysteine proteases, deubiquitinases, and deamidases. Upon delivery into insect larvae, PaTox alone without full-length PaTox had no toxic effects. Yet, PaTox expression in mammalian cells was toxic and enhanced the apoptotic phenotype induced by PaTox in HeLa cells. We propose that PaTox is a C58-like cysteine protease module that is essential for full PaTox activity.