Large bacterial protein toxins autotranslocate functional effector domains to the eukaryotic cell cytosol, resulting in alterations to cellular functions that ultimately benefit the infecting pathogen. Among these toxins, the clostridial glucosylating toxins (CGTs) produced by Gram-positive bacteria and the multifunctional-autoprocessing RTX (MARTX) toxins of Gram-negative bacteria have distinct mechanisms of post-translocation, but a shared mechanism of post-translocation autoprocessing that releases these functional domains from the large holotoxins. These toxins carry an embedded cysteine protease domain (CPD) that is regulated by a unique allosteric activation mechanism. Binding of the eukaryotic-specific small molecule inositol hexakisphosphate (InsP(6)) to a basic cleft within the CPD induces a structural rearrangement that exposes the protease active site to its substrates. Proteins containing this domain belong to the peptidase family C80 of clan CD PMID:20628577,PMID:17464284,PMID:18845756,PMID:19465933.

The MARTX CPD domain consists of a central beta-sheet that is surrounded by alpha-helices. Additional beta-strands at the C terminus form a subdomain known as the beta-flap, that is loosely attached to the core protease. The MARTX CPD catalytic dyad is composed of one His and one Cys residue. The distance between the catalytic residues indicates that the Cys is not activated by protonation from His, but rather suggests that the Cys is substrate-activated by close alignment of the scissile bond, while the His functions solely to protonate the leaving group PMID:20628577,PMID:18845756,PMID:19465933.

PMID:20628577 PMID:17464284 PMID:18845756 PMID:19465933 PMID:20628577 PMID:18845756 PMID:19465933