1.C.59 The Clostridium perfringens Enterotoxin (CPE) Family
C. perfringens uses an arsenal of 14 toxins to cause enteric and histotoxic infections in humans and domestic animals. One of these is CPE, also called heat-labile enterotoxin B chain precursor, possibly the most important of them from a medical standpoint. It forms complexes of variable sizes (~135, 155 and 200 kDa), but the 155 kDa complex alone causes 86Rb+ release and is probably responsible for the diarrheal and cramping symptoms of C. perfringens type A food poisoning. CPE also prevents tight junction formation, possibly by complexing the tight junction structural protein, occludin, in the 200 kDa complex. CPE is thus a bifunctional toxin which first generates pores (the 155 kDa complex) and then damages the tight junctions (the 200 kDa complex). It thus increases both cellular and paracellular permeability, thereby contributing to diarrhea of C. perfringens gastrointestinal disease. The Clostridium botulinum haemagglutinin/neurotoxin is 623 aas long (in contrast to CPE which is 309 aas long) and exhibits an internal repeat. Both repeats are about 24% identical to CPE. This toxin is composed of several subcomponents of ~53, 33, 23 and 17 kDa. Mature botulinum toxins are large and heterogeneous in size (Singh et al., 2001).
Upon its release from C. perfringens spores, CPE binds to its receptor, claudin, at the tight junctions between the epithelial cells of the gut wall and subsequently forms pores in the cell membranes. A number of different complexes between CPE and claudin have been observed. Briggs et al. (2011) have determined the three-dimensional structure of the soluble form of CPE in two crystal forms by X-ray crystallography, to a resolution of 2.7 and 4.0 Å, respectively, and found that the N-terminal domain shows structural homology with the aerolysin-like β-pore-forming family of proteins. They show that CPE forms a trimer in both crystal forms, and that this trimer is likely to be biologically relevant, although it is not the active pore form. The crystal structure of Clostridium perfringens enterotoxin displays features of beta-pore-forming toxins (Kitadokoro et al., 2011).
The soluble monomer of the β-barrel pore-forming toxin (PFTs), Monalysin, is cleaved to yield oligomeric pores. The structure of a cleaved form lacking the transmembrane domain has been solved by x-ray crystalography and cryo-EM (PDB#4MJT; Leone et al. 2015). The structure displays an elongated shape, resembling those of beta-pore-forming toxins such as aerolysin, but it lacks the receptor binding domain. Pro-monalysin forms a stable doughnut-like 18-mer complex composed of two disk-shaped nonamers held together by N-terminal swapping of the pro-peptides. This is in contrast with the monomeric pro-form of the other beta-PFTs that are receptor-dependent for membrane interaction. The membrane-spanning region of pro-monalysin is fully buried in the center of the doughnut, suggesting that upon pro-peptide cleavage, the two disk-shaped nonamers can - and have to - dissociate to leave the transmembrane segments free to deploy and lead to pore formation. In contrast with other toxins, the delivery of 18 subunits at once, nearby the cell surface, may be used to by-pass the requirement for a receptor-dependent concentration to reach the threshold for oligomerization into the pore forming complex (Leone et al. 2015).
The transport reactions catalyzed by members of the CPE familly is:
small molecules (in) small molecules (out)