Cysteine-Scanning Mutagenesis Supports the Importance of Clostridium perfringens Enterotoxin Amino Acids 80 to 106 for Membrane Insertion and Pore Formation

Clostridium perfringens enterotoxin (CPE) causes the gastrointestinal symptoms of the second most common bacterial food-borne illness. Previous studies suggested that a region named TM1, which has amphipathic characteristics and spans from amino acids 81 to 106 of the native CPE protein, forms a beta-hairpin involved in beta-barrel pore formation. To further explore the potential role of TM1 in pore formation, the single Cys naturally present in CPE at residue 186 was first altered to alanine by mutagenesis; the resultant rCPE variant, named C186A, was shown to retain cytotoxic properties. Cys-scanning mutagenesis was then performed in which individual Cys mutations were introduced into each TM1 residue of the C186A variant. When those Cys variants were characterized, three variants were identified that exhibit reduced cytotoxicity despite possessing binding and oligomerization abilities similar to those of the C186A variant from which they were derived. Pronase challenge experiments suggested that the reduced cytotoxicity of those two Cys variants, i.e., the F91C and F95C variants, which model to the tip of the beta-hairpin, was attributable to a lessened ability of these variants to insert into membranes after oligomerization. In contrast, another Cys variant, i.e., the G103C variant, with impaired cytotoxicity apparently inserted into membranes after oligomerization but could not form a pore with a fully functional channel. Collectively, these results support the TM1 region forming a beta-hairpin as an important step in CPE insertion and pore formation. Furthermore, this work identifies the first amino acid residues specifically involved in those two steps in CPE action.