The Identification and Structure of the Membrane-spanning Domain of the Clostridium septicum Alpha Toxin

Alpha toxin (AT) is a pore-forming toxin produced by Clostridium septicum that belongs to the unique aerolysin-like family of pore-forming toxins. The location and structure of the transmembrane domains of these toxins have remained elusive. Using deletion mutagenesis, cysteine-scanning mutagenesis and multiple spectrofluorimetric methods a membrane-spanning amphipathic β-hairpin of AT has been identified. Spectrofluorimetric analysis of cysteine-substituted residues modified with an environmentally sensitive fluorescent probe via the cysteine sulfydryl showed that the side chains of residues 203-232 alternated between the aqueous milieu and the membrane core when the AT oligomer was inserted into membranes, consistent with the formation of an amphipathic transmembrane β-hairpin. AT derivatives that contained deletions that removed up to 90% of the β-hairpin did not form a pore but were similar to native toxin in all other aspects of the mechanism. Furthermore, a mutant of AT that contained an engineered disulfide, predicted to restrict the movement of the β-hairpin, functioned similarly to native toxin except that it did not form a pore unless the disulfide bond was reduced. Together these studies revealed the location and structure of the membrane-spanning domain of AT.