Purification and characterization of procytotoxin of Pseudomonas aeruginosa. Dimer to monomer conversion of protoxin by proteolytic activation.

Cytotoxin of Pseudomonas aeruginosa is a cytolytic toxin that forms a pore on the target membrane by oligomerizing into a pentamer. This toxin is produced as an inactive precursor (proCTX) and is converted to an active form by proteolytic cleavage at the C terminus. We purified proCTX to apparent homogeneity and characterized it in a comparison with the active toxin. ProCTX bound to the erythrocyte membrane but did not form an oligomer on the membrane, hence the lack of hemolytic activity in proCTX. Circular dichroic experiments showed that active and proCTX have similar beta-sheet dominant structures. Intrinsic fluorescence analysis indicated that a molecule-buried tryptophan residue(s) of proCTX was exposed to the surface of the molecule as a result of conversion to the active form. In analytical gel filtration, chemical cross-linking, and analytical ultracentrifugation experiments, dimer to monomer conversion occurred with proteolytic activation.