Site-directed mutagenesis of a single tryptophan near the middle of the channel-forming toxin aerolysin inhibits its transfer across the outer membrane of Aeromonas salmonicida.

The channel-forming protein aerolysin must cross both the inner and outer bacterial membranes during its secretion from Aeromonas hydrophila or from Aeromonas salmonicida containing the cloned structural gene. We examined the fate of three mutant proteins in which Trp-227, near the middle of the amino acid chain, was replaced with glycine, leucine, or phenylalanine by site-directed mutagenesis. All three proteins crossed the inner membrane and entered the periplasm in the same way as wild-type, and in each case the signal sequence was removed correctly. Little or none of the proaerolysin substituted with glycine or leucine was released into the culture supernatant. Instead, significant amounts became associated with the outer membrane. The Phe-227 protoxin was secreted by the bacteria but at a reduced rate. The leucine and phenylalanine mutant proteins were purified and compared with native proaerolysin. They were processed correctly to the mature forms by treatment with trypsin, and like native aerolysin, both were resistant to further proteolysis. In each case, processing was followed by the formation of oligomers similar to those produced by native toxin. The hemolytic activity of the processed Phe-227 mutant was one-quarter that of wild-type toxin whereas Leu-227 aerolysin had less than one-hundredth the wild-type activity. These results are further evidence that aerolysin is secreted in at least two steps. As well, they show that the last step, crossing the outer membrane, can be blocked by an apparently small change in the structure of the protein.