HISTIDINE-RESIDUES NEAR THE N-TERMINUS OF STAPHYLOCOCCAL ALPHA-TOXIN AS REPORTERS OF REGIONS THAT ARE CRITICAL FOR OLIGOMERIZATION AND PORE FORMATION

Chemical modification of histidine residues in staphylococcal alpha-toxin leads to loss of functional activity. Site-directed mutants of the toxin in which each of the four histidine residues was replaced by several amino acids were therefore produced. The mutant proteins were purified and characterized. Exchange of H-259 or H-144 was sometimes tolerated without reduction in hemolytic activity. These histidine residues are thus not essential for toxin function. Exchange of H-35 and H-48, however, had marked effects. H-35 mutant toxins bound with high affinity to rabbit erythrocytes but displayed faulty oligomerization and were unable to form pores. H-48 mutant toxins also had severely impaired hemolytic activity due probably to faulty hexamerization. We interpret these results to indicate that the N-terminal domain of alpha-toxin in the region of H-35 and H-48 is involved in protomer-protomer interactions that underlie the hexamerization and pore-forming process.