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

Chemical modification of histidine residues in staphylococcal alpha-to xin leads to loss of functional activity. Site-directed mutants of the toxin in which each of the four histidine residues was replaced by se veral amino acids were therefore produced. The mutant proteins were pu rified and characterized. Exchange of H-259 or H-144 was sometimes tol erated without reduction in hemolytic activity. These histidine residu es are thus not essential for toxin function. Exchange of H-35 and H-4 8, however, had marked effects. H-35 mutant toxins bound with high aff inity to rabbit erythrocytes but displayed faulty oligomerization and were unable to form pores. H-48 mutant toxins also had severely impair ed hemolytic activity due probably to faulty hexamerization. We interp ret these results to indicate that the N-terminal domain of alpha-toxi n in the region of H-35 and H-48 is involved in protomer-protomer inte ractions that underlie the hexamerization and pore-forming process.