Activity of the mitogenic Pasteurella multocida toxin requires an essential C-terminal residue

Pasteurella multocida toxin (PMT) is a potent mitogen that also affects bon e resorption. PMT acts intracellularly and is therefore postulated to have several domains involved in different aspects of its function. The toxin co ntains eight cysteine residues. Mutants with individual substitutions for e ach of these residues were constructed, and the effects of these on the bio logical activity of the toxin were determined by cultured-cell assays. Only the most C-terminal of the eight cysteines (C1165) was essential for full activity, although mutation of the cysteine residue at position 1159 caused a slight but reproducible loss of potency. In animal challenge experiments , mutant toxin (C1165S) was not toxic to piglets, even at doses exceeding a lethal dose of active PMT 1,000-fold. The mutant and wild-type toxins disp layed identical purification characteristics, similar susceptibility to pro teolytic digestion, and circular dichroism profiles, which indicated that n o gross structural changes had taken place. The function of the essential C 1165 residue is not yet known, although its most likely role is an enzymati c one at or near the catalytic center of the toxin.