Target cell specificity of the Pasteurella haemolytica leukotoxin is unaffected by the nature of the fatty‐acyl group used to activate the toxin in vitro

The leukotoxin (LktA) of Pasteurella haemolytica is active only against cells of ruminant origin. It is synthesised as an inactive protoxin encoded by the lktA gene and post‐translationally modified to the active toxin by the product of the lktC gene. The LktA and LktC proteins were expressed separately in Escherichia coli and partially purified. Active LktA was produced in vitro in the presence of LktC and acyl‐acyl carrier protein (ACP) charged separately in vitro with a fatty‐acyl group. The toxic activity and target cell specificity of LktA and adenylate cyclase toxin (CyaA), a toxin active against a wide variety of mammalian cells, were investigated after activation with ACP charged with different fatty acids. Palmitoyl‐ACP produced the most active toxin in both cases and, although other fatty acids were also effective, the fatty acid preference was the same for the in vitro activation of both toxins. Activated LktA remained ruminant cell‐specific whichever acyl group was used to acylate the A protoxin.