Alkylation of cysteine 41, but not cysteine 200, decreases the ADP-ribosyltransferase activity of the S1 subunit of pertussis toxin.

Sulfhydryl-alkylating reagents are known to inactivate the NAD glycohydrolase and ADP-ribosyltransferase activities of the S1 subunit of pertussis toxin, a protein which contains two cysteines at positions 41 and 200. It has been proposed that NAD can retard alkylation of one of the two cysteines of this protein (Kaslow, H.R., and Lesikar, D.D. (1987) Biochemistry 26, 4397-4402). We now report that NAD retards the ability of these alkylating reagents to inactivate the S1 subunit. In order to determine which cysteine is protected by NAD, we used site-directed mutagenesis to construct analogs of the toxin with serines at positions 41 and/or 200. Sulfhydryl-alkylating reagents reduced the ADP-ribosyltransferase activity of the analog with a single cysteine at position 41; NAD retarded this inactivation. In contrast, sulfhydryl-alkylating reagents did not inactivate analogs with serine at position 41. An analog with alanine at position 41 possessed substantial ADP-ribosyltransferase activity. We conclude that alkylation of cysteine 41, and not cysteine 200, inactivates the S1 subunit of pertussis toxin, but that the sulfhydryl group of cysteine 41 is not essential for the ADP-ribosyltransferase activity of the toxin. These results suggest that the region near cysteine 41 contributes to features of the S1 subunit important for ADP-ribosyltransferase activity. Using site-directed mutagenesis, we found that changing aspartate 34 to asparagine, arginine 39 to lysine, and glutamine 42 to glutamate had little effect on ADP-ribosyltransferase activity. However, substituting an asparagine for the histidine at position 35 markedly decreased, but did not eliminate, ADP-ribosyltransferase activity. Chou-Fasman analysis predicted no significant modifications in secondary structure of the S1 peptide with the change of histidine 35 to asparagine. Thus, histidine 35 may interact with a substrate of the S1 subunit without being essential for catalysis.