Role of histidine 35 of the S1 subunit of pertussis toxin in the ADP-ribosylation of transducin.

Molecular modeling of the S1 subunit (S1) of pertussis toxin with other ADP-ribosylating bacterial exotoxins predicted that histidine 35 (His-35) would residue within the active site of S1. Recombinant derivatives of S1 (rS1 and the C180 peptide) which contained either a H35Q or H35P mutation were analyzed to determine the role of His-35 in ADP-ribosylation. C180 peptide is a recombinant peptide composed of the amino-terminal 180 amino acids of S1. Under linear velocity conditions, C180H35Q possessed 2% of wild type C180 peptide activity and C180H35P possessed no detectable activity in the ADP-ribosylation of transducin. The H35Q mutation did not change the affinity of recombinant peptides for NAD or two targets for ADP-ribosylation, transducin, or alpha i3C20, but did lower the kcat in the NAD glycohydrolase and ADP-ribosyltransferase reactions. Neither the H35Q nor H35P mutation reduced the ability of recombinant proteins to be photocross-linked with NAD which was consistent with the His-35 mutations not reducing the affinity for NAD. These data indicate that His-35 does not reduce the affinity of S1 for NAD or transducin, but functions as a catalytic residue in the ADP-ribosylation reaction possibly in a hydrogen bonding capacity.