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The site of interaction of NAD with the isolated S1 subunit of pertussis toxin was investigated by photoaffinity labelling. When S1 was irradiated at 254 nm in the presence of carbonyl-14C- or adenine-14CNAD, the uptake of radioactivity was equivalent to 0.75 and 0.1 mol/mol respectively, while the NAD glycohydrolase activity was abolished. Inactivation was thus accompanied by crosslinking of the nicotinamide portion of NAD to the protein. Sequence determination of purified radioactive peptides indicated that Glu-129 was a major site of labelling. This residue is therefore closely associated with either NAD binding or hydrolysis.
Stephen A. Cockle (Mon,) studied this question.