Requirement of ADP-ribosylation for the pertussis toxin-induced alteration in electrophoretic mobility of G-proteins

Pertussis toxin (PTX) catalyzes the ADP-ribosylation of the α-subunit of GTPbinding proteins (G-proteins) in the presence of NAD⁺. Pertussis toxin also decreases the electrophoretic mobility of the a-subunit on urea SDS PAGE. This effect of PTX has been suggested to be a property of the toxin different from its ability to catalyze ADP-ribosylation. However, the present report provides evidence to the contrary; ie, this mobility shift required the ADP-ribosylation of α-subunits. This conclusion was based on: (1) in the presence of increasing concentrations of NAD⁺ (0.026-1.3 μM), there was a linear increase in the formation of the slower migrating α-subunit as measured by immunoblotting with selective antisera, (2) addition of NADase to the incubation mixture completely eliminated the formation of this protein, and (3) increasing concentrations of nicotinamide (50-250 mM), which inhibits ADP-ribosylation, decreased the amount of the slower migrating a-subunit. Thus, in addition to PTX, NAD⁺ was required for the mobility shift and the slower migrating a-subunit is likely the ADP-ribosylated form.