Immunochemical evidence for a novel pertussis toxin substrate in human neutrophils

Pertussis toxin catalyzes incorporation of 20.2 pmol of ADP-ribose/mg of protein into approximately 40-kDa protein(s) in human neutrophil membranes compared with 14.1 pmol/mg in bovine brain membranes. Based on these measurements we estimate that pertussis toxin substrate(s) should represent at least 0.085% of total membrane protein in neutrophils. Both brain and neutrophil membranes show high concentrations (0.34 versus 0.16% of total membrane protein, respectively) of the common β subunit of guanine nucleotide binding proteins. Affinity purified antibodies specific for G(o)-α fail to detect any protein in immunoblots of neutrophil membranes (150 μg) under conditions where as little as 10 ng of purified G(o)-α is detectable, and G(o)-α is readily detected in brain membranes (100 μg). An antiserum against transducin that cross-reacts strongly with G(i)-α, detects as little as 5 ng of purified G(i)-α and readily detects G(i)-α in brain membranes, but in neutrophil membranes, the antiserum detects an approximately 40-kDa band that corresponds to less than 10% of the expected amount of pertussis toxin substrate(s). The results show that human neutrophil membranes contain relatively large amounts of pertussis toxin substrate(s), but that the predominant pertussis toxin substrate is immunochemically distinct from previously identified substrates, transducin, G(i), and G(o).