Purification and characterization of the 22,000-dalton GTP-binding protein substrate for ADP-ribosylation by botulinum toxin, G22K.

GTP-binding proteins were purified from human neutrophils, including a 40,000-Da pertussis toxin substrate (Gn) and 22,000-, 24,000-, and 26,000-Da proteins, termed G22K, G24K, and G26K, respectively. The latter proteins were shown to be immunologically unrelated to Gn. G22K cross-reacted with anti-ras monoclonal antibody 142-24EO5, but not with monoclonal antibody Y13-259. A single 22,000-Da substrate for botulinum toxin-catalyzed ADP-ribosylation present in neutrophil membranes co-migrated upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis with G22K. In the presence of a cytosolic factor, G22K could serve as a specific botulinum toxin substrate. The 22,000-Da botulinum toxin substrate in neutrophil membranes could be immunoprecipitated by antibody 142-24EO5, but not by antibody Y13-259. G22K appears to be a unique GTP-binding protein which serves as a substrate for ADP-ribosylation by a component of botulinum toxin and which may be involved in exocytotic secretion or cellular differentiation.