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A guanine nucleotide-binding regulatory protein (G protein), with subunits designated as alpha 40 beta gamma, was identified and partially resolved from two other purified G proteins, Go (alpha 39 beta gamma) and Gi (alpha 41 beta gamma), found in bovine brain. The alpha 40 G protein subunit served as a substrate for ADP-ribosylation catalyzed by Bordetella pertussis toxin, as did alpha 39 and alpha 41. alpha 40 was shown to be closely related to, but distinct from, alpha 41 by reaction with various peptide antisera. An antiserum generated against a peptide derived from the sequence of a Gi alpha clone isolated from a rat C6 glioma cDNA library (Itoh, H., Kozasa, T., Nagata, S., Nakamura, S., Katada, T., Ui, M., Iwai, S., Ohtsuka, E., Kawasaki, H., Suzuki, K., and Kaziro, Y. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 3776-3780) reacted with alpha 40 to the exclusion of all other alpha subunits tested. Another …