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The purification of a guanine nucleotide regulatory protein from rat brain membrane and the measurement of its GTPase.

A guanine nucleotide regulatory protein (G-protein) was purified to homogeneity from rat brain membrane though a series of chromatography runs, including DE-52 ion exchange, AcA-34 ultrogel, and octyl-Sepharose columns. The purified G-protein consisted of 3 subunits with molecular weights of 41000, 36000, and 8000, respectively. The alpha-subunit of the G-protein possessed high affinity GTP binding sites (k = 22.5 nmol/L) and GTPase activity (Km = 82.5 nmol/L, Vmax = 2.8 pmol/min/microgram protein), and it could be ADP-ribosylated by [adenylate-32P]-NAD in the presence of pertussis toxin. These results suggest that the purified protein was Gi-protein.