Isolation of two proteins with high affinity for guanine nucleotides from membranes of bovine brain.

Membranes from bovine brain bind relatively large quantities of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) with high affinity. The two proteins responsible for most of this activity were purified; they account for 1.5% of the membrane protein. The two proteins contain alpha subunits of either 39,000 or 41,000 Da, beta subunits of 36,000 or 35,000 Da, and a potential gamma subunit (11,000 Da). These structures are the same as a family of proteins that includes transducin and the regulatory proteins, GS and GI, of adenylate cyclase. The 41,000- and 39,000-Da polypeptides can be ADP-ribosylated with islet-activating protein from Bordetella pertussis, bind guanine nucleotides specifically, and migrate through polyacrylamide gels with rates similar to the alpha subunits of GI and transducin, respectively. The 36,000- and 35,000-Da polypeptides are similar to the beta subunits of GI and GS. The gamma subunit is found whenever beta subunits are present. The 41,000- and 39,000-Da polypeptides (with beta and gamma) are designated, respectively, GI and GO from brain. The alpha subunit of GO was isolated without the use of ligands known to dissociate other G proteins. GO alpha binds GTP gamma S reversibly in the absence of Mg2+ and is relatively stable in cholate. This isolated alpha subunit should be of great utility in elucidating the mechanism of action of this family of GTP-binding proteins.