We have examined the ability of the βγ subunits of guanine nucleotide binding regulatory proteins (G proteins) to support the pertussis toxin (PT) catalyzed ADP-ribosylation of G protein α subunits. Substoichiometric amounts of the βγ complex purified from either bovine brain G proteins or the bovine retinal G protein, Gt, are sufficient to support the ADP-ribosylation of the α subunits of Gi (the G protein that mediates inhibition of adenylyl cyclase) and Go (a G protein of unknown function) by PT. This observation indicates that ADP-ribosylated G protein oligomers can dissociate into their respective α and βγ subunits in the absence of activating regulatory ligands, i.e., nonhydrolyzable GTP analogues or fluoride. Additionally, the catalytic support of ADP-ribosylation by bovine brain βγ does not require Mg2+. Although the βγ subunit complexes purified from bovine brain G proteins and the βγ complex of Gt support equally the ADP-ribosylation of a subunits by PT, there is a marked difference in their abilities to interact with Gsα. The enhancement of deactivation of fluoride-activated Gsα requires 25-fold more βγ from Gt than from brain G proteins to produce a similar response. This difference in potency of βγ complexes from the two sources was also observed in the ability of βγ to produce an increase in the activity of recombinant Gsα produced in Escherichia coli.
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