G protein .beta..gamma. subunits from bovine brain and retina: equivalent catalytic support of ADP-ribosylation of .alpha. subunits by pertussis toxin but differential interactions with Gs.alpha.

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Abstract
We have examined the ability of the .beta..gamma. subunits of guanine nucleotide binding regulatory proteins (G proteins) to support the pertussis toxin (PT) catalyzed ADP-ribosylation of G protein .alpha. subunits. Substoichiometric amounts of the .beta..gamma. complex purified from either bovine brain G proteins or the bovine retinal G protein, Gt, are sufficient to support the ADP-ribosylation of the .alpha. subunits of Gi (the G protein that mediates inhibition of adenylyl cyclase) and G0 (a G protein of unknown function) by PT. This observation indicates that ADP-ribosylated G protein oligomers can dissociate into their respective .alpha. and .beta..gamma. 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 .beta..gamma. does not require Mg2+. Although the .beta..gamma. subunit complexes purified from bovine brain G proteins and the .beta..gamma. complex of Gt support equally the ADP-ribosylation of .alpha. subunits by PT, there is a marked difference in their abilities to interact with Gs.alpha.. The enhancement of deactivation of fluoride-activated Gs.alpha. requires 25-fold more .beta..gamma. from Gt than from brain G proteins to produce a similar response. This difference in potency of .beta..gamma. complexes from the two sources was observed in the ability of .beta..gamma. to produce an increase in the activity of recombinant Gs.alpha. produced in Escherichia coli.

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