Gβγ subunit combinations differentially modulate receptor and effector coupling in vivo

In vitro, little specificity is seen for modulation of effecters by different combinations of G beta gamma subunits from heterotrimeric G proteins. Here, we demonstrate that the coupling of specific combinations of G beta gamma subunits to different receptors leads to a differential ability to modulate effecters in vivo. We have shown that the beta (1)AR and beta (2)AR can activate homomultimers of the human inwardly rectifying potassium channel Kir 3.2 when coexpressed in Xenopus oocytes, and that this requires a functional mammalian Gs heterotrimer. Modulation was independent of cAMP production, suggesting a membrane-delimited mechanism. To analyze further the importance of different G beta gamma combinations, we have tested the facilitation of Kir 3.2 activation by PAR mediated by different G beta gamma subunits. The subunits tested were G beta (1,5) and G gamma (1,2,7,11) These experiments demonstrated significant variation between the ability of the G beta gamma combinations to activate the channels after receptor stimulation. This was in marked contrast to the situation in vitro where little specificity for binding of a Kir 3.1 C-terminal GST fusion protein by different G beta gamma combinations was detected. More importantly, neither receptor, although homologous both structurally and functionally, shared the same preference for G beta gamma subunits. In the presence of beta (1)AR, G beta (5)gamma (1) and G beta (5)gamma (11) activated Kir 3.2 to the greatest extent, while for the beta (2)AR, G beta (1)gamma (7), G beta (1)gamma (11), and G beta (5)gamma (2) produced the greatest responses. Interestingly, no preference was seen in the ability of different G beta gamma subunits to facilitate receptor-stimulated GTPase activity of the Gs alpha. These results suggest that it is not the receptor/G protein alpha subunit interaction or the G beta gamma /effector interaction that is altered by G beta gamma, but rather that the ability of the receptor to interact productively with the G beta gamma subunit directly and/or the G protein/effector complex is dependent on the specific G protein heterotrimer associated with the receptor. (C) 2000 Elsevier Science Inc. All rights reserved.