G beta gamma subunit combinations differentially modulate receptor and effector coupling in vivo

In vitro, little specificity is seen for modulation of effecters by differe nt combinations of G beta gamma subunits from heterotrimeric G proteins. He re, we demonstrate that the coupling of specific combinations of G beta gam ma subunits to different receptors leads to a differential ability to modul ate 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 funct ional mammalian Gs heterotrimer. Modulation was independent of cAMP product ion, suggesting a membrane-delimited mechanism. To analyze further the impo rtance of different G beta gamma combinations, we have tested the facilitat ion of Kir 3.2 activation by PAR mediated by different G beta gamma subunit s. The subunits tested were G beta (1,5) and G gamma (1,2,7,11) These exper iments demonstrated significant variation between the ability of the G beta gamma combinations to activate the channels after receptor stimulation. Th is was in marked contrast to the situation in vitro where little specificit y for binding of a Kir 3.1 C-terminal GST fusion protein by different G bet a gamma combinations was detected. More importantly, neither receptor, alth ough homologous both structurally and functionally, shared the same prefere nce for G beta gamma subunits. In the presence of beta (1)AR, G beta (5)gam ma (1) and G beta (5)gamma (11) activated Kir 3.2 to the greatest extent, w hile for the beta (2)AR, G beta (1)gamma (7), G beta (1)gamma (11), and G b eta (5)gamma (2) produced the greatest responses. Interestingly, no prefere nce was seen in the ability of different G beta gamma subunits to facilitat e receptor-stimulated GTPase activity of the Gs alpha. These results sugges t 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 rathe r 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) 20 00 Elsevier Science Inc. All rights reserved.