Setting Gβγ Free Without a Receptor

Abstract

Rishal et al . have described a novel mode of heterotrimeric guanine nucleotide-binding protein (G protein) regulation that points to a possible role for Na + as a second messenger. G protein-activated inward rectifier K + channels (GIRKs) play a role in regulating the heartbeat and mediate the effects of many inhibitory neurotransmitters. These channels are activated by the G protein βγ heterodimer (Gβγ), which is released from the heterotrimer after stimulation of G protein-coupled receptors (GPCRs) and GTP binding to the Gα subunit. GIRKs are also rapidly activated by intracellular Na + independently of G proteins. Rishal et al . used inside-out patch clamp analysis to investigate activation of GIRK channels expressed in Xenopus oocytes. They observed a slow phase of Na + -dependent activation in both wild-type channels and mutant channels lacking fast Na + -dependent activation. Although slow Na + activation of GIRK was inhibited by a protein that bound Gβγ, it did not require GTP in the medium. High Na + reduced the association of radiolabeled Gβγ with a GDP-bound fusion protein of Gα with glutathione S -transferase (GST-Gα GDP ) and decreased Gβγ binding to GST-Gα GDP as assessed by surface plasmon resonance. Mild overexpression of Gα (which should increase the fraction of total Gβγ bound to Gα GDP ) enhanced slow Na + -dependent activation of GIRK. These data suggest that Na + regulates the resting equilibrium between free Gβγ and Gβγ bound to Gα GDP and may, under some conditions, act as a second messenger coupling electrical activity in excitable cells to GPCR-independent effects of Gβγ on target proteins. I. Rishal, T. Keren-Raifman, D. Yakubovich, T. Ivanina, C. W. Dessauer, V. Z. Slepak, N. Dascal, Na + promotes the dissociation between Gα GDP and Gβγ, activating G protein-gated K + channels. J. Biol. Chem . 278 , 3840-3845 (2003). [Abstract] [Full Text]