Na+ promotes the dissociation between Galpha GDP and Gbeta gamma, activating G protein-gated K+ channels.
G protein-gated K(+) channels (GIRK, or Kir3) are activated by the direct binding of Gbetagamma or of cytosolic Na(+). Na(+) activation is fast, Gbetagamma-independent, and probably via a direct, low affinity (EC(50), 30-40 mm) binding of Na(+) to the channel. Here we demonstrate that an increase in intracellular Na(+) concentration, [Na(+)](in), within the physiological range (5-20 mm), activates GIRK within minutes via an additional, slow mechanism. The slow activation is observed in GIRK mutants lacking the direct Na(+) effect. It is inhibited by a Gbetagamma scavenger, hence it is Gbetagamma-dependent; but it does not require GTP. We hypothesized that Na(+) elevates the cellular concentration of free Gbetagamma by promoting the dissociation of the Galphabetagamma heterotrimer into free Galpha(GDP) and Gbetagamma. Direct biochemical measurements showed that Na(+) causes a moderate decrease (approximately 2-fold) in the affinity of interaction between Galpha(GDP) and Gbetagamma. Furthermore, in accord with the predictions of our model, slow Na(+) activation was enhanced by mild coexpression of Galpha(i3). Our findings reveal a previously unknown mechanism of regulation of G proteins and demonstrate a novel Gbetagamma-dependent regulation of GIRK by Na(+). We propose that Na(+) may act as a regulatory factor, or even a second messenger, that regulates effectors via Gbetagamma.
Rishal
Ida
I
Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel.
Keren-Raifman
Tal
T
Yakubovich
Daniel
D
Ivanina
Tatiana
T
Dessauer
Carmen W
CW
Slepak
Vladlen Z
VZ
Dascal
Nathan
N
D000818:Animals; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007150:Immunohistochemistry; D015640:Ion Channel Gating; D015221:Potassium Channels / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014981:Xenopus
D015221:Potassium Channels; D012964:Sodium; D019204:GTP-Binding Proteins