Interaction between the RGS domain of RGS4 with G protein α subunits mediates the voltage‐dependent relaxation of the G protein‐gated potassium channel

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Abstract
1 In native cardiac myocytes, there is a time dependence to the G protein‐gated inwardly rectifying K+ (KG) channel current during voltage steps that accelerates as the concentration of acetylcholine is increased. This phenomenon has been called ‘relaxation’ and is not reproduced in the reconstituted Kir3.1/Kir3.4 channel in Xenopus oocytes. We have shown that RGS4, a regulator of G protein signalling, restores relaxation to the reconstituted Kir3.1/Kir3.4 channel. In this study, we examined the mechanism of this phenomenon by expressing various combinations of membrane receptors, G proteins, Kir3.0 subunits and mutants of RGS4 in Xenopus oocytes. 2 RGS4 restored relaxation to KG channels activated by the pertussis toxin (PTX)‐sensitive G protein‐coupled m2‐muscarinic receptor but not to those activated by the Gs protein‐coupled β2‐adrenergic receptor. 3 RGS4 induced relaxation not only in heteromeric KG channels composed of Kir3.1 and Kir3.4 but also in homomeric assemblies of either an active mutant of Kir3.1 (Kir3.1/F137S) or an isoform of Kir3.2 (Kir3.2d). 4 Truncation mutants of RGS4 showed that the RGS domain itself was essential to reproduce the effect of wild‐type RGS4 on the KG channel. 5 The mutation of residues in the RGS domain which interact with the α subunit of the G protein (Gα) impaired the effect of RGS4. 6 This study therefore shows that interaction between the RGS domain and PTX‐sensitive Gα subunits mediates the effect of RGS4 on the agonist concentration‐dependent relaxation of KG channels.

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