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

1. In native cardiac myocytes, there is a time dependence to the G protein- gated inwardly rectifying K+ (K-G) channel current during voltage steps tha t accelerates as the concentration of acetylcholine is increased. This Phen omenon has been called 'relaxation' and is not reproduced in the reconstitu ted Kir3.1/Kir3.4 channel in Xenopus oocytes. We have shown that RGS4, a re gulator of G protein signalling, restores relaxation to the reconstituted K ir3.1/Kir3.4 channel. In this study, we examined the mechanism of this phen omenon by expressing various combinations of membrane receptors, G proteins , Kir3.0 subunits and mutants of RGS4 in Xenopus oocytes. 2. RGS4 restored relaxation to K-G channels activated by the pertussis toxi n (PTX)-sensitive G protein-coupled m(2)-muscarinic receptor but not to tho se activated by the G(s) protein-coupled beta (2)-adrenergic receptor. 3. RGS4 induced relaxation not only in heteromeric K-G channels composed of Kir3.1 and Kir3.4 but also in homomeric assemblies of either an active mut ant 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 essenti al to reproduce the effect of wild-type RGS4 on the K-G channel. 5. The mutation of residues in the RGS domain which interact with the alpha subunit of the G protein impaired the effect of RGS4. 6. This study therefore shows that interaction between the RGS domain and P TX-sensitive G(alpha) subunits mediates the effect of RGS4 on the agonist c oncentration-dependent relaxation of K-G channels.