Interaction of G Protein β Subunit with Inward Rectifier K⁺ Channel Kir3

Abstract

G protein βγ subunits bind and activate G protein-coupled inward rectifier K⁺ (GIRK) channels. This protein-protein interaction is crucial for slow hyperpolarizations of cardiac myocytes and neurons. The crystal structure of Gβ shows a seven-bladed propeller with four β strands in each blade. The Gβ/Gα interacting surface contains sites for activating GIRK channels. Furthermore, our recent investigation using chimeras between Gβ₁ and yeast β (STE4) suggested that the outer strands of blades 1 and 2 of Gβ1 could be an interaction area between Gβ1 and GIRK. In this study, we made point mutations on suspected residues on these outer strands and investigated their ability to activate GIRK1/GIRK2 channels. Mutations at Thr-86, Thr-87, and Gly-131, all located on the loops between β-strands, substantially reduced GIRK channel activation, suggesting that these residues are Gβ/GIRK interaction sites. These mutations did not affect the expression of Gβ1 or its ability to stimulate PLCβ2. These residues are surface-accessible and located outside Gβ/Gα interaction sites. These results suggest that the residues on the outer surface of blades 1 and 2 are involved in the interaction of Gβγ with GIRK channels. Our study suggests a mechanism by which different effectors use different blades to achieve divergence of signaling. We also observed that substitution of alanine for Trp-332 of Gβ1 impaired the functional interaction of Gβ1 with GIRK, in agreement with the data on native neuronal GIRK channels. Trp-332 plays a critical role in the interaction of Gβ1 with Gα as well as all effectors so far tested.