Interaction of G protein beta subunit with inward rectifier K(+) channel Kir3.

G protein betagamma 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 Gbeta shows a seven-bladed propeller with four beta strands in each blade. The Gbeta/Galpha interacting surface contains sites for activating GIRK channels. Furthermore, our recent investigation using chimeras between Gbeta1 and yeast beta (STE4) suggested that the outer strands of blades 1 and 2 of Gbeta1 could be an interaction area between Gbeta1 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 beta-strands, substantially reduced GIRK channel activation, suggesting that these residues are Gbeta/GIRK interaction sites. These mutations did not affect the expression of Gbeta1 or its ability to stimulate PLCbeta2. These residues are surface-accessible and located outside Gbeta/Galpha interaction sites. These results suggest that the residues on the outer surface of blades 1 and 2 are involved in the interaction of Gbetagamma 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 Gbeta1 impaired the functional interaction of Gbeta1 with GIRK, in agreement with the data on native neuronal GIRK channels. Trp-332 plays a critical role in the interaction of Gbeta1 with Galpha as well as all effectors so far tested.