Weinzinger, A. S. | bioRxiv Science

bioRxiv2025

Live-cell quantitative monitoring reveals distinct, high-affinity Gβγ regulations of GIRK2 and GIRK1/2 channels

Gi/o protein-coupled receptors (GPCRs) inhibit cardiac and neuronal excitability via G protein-activated K+ channels (GIRK), assembled by combinations of GIRK1 - GIRK4 subunits. GIRKs are activated by direct binding of the G{beta}{gamma} dimer of inhibitory Gi/o proteins. However, key aspects of this textbook signaling pathway remain debated. Recent studies suggested no Gi/o-GIRK pre-coupling and low (>250 {micro}M) G{beta}{gamma}-GIRK interaction affinity, contradicting earlier sub-{micro}M estimates and implying low signaling efficiency. We show that G{gamma} prenylation, which mediates G{beta}{gamma} membrane attachment required for GIRK activation, also contributes to the G{beta}{gamma}-GIRK interaction, explaining the poor affinity obtained with non-prenylated G{beta}{gamma}. Using quantitative protein titration and electrophysiology in live Xenopus oocytes, G{beta}{gamma} affinity for homotetrameric GIRK2 ranged from 4-30 {micro}M. Heterotetrameric GIRK1/2 showed a higher G{beta}{gamma} apparent affinity due to unique G{beta}{gamma}-docking site (anchor) in GIRK1, which enriches G{beta}{gamma} at the channel. Biochemical approaches and molecular dynamic simulations revealed that the G{beta}{gamma} anchor is formed by interacting N-terminal and distal C-terminal domains of the GIRK1 subunits, distinct from the G{beta}{gamma}-binding "activation" site(s) underlying channel opening. Thus, the affinity of G{beta}{gamma}-GIRK interaction is within the expected physiological range, while dynamic pre-coupling of G{beta}{gamma} to GIRK1-containing channels through high-affinity interactions further enhances the GPCR-Gi/o-GIRK signaling efficiency.

physiology↗

bioRxiv2021

Simulating PIP2-induced gating transitions in Kir6.2 channels

ATP-sensitive potassium (KATP) channels consist of an inwardly rectifying K+ channel (Kir6.2) pore, to which four ATP-sensitive sulfonylurea receptor (SUR) domains are attached, thereby coupling K+ permeation directly to the metabolic state of the cell. Dysfunction is linked to neonatal diabetes and other diseases. K+ flux through these channels is controlled by conformational changes in the helix bundle region, which acts as a physical barrier for K+ flux. In addition, the G-loop, located in the cytoplasmic domain, and the selectivity filter might contribute to gating, as suggested by different disease-causing mutations. Gating of Kir channels is regulated by different ligands, like G{beta}{gamma}, H+, Na+, adenosine nucleotides and the signaling lipid phosphatidyl-inositol 4,5-bisphosphate (PIP2), which is an essential activator for all eukaryotic Kir family members. Although molecular determinants of PIP2 activation of KATP channels have been investigated in functional studies, structural information of the binding site is still lacking as PIP2 could not be resolved in Kir6.2 cryo-EM structures. In this study, we used Molecular Dynamics (MD) simulations to examine the dynamics of residues associated with gating in Kir6.2. By combining this structural information with functional data, we investigated the mechanism underlying Kir6.2 channel regulation by PIP2.

biophysics↗