1.A.2.1.10
G-protein-activated inward rectifying K⁺ channel, Kir3.2, KATP2, KCNJ6, KCNJ7 or GIRK2 of 423 aas and 2 TMSs (Inanobe et al., 2011; Yokogawa et al. 2011). Mutations cause the Keppen-Lubinsky syndrome (Gao et al. 2022). It functions in electrical signaling in neurons and muscle cells (Weng et al. 2021), being important in regulating heart rate and neuronal excitability.  It is activated by binding of the βγ-subunit complex to the cytoplasmic pore gate (Yokogawa et al. 2011). Chen et al. 2017 found that GIRK channels are activated by Ivermectin (IVM). Cholesterol binds to and upregulates GIRK channels (GIRK2 and 4), and the binding sites have been determined (Rosenhouse-Dantsker 2018). An inherited gain-of-function mutation in the human GIRK3.4 causes familial human sinus node dysfunction (SND). The increased activity of GIRK channels likely leads to a sustained hyperpolarization of pacemaker cells and thereby reduces heart rate (Kuß et al. 2019). GIRK2 channels are abundantly expressed in the heart and require that phosphatidylinositol bisphosphate (PIP2) is present so that intracellular channel-gating regulators such as Gbetagamma (Gβγ) and Na⁺ ions maintain the channel-open state. Li et al. 2019 determined how each regulator uses  channel domain movements to control gate transitions. Na⁺ controls the cytosolic gate of the channel through an anti-clockwise rotation, whereas Gβγ stabilizes the transmembrane gate in the open state through a rocking movement of the cytosolic domain. Both effects altered the way by which the channel interacts with PIP2 and thereby stabilizes the open states of the respective gates (Li et al. 2019). The protein plays a role in heart atrial fibrillation-valvular heart disease (VHD) (Zhao et al. 2021). Measurements of ligand binding and channel current have been made (Usher et al. 2021). CryoEM structures of GIRK2 in the presence and absence of the cholesterol analog cholesteryl hemisuccinate (CHS) and phosphatidylinositol 4,5-bisphosphate (PIP2) reveal that CHS binds near PIP2 in lipid-facing hydrophobic pockets of the transmembrane domain, suggesting that CHS stabilizes the PIP2 interaction with the channel to promote engagement of the cytoplasmic domain with the transmembrane region (Mathiharan et al. 2021). It may play a role in Parkinson's Disease (Zhou et al. 2023).