ATP-activated inward rectifier K⁺ channel, IRK1 (also called ROMK or KIR1.1) (regulated by Sur1, allowing ATP sensitivity; also activated by phosphatidylinositol 4,5-bisphosphate (PIP) with affinity to PIP controlled by protein kinase A phosphorylation (which increases affinity for PIP) and protein kinase C phosphorylation (which decreases affinity for PIP (Zeng et al., 2003). The mechanism of voltage sensitivity of IRK1 inward-rectifier K⁺ channel block by the polyamine, spermine, has been proposed (Shin and Lu 2005). A putative pH sensor has been identified (Rapedius et al. 2006). Closure of the Kir1.1 pH gate results from steric occlusion of the permeation path by the convergence of four leucines (or phenylalanines) at the cytoplasmic apex of the inner transmembrane helices. In the open state, K⁺ crosses the pH gate together with its hydration shell (Sackin et al. 2005). Alternariol (AOH), the most important mycotoxin produced by Alternaria species, which are the most common mycoflora infecting small grain cereals worldwide, causes loss of cell viability by inducing apoptosis. AOH-induced apoptosis through a mitochondria-dependent pathway is characterized by p53 activation, an opening of the mitochondrial permeability transition pore (PTP), loss of mitochondrial transmembrane potential (ΔΨm), a downstream generation of O₂^- and caspase 9 and 3 activation (Bensassi et al., 2012). Pharmacological inhibition of renal ROMK causes diuresis and natriuresis in the absence of kaliuresis (Garcia et al. 2013). Cholesterol binding sites in KIR channels have been identified (Rosenhouse-Dantsker 2019). The ubiquitously expressed family of inward rectifier potassium (KIR) channels, encoded by KCNJ genes, is primarily involved in cell excitability and potassium homeostasis. Disease-associated mutations in KIR proteins have been linked to aberrant inward rectifier channel trafficking (Zangerl-Plessl et al. 2019). Interfacial binding Ssites for cholesterol on Kir, Kv, K2P, and related potassium channels have been identified (Lee 2020). Decreasing pH(in) increases the sensitivity of ROMK2 channels to K⁺(out) by altering the properties of the selectivity filter (Dahlmann et al. 2004).

Animals

IRK1 of Homo sapiens (P48048)

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).

Animals

Kir3.2 of Homo sapiens (P48051)

Inward rectifying potassium channel 16, Kir5.1 or KCNJ16. (Potassium channel subfamily J member 16).  Involved in pH and fluid regulation.  Forms heteromers with Kir4.1/KCNJ10 or Kir2.1/KCNJ2. MAGI-1 anchors Kir4.1 channels (Kir4.1 homomer and Kir4.1/Kir5.1 heteromer) and contributes to basolateral K⁺ recycling. The Kir4.1 A167V mutation is associated with EAST/SeSAME syndrome caused by mistrafficking of the mutant channels and inhibiting their expression on the basolateral surface of tubular cells. These findings suggest that mislocalization of the Kir4.1 channels contributes to renal salt wasting. (Tanemoto et al. 2014).

Animals

KCNJ16 of Homo sapiens

G protein-activated inward rectifying K⁺ channel 1 (Kir3.1; IRK3; KCNJ3; GIRK1). Regulates the heartbeat in humans. Phosphatidylinositol bisphosphate (PIP2) activates by opening the intracelluar G-loop gate (Meng et al., 2012).  Along the ion permeation pathway, three relatively narrow regions (the selectivity filter, the inner helix bundle crossing, and the cytosolic G loop) may serve as gates to control ion permeation (Meng et al. 2016). Cholesterol up-regulates neuronal GIRK channel activity (Bukiya et al. 2017). Changes in the levels of cholesterol and PI(4,5)P2 may act in concert to provide fine-tuning of Kir3 channel function (Bukiya et al. 2017). Kir3.1 forms oligomers with Kir3.4 (TC# 1.A.2.1.3) and transporters Rb⁺ and spermine.  It has been suggested that the selectivity filter is responsible for inward rectification and agonist activation as well as permeation and block (Makary et al. 2006). Cytoplasmic domain structures of Kir2.1 and Kir3.1 show sites for modulating gating and rectification (Pegan et al. 2005). A key basic residue that coordinates PIP2 to stabilize the pre-open and open states of the transmembrane gate flips in the inhibited state to form a direct salt-bridge interaction with the cytosolic gate and destabilize its open state (Gazgalis et al. 2022).

Animals

Kir3.1 (IRK3) of Homo sapiens (P48549)

ATP-sensitive inward rectifying K⁺ channel 8, KCNJ8 or Kir6.1. It acts with Sur2B (3.A.1.208.23). Channel activity is inhibited in oxidative stress via S-glutathionylation (Yang et al., 2011). Oxidative sensitivity is dependent on Cys176 (Yang et al., 2011).  These proteins comprise part of a glucose sensing mechanism (Rufino et al. 2013). It may play a role in limb wound repair and regeneration (Zhang et al. 2020). It is inhibited by glibenclamide (glyburide), an antidiabetic sulfonylurea used in the treatment of type II diabetes (Fernandes et al. 2004). Gain-of-function mutations in Kir6.1 and regulatory (SUR1) subunits of K_ATP channels can cause human neonatal diabetes mellitus by altering insulin secretion (Remedi et al. 2017).

Animals

Kir6.1 of Homo sapiens (Q15842)

Inward rectifying potassium (K⁺) (IRK) channel of 426 aas and 2 TMSs, AgaP.

AgaP of Anopheles gambiae (African malaria mosquito)

Kir1 (AgaP) K⁺ channel of 444 aas and 2 TMSs. Kir channels play a role in mosquito fecundity and may be promising molecular targets for the development of a new class of mosquitocides (Raphemot et al. 2014).

Kir1 of Anopheles gambiae (African malaria mosquito)

Inward rectifying K⁺ channel, Kir4.1, encoded by the KCNJ10 gene, of 379 aas and 2 TMSs.  It is inhibited by chloroethylclonidine and pentamidine which bind in the channel (Rodríguez-Menchaca et al. 2016; Aréchiga-Figueroa et al. 2017). It is also inhibited by chloropuine which inhibits by an open pore blocking mecnanism (Marmolejo-Murillo et al. 2017). Loss-of-function mutations in the pore-forming Kir4.1 subunit cause an autosomal recessive disorder characterized by epilepsy, ataxia, sensorineural deafness and tubulopathy (SeSAME/EST syndrome) Pentamidine potently inhibited Kir4.1 channels when applied to the cytoplasmic side under inside-out patch clamp configuration (IC₅₀ = 97nM). The block was voltage dependent. Molecular modeling predicted the binding of pentamidine to the transmembrane pore region of Kir4.1 at amino acids T127, T128 and E158. Mutation of each of these residues reduced the potency of pentamidine to block Kir4.1 channels (Aréchiga-Figueroa et al. 2017). Mutations in the KCNJ10 gene are associated with a distinctive ataxia, sensorineural hearing loss and a spasticity (Morin et al. 2020). It is regulated by kidins220 (TC# 8.A.28.1.8) (Jaudon et al. 2021). Pentamidine is a potent inhibitor of Kir4.1 (Zhang et al. 2022).

Kir4.1 of Homo sapiens

KCNJ11 or Kir6.2 or KATP of 390 aas; 96% identical to the rat homologue, TC# 1.A.2.1.7. Congenital hyperinsulinism (CHI) is characterized by persistent insulin secretion despite severe hypoglycemia. Mutations in the pancreatic ATP-sensitive K⁺ (K(ATP)) channel proteins sulfonylurea receptor 1 (SUR1) and Kir6.2, encoded by ABCC8 and KCNJ11, respectively, is the most common cause of the disease. Many mutations in SUR1 render the channel unable to traffic to the cell surface, thereby reducing channel function. Many studies have shown that for some SUR1 trafficking mutants, the defects could be corrected by treating cells with sulfonylureas or diazoxide (Yan et al. 2007). Inward rectifier potassium channels are characterized by a greater tendency to allow potassium to flow into the cell rather than out of it. Their voltage dependence is regulated by the concentration of extracellular potassium; as external potassium is raised, the voltage range of the channel opening shifts to more positive voltages. The inward rectification is mainly due to the blockage of outward current by internal magnesium (Tammaro and Ashcroft 2007). Kir6.2 is an ATP-sensitive potassium (K_ATP) channel coupling cell metabolism to electrical activity by regulating K⁺ fluxes across the plasma membrane. Channel closure is facilitated by ATP, which binds to the pore-forming subunit (Kir6.2). Conversely, channel opening is potentiated by phosphoinositol bisphosphate (PIP₂), which binds to Kir6.2 and reduces channel inhibition by ATP.  The PIP₂ binding site has been identified (Haider et al. 2007).  KATP channels are metabolic sensors that couple cell energetics to membrane excitability. In pancreatic beta-cells, channels formed by SUR1 and Kir6.2 regulate insulin secretion and are the targets of antidiabetic sulfonylureas. Martin et al. 2017 used cryo-EM to elucidate the structural basis of channel assembly and gating. The structure, determined in the presence of ATP and the sulfonylurea, glibenclamide, at ~6 Å resolution, revealed a closed Kir6.2 tetrameric core with four peripheral SUR1s, each anchored to a Kir6.2 by its N-terminal transmembrane domain (TMD0). Intricate interactions between TMD0, the loop following TMD0, and Kir6.2 near the proposed PIP2 binding site, and where ATP density is observed, suggest that SUR1 may contribute to ATP and PIP2 binding to enhance Kir6.2 sensitivity to both. The SUR1-ABC core is found in an unusual inward-facing conformation whereby the two nucleotide binding domains are misaligned along a two-fold symmetry axis, revealing a possible mechanism by which glibenclamide inhibits channel activity (Martin et al. 2017). a cryo-EM structure of a hamster SUR1/rat Kir6.2 channel bound to a high-affinity sulfonylurea drug glibenclamide and ATP has been solved at 3.63 Å resolution. The structure shows that glibenclamide is lodged in the transmembrane bundle of the SUR1-ABC core connected to the first nucleotide binding domain near the inner leaflet of the lipid bilayer (Martin et al. 2017). The activation of K(ATP) channels contributes to the shortening of action potential duration but is not the primary cause of extracellular K⁺ accumulation during early myocardial ischemia (Saito et al. 2005). KATP binds nucleotides (Usher et al. 2021). Mitochondrial KATP channels stabilize intracellular Ca²⁺ during hypoxia in retinal horizontal cells of goldfish (Carassius auratus) (Country and Jonz 2021). Medicinal plant products can interact with KATP (Rajabian et al. 2022). Remodeling of excitation-contraction coupling in transgenic mice expressing ATP-insensitive sarcolemmal KATP channels has been observed (Flagg et al. 2004). Thus, a compensatory increase in I(Ca) counteracts a mild activation of ATP-insensitive K(ATP) channels. Pharmacological inhibitors and ATP enrich a channel conformation in which the Kir6.2 cytoplasmic domain is closely associated with the transmembrane domain, while depleting one where the Kir6.2 cytoplasmic domain is extended away into the cytoplasm. This conformational change remodels a network of intra- and inter-subunit interactions as well as the ATP and PIP2 binding pockets. The structures resolved key contacts between the distal N-terminus of Kir6.2 and SUR1's ABC module involving residues implicated in channel function and showed a SUR1 residue, K134, participates in PIP2 binding. Molecular dynamics simulations revealed two Kir6.2 residues, K39 and R54, that mediate both ATP and PIP2 binding, suggesting a mechanism for competitive gating by ATP and PIP2 (Sung et al. 2022). The natural product, 7-hydroxycoumarin (7-HC), exhibits pharmacological properties linked to antihypertensive mechanisms of action. This relaxant effect induced by 7-HC relies on K⁺-channels (K_ATP, BK_Ca, and, to a lesser extent, K_v) activation and also on Ca²⁺ influx from sarcolemma and sarcoplasmic reticulum mobilization (inositol 1,4,5-triphosphate (IP3) and ryanodine receptors) (Jesus et al. 2022). Lymphatic contractile dysfunction in mouse models of Cantú Syndrome is oberved with K_ATP channel gain-of-function mutations (Davis et al. 2023). The structure of an open K (ATP) channel has revealed tandem PIP binding sites mediating the Kir6.2 and SUR1 regulatory interface (Driggers et al. 2023).

Insulin secretion is regulated by ATP-sensitive potassium (K_ATP) channels in pancreatic β-cells. Peroxisome proliferator-activated receptors (PPAR)α ligands are used to treat dyslipidemia. A PPARα ligand, fenofibrate, and PPARγ ligands troglitazone and 15-deoxy-∆^12,14-prostaglandin J2 close K_ATP channels and induce insulin secretion. The PPARα ligand, pemafibrate, is used to treat dyslipidemia and improves glucose intolerance in mice treated with a high fat diet and a novel selective PPARα modulator, it may affect K_ATP channels or insulin secretion. The effect of fenofibrate and pemafibrate (both at 100 µM) on insulin secretion was measured. Addition of fenofibrate for 10 min increased insulin secretion in low glucose conditions. The K_ATP channel activity was measured. Although fenofibrate (100 µM) reduced the K_ATP channel current, it had no effect on insulin mRNA expression (Kitamura et al. 2023).

Kir6.2 or KATP of Homo sapiens

Inward rectifier potassium channel 4, KCNJ4, IRK3 or 4, of 445 aas. Its voltage dependence is regulated by the concentration of extracellular potassium; as external potassium is raised, the voltage range of the channel opening shifts to more positive voltages. The inward rectification is mainly due to the blockage of outward current by internal magnesium, and it can be blocked by extracellular barium and cesium. It may play a role in the control of polyamine-mediated channel gating and in the blocking by intracellular magnesium. Overexpression of KCNJ4 correlates with cancer progression and nfavorable prognosis in lung adenocarcinoma (Wu and Yu 2019).

KMCJ4 of Homo sapiens

G protein-activated inward rectifier potassium channel 3, GIRK3 or KCNJ9 of 393 aas and 2 TMSs. It is expressed in sensory neurons and spinal cord and has uses both anterograde and retrograde axonal transport (Lyu et al. 2020).

GIRK3 of Homo sapiens

G-protein enhanced inward rectifier K channel 2, IRK1, IRK2, KCNJ2, KCNJ5, Kir2.1 (Andersen-Tawil Syndrome (ATS-1) protein; the V302M mutation causing the syndrome, alters the G-loop cytoplasmic K conduction pathway) (Bendahhou et al., 2003; Ma et al., 2007). (Blocked by chloroquine which binds in the cytoplasmic pore domain (Rodriguez-Menchaca et al., 2008)). Forms heteromultimers with Kir3.1 and Kir3.4 (Ishihara et al., 2009). A C-terminal domain is critical for the sensitivity of Kir2.1 to cholesterol (Epshtein et al., 2009). Flecainide increases Kir2.1 currents by interacting with cysteine 311, decreasing the polyamine-induced rectification (Caballero et al., 2010).  The inhibitory cholesterol binding site has been identified (Fürst et al. 2014).  Polyamines and Mg²⁺ block ion flux synergistically (Huang and Kuo 2016). Long polyamines serve a dual role as both blockers and coactivators (with PIP2) of Kir2.1 channels (Xie et al. 2005).  Cytoplasmic domain structures of Kir2.1 and Kir3.1 show sites for modulating gating and rectification (Pegan et al. 2005). Loss-of-function mutations are a rare cause of long QT syndrome (Fodstad et al. 2004). Fibroblast growth factor 21 ameliorates NaV1.5 and Kir2.1 channel dysregulation in human AC16 cardiomyocytes (Li et al. 2021). The trafficking of Kir2.1 and its role in development have been reviewed (Hager et al. 2021). Cholesterol-induced suppression of Kir2 channels is mediated by decoupling at the inter-subunit interfaces (Barbera et al. 2022). CryoEM studies have revealed a well-connected network of interactions between the PIP2-binding site and the G-loop through residues R312 and H221.Moreover, the intrinsic tendency of the CTD to tether to the TMD and a movement of the secondary anionic binding site to the membrane even without PIP2 was revealed (Fernandes et al. 2022). The results revealed structural features unique to human Kir2.1. Individual protonation events change the electrostatic microenvironment of the pore, resulting in distinct, uncoordinated, and relatively long-lasting conductance states, which depend on levels of ion pooling in the pore and the maintenance of pore wetting (Maksaev et al. 2023). Subunit gating results from individual protonation events in Kir2 channels (Maksaev et al. 2023).

Animals

IRK2 of Homo sapiens (P63252)

G-protein activated IRK5 (Kir3.4, KCNJ5, GIRK4) channel. The p75 neurotrophin receptor mediates cell death by activating GIRK channels through phosphatidylinositol 4,5-bisphosphate (Coulson et al., 2008). Cholesterol up-regulates neuronal GIRK channel activity (Bukiya et al. 2017). It forms an oligomeric channel with Kir3.1, transporting K⁺, Rb⁺ and spermine.  The selectivity filter may be responsible for inward rectification and agonist activation as well as permeation and block by Cs⁺ (Makary et al. 2006). Ivermictin activates GIRK channels in a PIP₂-dependent manner (Chen et al. 2017). GIRK channels function as either homomeric (i.e., GIRK2 and GIRK4) or heteromeric (e.g., GIRK1/2, GIRK1/4, and GIRK2/3) tetramers (Cui et al. 2022). Activators, such as ML297, ivermectin, and GAT1508, activate heteromeric GIRK1/2 channels better than GIRK1/4 channels with varying degrees of selectivity but not homomeric GIRK2 and GIRK4 channels. VU0529331 was the first homomeric GIRK channel activator, but it shows weak selectivity for GIRK2 over GIRK4 homomeric channels. The first highly selective small-molecule activator targeting GIRK4 homomeric channels is 3hi2one-G4 (3-[2-(3,4-dimethoxyphenyl)-2-oxoethyl]-3-hydroxy-1-(1-naphthylmethyl)-1,3-dihydro-2H-indol-2-one). 3hi2one-G4 does not activate GIRK2, GIRK1/2, or GIRK1/4 channels. The binding site of 3hi2one-G4 is formed by TMSs 1 and 2, and slide helix regions of the GIRK4 channel, near the phosphatidylinositol-4,5-bisphosphate binding site; it causes channel activation by strengthening channel-phosphatidylinositol-4,5-bisphosphate interactions. Slide helix residue L77 in GIRK4, corresponding to residue I82 in GIRK2 is a major determinant of isoform-specific selectivity (Cui et al. 2022). Cardiovascular and metabolic characteristics of KCNJ5 somatic mutations are important for primary aldosteronism (Chang et al. 2023).

Animals

IRK5 or GIRK4 of Homo sapiens (P48544)

Hepatocyte basolateral inwardly rectifying K⁺ channel, Kir4.2, involved in bile secretion (Hill et al., 2002).  This protein is 96% identical to the human KCNJ14 or KCNJ15 of 375 aas (Q99712).

Animals

Kir4.2 of Rattus norvegicus (Q91ZF1)

Cranial nerve inward rectifying K⁺ channel, Kir2.4 (IRK4) (Töpert et al., 1998).  The human ortholog, of 436 aas, is 94% identical and is called KCMJ14 or IRK4.

Animals

Kir2.4 of Rattus norvegicus (O70596)

Kidney/pancreas/muscle ATP-senstive, ER/Golgi K⁺ channel, KATP or ROMK (Kir6.2) (Boim et al., 1995) (three alternatively spliced isoforms are called ROMK1-3). Involved in congenital hyperinsulinism (Lin et al., 2008). Regulated by Ankyrin-B (Li et al., 2010). ATP activates ATP-sensitive potassium channels composed of mutant sulfonylurea receptor 1 (SUR1) and Kir6.2 with diminished PIP2 sensitivity (Pratt and Shyng, 2011). This channel protects the myocardium from hypertrophy induced by pressure-overloading (Alvin et al., 2011). Domain organization studies show which domains in Sur and Kir6.2 interact (Wang et al. 2012).  KATP channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion in pancreatic beta cells, and mutations in the former protein can compensate for mutations in the latter (Zhou et al. 2013).  Mutations cause inactivation of channel function by disrupting PIP2-dependent gating (Bushman et al. 2013). Thus, these proteins comprise part of the glucose sensing mechanism (Rufino et al. 2013).  A single point mutation can confer voltage sensitivity (Kurata et al. 2010).  Its involvement in type II diabetes has been reviewed by Bonfanti et al. 2015. KATP channels (Kir6.2/SUR1) in the brain and endocrine pancreas  couple metabolic status to the membrane potential. In beta-cells, increases in cytosolic [ATP/ADP] inhibit KATP channel activity, leading to membrane depolarization and exocytosis of insulin granules. Mutations in ABCC8 (SUR1) or KCNJ11 (Kir6.2) can result in gain or loss of channel activity and cause neonatal diabetes (ND) or congenital hyperinsulinism (CHI), respectively.  Nucleotide binding without hydrolysis switches SUR1 to stimulatory conformations.  Increased affinity for ATP gives rise to ND while decreased affinty gives rise to CHI (Ortiz and Bryan 2015).  Kir6.2 can associate with either SUR1 (TC# 3.A.1.208.4) or SUR2A (TC# 3.A.1.208.23) to form heteroctamers, leading to different locations and consequences (Principalli et al. 2015). IATP channels and Ca²⁺ influx play roles in purinergic vasotoxicity and cell death (Shibata et al. 2018).
       A cryo-EM structure of a hamster SUR1/rat Kir6.2 channel bound to a high-affinity sulfonylurea drug, glibenclamide, and ATP at 3.63 A resolution revealed details of the ATP and glibenclamide binding sites (Martin et al. 2017). The structure showed that glibenclamide lodges in the transmembrane bundle of the SUR1-ABC core connected to the first nucleotide binding domain near the inner leaflet of the lipid bilayer. Mutation of residues predicted to interact with glibenclamide led to reduced sensitivity to this drug (Martin et al. 2017).

Animals

ROMK of Rattus norvegicus (P70673)

The inward rectifier potassium channel 13, Kir 7.1, Kir1.4, or KCNJ13, of 360 aas and 2 TMSs. A splice variant expressed in mouse tissues shares organisational and functional properties with human leber amaurosis-causing mutations of this channel (Vera et al. 2019). In fact, mutations in KCNJ13 are associated with two retinal disorders; Leber congenital amaurosis (LCA) and snowflake vitreoretinal degeneration (SVD) (Toms et al. 2019). Pinacidil is a channel opener (Sun et al. 2019). It may play a role in the control of polyamine-mediated channel gating and in the blocking by intracellular magnesium. A Kir7.1 disease mutant T153I within the inner pore affects K⁺ conduction (Beverley et al. 2022). Kir7.1 exhibits small unitary conductance and low dependence on external potassium. Kir7.1 channels also show a phosphatidylinositol 4,5-bisphosphate (PIP(2)) dependence for opening (Hernandez et al. 2023). Retinopathy- associated Kir7.1 mutations map at the binding site for PIP(2), resulting in channel gating defects, leading to channelopathies such as snowflake vitreoretinal degeneration and Leber congenital amaurosis in blind patients. These properties may be due to its unusual structure (Hernandez et al. 2023).

Animals

Kir 7.1 or KCNJ13 of Homo sapiens (O60928)

The inward-rectifier K⁺ channel, Kir2.2, KCNJ12, KCNJN1, KCNJ18, IRK2, of 433 aas and 2 TMSs. The 3-d structure at 3.1 Å resolution is available (Tao et al., 2009). (It is 70% identical to Kir2.1 (TC # 1.A.2.1.2)). The structural basis of PIP2 activation of Kir2.2 has been presented (Hansen et al., 2011). Inward rectifier potassium channels (Kir channels) exist in a variety of cells and are involved in maintaining resting membrane potential and signal transduction in most cells, as well as connecting metabolism and membrane excitability of body cells. It is closely related to normal physiological functions of body and the occurrence and development of some diseases. The functional expression of Kir channels in vascular endothelial cells and smooth muscle cells and their changes in disease states were reviewed, especially the recent research progress of Kir channels in stem cells was introduced, in order to have a deeper understanding of Kir channels in vascular tissues and provide new ideas and directions for the treatment of related ion channel diseases (Li and Yang 2023).

Animals

Kir2.2 of Homo sapiens (Q14500)

Prokaryotic K⁺-selective Kir channel KirBac1.1 (selectivity: K⁺ = Rb⁺ = Cs⁺ >> Li⁺, Na⁺ or NMGM) (Enkvetchakul et al., 2004), inward rectifying (Cheng et al., 2009). Closure of the Kir1.1 pH gate results from steric occlusion of the permeation path by the convergence of four leucines (or phenylalanines) at the cytoplasmic apex of the inner transmembrane helices. In the open state, K⁺ crosses the pH gate together with its hydration shell (Sackin et al. 2005). An inhibitory cholesterol binding site has been identified (Fürst et al. 2014). Conformational changes associated with an open activation gate have been identified, and these suggest an allosteric pathway that ties the selectivity filter to the activation gate through interactions between both transmembrane helices, the turret, the selectivity filter loop, and the pore helix. Specific residues involved in this conformational exchange that are highly conserved among human Kir channels have also been identified (Amani et al. 2020). Anionic lipids, especially cardiolipin, initiate a concerted rotation of the cytoplasmic domain subunits. This action buries ionic side chains away from the bulk water, while allowing water greater access to the K⁺ conduction pathway (Borcik et al. 2020). Kv1.5 channels are regulated by PKC-mediated endocytic degradation (Du et al. 2021). Pore-forming TMSs control ion selectivity and the selectivity filter conformation in the KirBac1.1 channel (Matamoros and Nichols 2021). Key functional residues involved in gating and lipid allostery of K⁺ Kir channels have been identified (Yekefallah et al. 2022).

Proteobacteria

KirBac1.1 OF Burkholderia pseudomallei (IP7BA; gi33357898)

The KirBac3.1 K⁺ channel (a dimer of dimers with gating visualized by atomic force microscopy (Jaroslawski et al., 2007) (regulated by binding lipids, G-proteins, nucleotides, and ions (H⁺, Ca²⁺, and Mg²⁺)). The 3-D structure is available (1XL6_A).  The inhibitory cholesterol binding site has been identified (Fürst et al. 2014). The constricted opening in this, and presumably other, Kir channels does not impede potassium conduction (Black et al. 2020). The structural and dynamic properties of a KirBac3.1 mutant revealed the function of a highly conserved tryptophan in the transmembrane domain (Fagnen et al. 2021).

Proteobacteria

KirBac3.1 of Magnetospirillum magnetotacticum  (D9N164)

ATP-sensitive inward rectifying Kir K channel (Choi et al. 2010).

Bacteria

Kir K⁺ channel of Chromobacterium violaceum

Putative K⁺ channel

Cyanobacteria

K⁺ channel of Synechocystis PCC 6803

Inward rectifier potassium channel 

Proteobacteria

K⁺ channel of Burkholderia xenovorans

Uncharacterized algal protein of 886 aas, largely hydrophilic with a VIC-type 2 TMS channel domain near its C-terminus.

UP of Klebsormidium nitens