Kir2.1

Description: potassium inwardly-rectifying channel, subfamily J, member 2
Gene: Kcnj2
Alias: Kir2.1, IRK1, LQT7, SQT3, HHIRK1, HHBIRK1, KCNJ2

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Introduction

KCNJ2 (also known as IRK1; LQT7; SQT3; ATFB9; HHIRK1; KIR2.1; HHBIRK1) encodes member 2 of subfamily J of potassium inwardly-rectifying channels, which is called Kir2.1. This channel has a greater tendency to allow potassium to flow into a cell rather than out of a cell, probably participates in establishing action potential waveform and excitability of neuronal and muscle tissues. Mutations in this gene have been associated with Andersen syndrome, which is characterized by periodic paralysis, cardiac arrhythmias, and dysmorphic features. http://www.ncbi.nlm.nih.gov/gene/3759

Kir2.1, along with Kir2.2 and Kir2.3, is thought to underlie the background inward rectifier K+ current I K,ACh (Liu et al. 2001; [909] Zaritsky et al. 2001 [910]; Zobel et al. 2003 [911]). Kir2.1 channel underlies the cardiac current I K,1. Makary [182]

Experimental data

Rat Kir2.1 gene in CHO host cells
	Click for details 25 °C show 48 cells	Click for details 35 °C show 11 cells

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Gene

Species	NCBI gene ID	Chromosome	Position
Human	3759	17	10512
Mouse	16518	11	10661
Rat	29712	10	10552

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Transcript

Species	NCBI accession	Length (nt)
Human	NM_000891.3	5391
Mouse	NM_008425.4	5468
Rat	NM_017296.1	1284

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Protein Isoforms

Species	Uniprot ID	Length (aa)
Human	P63252	427
Mouse	P35561	428
Rat	Q64273	427

Isoforms

Transcript

Length (nt)

Protein

Length (aa)

Variant

Isoform

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Post-Translational Modifications

PTM

Position

Type

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Structure

Cytoplasmic pore region of Kir2.1

Analysis of the crystal structure of the cytoplasmic domain of Kir2.1 has recently identified an intrinsically flexible loop around the membrane face of the cytoplasmic pore. The loop constricts the cytoplasmic pore to ∼3 Å and forms a girdle around the central pore axis. The girdle, which consists of a loop between βH and βI strands and is called the “G-loop,” forms the narrowest portion of the ion conduction pathway in the cytoplasmic region. The narrowest part of the G-loop is made up by A306 and to a lesser extent by E299, G300, M301, and M307. A306 is localized at the apex of the G-loop. The substitution of Glu, Cys, or Thr for A306 abolished Kir2.1 current. Because the side chain of these residues is larger than that of Ala, these substitutions would result in the physical occlusion of the G-loop without changing its backbone conformation. When another constituent of the G-loop M301 was mutated to Ala, an enhancement of inward rectification was observed [1861]

In Kir2.1, a number of residues within the pore lining second transmembrane domain and proximal C terminus have been shown to be important for inward rectification (Lu & MacKinnon, 1994 [912]; Stanfield et al. 1994 [913]; Yang et al. 1995 [919]; Kubo & Murata, 2001 [914]; Fujiwara & Kubo, 2002 [915]). Recent evidence suggests that these may not be the site of block, but instead shuttle the polyamines to their eventual binding site deeper within the pore (Kubo & Murata, 2001 [914]; Guo et al. 2003 [916]; Xie et al. 2003 [917]; Chang et al. 2003 [918]).

Kir2.1 predicted AlphaFold size

Species	Area (Å²)	Reference
Human	4925.70	source
Mouse	4966.83	source
Rat	5007.31	source

Methodology for AlphaFold size prediction and disclaimer are available here

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Kinetics

Single Channel Conductance of Kir2.1

Representative current traces from -40 mV and -80 mV from HEK293T cells co-transfected with Kir2.1+GFP. In the control condition (absence of caveolin) the conductance of Kir was 21 pS [1862] Typical single channel behaviour of WT Kir2.1 channels in recordings from an inside-out patch at various holding potentials with the unitary conductance (g) averaging 29.1 ± 1.6 pS has been recorded [1856]

Rat Kir2.1 Skeletal Muscle Current Expressed in CHO

There was little outward current in response to small depolarizing pulses, but hyperpolarization resulted in large inward currents. Steps to voltages more positive than about 0 mV resulted in the additional activation of delayed rectifier currents. Qualitatively, control inward rectifier currents tended to saturate negative to about −150 mV [1865]

Mouse Kir2.1 Kinetics in CHO cells

The channels were heterologously expressed in CHO cells, a null cell line that almost completely lacks inward K1 current. A two-pulse voltage pro- tocol was used: 500-ms voltage steps from ÿ180 to 160 mV with increments of 10 mV and immediately followed by 10-ms test pulses to ÿ160 mV [1866]

Compared to Kir3

Kir3.1/Kir3.4 exhibits weaker inward rectification than Kir2.1. Makary [182]

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Biophysics

Markov Model of Kir2.1 interaction with PIP2

The values of the rate constants (α, β, δ, γ) were obtained by fitting to the mean open and closed times for the fully open state. To account for sublevels, we linked the fully open state (O1) to series of partially open states (Graphic), leading to a closed state (Cn) representing the unavailable mode. We assumed that the rate constants (α, β, δ) were identical for the sub level transitions [1856]

Model Kir21 (ID=44)

Animal	Chinese Hamster
CellType	CHO
Age	0 Days
Temperature	28.0°C
Reversal	-70.6 mV
Ion	K +
Ligand ion
Reference	[182] Samy M Y Makary et. al; J. Physiol. (Lond.) 2005 Nov 1
mpower	1.0
m Inf	1 /(1+exp((v-(-96.48))/23.26))
m Tau	3.7 +( -3.37 / (1 + exp((v - -32.9)/27.93)))
hpower	2.0
h Inf	1 /(1+exp((v-(-168.28))/-44.13))
h Tau	0.85 + (306.3 / (1 + exp((v - -118.29)/-27.23)))

MOD - xml - channelML

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Expression and Distribution

Kir2.1 is an inwardly rectifying K+ channel, being expressed in the heart. Makary [182] More Kir2.1 in atrial myocytes compared with ventricular cells. Panama [183]

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Function

AP Repolarization

I K1 - a current for which Kir2.1 mediates - regulates the late phase of action potential (AP) repolarization and stabilizes the resting membrane potential. In most species, the inward current density of atrial I K1 is significantly smaller than that of ventricles (Dhamoon [920], Giles [921], Melnik [922]).

QT Syndrome/Autism/Epilepsy

Genetically induced dysfunctions of Kir2.1 channels: implications for short QT3 syndrome and autism-epilepsy phenotype [1864]

ERAD

altered Kir2.1 levels lead to human disease and Kir2.1 restores growth on low-potassium medium in yeast mutated for endogenous potassium channels. Using this system, first we find that Kir2.1 is targeted for endoplasmic reticulum–associated degradation (ERAD).

Cardiac Channel

There is an increasing body of evidence that heteromeric assembly of Kir2.1, Kir2.2 and Kir2.3 potassium channels is the molecular basis of cardiac IK1 current [1869]

Golgi Export

Here, we show that the potassium channel Kir2.1, mutations in which are associated with Andersen-Tawil syndrome, is selected as cargo into Golgi export carriers in an unusual signal-dependent manner. Unlike conventional trafficking signals, which are typically comprised of short linear peptide sequences, Golgi exit of Kir2.1 is dictated by residues that are embedded within the confluence of two separate domains. This signal patch forms a recognition site for interaction with the AP1 adaptor complex, thereby marking Kir2.1 for incorporation into clathrin-coated vesicles at the trans-Golgi

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Interaction

Caveolin-1 (cholesterol) suppresses Kir2.1

We whether Cav-1 regulates the function of Kir2.1 channels that play major roles in the regulation of membrane potential of numerous mammalian cells. Our earlier studies demonstrated that Kir2.1 channels are cholesterol sensitive. In this study, we show that Kir2.1 channels co-immunoprecipitate with Cav-1 and that co-expression of Kir2.1 channels with Cav-1 in HEK293 cells results in suppression of Kir2 current indicating that Cav-1 is a negative regulator of Kir2 function [1862]

Extracellular Spermine block

Kir3.1/Kir3.4 is more sensitive to extracellular spermine block than Kir2.1, and that intracellular and extracellular polyamines can permeate Kir3.1/Kir3.4, but not Kir2.1, to a limited extent. Makary [182]

Kir2.3

Inclusion of only one Kir2.3 subunit to a Kir2.1 channel led to an approximate threefold slowing of activation kinetics, with greater slowing on subsequent additions of Kir2.3 subunits. Panama [183]

P639Amiodarone and Dronedarone

P639Amiodarone and dronedarone inhibit inwardly rectifying Kir2.1 channels, but not Kir2.2 and Kir2.3 channels [1863]

chloroethylclonidine

Chloroethylclonidine (CEC). The degree of current inhibition by CEC was found to vary with the membrane potential (approximately 70% block at -50 mV c.f. approximately 10% block at -190 mV). The kinetics of this voltage dependence were further investigated using recombinant inward rectifier K+ channels (Kir2.1) expressed in the MEL cell line [1865]

Mutation increases binding affinity

We find that a single mutation of tertiapin-Q increases the binding affinity for Kir2.1 by 5 orders of magnitude (K(d) = 0.7 nM). This potent blocker of Kir2.1 may serve as a structural template from which potent compounds for the treatment of various diseases mediated by this channel subfamily, such as cardiac arrhythmia, can be developed.

Kir3.1/Kir3.4

We examined if the subunits belonging to different subfamilies Kir2 and Kir3 can co-assemble to form heteromultimers in heterologous expression systems. We observed co-immunoprecipitation of Kir2.1 and Kir3.1 as well as Kir2.1 and Kir3.4 in HEK293T cells. Furthermore, analyses of subcellular localization using confocal microscopy revealed that co-expression of Kir2.1 promoted the cell surface localization of Kir3.1 and Kir3.4 in HEK293T cells. In electrophysiological experiments, co-expression of Kir2.1 with Kir3.1 and/or Kir3.4 in Xenopus oocytes and HEK293T cells did not yield currents with distinguishable features. However, co-expression of a dominant-negative Kir2.1 with the wild-type Kir3.1/3.4 decreased the Kir3.1/3.4 current amplitude in Xenopus oocytes. The results indicate that Kir2.1 is capable of forming heteromultimeric channels with Kir3.1 and with Kir3.4 [966]