Cryo-EM Reveals How Kir4.1/5.1 Channels Block Potassium Flow
First atomic-resolution structures of the Kir4.1/5.1 heteromeric channel expose a novel inner-ring pore-blocking mechanism relevant to brain and kidney disease.
Summary
Scientists used cryo-electron microscopy to capture the first high-resolution structures of the Kir4.1/5.1 potassium channel—critical for brain, kidney, and inner-ear function—in its resting state and bound to three different blockers: spermine (a natural polyamine), VU0134992 (a known channel blocker), and EHop-016 (a newly identified inhibitor). The structures, resolved at 3.1–3.5 Å, reveal that two spermine molecules, two VU0134992 molecules, or one EHop-016 molecule bind horizontally inside the channel's central cavity, forming an 'inner ring' that physically plugs the pore. These findings clarify the long-debated structural mechanism of inward rectification and open new avenues for drug development targeting conditions like EAST/SeSAME syndrome, hypertension, and depression.
Detailed Summary
Inwardly rectifying potassium (Kir) channels govern membrane potential and potassium homeostasis across heart, brain, kidney, and inner ear. Among them, the heteromeric Kir4.1/5.1 channel—formed by KCNJ10 and KCNJ16 gene products—is especially important: mutations in KCNJ10 cause EAST/SeSAME syndrome (epilepsy, ataxia, sensorineural hearing loss, renal tubulopathy), and pharmacological inhibition of Kir4.1/5.1 produces diuresis and antidepressant effects in animal models. Despite decades of electrophysiology and mutagenesis work, the atomic-level mechanism by which polyamines like spermine block these channels to produce inward rectification had remained unresolved.
The research team expressed truncated but functionally validated Kir4.1 (residues 25–368) and Kir5.1 (residues 20–355) constructs, confirmed their electrophysiological properties matched full-length channels, and purified them in glyco-diosgenin (GDN) detergent micelles without PIP2. Using single-particle cryo-EM, they determined four structures: the apo channel and complexes with spermine, VU0134992, and EHop-016, all at 3.1–3.5 Å resolution. The channel assembles as a 2:2 heterotetramer in an alternating (opposite) arrangement—not the previously assumed 4-5-4-5 order—measuring approximately 115 × 65 × 70 Å with a conserved transmembrane domain and cytoplasmic beta-strand-rich domain.
The most striking finding is an 'inner-ring blockage' mechanism. Rather than threading linearly along the pore axis as previously hypothesized, spermine (two molecules), VU0134992 (two molecules), and EHop-016 (one molecule) all adopt a membrane-parallel orientation at the upper site of the transmembrane central cavity. They form a ring-like barrier that occludes ion flow. The key acidic residue E158 in Kir4.1 (the 'rectification controller') coordinates these ligands, explaining why mutation of this residue abolishes strong rectification. The structural data are corroborated by electrophysiology and molecular dynamics simulations, which confirm that blocker occupancy at this site prevents outward K+ current.
EHop-016, previously known only as a Rac GTPase inhibitor with anticancer properties, was newly identified here as a potent Kir channel inhibitor with IC50 values of 0.80 ± 0.05 μM for Kir4.1 and 3.20 ± 0.18 μM for Kir4.1/5.1. Its single-molecule occupancy at the inner-ring site contrasts with the two-molecule binding of spermine and VU0134992, illustrating how molecular geometry determines stoichiometry of blockade. The two distinct cytoplasmic subunit interfaces (canonical Kir4/5 and atypical Kir5/4) also differ structurally and likely underlie the heteromer's unique pH sensitivity and stronger rectification compared to homomeric Kir4.1.
These structures establish a unified framework for understanding Kir channel pharmacology and provide a template for rational design of drugs targeting Kir4.1/5.1 in neurological, cardiovascular, and renal diseases.
Key Findings
- Kir4.1/5.1 assembles as an alternating 2:2 heterotetramer, resolved by cryo-EM at 3.1–3.5 Å in four states.
- Spermine, VU0134992, and EHop-016 all block the pore via a membrane-parallel 'inner-ring' mechanism in the central cavity.
- Two spermine or two VU0134992 molecules, but only one EHop-016, occupy the inner-ring blocking site.
- E158 in Kir4.1 (the rectification controller) directly coordinates blocker binding and is essential for strong inward rectification.
- EHop-016, a known anticancer compound, is identified as a new Kir4.1/5.1 inhibitor with sub-micromolar potency on Kir4.1.
Methodology
Single-particle cryo-EM was used to determine four structures of truncated Kir4.1/5.1 in GDN micelles (apo, spermine-, VU0134992-, and EHop-016-bound) at 3.1–3.5 Å resolution without PIP2. Findings were validated by whole-cell patch-clamp electrophysiology in CHO cells and molecular dynamics simulations.
Study Limitations
Structures were determined in detergent micelles without PIP2, meaning the channel is likely in a non-conducting closed state rather than the physiologically relevant open state, which may affect precise blocker positioning. Truncated rather than full-length constructs were used, though functional equivalence was confirmed electrophysiologically. In vivo validation of EHop-016 as a Kir4.1/5.1 inhibitor has not yet been demonstrated.
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