Affinage

KCNU1

Potassium channel subfamily U member 1 · UniProt A8MYU2

Length
1149 aa
Mass
129.5 kDa
Annotated
2026-04-28
27 papers in source corpus 22 papers cited in narrative 22 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

KCNU1 (SLO3) is a sperm-specific, voltage- and pH/Ca2+-gated potassium channel that establishes the resting membrane potential in sperm and drives the capacitation-associated hyperpolarization required for hyperactivated motility and the acrosome reaction. In mouse sperm, SLO3 is activated by intracellular alkalinization and membrane depolarization, whereas human SLO3 is more strongly activated by intracellular Ca2+; in both species it constitutes the sole K+ conductance (KSper) responsible for capacitation-induced hyperpolarization, operating in dynamic negative-feedback interplay with CatSper to regulate Ca2+ influx and membrane potential (PMID:9452476, PMID:21427226, PMID:24670955, PMID:36649421). The auxiliary subunit LRRC52 co-assembles with SLO3 and shifts its gating to physiologically relevant voltages and pH, while PIP2, intracellular Zn2+, and cSrc kinase further tune channel activity (PMID:22084117, PMID:25675513, PMID:20392696, PMID:26060254). Bi-allelic loss-of-function variants in human KCNU1 cause male infertility characterized by impaired acrosome reactions, abnormal capacitation-associated membrane potential changes, and asthenoteratozoospermia (PMID:35551387, PMID:34980136).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 1998 High

    Identification of SLO3 as a novel Slo-family K+ channel expressed specifically in spermatocytes and gated by intracellular pH and voltage established a candidate molecular identity for sperm K+ conductance.

    Evidence Cloning, heterologous expression in Xenopus oocytes with electrophysiology, tissue expression profiling by RT-PCR and in situ hybridization

    PMID:9452476

    Open questions at the time
    • Native sperm current identity not yet linked to SLO3
    • Auxiliary subunit composition unknown
    • Human channel properties not characterized
  2. 2006 High

    Quantitative allosteric modeling of SLO3 gating revealed weak intrinsic voltage dependence and weak voltage-sensor/gate coupling compared to SLO1, explaining why pH is the dominant gating stimulus and setting the biophysical framework for understanding how capacitation signals open the channel.

    Evidence Two-electrode voltage clamp in Xenopus oocytes with Horrigan-Aldrich allosteric model fitting

    PMID:16940555

    Open questions at the time
    • Structural basis of pH sensing not identified
    • Role of cytoplasmic gating ring not resolved
  3. 2009 High

    Structure-function analysis identified the RCK1 domain loop as a critical determinant of SLO3's voltage range and kinetics, providing the first molecular mapping of gating elements, while KCNMB4 (β4) was identified as a potential auxiliary subunit enhancing surface expression.

    Evidence Species-swap mutagenesis of bovine vs. mouse SLO3 (electrophysiology); co-expression of β4 with SLO3 in oocytes with surface biotinylation and RT-PCR in testis

    PMID:19473978 PMID:19578543

    Open questions at the time
    • Whether β4 is the physiological partner in vivo unresolved
    • LRRC52 not yet discovered
    • pH sensor residues not mapped
  4. 2010 High

    Genetic knockout demonstrated that SLO3 is essential for male fertility, being the principal K+ channel driving capacitation-induced hyperpolarization and the acrosome reaction, while PIP2 was established as a direct lipid activator and its pharmacological profile was defined.

    Evidence Slo3-null mouse with membrane potential recordings and acrosome reaction assays with valinomycin rescue; inside-out patch with PIP2 and mutagenesis; pharmacological profiling with quinidine, 4-AP, TEA, charybdotoxin

    PMID:19934650 PMID:20138882 PMID:20392696

    Open questions at the time
    • Residual outward current in KO sperm unexplained
    • Whether SLO3 is sole K+ channel in human sperm unknown
    • PIP2 regulation in native sperm not confirmed
  5. 2011 High

    Electrophysiology in Slo3-null sperm proved SLO3 is the sole pH-dependent K+ conductance (KSper) and revealed that alkalization paradoxically depolarizes Slo3-null sperm via unopposed CatSper, establishing the functional interplay between these two channels; LRRC52 was identified as the physiological auxiliary subunit that shifts SLO3 gating to match native KSper.

    Evidence Patch-clamp in Slo3-KO sperm with clofilium pharmacology; co-IP, heterologous co-expression electrophysiology, and LRRC52 dependence on SLO3 in KO tissue

    PMID:21427226 PMID:22084117

    Open questions at the time
    • LRRC52-KO phenotype not yet tested
    • Crystal structure of SLO3/LRRC52 complex lacking
    • Human SLO3 gating mechanism unclear
  6. 2012 High

    Crystal structure of the human SLO3 gating ring revealed a conformation distinct from SLO1 (potentially open state), providing the first structural framework for understanding pH-dependent gating.

    Evidence X-ray crystallography of cytoplasmic gating ring domain with heterologous electrophysiology validation

    PMID:23129643

    Open questions at the time
    • Full-length channel structure unavailable
    • Mechanism of pH sensing at atomic level unresolved
    • LRRC52 binding site unknown
  7. 2013 High

    Double knockout of Slo3 and CatSper1 abolished all voltage-activated outward current in mouse sperm, proving these two channels constitute the complete voltage/pH-activated conductance complement controlling sperm membrane potential.

    Evidence Slo3/CatSper1 double-KO mouse with patch-clamp electrophysiology

    PMID:23980198

    Open questions at the time
    • Whether this minimalist model applies to human sperm untested
    • Signaling cross-talk mechanisms between SLO3 and CatSper not defined
  8. 2014 High

    Translation to human sperm revealed a key species difference: human SLO3 is activated more strongly by Ca2+ than pH, and pharmacological inhibition confirmed SLO3 as the carrier of human IKSper and a negative-feedback regulator limiting CatSper-mediated Ca2+ influx triggered by progesterone.

    Evidence Immunolocalization in human sperm flagellum, comparative electrophysiology of human vs. mouse SLO3, selective pharmacological inhibition in native human sperm

    PMID:24670955

    Open questions at the time
    • Molecular basis of Ca2+ vs. pH selectivity unknown
    • Human SLO3 full-length structure unavailable
    • Whether species differences affect contraceptive targeting unclear
  9. 2015 High

    LRRC52 knockout confirmed its essential role in vivo—shifting native KSper gating to impractical voltages/pH and severely impairing fertility—while cSrc kinase was placed downstream of PKA and upstream of SLO3 in the capacitation signaling cascade, and the pore architecture was refined by mutagenesis defining the activation gate between F304 and the selectivity filter.

    Evidence LRRC52-KO mouse with sperm electrophysiology and IVF; cSrc pharmacological inhibition in sperm and heterologous SLO3; mutagenesis/docking for quinidine binding site

    PMID:25675513 PMID:26045093 PMID:26060254

    Open questions at the time
    • Direct phosphorylation sites on SLO3 not mapped
    • Structural basis of LRRC52 modulation unresolved
    • Short somatic isoform functions unknown
  10. 2022 High

    Human genetics established that bi-allelic KCNU1 loss-of-function variants cause male infertility with impaired acrosome reactions and abnormal capacitation, confirmed by knock-in mouse models and patient sperm analysis showing reduced LRRC52 co-expression.

    Evidence WES in infertile men, Sanger sequencing, RT-PCR for splicing, KI mouse model with IVF, electron microscopy, acrosome reaction and membrane potential assays in patient sperm

    PMID:34980136 PMID:35551387

    Open questions at the time
    • Prevalence of KCNU1 variants among infertile men unknown
    • Whether heterozygous carriers have subfertility not tested
    • Therapeutic rescue strategies not explored
  11. 2023 High

    Development of VU0546110 as the first selective SLO3 inhibitor demonstrated that SLO3 is the sole K+ channel responsible for hyperpolarization in human sperm, validating it as a non-hormonal male contraceptive target.

    Evidence High-throughput screen, patch-clamp validation in heterologous and native human sperm, membrane potential, motility, and acrosome reaction assays

    PMID:36649421

    Open questions at the time
    • In vivo contraceptive efficacy not tested
    • Binding site on SLO3 not determined
    • Off-target effects in vivo not characterized

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include the full-length cryo-EM structure of the SLO3/LRRC52 complex, the molecular basis for the Ca2+-versus-pH gating divergence between human and mouse SLO3, the physiological significance of somatic SLO3 isoforms, and whether SLO3 inhibitors can serve as effective non-hormonal male contraceptives in vivo.
  • No full-length SLO3/LRRC52 structure
  • Ca2+ vs pH sensing residues not mapped
  • Somatic isoform function completely uncharacterized
  • In vivo contraceptive proof-of-concept lacking

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005215 transporter activity 6
Localization
GO:0005886 plasma membrane 2 GO:0005929 cilium 1
Pathway
R-HSA-1474165 Reproduction 5 R-HSA-382551 Transport of small molecules 5
Partners
CATSPER1KCNMB4LRRC52
Complex memberships
SLO3/LRRC52 channel complex

Evidence

Reading pass · 22 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1998 SLO3 (KCNU1) was cloned and identified as a novel potassium channel primarily expressed in mammalian spermatocytes, regulated by both intracellular pH (alkalization activates) and membrane voltage, representing a distinct member of the Slo channel multigene family. Cloning, RT-PCR, Northern analysis, in situ hybridization, heterologous expression with electrophysiology The Journal of biological chemistry High 9452476
2006 Slo3 macroscopic conductance exhibits pH-dependent maximum open probability at positive potentials, weak intrinsic voltage dependence (z_L) and weak voltage-sensor coupling (D) compared to Slo1, and biexponential activation/deactivation kinetics that are only weakly voltage-dependent; a Horrigan-Aldrich allosteric model was applied to describe Slo3 gating. Heterologous expression in Xenopus oocytes, two-electrode voltage clamp, macroscopic current analysis, allosteric modeling The Journal of general physiology High 16940555
2009 The RCK1 domain loop linking the intermediate RCK1 subdomain to the C-terminal subdomain is a critical structural determinant of Slo3 voltage range of activation and gating kinetics, as shown by functional divergence between bovine and mouse SLO3 orthologs and species-swapping analysis. Comparative electrophysiology of bovine and mouse SLO3 in heterologous expression, domain-swap mutagenesis analysis The Journal of biological chemistry Medium 19473978
2009 The beta4 subunit (KCNMB4) of the KCNMB family selectively co-assembles with Slo3, producing an 8–10-fold enhancement of Slo3 surface expression and current in Xenopus oocytes; beta4 mRNA is expressed in spermatocytes at levels comparable to Slo3. Co-expression in Xenopus oocytes, electrophysiology, YFP-tagged surface expression, biotin labeling, fluorescence microscopy, quantitative RT-PCR, biochemical co-assembly assays PloS one High 19578543
2010 Slo3 knockout male mice are infertile; in wild-type sperm, Slo3 is the principal potassium channel responsible for capacitation-induced membrane hyperpolarization, and this hyperpolarization is required for the acrosome reaction (rescued by valinomycin in Slo3-null sperm). Slo3 gene knockout mouse, membrane potential recordings, acrosome reaction assay, pharmacological rescue with valinomycin FEBS letters High 20138882
2010 Phosphatidylinositol 4,5-bisphosphate (PIP2) activates Slo3 currents, and receptor-mediated PIP2 hydrolysis (via EGF receptor activation) inhibits Slo3 channel activity; positively charged residues in the channel are required for PIP2 interaction. Inside-out macropatches from Xenopus oocytes, whole-cell recording from sperm, EGF receptor co-expression, PIP2 depletion, charge-neutralizing mutagenesis The Journal of biological chemistry High 20392696
2010 Slo3 is resistant to charybdotoxin, iberiotoxin, and extracellular TEA; quinidine blocks Slo3 preferentially in the closed state (block relieved by depolarization) by binding in a hydrophobic pocket, while cytosolic 4-AP produces open-channel block of Slo3 with ~10–15-fold greater potency than Slo1. Heterologous expression in Xenopus oocytes, two-electrode voltage clamp with pharmacological agents, mutagenesis to identify blocking determinants Channels (Austin, Tex.) High 19934650
2011 Genetic deletion of Slo3 abolishes all pH-dependent K+ current (KSper) in mouse spermatozoa at physiological membrane potentials, demonstrating that KSper/Slo3 is the sole pH-dependent K+ conductance; alkalization depolarizes Slo3-null sperm (versus hyperpolarization in WT) due to unopposed CatSper activation. Slo3-null mice are infertile with motility defects and morphological abnormalities. Slo3 knockout mouse, patch-clamp electrophysiology in isolated sperm, pharmacological dissection with clofilium Proceedings of the National Academy of Sciences of the United States of America High 21427226
2011 LRRC52 is a testis-specific auxiliary subunit that co-assembles with Slo3, shifting its gating to voltages and pH values matching native KSper current; LRRC52 expression is critically dependent on Slo3 (absent from Slo3-null sperm); LRRC52 is a stronger modifier of Slo3 than related LRRC26. Co-immunoprecipitation, heterologous expression with electrophysiology, quantitative RT-PCR, Western blot in Slo3-null tissue Proceedings of the National Academy of Sciences of the United States of America High 22084117
2012 Human SLO3 is activated by intracellular pH increase; its expression and gating properties are modulated by auxiliary subunit LRRC52; crystal structure of the human SLO3 gating ring suggests it may represent an open-state conformation compared to SLO1. Heterologous electrophysiology, crystal structure determination of the cytoplasmic gating ring domain, structural comparison with SLO1 Proceedings of the National Academy of Sciences of the United States of America High 23129643
2013 Simultaneous knockout of Slo3 and CatSper1 abolishes all alkalization- and voltage-activated outward current in mouse sperm, demonstrating that the residual current in Slo3-null sperm arises from CatSper and that KSPER and CatSper together constitute the complete ion channel complement regulating membrane potential in mouse sperm. Double knockout mouse (Slo3-/-; CatSper1-/-), patch-clamp electrophysiology in isolated sperm The Journal of general physiology High 23980198
2014 In human sperm, IKSper is carried by SLO3 (KCNU1): Slo3 protein is localized to the sperm flagellum; human SLO3 (unlike mouse) is activated more strongly by Ca2+ than by alkaline pHi; selective SLO3 inhibitors suppress endogenous human IKSper; human SLO3 controls membrane potential in a Ca2+-dependent manner, limiting progesterone-evoked CatSper-mediated Ca2+ influx. Immunolocalization, heterologous expression of human vs. mouse SLO3 with patch clamp, pharmacological inhibition in human sperm eLife High 24670955
2015 Genetic knockout of LRRC52 severely impairs mouse fertility; KSPER current in LRRC52-null sperm requires more positive voltages and higher pH for activation than WT, establishing that LRRC52 is required for physiological gating of native SLO3/KSPER channels and that in vitro fertilization competence correlates with net KSPER conductance available under physiological conditions. LRRC52 knockout mouse, patch-clamp electrophysiology in isolated sperm, IVF assays across multiple genotypes Proceedings of the National Academy of Sciences of the United States of America High 25675513
2015 cSrc kinase acts downstream of PKA and upstream of SLO3 in the capacitation signaling cascade; cSrc inhibition blocks capacitation-induced hyperpolarization and acrosome reaction without blocking tyrosine phosphorylation; cSrc inhibitors directly reduce heterologously expressed SLO3 currents, analogous to its regulation of SLO1. Anti-phospho-Tyr416-cSrc antibody kinetics, pharmacological inhibition in sperm (membrane potential assay, acrosome reaction), pharmacological rescue with valinomycin, patch clamp of heterologously expressed SLO3 The Journal of biological chemistry Medium 26060254
2015 Extracellular barium blocks Slo3 outside the cell by interacting with the selectivity filter (prevented by elevated external K+); quinine and quinidine block Slo3 from the intracellular side in a state-independent manner by binding in a hydrophobic pocket formed by S6 segment residues; F304Y in S6 increases quinine/quinidine potency ~10-fold, defining the activation gate as lying deep in the pore between F304 and the selectivity filter. Xenopus oocyte two-electrode voltage clamp, gain-of-function mutations (R196Q, F304Y), in silico docking of quinidine to Slo3 structure British journal of pharmacology High 26045093
2019 Short cytoplasmic isoforms of Slo3 lacking the transmembrane/pore domain are expressed in somatic tissues (brain, kidney, eye) from alternative transcription start sites; full-length Slo3 channel is exclusively expressed in testis/sperm. Computational isoform prediction, RT-PCR, Western blot in multiple mouse tissues Molecular biology reports Medium 31270758
2020 The Slo3/LRRC52 complex retains PIP2 sensitivity comparable to Slo3 alone; voltage-sensing phosphatase (VSP)-mediated PIP2 depletion inhibits Slo3/LRRC52 current in Xenopus oocytes, consistent with VSP-regulated PIP2 distribution modulating Slo3 activity in native sperm flagellum. Co-expression of Slo3 + LRRC52 with VSP in Xenopus oocytes, two-electrode voltage clamp Channels (Austin, Tex.) Medium 32564653
2022 Bi-allelic loss-of-function variants in human KCNU1 (a missense reducing protein levels and a splice-site causing frameshift) cause male infertility with impaired acrosome reactions; knock-in mouse model recapitulates reduced KCNU1 protein, impaired acrosome reaction, abnormal membrane potential during capacitation, and reduced LRRC52 levels. Whole-exome sequencing, Sanger sequencing, RT-PCR for splicing, Western blot, immunofluorescence, acrosome reaction assay, KI mouse model, IVF/ICSI Human reproduction (Oxford, England) High 35551387
2022 A homozygous missense variant (p.Ile413Phe) in SLO3 reduces SLO3 mRNA and protein in human sperm and causes severe asthenoteratozoospermia with acrosome hypoplasia, mitochondrial sheath malformations, impaired acrosome reaction, altered membrane potential during capacitation, and reduction of LRRC52 auxiliary subunit. Whole-exome sequencing, RT-PCR, Western blot, immunofluorescence, electron microscopy, acrosome reaction assay, mitochondrial membrane potential measurement, ICSI rescue Reproductive biology and endocrinology High 34980136
2023 VU0546110 is the first selective inhibitor of human SLO3; it completely blocks heterologous SLO3 currents and endogenous K+ current in human sperm, prevents membrane hyperpolarization, and inhibits hyperactivated motility and acrosome reaction, establishing SLO3 as the sole K+ channel responsible for hyperpolarization in human sperm. High-throughput screen, heterologous expression with patch clamp, whole-cell patch clamp of human sperm, membrane potential assay, motility analysis, acrosome reaction assay Proceedings of the National Academy of Sciences of the United States of America High 36649421
2024 Intracellular Zn2+ directly inhibits mouse Slo3 currents in a dose-dependent manner at micromolar concentrations with exceptionally slow dissociation; specific amino acid residues contributing to slow Zn2+ dissociation were identified by MD simulations and confirmed by mutagenesis; Zn2+ levels in sperm decrease dynamically during capacitation, linking Zn2+ release to alkalization-induced SLO3-mediated hyperpolarization. Xenopus oocyte expression with two-electrode voltage clamp, intracellular Zn2+ measurements in sperm, molecular dynamics simulations, mutagenesis bioRxivpreprint Medium bio_10.1101_2024.12.12.628223
2025 SLO3 sets the resting membrane potential (~−65 mV) in human sperm; progesterone-evoked CatSper-mediated Ca2+ influx depolarizes sperm away from this SLO3-established resting potential; Ca2+- and Vm-dependent negative feedback limits CatSper activity and promotes repolarization via K+ efflux through SLO3, restoring resting Vm. Quantitative kinetic fluorimetry with voltage-sensitive fluorescent indicators (FAST M technique), simultaneous millisecond-resolution Vm and Ca2+ recording in human sperm bioRxivpreprint Medium bio_10.1101_2025.09.14.675619

Source papers

Stage 0 corpus · 27 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2010 The SLO3 sperm-specific potassium channel plays a vital role in male fertility. FEBS letters 201 20138882
1998 Slo3, a novel pH-sensitive K+ channel from mammalian spermatocytes. The Journal of biological chemistry 193 9452476
2011 Deletion of the Slo3 gene abolishes alkalization-activated K+ current in mouse spermatozoa. Proceedings of the National Academy of Sciences of the United States of America 155 21427226
2014 The Ca2+-activated K+ current of human sperm is mediated by Slo3. eLife 100 24670955
2011 LRRC52 (leucine-rich-repeat-containing protein 52), a testis-specific auxiliary subunit of the alkalization-activated Slo3 channel. Proceedings of the National Academy of Sciences of the United States of America 72 22084117
2010 Block of mouse Slo1 and Slo3 K+ channels by CTX, IbTX, TEA, 4-AP and quinidine. Channels (Austin, Tex.) 55 19934650
2012 Functional and structural analysis of the human SLO3 pH- and voltage-gated K+ channel. Proceedings of the National Academy of Sciences of the United States of America 54 23129643
2006 Slo3 K+ channels: voltage and pH dependence of macroscopic currents. The Journal of general physiology 50 16940555
2015 SLO3 auxiliary subunit LRRC52 controls gating of sperm KSPER currents and is critical for normal fertility. Proceedings of the National Academy of Sciences of the United States of America 48 25675513
2013 Simultaneous knockout of Slo3 and CatSper1 abolishes all alkalization- and voltage-activated current in mouse spermatozoa. The Journal of general physiology 48 23980198
2015 Src Kinase Is the Connecting Player between Protein Kinase A (PKA) Activation and Hyperpolarization through SLO3 Potassium Channel Regulation in Mouse Sperm. The Journal of biological chemistry 34 26060254
2023 A selective inhibitor of the sperm-specific potassium channel SLO3 impairs human sperm function. Proceedings of the National Academy of Sciences of the United States of America 32 36649421
2009 Interactions between beta subunits of the KCNMB family and Slo3: beta4 selectively modulates Slo3 expression and function. PloS one 30 19578543
2010 Phosphatidylinositol 4,5-bisphosphate activates Slo3 currents and its hydrolysis underlies the epidermal growth factor-induced current inhibition. The Journal of biological chemistry 28 20392696
2022 Homozygous mutation in SLO3 leads to severe asthenoteratozoospermia due to acrosome hypoplasia and mitochondrial sheath malformations. Reproductive biology and endocrinology : RB&E 22 34980136
2015 Mechanism of inhibition of mouse Slo3 (KCa 5.1) potassium channels by quinine, quinidine and barium. British journal of pharmacology 20 26045093
2009 Bovine and mouse SLO3 K+ channels: evolutionary divergence points to an RCK1 region of critical function. The Journal of biological chemistry 20 19473978
2022 Bi-allelic variants in KCNU1 cause impaired acrosome reactions and male infertility. Human reproduction (Oxford, England) 18 35551387
2023 SLO3: A Conserved Regulator of Sperm Membrane Potential. International journal of molecular sciences 14 37446382
2017 Premammalian origin of the sperm-specific Slo3 channel. FEBS open bio 9 28286733
2019 Onion peel extract and its constituent, quercetin inhibits human Slo3 in a pH and calcium dependent manner. The Korean journal of physiology & pharmacology : official journal of the Korean Physiological Society and the Korean Society of Pharmacology 8 31496875
2018 Slo3 K+ channel blocker clofilium extends bull and mouse sperm-fertilizing competence. Reproduction (Cambridge, England) 7 30422808
2018 Analyzing the functional divergence of Slo1 and Slo3 channel subfamilies. Molecular phylogenetics and evolution 7 30586650
2019 A cytoplasmic Slo3 isoform is expressed in somatic tissues. Molecular biology reports 4 31270758
2020 The Slo3/Lrrc52 complex is sensitive to phosphoinositides. Channels (Austin, Tex.) 2 32564653
2023 Pharmacological Evidence Suggests That Slo3 Channel Is the Principal K+ Channel in Boar Spermatozoa. International journal of molecular sciences 1 37175513
2025 Differential molecular interactions between iberiotoxin and human SLO3 and SLO1 potassium channels. Journal of molecular modeling 0 40358624