FXYD3

FXYD3 is a single-pass transmembrane protein that functions as a regulatory subunit of P-type ATPases, modulating ion transport, epithelial homeostasis, and cell signaling across diverse tissues. It associates with Na,K-ATPase to alter apparent Na⁺ and K⁺ affinities and maximal pump activity, thereby controlling transepithelial Na⁺ and liquid absorption in airway and intestinal epithelia, and it interacts with SERCA2 in goblet cells to enhance ER Ca²⁺ homeostasis and mucin glycosylation required for mucus barrier integrity (PMID:17077088, PMID:19109419, PMID:35993520, PMID:41187059). FXYD3 protects the Na,K-ATPase β1 subunit from oxidative glutathionylation-mediated inhibition, and a conserved Gly41 residue in its transmembrane domain is indispensable for pump association and plasma membrane targeting (PMID:17409496, PMID:26740212). Beyond ion transport, FXYD3 participates in signaling by competitively sequestering TRAF3 to promote IL-17R/ACT1-dependent NF-κB activation in keratinocytes, by forming a Src/ERα complex that drives non-genomic estrogen signaling via a SOX9-dependent transcriptional feedback loop in breast cancer, and by binding IRF7 to sustain cGAS/STING-JAK2/STAT5 signaling in cholangiocarcinoma (PMID:36693922, PMID:30206184, PMID:41164952).

1. 1995 High

   The initial functional identity of FXYD3 was established as a membrane protein capable of inducing hyperpolarization-activated chloride currents, placing it in the same functional family as phospholemman but with a distinct regulatory domain.

   Evidence Electrophysiology in Xenopus oocyte heterologous expression system

   PMID:7836447

   Open questions at the time

   • Whether FXYD3 is itself a chloride channel or a regulator of endogenous channels was not resolved
   • Native tissue function unknown
2. 2005 High

   The chloride-channel model was superseded when FXYD3 was shown to be a bona fide Na,K-ATPase-associated regulatory subunit that decreases apparent Na⁺ and K⁺ affinities, redefining its primary molecular function as an ion pump modulator.

   Evidence Xenopus oocyte co-expression with Na,K-ATPase, electrophysiology, glycosylation analysis, and co-immunoprecipitation

   PMID:15743908 PMID:17077088

   Open questions at the time

   • Whether splice-variant-specific modulation occurs in native tissues was not demonstrated
   • Structural basis of differential affinity effects between short and long isoforms unresolved
3. 2007 High

   Structure–function dissection identified Gly41 in the transmembrane domain as indispensable for Na,K-ATPase association and plasma membrane targeting, establishing the molecular determinants of the FXYD3–pump interaction.

   Evidence Site-directed mutagenesis, co-immunoprecipitation, and live-cell imaging in colorectal cancer cells

   PMID:17409496

   Open questions at the time

   • No crystal or cryo-EM structure of the FXYD3–Na,K-ATPase complex exists
   • Whether Gly41 is also required for interactions with other ATPases was not tested
4. 2008 High

   Loss-of-function studies demonstrated that FXYD3 is required for intestinal epithelial differentiation, cell survival, and proper Na,K-ATPase turnover number, linking pump regulation to broader epithelial biology.

   Evidence siRNA knockdown in Caco-2 cells with transepithelial resistance, Na,K-ATPase activity assays, and differentiation marker expression

   PMID:19109419

   Open questions at the time

   • In vivo intestinal phenotype of FXYD3 loss not yet established at this point
   • Mechanism connecting Na,K-ATPase modulation to apoptosis/differentiation unclear
5. 2010 Medium

   The discovery that the Pseudomonas aeruginosa effector ExoS directly binds FXYD3's transmembrane domain — the same interface used for Na,K-ATPase — revealed that pathogens exploit FXYD3 to compromise epithelial barrier function.

   Evidence Bacterial two-hybrid screen, pulldown assay, domain mapping

   PMID:20805335

   Open questions at the time

   • Functional consequence of ExoS–FXYD3 interaction on Na,K-ATPase activity was inferred, not directly measured
   • Relevance in mammalian infection models not confirmed
6. 2011 Medium

   Transcriptional regulation of FXYD3 was placed downstream of TGF-β/Smad3/ZEB1, explaining how epithelial-to-mesenchymal transition signals silence FXYD3.

   Evidence siRNA knockdown, TGF-β treatment, pathway inhibitors, and RT-PCR in MCF-10A cells

   PMID:21372379

   Open questions at the time

   • Direct ZEB1 binding to the FXYD3 promoter not shown by ChIP
   • In vivo relevance to EMT programs not established
7. 2014 High

   Epigenetic regulation of FXYD3 was demonstrated in pancreatic β-cells, where gluco-incretin-driven promoter CpG methylation silences FXYD3, and its overexpression reduces glucose-stimulated insulin secretion downstream of Ca²⁺ influx.

   Evidence Overexpression, knockdown, insulin secretion assays, promoter methylation analysis, ChIP for H3K4me3

   PMID:25058609

   Open questions at the time

   • The specific ion transport target (Na,K-ATPase or other) mediating insulin secretion suppression was not identified
   • Phenotype of Fxyd3 deletion in β-cells in vivo not shown
8. 2016 Medium

   A cytoprotective mechanism was uncovered: FXYD3 facilitates reversal of oxidative glutathionylation on the Na,K-ATPase β1 subunit, protecting pump activity under oxidative stress and conferring resistance to doxorubicin- and radiation-induced cell death.

   Evidence siRNA knockdown, Na,K-ATPase activity assay, glutathionylation measurement, caspase activation in MCF-7 breast cancer cells

   PMID:26740212

   Open questions at the time

   • Whether FXYD3 directly catalyzes deglutathionylation or recruits a thiol reductase is unknown
   • Relevance to therapy resistance in vivo not tested
9. 2018 Medium

   FXYD3 was shown to function beyond ion transport as a signaling scaffold: it forms a complex with Src and ERα for non-genomic estrogen signaling, and participates in a positive feedback loop with SOX9 that maintains breast cancer stem cells and tamoxifen resistance.

   Evidence Co-immunoprecipitation, siRNA, reporter assays, subcellular fractionation in ER+ breast cancer cells

   PMID:30206184

   Open questions at the time

   • Direct binding interface between FXYD3 and Src not mapped
   • SOX9 binding site on FXYD3 promoter confirmed by reporter but not by endogenous ChIP-seq
   • Independent replication in other ER+ models lacking
10. 2022 High

    Physiological relevance of FXYD3 as a Na,K-ATPase modulator was demonstrated in intact human airway epithelia, where FXYD3 knockdown reduced Na,K-ATPase transport capacity, amiloride-sensitive Na⁺ absorption, and transepithelial liquid clearance.

    Evidence scRNA-seq, immunohistochemistry, siRNA knockdown, short-circuit current measurements with nystatin permeabilization, liquid absorption assay

    PMID:35993520

    Open questions at the time

    • In vivo airway phenotype of FXYD3 deletion not tested
    • Whether FXYD3 modulates ENaC directly or only via Na,K-ATPase is unclear
11. 2023 High

    An entirely new signaling axis was revealed: FXYD3 competitively binds TRAF3 to relieve its inhibition of IL-17R, promoting ACT1 recruitment and NF-κB/MAPK activation in keratinocytes, with genetic deletion attenuating psoriasis-like inflammation in vivo.

    Evidence Co-immunoprecipitation, competitive binding assays, genetic KO in keratinocytes, imiquimod psoriasis model

    PMID:36693922

    Open questions at the time

    • Structural basis of FXYD3–TRAF3 interaction not determined
    • Whether this mechanism operates in other IL-17-responsive cell types is unknown
12. 2025 High

    FXYD3 was shown to interact with SERCA2 in intestinal goblet cells, enhancing ER Ca²⁺ pump activity to support mucin glycosylation and mucus barrier integrity, establishing a second P-type ATPase partnership beyond Na,K-ATPase.

    Evidence Intestinal epithelium-specific genetic KO in mice, Co-IP with SERCA2, ER Ca²⁺ measurements, colitis model

    PMID:41187059

    Open questions at the time

    • Whether FXYD3 modulates SERCA2 affinity parameters analogously to Na,K-ATPase is not resolved
    • Direct structural evidence for the FXYD3–SERCA2 interface absent
13. 2025 Medium

    FXYD3 was found to bind IRF7 via its 60–87 amino acid domain, initiating a cGAS/STING-mediated positive feedback loop that sustains JAK2/STAT5 signaling and drives intrahepatic cholangiocarcinoma progression.

    Evidence Co-immunoprecipitation with domain mapping, single-cell and spatial transcriptomics, in vivo tumor models

    PMID:41164952

    Open questions at the time

    • Independent validation of FXYD3–IRF7 interaction in a second lab awaited
    • Whether this axis operates in normal cholangiocytes or only in cancer is unclear
    • Causal chain from FXYD3–IRF7 binding to cGAS/STING activation not fully delineated

• Key unresolved questions include the structural basis of FXYD3 interactions with its multiple ATPase and signaling partners, whether its ion-transport and signaling functions are mechanistically linked or independent, and how tissue-specific expression patterns dictate which of its diverse functions predominate.
• No high-resolution structure of FXYD3 in complex with any partner
• Whether FXYD3's signaling roles (TRAF3, Src, IRF7) require or are independent of ATPase association is unknown
• Comprehensive in vivo phenotyping across tissues using conditional knockouts is incomplete