CUBN

Cubilin (CUBN) is a large peripheral membrane endocytic receptor of intestinal and renal proximal tubule epithelia that mediates clathrin-dependent uptake of filtered and luminal ligands (PMID:9153271, PMID:7664754). It was first identified as the intrinsic factor-cobalamin (IF-Cbl) receptor (gp280/IFCR), binding IF-cobalamin with high affinity in a calcium- and cobalamin-dependent manner and internalizing the complex at apical membranes of intestinal, renal, and yolk sac cells (PMID:9153271, PMID:9572993). The mature protein is a ~3597-residue peripheral membrane protein whose N-terminal signal sequence is cleaved by the trans-Golgi protease furin during maturation, and which is targeted to the plasma membrane in an immature, endoglycosidase H-sensitive form that completes glycosylation by recycling through the Golgi (PMID:9572993, PMID:7626048). Cubilin concentrates in clathrin-coated intermicrovillar membrane domains, where antibody-mediated blockade inhibits endocytic tracer uptake, establishing its direct role in receptor-mediated endocytosis (PMID:7664754). Beyond IF-cobalamin, it serves as a receptor for myeloma immunoglobulin light chains in the proximal tubule (PMID:9691015) and for albumin and other proteins reabsorbed from the glomerular filtrate (PMID:31613795). Membrane expression and ligand uptake require its partner amnionless (AMN), with which cubilin forms the apical cubam complex; variants that disrupt cubilin–AMN association cause AMN to mislocalize to the cytoplasm and abolish surface cubam (PMID:36266725). Domain function is regionally separable: N-terminal regions mediate IF-cobalamin binding, while C-terminal CUB domains are specifically required for renal albumin reabsorption (PMID:31613795). Loss-of-function CUBN mutations cause hereditary megaloblastic anaemia 1 (Imerslund-Gräsbeck syndrome) with selective intestinal B12 malabsorption (PMID:10080186), whereas C-terminal biallelic variants cause isolated chronic proteinuria without B12 malabsorption (PMID:31613795); this dissociation is explained by a truncated CUBN isoform expressed in intestine that lacks the C-terminal region, sparing cobalamin absorption when C-terminal variants are present (PMID:41624452).

1. 1995 Medium

   Establishing where cubilin acts and whether it is functionally required for endocytosis defined its cellular role before its ligands were known.

   Evidence Immunofluorescence and immunoelectron microscopy with antibody-mediated endocytosis inhibition in yolk sac and proximal tubule epithelia

   PMID:7664754

   Open questions at the time

   • Ligand identity not yet established
   • Did not define the molecular interactions driving clathrin recruitment
2. 1995 Medium

   Characterizing cubilin's unusual maturation showed it reaches the plasma membrane in an immature form and completes processing by Golgi recycling, distinguishing its trafficking from related receptors.

   Evidence Endoglycosidase H sensitivity assay, biosynthetic labeling and subcellular fractionation of cultured yolk sac epithelial cells

   PMID:7626048

   Open questions at the time

   • Single method type from one lab
   • Functional consequence of the recycling route not tested
3. 1997 High

   Identifying gp280 as the IF-cobalamin receptor assigned cubilin its first defined ligand and physiological function.

   Evidence Radiolabeled IF-[57Co]Cbl binding and internalization assays, immunoprecipitation, and immunoelectron microscopy in rat kidney and yolk sac

   PMID:9153271

   Open questions at the time

   • Did not define the binding domain
   • No partner protein for membrane anchoring identified
4. 1998 High

   Cloning human cubilin and mapping its furin cleavage site defined the mature receptor and confirmed calcium/cobalamin-dependent high-affinity IF-Cbl binding.

   Evidence Surface plasmon resonance on affinity-purified protein, full cDNA cloning, N-terminal sequencing, FISH and radiation hybrid mapping

   PMID:9572993

   Open questions at the time

   • No membrane anchor or transmembrane domain identified, leaving membrane association unexplained
   • Other ligands not yet defined
5. 1998 Medium

   Demonstrating that cubilin binds myeloma immunoglobulin light chains broadened its role beyond cobalamin to proximal tubule protein reabsorption.

   Evidence Immunoaffinity co-elution, antibody displacement from brush-border membranes, surface plasmon resonance, and endocytosis inhibition in yolk sac cells

   PMID:9691015

   Open questions at the time

   • Endocytosis inhibition was only partial
   • Additional light-chain receptors not excluded
6. 1999 High

   Identifying disease-causing CUBN mutations established the gene as causative for hereditary megaloblastic anaemia 1, linking receptor loss to selective B12 malabsorption in humans.

   Evidence Linkage disequilibrium mapping and mutation sequencing in 17 Finnish MGA1 families

   PMID:10080186

   Open questions at the time

   • Did not address renal protein-reabsorption phenotypes
   • Mechanism by which mutations impair receptor function not defined
7. 2013 High

   An animal frameshift model showed that CUBN loss-of-function via nonsense-mediated decay abrogates cubam and causes both cobalamin malabsorption and proteinuria, linking the two functions to one receptor.

   Evidence Whole-genome resequencing, mRNA/protein quantification, radiolabeled cobalamin uptake and urine SDS-PAGE in Border Collies

   PMID:23746554

   Open questions at the time

   • Did not separate cobalamin and albumin functions by domain
   • Mechanism of AMN dependence not addressed
8. 2020 High

   Domain-specific genotype–phenotype dissection showed C-terminal CUB domains are required for renal albumin reabsorption independently of N-terminal IF-cobalamin binding.

   Evidence Kidney disease gene-panel sequencing, structural modeling, and clinical/population cohort analysis in 39 patients

   PMID:31613795

   Open questions at the time

   • Did not demonstrate the molecular mechanism of C-terminal variant dysfunction
   • Why intestinal B12 uptake is spared not explained
9. 2022 Medium

   Linking C-terminal variants to loss of cubilin–AMN association explained how they abolish surface cubam through AMN cytoplasmic mislocalization.

   Evidence Exome sequencing, HEK293T expression, co-immunoprecipitation, AMN immunofluorescence localization, and mouse in vivo experiments

   PMID:36266725

   Open questions at the time

   • Single lab with co-IP not reciprocally validated
   • Quantitative contribution of AMN mislocalization to proteinuria not measured
10. 2025 Medium

    Identifying a truncated, intestine-expressed CUBN isoform resolved why C-terminal variants cause renal proteinuria without affecting cobalamin absorption.

    Evidence Targeted panel sequencing, cDNA transcript analysis, tissue expression profiling (kidney vs intestine), and renal biopsy electron microscopy in 52 patients

    PMID:41624452

    Open questions at the time

    • Isoform identified by cDNA analysis in one lab
    • Functional reconstitution of the truncated isoform not performed

• How cubilin, a protein lacking its own transmembrane anchor, couples ligand binding to clathrin-mediated internalization at the molecular level remains incompletely defined.
• No structural model of the full cubam endocytic complex in the corpus
• Mechanism of clathrin recruitment and cargo handoff not defined
• Binding sites for albumin and transferrin not mapped to specific CUB domains