CANX (Calnexin, P27824) curation notes

Overview

CANX is a type I single-pass ER membrane lectin chaperone, one of the two central
ER lectins of the calnexin/calreticulin cycle. It binds monoglucosylated N-glycans
(Glc1Man9GlcNAc2) on nascent glycoproteins, recruits ERp57 (PDIA3) for oxidative
folding, retains misfolded glycoproteins for ER quality control, and binds calcium.

Core function evidence

• UniProt FUNCTION [file:human/CANX/CANX-uniprot.txt]:
  "Calcium-binding protein that interacts with newly synthesized monoglucosylated
  glycoproteins in the endoplasmic reticulum. It may act in assisting protein
  assembly and/or in the retention within the ER of unassembled protein subunits.
  It seems to play a major role in the quality control apparatus of the ER by the
  retention of incorrectly folded proteins."

• Topology: lumenal (21-481), transmembrane (482-502), cytoplasmic (503-592). The
  lumenal globular lectin domain + extended P domain (276-409) recruit ERp57 and PPIB.
  Ca2+ binding sites at residues 74, 117, 436; alpha-D-glucoside (glycan) binding at
  164, 166, 185, 192, 425. [file:human/CANX/CANX-uniprot.txt]

• Palmitoylation/translocon: PMID:22314232 and "palmitoylation mediates the association of calnexin
  with the ribosome-translocon complex (RTC)". When supercomplex formation was disrupted,
  "folding of glycoproteins was impaired." This is direct experimental support for ER
  membrane localization and the glycoprotein-folding core function.

• ERAD / quality control: calnexin retains misfolded glycoproteins; the IBA ERAD pathway
  annotation reflects the well-established role of the CNX/CRT cycle feeding terminally
  misfolded clients to ERAD.

• Client-specific chaperone examples (all consistent with the core lectin-chaperone role,
  but client-specific, not the generic function):

• CHRNA7/α7 nicotinic receptor assembly via NACHO: PMID:32783947
• hERG/KCNH2 PMID:16361248, CFTR [PMID:16546175, 26618866, 31324722, 36012204],
  SERPINA1/SERPINA2 PMID:23826168, delta opioid receptor PMID:20528919, NPC2/Nogo-B
  receptor PMID:19723497.

Localization

• ER membrane is the core compartment (IDA, EXP PMID:22314232, multiple IDA).
• Melanosome membrane (PMID:12643545, 17081065): mass-spec detection in melanosome
  fractions; secondary localization of an ER protein, non-core.
• Mitochondrial membrane / MAM contact site (PMID:23455425): MAM localization is real;
  the bare "mitochondrial membrane" subcell mapping is an over-call (calnexin is at the
  ER side of ER-mitochondria contact sites).
• Extracellular exosome (HDA PMID:23533145): high-throughput proteomic; non-core. Note a
  NEGATED exosome annotation from PMID:21235781.
• Presynapse / synaptic vesicle endocytosis / clathrin-dependent endocytosis: derived from
  ortholog (rodent) SGIP1-related receptor-mediated endocytosis role; UniProt notes "may
  play a role in receptor-mediated endocytosis at the synapse." Speculative for human,
  non-core.
• Nuclear membrane (IEA Ensembl): over-call, the ER membrane is continuous with the nuclear
  envelope so this is a propagated contiguity artifact.

Over-annotations / uninformative

• The large block of GO:0005515 "protein binding" (IPI) annotations come from interactome
  maps (PMID:26496610, 28514442, 29568061, 33961781, 35271311, 40205054, 30021884, 32296183
  via BioPlex/OpenCell/etc.), virus host factor screens (HIV PMID:22190034, 25170080),
  and individual client studies. Per curation guidelines, bare "protein binding" is
  uninformative -> MARK_AS_OVER_ANNOTATED (a few that pinpoint the chaperone-client
  relationship could in principle be MODIFY to unfolded protein binding, but most are
  generic interactome hits).
• RNA binding (HDA PMID:22658674, 22681889): from global mRNA-interactome capture; calnexin
  is not a bona fide RNA-binding protein in a functional sense -> over-annotated.
• Viral protein processing (TAS Reactome, SARS-CoV-2 spike): client-specific application of
  the chaperone cycle; non-core.

Calcium

• Ca2+ binding (IBA, IEA, TAS PMID:8136357): supported; calnexin contributes to ER Ca2+
  via high-capacity Ca2+ binding in its acidic regions. Core ER property. [PMID:8136357
  cDNA cloning "identification of potential calcium binding motifs"]

Summary of core functions

1. MF: carbohydrate (monoglucosylated N-glycan) binding lectin / unfolded protein binding
   chaperone activity in the ER.
2. MF: calcium ion binding (ER Ca2+ buffering).
3. BP: protein folding in the ER / glycoprotein quality control (calnexin/calreticulin cycle).
4. BP: ERAD pathway (retention/triage of terminally misfolded clients).
5. CC: ER membrane (lumenal-facing lectin domain).

GO:0051082 migration (2026-09-27)

The quality-control retention/triage core function carries no molecular-function term. GO:0051082 unfolded protein binding is obsolete; the glycan-recognition and folding-chaperone activities that underlie retention are captured in the lectin and protein-folding-chaperone core functions, and no GO term describes the retention decision itself.