CRTAP (Cartilage-associated protein) — review notes

UniProt: O75718 (CRTAP_HUMAN), 401 aa precursor; gene CRTAP (synonym CASP); chr 3.
HGNC:2379. Member of the leprecan family (SIM, UniProt). Signal peptide 1-26;
secreted/ER-resident glycoprotein (N-glyc at 87, 363).

Core biology

CRTAP is a non-catalytic, essential component of the endoplasmic reticulum
collagen prolyl 3-hydroxylation complex, together with prolyl 3-hydroxylase 1
(P3H1/LEPRE1, UniProt Q32P28) and cyclophilin B / peptidyl-prolyl isomerase B
(PPIB/CyPB). This ternary complex 3-hydroxylates a single specific proline,
Pro986, of the α1(I) and α1(II) fibrillar (pro)collagen chains in the ER lumen.
- PMID:19846465
- PMID:20089953

P3H1 carries the catalytic prolyl 3-hydroxylase activity and is the only complex
member with a KDEL ER-retrieval signal; CRTAP acts as the "helper protein" /
non-catalytic subunit, NOT the catalytic hydroxylase.
- PMID:19846465

Mutual stabilization & chaperone role

CRTAP and P3H1 mutually stabilize each other in the ER: a null mutation in one
gene causes loss/reduction of BOTH proteins despite normal transcript levels of
the non-mutated gene. Proteasome inhibition partially rescues P3H1 in CRTAP-null
cells, indicating destabilized partner is degraded.
- PMID:19846465
- PMID:19846465

The complex also functions as a collagen chaperone/foldase: loss of P3H1 or CRTAP
delays collagen helix folding, causing overmodification (excess 4-hydroxylation /
lysyl hydroxylation) of the helical region. The complex has classical chaperone
activity in vitro (inhibits citrate synthase aggregation, rhodanese refolding) and
modulates collagen fibril formation.
- PMID:19846465
- PMID:20089953

Note (negative regulation of PTM): the IMP annotation GO:1901874 "negative
regulation of post-translational protein modification" reflects that the complex
LIMITS overmodification by P4H/LH — i.e., its absence causes overmodification.
This is an indirect/consequential phenotype of the chaperone/foldase role, not a
direct enzymatic activity of CRTAP. Best regarded as non-core.

Subcellular location

Primary functional location is the ER lumen. Endogenous CRTAP detected in ER by
IDA (PMID:19846465, PMID:20089953). A minor fraction is secreted to the
extracellular space/matrix: in normal cells ~10-12% of CRTAP is secreted, rising
to 15-20% in LEPRE1-null cells (because P3H1's KDEL normally retains CRTAP).
- PMID:19846465
The UniProt SUBCELLULAR LOCATION line lists "Secreted, extracellular space,
extracellular matrix" (by similarity, ECO:0000250); the dominant biology is ER.

Structure

Cryo-EM structures of the human P3H1/CRTAP/PPIB ternary complex (PDB 8K0E, 8K0F,
8K0I, 8K0M, 8K17, 8KC9). The active sites of P3H1 and PPIB form a face-to-face
bifunctional reaction center; the complex binds collagen peptide across multiple
sites forming a "substrate interacting zone"; a dual-ternary state is observed.
CRTAP is a TPR-like α-helical (leprecan-domain) scaffold subunit.
- PMID:39245686

Disease

Biallelic loss-of-function CRTAP variants cause autosomal recessive osteogenesis
imperfecta type VII (OI7, MIM:610682), a severe-to-lethal bone dysplasia with
rhizomelia, overmodified type I collagen, reduced bone mass, fractures.
- [PMID:17055431 "CRTAP is required for prolyl 3-hydroxylation and mutations cause
recessive osteogenesis imperfecta." (title; UniProt FUNCTION + DISEASE)]
- UniProt DISEASE: OI7 autosomal recessive (ECO:0000269|PubMed:17055431,
18566967, 19550437, 21955071).
Crtap-knockout mice have recessive osteochondrodystrophy with loss of prolyl
3-hydroxylation in cartilage and bone.
- PMID:19846465

Interaction evidence supporting "protein binding" (GO:0005515) IPI annotations

All three IPI GO:0005515 annotations (PMID:30021884, PMID:33961781, PMID:39245686)
use WITH/FROM UniProtKB:Q32P28 = P3H1, i.e. they capture the CRTAP–P3H1
interaction. PMID:39245686 (cryo-EM ternary complex) is direct, specific
structural evidence; PMID:33961781 (BioPlex affinity-MS interactome) and
PMID:30021884 (XL-MS) are high-throughput. Bare "protein binding" is
uninformative; the meaningful, specific statement is membership in the
P3H1/CRTAP/PPIB complex.

Annotation assessment summary

• GO:0005518 collagen binding (IBA, contributes_to): the complex binds collagen
  substrate; CRTAP contributes to substrate binding within the complex (substrate
  interacting zone, PMID:39245686). Supportable as contributes_to; keep
  non-core (it is the complex, not CRTAP alone, that binds — but reasonable).
• GO:0005783 endoplasmic reticulum (multiple IBA/IEA/IDA): well supported, core
  location. ACCEPT IDA; IBA/IEA redundant but correct.
• GO:0005788 endoplasmic reticulum lumen (TAS Reactome x4): more precise than ER;
  correct. ACCEPT (one representative); others redundant.
• GO:0030199 collagen fibril organization (IBA): supported (chaperone modulates
  fibril formation; OI collagen disorganized). Keep non-core / accept.
• GO:0005515 protein binding (IPI x3): bare term — MARK_AS_OVER_ANNOTATED; the
  specific content is P3H1 binding / complex membership.
• GO:0005783 IEA (GO_REF:0000120) — redundant with IDA, over-annotated/non-core.
• GO:0007283 spermatogenesis (IEA, Ensembl from mouse): broad ortholog transfer,
  no human evidence; MARK_AS_OVER_ANNOTATED / non-core.
• GO:0050821 protein stabilization (IMP, PMID:19846465): supported — CRTAP
  stabilizes P3H1 (mutual stabilization). ACCEPT, but specific to complex partner.
• GO:0006457 protein folding (ISS): supported (complex is collagen chaperone).
  Keep; could be more specific (protein folding in ER / chaperone).
• GO:0032991 protein-containing complex (ISS): correct but generic; the specific
  complex is the prolyl 3-hydroxylase / collagen-modifying complex. MODIFY toward
  more specific if available, else keep non-core.
• GO:0005576 extracellular region (IDA, PMID:19846465): a minor secreted fraction
  is real but the dominant biology is ER; KEEP_AS_NON_CORE.
• GO:1901874 negative regulation of post-translational protein modification (IMP):
  indirect consequence (loss → overmodification); KEEP_AS_NON_CORE.