# 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 "CRTAP and P3H1 form a complex with cyclophilin B (CyPB) in the endoplasmic reticulum (ER) which 3-hydroxylates the Pro986 residue of alpha1(I) and alpha1(II) collagen chains."] - [PMID:20089953 "the collagen prolyl 3-hydroxylation complex, consisting of P3H1 ... CRTAP, and CyPB, modifies the α1(I)Pro986 residue."] 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 "P3H1 ... contains the enzymatic activity of the complex and is the only complex component with a KDEL ER-retrieval signal ... CRTAP ... acts as the helper protein of the complex."] ## 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 "a null mutation for one of these proteins results in absence or substantial decrease of both proteins ... despite normal levels of transcript ... These data indicate that CRTAP and P3H1 are mutually stabilized in the collagen prolyl 3-hydroxylation complex."] - [PMID:19846465 "Proteasomal inhibitors partially rescue P3H1 protein in CRTAP-null cells."] 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 "The prolyl 3-hydroxylation complex has recently been reported ... to function as a chaperone in both classical chaperone assays inhibiting citrate synthase aggregation and rhodanese refolding and aggregation, as well as in a fibril formation assay of type I collagen ... the complex also has a general function to assist protein folding."] - [PMID:20089953 "In the absence of P3H1 or CRTAP, α1(I)Pro986 3-hydroxylation is decreased and collagen folding is delayed, resulting in overmodification of the helical lysine and proline residues."] 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 "in conditioned media from control cells ... about 10-12% of total CRTAP is secreted ... in LEPRE1-null cells, P3H1 is not present to retain CRTAP in the ER, and the proportion of CRTAP secreted ... is more than tripled, accounting for 15-20%"] 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 "We determine cryo-EM structures of the P3H1/CRTAP/PPIB complex. The active sites of P3H1 and PPIB form a face-to-face bifunctional reaction center ... The structure of the P3H1/CRTAP/PPIB/collagen peptide complex reveals multiple binding sites, suggesting a substrate interacting zone."] ## 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 "Crtap knock-out mice have a recessive osteochondrodystrophy associated with lack of the prolyl 3-hydroxylation modification in cartilage and bone."] ## 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.