Affinage

CRTAP

Cartilage-associated protein · UniProt O75718

Length
401 aa
Mass
46.6 kDa
Annotated
2026-06-09
23 papers in source corpus 12 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

CRTAP is an essential ER-resident component of the collagen prolyl 3-hydroxylation machinery, where it assembles with the P3H1 hydroxylase and the PPIB (cyclophilin B) isomerase into a ternary complex that modifies fibrillar collagen during biosynthesis (PMID:18566967, PMID:19862557). Cryo-EM of the P3H1/CRTAP/PPIB complex shows the P3H1 and PPIB active sites arranged face-to-face into a bifunctional reaction center with a multi-site collagen substrate-interacting zone, defining a coupled hydroxylation–isomerization mechanism (PMID:39245686). The complex 3-hydroxylates Pro986 of clade A (type I, II, V) collagen α1 chains, and loss of CRTAP abolishes this site-specific modification while leaving non-fibrillar α1(IV) sites unaffected, establishing its substrate specificity (PMID:20485499). CRTAP and P3H1 mutually stabilize one another post-translationally: absence of either triggers proteasome-mediated degradation of the partner, and re-expression restores both protein levels and Pro986 hydroxylation (PMID:19846465). Beyond hydroxylation, CRTAP acts as a collagen chaperone; its loss causes intracellular collagen overmodification and delayed helix folding, ER retention and aggregation, deficient extracellular matrix deposition, and disorganized fibrils of altered diameter (PMID:19862557, PMID:21955071, PMID:32173581). These defects perturb bone mineralization and osteoblast-driven bone formation, and biallelic CRTAP mutations produce a severe recessive osteogenesis imperfecta phenotype (PMID:19895918, PMID:38214665).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 1999 Medium

    Established CRTAP as a novel cartilage-expressed protein and defined it as a new gene/protein family, providing the molecular entity later linked to collagen biology.

    Evidence cDNA cloning, Northern blot, FISH mapping to 3p22, and immunohistochemistry across tissues

    PMID:10702664

    Open questions at the time
    • No biochemical function assigned at this stage
    • No interaction partners identified
    • Relationship to collagen modification unknown
  2. 2008 Medium

    Showed CRTAP is not an isolated protein but part of a complex with P3H1 that 3-hydroxylates Pro986 in α1(I)/α1(II) collagen and recruits cyclophilin B, defining its catalytic context.

    Evidence Biochemical collagen analysis of patient fibroblasts carrying CRTAP loss-of-function mutations

    PMID:18566967

    Open questions at the time
    • CRTAP's own enzymatic role within the complex not resolved
    • Stoichiometry and assembly mechanism unknown
    • No structural basis for catalysis
  3. 2009 High

    Resolved how CRTAP and P3H1 depend on each other by demonstrating reciprocal post-translational stabilization, explaining why mutation in one gene eliminates both proteins.

    Evidence Western blot, immunofluorescence, transfection rescue, and proteasome inhibition in patient fibroblasts; biochemical collagen overmodification analysis in patient cells and Crtap-/- mice

    PMID:19846465 PMID:19862557

    Open questions at the time
    • The degradation signal/machinery targeting the unstabilized partner not defined
    • Quantitative contribution of hydroxylation vs folding delay to overmodification unclear
  4. 2010 High

    Defined the substrate range of the CRTAP-containing complex by mapping which collagen Pro residues require CRTAP, establishing specificity for fibrillar (clade A) collagens.

    Evidence Mass spectrometry hydroxyproline mapping in Crtap-/- mouse tissues and human OI fibroblasts

    PMID:20485499

    Open questions at the time
    • Determinants of site selectivity within collagen sequences not defined
    • Whether CRTAP contributes binding specificity vs P3H1 unknown
  5. 2011 Medium

    Extended CRTAP's role beyond hydroxylation to matrix assembly, showing its loss reduces ECM collagen deposition and disorganizes the fibrillar network, implicating a chaperone function.

    Evidence Immunofluorescence, quantitative pulse-chase, and electron microscopy in patient fibroblasts

    PMID:21955071

    Open questions at the time
    • Single patient cell line
    • Molecular basis of chaperone activity not defined
    • Whether matrix defect is direct or secondary to overmodification unclear
  6. 2020 High

    Confirmed the chaperone-dominant role in a vertebrate genetic model, showing intracellular collagen overmodification, ER retention, and disorganized extracellular fibrils upon crtap loss.

    Evidence CRISPR/Cas9 crtap-knockout zebrafish with TEM and collagen biochemical analysis

    PMID:32173581

    Open questions at the time
    • Relative weighting of hydroxylase vs chaperone contributions to phenotype not quantified
    • Mechanism of ER retention not defined
  7. 2023 Medium

    Identified a degron-based mechanism by which deep intronic mutations destabilize CRTAP, linking protein instability to collagen aggregation, senescence, and autophagic clearance.

    Evidence Genome sequencing, minigene splicing assays, western blot, collagen aggregation and autophagy inhibition experiments in patient cells

    PMID:37146916

    Open questions at the time
    • Single patient cell line, not independently replicated
    • Generality of the GWxxI degron to other variants unknown
    • Mechanistic basis of senescence not defined
  8. 2024 High

    Provided the structural basis of CRTAP function, showing it scaffolds P3H1 and PPIB active sites into a face-to-face bifunctional center, revealing a coupled hydroxylation–isomerization mechanism.

    Evidence Cryo-EM of the P3H1/CRTAP/PPIB ternary complex with collagen peptide co-complex and active-site mutagenesis

    PMID:39245686

    Open questions at the time
    • Functional role of the observed dual-ternary complex equilibrium not established
    • Catalytic order/coupling kinetics not measured
    • CRTAP's direct contribution to substrate binding vs scaffolding not separated
  9. 2024 Medium

    Tied CRTAP function directly to bone-forming cells, showing reduced osteoblast hydroxylation activity, osteoid volume, and osteoblast number in patient bone.

    Evidence Bone biopsy immunostaining, RT-PCR, western blot, and mass spectrometry of collagen hydroxylation in patient osteoblasts

    PMID:38214665

    Open questions at the time
    • Single patient sample
    • Causal chain from collagen defect to reduced osteoblast number not resolved
    • Cell-autonomous vs systemic effects not separated

Open questions

Synthesis pass · forward-looking unresolved questions
  • How CRTAP's distinct activities — scaffolding the bifunctional reaction center, mutual stabilization of P3H1, and collagen chaperoning — are mechanistically separable and individually contribute to the osteogenesis imperfecta phenotype remains unresolved.
  • No separation-of-function alleles isolating hydroxylase vs chaperone roles
  • Functional meaning of the dual-ternary complex equilibrium unknown
  • Mechanism linking collagen defect to mineralization abnormality undefined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0044183 protein folding chaperone 2 GO:0060090 molecular adaptor activity 1
Localization
GO:0005783 endoplasmic reticulum 2 GO:0031012 extracellular matrix 1
Pathway
R-HSA-392499 Metabolism of proteins 3 R-HSA-1474244 Extracellular matrix organization 2 R-HSA-1643685 Disease 2
Partners
P3H1PPIB
Complex memberships
P3H1/CRTAP/PPIB collagen prolyl 3-hydroxylation complex

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2024 Cryo-EM structures of the P3H1/CRTAP/PPIB ternary complex reveal that the active sites of P3H1 and PPIB form a face-to-face bifunctional reaction center, indicating a coupled collagen modification mechanism. A structure with bound collagen peptide reveals multiple substrate-binding sites forming a substrate-interacting zone. An unexpected dual-ternary complex was also observed, whose equilibrium with the ternary state can be altered by mutations at the P3H1/PPIB active site or addition of PPIB inhibitors. Cryo-EM structure determination, active-site mutagenesis, collagen peptide co-complex Nature communications High 39245686
2009 CRTAP and P3H1 are mutually stabilized at the protein level within the ER collagen prolyl 3-hydroxylation complex: null mutations in either gene cause loss of both proteins (despite normal mRNA of the unaffected gene), and stable transfection of the missing cDNA restores both protein levels and Pro986 3-hydroxylation activity. Proteasomal inhibitors partially rescue P3H1 in CRTAP-null cells, indicating proteasome-mediated degradation of the unstabilized partner. Western blot, immunofluorescence, stable transfection rescue, proteasomal inhibitor treatment in patient fibroblasts Human molecular genetics High 19846465
2008 CRTAP and P3H1 (LEPRE1) form a complex that catalyzes 3-hydroxylation of Pro986 in the α1(I) and α1(II) collagen chains and recruits cyclophilin B (CYPB) to unfolded collagen. Loss-of-function mutations in CRTAP abolish this hydroxylation. Biochemical analysis of collagen from patient fibroblasts with CRTAP mutations; complex composition determined by prior biochemical studies cited Human mutation Medium 18566967
2009 CRTAP, P3H1, and cyclophilin B (PPIB) comprise the collagen prolyl 3-hydroxylation complex in the ER. Absence of CRTAP or P3H1 leads to overmodification of the type I collagen helical region (by lysyl hydroxylase and prolyl 4-hydroxylase), indicating substantially delayed collagen helix folding. Biochemical collagen analysis from patient cells and Crtap-/- mice; SDS-PAGE migration shift assay for collagen overmodification Cell and tissue research High 19862557
2010 CRTAP and P3H1 are required to maintain a stable complex that 3-hydroxylates Pro986 in clade A collagen chains (types I, II, and V). Loss of CRTAP also abolishes 3-hydroxylation at Pro986 in α2(V) chains, but does not affect 3-hydroxylation at two known sites in α1(IV) chains, indicating substrate specificity of the complex for fibrillar collagens. Mass spectrometry-based collagen hydroxyproline analysis from Crtap-/- mouse tissues (lung, kidney) and human OI fibroblasts PloS one High 20485499
2011 CRTAP deficiency reduces deposition of type I collagen into the extracellular matrix (to 10–15% of control) and causes disorganization of the fibrillar network and increased collagen fibril diameters, demonstrating a chaperone/matrix-assembly role for CRTAP beyond intracellular hydroxylation. Immunofluorescence of long-term fibroblast cultures, quantitative pulse-chase experiments, electron microscopy of dermal collagen fibrils Clinical genetics Medium 21955071
2020 In zebrafish lacking crtap (CRISPR/Cas9 knockout), type I collagen is intracellularly overmodified and partially retained in enlarged ER cisternae, and extracellular collagen assembles into disorganized fibers with altered fibril diameter, supporting the complex's primary role as a collagen chaperone rather than solely a hydroxylase. CRISPR/Cas9 knockout zebrafish, transmission electron microscopy, collagen biochemical analysis Matrix biology High 32173581
2023 Deep intronic CRTAP mutations generate unstable truncated/aberrant isoforms containing a 'GWxxI' degron sequence, leading to protein instability, loss of Pro986 hydroxylation, type I collagen aggregation, and cell death by senescence. Collagen aggregates are partially cleared by autophagy. Genome sequencing, minigene splicing assay, western blot of patient cells, collagen aggregation assay, autophagy inhibition experiments Biochimica et biophysica acta. Molecular basis of disease Medium 37146916
1999 Human CRTAP (CASP) encodes a novel cartilage-associated protein with high sequence identity to mouse and lower identity to chick orthologs; the gene is expressed in cartilage and maps to chromosome 3p22; CRTAP protein is related to but distinct from certain nuclear proteins, defining a new protein family. cDNA cloning, Northern blot, FISH chromosome mapping, immunohistochemistry Cytogenetics and cell genetics Medium 10702664
2009 CRTAP deficiency (in Crtap-/- mice and OI type VII patients) results in significantly elevated bone matrix mineral content and altered mineralization kinetics, with increased proportions of highly mineralized matrix and primary bone persistence, indicating that abnormal collagen modification secondary to CRTAP loss directly perturbs bone mineralization. Quantitative backscattered electron imaging (qBEI) of mouse femurs and human bone biopsies Bone Medium 19895918
2021 Crtap-/- mouse tendons show reduced collagen fibril size, increased collagen cross-links, and altered progenitor cell populations, with dysregulated TGF-β, inflammatory, and metabolic signaling by RNA-seq, and increased αSMA, MMP2, and phospho-NFκB staining indicating excess matrix remodeling and tissue inflammation. Mechanical testing of isolated tendons, electron microscopy, RNA-seq, immunostaining, flow cytometry of tendon progenitor cells, behavioral motor testing eLife Medium 34036937
2024 Biallelic CRTAP mutations reduce CRTAP mRNA and protein in osteoblasts, leading to significantly reduced prolyl 3-hydroxylation at Pro986 of the α1(I) collagen chain, reduced osteoid volume, and reduced osteoblast numbers, directly linking CRTAP function in osteoblasts to active bone formation. Bone biopsy immunostaining, RT-PCR, western blot, mass spectrometry of collagen hydroxylation in patient osteoblasts The Journal of clinical endocrinology and metabolism Medium 38214665

Source papers

Stage 0 corpus · 23 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2008 CRTAP and LEPRE1 mutations in recessive osteogenesis imperfecta. Human mutation 173 18566967
2009 Null mutations in LEPRE1 and CRTAP cause severe recessive osteogenesis imperfecta. Cell and tissue research 85 19862557
2016 Sclerostin Antibody Treatment Improves the Bone Phenotype of Crtap(-/-) Mice, a Model of Recessive Osteogenesis Imperfecta. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research 68 26716893
2009 Prolyl 3-hydroxylase 1 and CRTAP are mutually stabilizing in the endoplasmic reticulum collagen prolyl 3-hydroxylation complex. Human molecular genetics 62 19846465
2009 CRTAP deficiency leads to abnormally high bone matrix mineralization in a murine model and in children with osteogenesis imperfecta type VII. Bone 57 19895918
2010 Generalized connective tissue disease in Crtap-/- mouse. PloS one 52 20485499
2009 CRTAP mutations in lethal and severe osteogenesis imperfecta: the importance of combining biochemical and molecular genetic analysis. European journal of human genetics : EJHG 40 19550437
2020 Crtap and p3h1 knock out zebrafish support defective collagen chaperoning as the cause of their osteogenesis imperfecta phenotype. Matrix biology : journal of the International Society for Matrix Biology 34 32173581
2011 Deficiency of CRTAP in non-lethal recessive osteogenesis imperfecta reduces collagen deposition into matrix. Clinical genetics 24 21955071
2011 Severe osteogenesis imperfecta caused by a small in-frame deletion in CRTAP. American journal of medical genetics. Part A 20 21964860
2023 Deep intronic mutation in CRTAP results in unstable isoforms of the protein to induce type I collagen aggregation in a lethal type of osteogenesis imperfecta type VII. Biochimica et biophysica acta. Molecular basis of disease 19 37146916
2020 Dental and craniofacial defects in the Crtap-/- mouse model of osteogenesis imperfecta type VII. Developmental dynamics : an official publication of the American Association of Anatomists 18 32133710
1999 cDNA cloning, characterization and chromosome mapping of Crtap encoding the mouse cartilage associated protein. Matrix biology : journal of the International Society for Matrix Biology 18 10429950
2015 CRTAP mutation in a patient with Cole-Carpenter syndrome. American journal of medical genetics. Part A 17 25604815
2021 Tendon and motor phenotypes in the Crtap mouse model of recessive osteogenesis imperfecta. eLife 16 34036937
2015 Mutational characterization of the P3H1/CRTAP/CypB complex in recessive osteogenesis imperfecta. Genetics and molecular research : GMR 16 26634552
2024 The structural basis for the collagen processing by human P3H1/CRTAP/PPIB ternary complex. Nature communications 11 39245686
2009 Common variants in FLNB/CRTAP, not ARHGEF3 at 3p, are associated with osteoporosis in southern Chinese women. Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA 11 19727905
1999 cDNA cloning, characterization and chromosome mapping of the gene encoding human cartilage associated protein (CRTAP). Cytogenetics and cell genetics 11 10702664
2020 Novel Compound Heterozygous Mutations in CRTAP Cause Rare Autosomal Recessive Osteogenesis Imperfecta. Frontiers in genetics 8 32922437
2024 Genetic Analysis, Phenotypic Spectrum and Functional Study of Rare Osteogenesis Imperfecta Caused by CRTAP Variants. The Journal of clinical endocrinology and metabolism 5 38214665
2022 Craniofacial and dental phenotype of two girls with osteogenesis imperfecta due to mutations in CRTAP. Bone 4 35970273
2025 CRTAP-Related Osteogenesis Imperfecta: Clinical Variability and a Potential Founder Variant in CRTAP. Molecular syndromology 0 41064055

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