id: P50454
gene_symbol: SERPINH1
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: SERPINH1 (Serpin H1, better known as HSP47, also called colligin / gp46) is an endoplasmic reticulum-resident, collagen-specific molecular chaperone. Although it belongs to the serpin superfamily by fold, it is non-inhibitory and does not act as a protease inhibitor. In the ER lumen HSP47 binds specifically to the folded triple-helical region of procollagen, stabilizing the nascent triple helix, preventing local unfolding and premature aggregation, and serving as a quality-control factor in collagen biosynthesis. HSP47 accompanies procollagen from the ER to the ER-Golgi intermediate compartment/cis-Golgi, where the lower pH triggers its release; it then recycles back to the ER through its C-terminal RDEL retrieval signal. It is heat-shock inducible. Loss-of-function variants cause autosomal-recessive osteogenesis imperfecta type X (OI10), underscoring its essential role in collagen maturation.
existing_annotations:
- term:
    id: GO:0004867
    label: serine-type endopeptidase inhibitor activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Phylogenetic transfer of serpin protease-inhibitor activity. HSP47 is a well-documented non-inhibitory serpin that functions as a collagen chaperone, not a protease inhibitor.
    action: MARK_AS_OVER_ANNOTATED
    reason: Inferred from the serpin fold/family, but HSP47/SERPINH1 lacks functional protease-inhibitory activity; its characterized role is collagen binding/chaperoning.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen. Could be involved as a chaperone in the biosynthetic pathway of collagen.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: HSP47 acts in the endoplasmic reticulum, where it chaperones procollagen. ER localization is well established.
    action: ACCEPT
    reason: Consistent with UniProt subcellular location (ER lumen) and multiple experimental annotations; the ER is the genuine site of HSP47 action.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0030199
    label: collagen fibril organization
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: HSP47 contributes to collagen biogenesis; proper fibril organization downstream depends on correctly matured procollagen. HSP47 itself acts in the ER, upstream of extracellular fibril assembly.
    action: KEEP_AS_NON_CORE
    reason: Collagen fibril organization is a downstream/extracellular consequence of HSP47's ER chaperone activity; a reasonable process annotation but not its direct molecular role.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen. Could be involved as a chaperone in the biosynthetic pathway of collagen.
- term:
    id: GO:0004867
    label: serine-type endopeptidase inhibitor activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro serpin-family transfer of protease-inhibitor activity. HSP47 is non-inhibitory.
    action: MARK_AS_OVER_ANNOTATED
    reason: Domain-based transfer; HSP47/SERPINH1 does not function as a serine protease inhibitor despite its serpin fold.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen. Could be involved as a chaperone in the biosynthetic pathway of collagen.
- term:
    id: GO:0005518
    label: collagen binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: HSP47 binds specifically to collagen (folded triple helix); this is its defining, core molecular function.
    action: ACCEPT
    reason: Directly supported by UniProt FUNCTION and by extensive literature; collagen binding is the central molecular activity of HSP47.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen.
- term:
    id: GO:0005788
    label: endoplasmic reticulum lumen
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Automated (UniProt SubCell) annotation of ER lumen, the precise compartment where HSP47 chaperones procollagen.
    action: ACCEPT
    reason: Matches UniProt subcellular location exactly; ER lumen is the genuine and specific HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  qualifier: involved_in
  review:
    summary: Process annotation inferred from the protein folding chaperone MF. HSP47 stabilizes already-folded triple-helical procollagen and prevents aggregation rather than catalyzing folding de novo.
    action: KEEP_AS_NON_CORE
    reason: HSP47 is a holdase-type collagen chaperone, not a foldase; protein folding is a reasonable downstream process but non-core relative to collagen binding.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Could be involved as a chaperone in the biosynthetic pathway of collagen.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32814053
  qualifier: enables
  review:
    summary: Large-scale neurodegeneration interactome screen capturing many HSP47 interactions with diverse partners (e.g. CDH1, ETS2), none of which are collagen. Bare protein binding is uninformative and does not reflect HSP47's collagen-specific function.
    action: KEEP_AS_NON_CORE
    reason: High-throughput interactome partners unrelated to HSP47's characterized collagen-chaperone role; uninformative protein binding term.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen.
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: Ensembl ortholog-based cytoplasm annotation. HSP47 is an ER-lumenal protein; the cytoplasm term is a coarse parent localization inconsistent with its specific ER-lumen residence.
    action: MARK_AS_OVER_ANNOTATED
    reason: Conflicts with the well-established ER-lumen localization; cytoplasm is an over-general/ortholog-transfer localization for this secretory-pathway protein.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: Ensembl ortholog-based ER localization, consistent with HSP47's documented ER residence.
    action: ACCEPT
    reason: Agrees with UniProt and experimental ER annotations; the genuine compartment of HSP47.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0044183
    label: protein folding chaperone
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: HSP47 is a collagen-specific molecular chaperone; the generic chaperone MF is correct but the precise, informative MF is collagen binding.
    action: ACCEPT
    reason: HSP47 functions as a molecular chaperone for procollagen; supported by UniProt FUNCTION. Captured more specifically by collagen binding in core_functions.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Could be involved as a chaperone in the biosynthetic pathway of collagen.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) evidence for ER localization, consistent with HSP47's role as an ER collagen chaperone.
    action: ACCEPT
    reason: IDA-supported ER localization corroborating UniProt; genuine HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0031012
    label: extracellular matrix
  evidence_type: HDA
  original_reference_id: PMID:28327460
  qualifier: colocalizes_with
  review:
    summary: High-throughput matrisome/ECM proteomics detection. HSP47 is ER-resident; ECM detection likely reflects co-secretion with collagen or proteomic carryover, not a core localization.
    action: KEEP_AS_NON_CORE
    reason: HDA ECM detection is peripheral to HSP47's documented ER-lumen site of action.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0031012
    label: extracellular matrix
  evidence_type: HDA
  original_reference_id: PMID:28675934
  qualifier: located_in
  review:
    summary: High-throughput ECM proteomics detection of HSP47, peripheral to its ER chaperone role.
    action: KEEP_AS_NON_CORE
    reason: HDA matrisome detection; not the core ER-lumen localization of HSP47.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0031012
    label: extracellular matrix
  evidence_type: HDA
  original_reference_id: PMID:25037231
  qualifier: located_in
  review:
    summary: High-throughput ECM proteomics detection of HSP47, peripheral to its ER chaperone role.
    action: KEEP_AS_NON_CORE
    reason: HDA matrisome detection; not the core ER-lumen localization of HSP47.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0045121
    label: membrane raft
  evidence_type: IDA
  original_reference_id: PMID:25204797
  qualifier: located_in
  review:
    summary: Reported cell-surface/membrane-raft pool of HSP47 in a specific context. A minor, non-canonical localization relative to its predominant ER-lumen residence.
    action: KEEP_AS_NON_CORE
    reason: A specialized, context-dependent localization; peripheral to HSP47's core ER collagen-chaperone function.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0003723
    label: RNA binding
  evidence_type: HDA
  original_reference_id: PMID:22658674
  qualifier: enables
  review:
    summary: mRNA-interactome-capture detection of HSP47 as an RNA-binder. There is no characterized RNA-dependent function for HSP47; this is a generic proteome-wide capture result.
    action: MARK_AS_OVER_ANNOTATED
    reason: High-throughput RNA-interactome capture without a validated functional role; not part of HSP47's collagen-chaperone function.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen.
- term:
    id: GO:0005788
    label: endoplasmic reticulum lumen
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-2022073
  qualifier: located_in
  review:
    summary: Curated (Reactome) ER lumen localization, matching the precise compartment of HSP47.
    action: ACCEPT
    reason: Consistent with UniProt ER-lumen location; the genuine and specific HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0005788
    label: endoplasmic reticulum lumen
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-2089971
  qualifier: located_in
  review:
    summary: Curated (Reactome) ER lumen localization, matching the precise compartment of HSP47.
    action: ACCEPT
    reason: Consistent with UniProt ER-lumen location; the genuine and specific HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0004867
    label: serine-type endopeptidase inhibitor activity
  evidence_type: TAS
  original_reference_id: PMID:1309665
  qualifier: enables
  review:
    summary: Author-stated serpin-family inhibitor classification. HSP47 is a non-inhibitory serpin; the inhibitory activity is a historical fold-based attribution.
    action: MARK_AS_OVER_ANNOTATED
    reason: HSP47/SERPINH1 does not function as a protease inhibitor despite the serpin fold; its characterized function is collagen chaperoning.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen.
- term:
    id: GO:0005518
    label: collagen binding
  evidence_type: NAS
  original_reference_id: PMID:1309665
  qualifier: enables
  review:
    summary: Author-stated collagen binding, HSP47's defining molecular function.
    action: ACCEPT
    reason: Collagen binding is the core, well-established molecular activity of HSP47; supported by UniProt and literature.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: TAS
  original_reference_id: PMID:1309665
  qualifier: located_in
  review:
    summary: Author-stated ER localization, consistent with HSP47's documented ER residence.
    action: ACCEPT
    reason: Agrees with UniProt and experimental evidence; genuine HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: TAS
  original_reference_id: PMID:7656593
  qualifier: located_in
  review:
    summary: Author-stated ER localization, consistent with HSP47's documented ER residence.
    action: ACCEPT
    reason: Agrees with UniProt and experimental evidence; genuine HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0006986
    label: response to unfolded protein
  evidence_type: TAS
  original_reference_id: PMID:1309665
  qualifier: involved_in
  review:
    summary: HSP47 is heat-shock inducible, historically framed as a stress/UPR-associated chaperone. However it is collagen-specific and not a general unfolded-protein-response chaperone.
    action: KEEP_AS_NON_CORE
    reason: HSP47 is stress-inducible but its substrate specificity is collagen, not general unfolded proteins; this process annotation is peripheral to its core role.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'INDUCTION: By heat shock.'
- term:
    id: GO:0005793
    label: endoplasmic reticulum-Golgi intermediate compartment
  evidence_type: IDA
  original_reference_id: PMID:15308636
  qualifier: located_in
  review:
    summary: Direct evidence that HSP47 localizes to the ER-Golgi intermediate compartment, consistent with its pH-dependent procollagen escort and recycling itinerary.
    action: ACCEPT
    reason: IDA-supported ERGIC localization matching the established HSP47 trafficking cycle (procollagen escort to ERGIC/cis-Golgi, pH-triggered release, RDEL-mediated return).
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-goa.tsv
      supporting_text: GO:0005793 endoplasmic reticulum-Golgi intermediate compartment
- term:
    id: GO:0005518
    label: collagen binding
  evidence_type: NAS
  original_reference_id: PMID:7656593
  qualifier: enables
  review:
    summary: Author-stated collagen binding, HSP47's defining molecular function.
    action: ACCEPT
    reason: Collagen binding is the core, well-established molecular activity of HSP47.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: Binds specifically to collagen.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: NAS
  original_reference_id: PMID:7656593
  qualifier: located_in
  review:
    summary: Author-stated ER localization, consistent with HSP47's documented ER residence.
    action: ACCEPT
    reason: Agrees with UniProt and experimental evidence; genuine HSP47 compartment.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum lumen.'
- term:
    id: GO:0006986
    label: response to unfolded protein
  evidence_type: TAS
  original_reference_id: PMID:10023073
  qualifier: involved_in
  review:
    summary: Heat-shock/stress-associated chaperone framing of HSP47. HSP47 is collagen-specific rather than a general UPR chaperone.
    action: KEEP_AS_NON_CORE
    reason: Stress-inducible but collagen-specific; this process annotation is peripheral to HSP47's core collagen-chaperone role.
    supported_by:
    - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
      supporting_text: 'INDUCTION: By heat shock.'
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB Subcellular Location vocabulary mapping
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to orthologs by Ensembl Compara
  findings: []
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on on inter-ontology links
  findings: []
- id: PMID:10023073
  title: 'The human genome has only one functional hsp47 gene (CBP2) and a pseudogene (pshsp47).'
  findings: []
- id: PMID:1309665
  title: 'Cloning of a human collagen-binding protein, and its homology with rat gp46, chick hsp47 and mouse J6 proteins.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: GOA-anchored (this PMID supports GO:0005518 collagen binding and GO:0005783 ER in SERPINH1-goa.tsv); early characterization of Serpin H1/HSP47 as the collagen-binding ER stress protein, supporting the collagen-binding core molecular function.
- id: PMID:15308636
  title: Proteomics of endoplasmic reticulum-Golgi intermediate compartment (ERGIC) membranes from brefeldin A-treated HepG2 cells identifies ERGIC-32, a new cycling protein that interacts with human Erv46.
  findings:
  - statement: HSP47 localizes to the ER-Golgi intermediate compartment, consistent with pH-dependent escort and recycling of procollagen.
    reference_section_type: RESULTS
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: ERGIC proteomics study supporting HSP47's ER/ERGIC cycling localization during collagen biosynthesis. Title corrected to verbatim PubMed (previously an HSP47-specific paraphrase).
- id: PMID:22658674
  title: Insights into RNA biology from an atlas of mammalian mRNA-binding proteins.
  findings: []
- id: PMID:25037231
  title: Extracellular matrix signatures of human primary metastatic colon cancers and their metastases to liver.
  findings: []
- id: PMID:25204797
  title: "Flotillin-1 facilitates toll-like receptor 3 signaling in human endothelial cells."
  findings: []
- id: PMID:28327460
  title: Comprehensive proteomic characterization of stem cell-derived extracellular matrices.
  findings: []
- id: PMID:28675934
  title: Characterization of the Extracellular Matrix of Normal and Diseased Tissues Using Proteomics.
  findings: []
- id: PMID:32814053
  title: Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.
  findings: []
- id: PMID:7656593
  title: 'Isolation, characterization and chromosomal assignment of human colligin-2 gene (CBP2).'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: GOA-anchored (this PMID supports GO:0005518 collagen binding and GO:0005783 ER in SERPINH1-goa.tsv); seminal evidence establishing HSP47/SERPINH1 as the ER-resident collagen-specific molecular chaperone - its core function.
- id: Reactome:R-HSA-2022073
  title: Collagen biosynthesis and modifying enzymes
  findings: []
- id: Reactome:R-HSA-2089971
  title: Collagen biosynthesis pathway
  findings: []
- id: file:human/SERPINH1/SERPINH1-uniprot.txt
  title: UniProt entry P50454 (SERPH_HUMAN), Serpin H1 / HSP47
  findings:
  - statement: HSP47 binds specifically to collagen and acts as a chaperone in collagen biosynthesis; it resides in the ER lumen, is heat-shock inducible, belongs to the serpin family (non-inhibitory), and its loss causes osteogenesis imperfecta type 10.
    reference_section_type: OTHER
core_functions:
- description: ER-resident collagen-specific molecular chaperone that binds the folded triple-helical region of procollagen, stabilizing it and preventing premature aggregation during collagen biosynthesis.
  molecular_function:
    id: GO:0005518
    label: collagen binding
  locations:
  - id: GO:0005788
    label: endoplasmic reticulum lumen
  supported_by:
  - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
    supporting_text: Binds specifically to collagen.
  - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
    supporting_text: Could be involved as a chaperone in the biosynthetic pathway of collagen.
- description: Collagen-dedicated chaperone activity within the early secretory pathway; HSP47 escorts procollagen from the ER to the ER-Golgi intermediate compartment, where pH-dependent release allows HSP47 to recycle back to the ER.
  molecular_function:
    id: GO:0044183
    label: protein folding chaperone
  locations:
  - id: GO:0005788
    label: endoplasmic reticulum lumen
  - id: GO:0005793
    label: endoplasmic reticulum-Golgi intermediate compartment
  supported_by:
  - reference_id: file:human/SERPINH1/SERPINH1-uniprot.txt
    supporting_text: Could be involved as a chaperone in the biosynthetic pathway of collagen.
  - reference_id: file:human/SERPINH1/SERPINH1-goa.tsv
    supporting_text: GO:0005793 endoplasmic reticulum-Golgi intermediate compartment
proposed_new_terms: []
suggested_questions:
- question: What is the precise structural basis and pH threshold for HSP47 release from procollagen in the ERGIC/cis-Golgi, and how is this coupled to its RDEL-mediated ER retrieval?
- question: Beyond fibrillar collagens, which collagen types and other ER clients (if any) does HSP47 chaperone, and how does substrate specificity arise from triple-helix recognition?
- question: How do OI10-causing SERPINH1 variants mechanistically impair collagen maturation (loss of binding, mislocalization, or destabilization)?
suggested_experiments:
- description: Quantitative in vitro binding/turbidity assays with purified HSP47 and triple-helical vs unfolded collagen peptides across a pH gradient to map binding affinity and the pH-dependent release transition.
- description: Knock-in of OI10 patient variants in osteoblast or fibroblast models followed by collagen secretion, triple-helix stability, and ER-stress assays.
- description: Proximity-labeling (BioID/APEX) of HSP47 in the secretory pathway to define its client repertoire and trafficking-machinery partners during collagen transport.