id: Q96CD0
gene_symbol: FBXL8
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  FBXL8 (F-box/LRR-repeat protein 8, FBL8) is a member of the FBXL subfamily of
  F-box proteins. It has an N-terminal F-box domain through which it docks onto
  the SKP1 adaptor and, via SKP1, onto CUL1 and the catalytic RING subunit RBX1,
  assembling a Cullin-RING (SCF) E3 ubiquitin ligase in which the F-box protein
  serves as the substrate-recognition subunit. Despite the "LRR" in its name,
  UniProt notes that FBXL8 does not actually contain canonical leucine-rich
  repeats; nonetheless its C-terminal substrate-binding region is functionally
  required for substrate engagement (deletion of either the F-box or the
  C-terminal region abolishes substrate turnover). FBXL8 acts as the
  substrate-recognition subunit of SCF-FBXL8 and targets several substrates in a
  strongly context-dependent manner: it promotes ubiquitination and proteasomal
  degradation of the tumor suppressor p53 (in colorectal cancer), of Thr283-
  phosphorylated cyclin D3 (CCND3) (in lymphoma), of Snail1 (in post-myocardial-
  infarction cardiac fibroblasts, dampening RhoA signaling and myofibroblast
  differentiation), and of unphosphorylated c-MYC (a distinct c-MYC pool from
  that controlled by FBXW7, with reported heterotypic K48/K63 chains); CCND2 and
  IRF5 are additional candidate substrates accumulating upon FBXL8 knockdown.
  Reported localization is predominantly cytoplasmic (loss of FBXL8 causes nuclear
  c-MYC accumulation), and in heart FBXL8 is enriched in cardiac fibroblasts. Its
  net effect is context-dependent rather than uniformly oncogenic or
  tumor-suppressive: oncogenic in colorectal and breast cancer, but
  tumor-suppressive in lymphoma and anti-fibrotic after myocardial infarction.
existing_annotations:
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19159283
  qualifier: enables
  review:
    summary: IntAct interaction with SKP1 (WITH/FROM UniProtKB:P63208) captured by the Array MAPPIT high-throughput screen. The SKP1 interaction is the defining F-box-domain partnership, but the bare protein binding term is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records the functionally meaningful FBXL8-SKP1 interaction (the basis of SCF assembly), but bare protein binding is uninformative per curation guidelines; the SCF/SCF-degradation annotations capture this relationship.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P63208: SKP1; NbExp=18; IntAct=EBI-2321097, EBI-307486;'
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  qualifier: enables
  review:
    summary: IntAct interaction with SKP1 (WITH/FROM UniProtKB:P63208) captured by a proteome-scale interactome map. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records the FBXL8-SKP1 interaction underlying SCF assembly, but bare protein binding is uninformative.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P63208: SKP1; NbExp=18; IntAct=EBI-2321097, EBI-307486;'
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:26496610
  qualifier: enables
  review:
    summary: IntAct interaction with SKP1 (WITH/FROM UniProtKB:P63208) captured by a quantitative-interactome study. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records the FBXL8-SKP1 interaction, but bare protein binding is uninformative.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P63208: SKP1; NbExp=18; IntAct=EBI-2321097, EBI-307486;'
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:27705803
  qualifier: enables
  review:
    summary: IntAct interaction with SKP1 (WITH/FROM UniProtKB:P63208) captured by the Polycomb complexome AP-MS study. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records the FBXL8-SKP1 interaction, consistent with SCF assembly rather than a Polycomb-specific function; bare protein binding is uninformative.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P63208: SKP1; NbExp=18; IntAct=EBI-2321097, EBI-307486;'
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: IntAct binary-interactome interactions captured here include ALAS1 (P13196), PICK1 (Q9NRD5) and SKP1 (P63208). SKP1 is the canonical F-box partner; ALAS1/PICK1 are candidate substrates/partners but not experimentally validated as FBXL8 substrates. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records real binary interactions (including the SCF-defining SKP1 partner and candidate substrates ALAS1/PICK1), but bare protein binding is uninformative and no substrate relationship is experimentally established.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P13196: ALAS1; NbExp=3; IntAct=EBI-2321097, EBI-3905054;'
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    summary: IntAct interaction with SKP1 (WITH/FROM UniProtKB:P63208) captured by the BioPlex dual proteome-scale interactome. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records the FBXL8-SKP1 interaction, but bare protein binding is uninformative.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P63208: SKP1; NbExp=18; IntAct=EBI-2321097, EBI-307486;'
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:40205054
  qualifier: enables
  review:
    summary: IntAct interaction with SKP1 (WITH/FROM UniProtKB:P63208) captured by a multimodal cell-map interactome study. Bare protein binding is uninformative.
    action: KEEP_AS_NON_CORE
    reason: Records the FBXL8-SKP1 interaction, but bare protein binding is uninformative.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: 'Q96CD0; P63208: SKP1; NbExp=18; IntAct=EBI-2321097, EBI-307486;'
- term:
    id: GO:0031146
    label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process
  evidence_type: NAS
  original_reference_id: PMID:33234069
  qualifier: involved_in
  review:
    summary: ComplexPortal/NAS assignment that FBXL8, as an SCF substrate receptor, participates in SCF-dependent proteasomal degradation. This is the core biological process for an F-box protein, and FBXL8-specific degradative substrates are now reported in the primary literature (p53 in colorectal cancer, Thr283-phospho-cyclin D3 in lymphoma, Snail1 in cardiac fibroblasts, and unphosphorylated c-MYC).
    action: ACCEPT
    reason: Consistent with FBXL8 being a substrate-recognition subunit of an SCF E3 ligase (documented SKP1/CUL1 interaction); SCF-dependent degradation is the canonical core process for F-box proteins, and multiple FBXL8 substrates whose turnover requires the F-box are now experimentally documented.
    additional_reference_ids:
    - file:human/FBXL8/FBXL8-deep-research-falcon.md
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
    - reference_id: file:human/FBXL8/FBXL8-deep-research-falcon.md
      supporting_text: "Across multiple primary studies, FBXL8 functions as an **SCF E3 ligase adaptor** that promotes **substrate ubiquitination**, frequently leading to **proteasome-dependent degradation** of specific targets"
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8952618
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation, propagated across the generic CRL1/neddylation reaction set. The cytosol is a plausible compartment for an SCF substrate receptor.
    action: KEEP_AS_NON_CORE
    reason: Plausible localization for a cytosolic SCF component, but derived from generic CRL pathway membership (one of many identical Reactome reactions) rather than FBXL8-specific evidence.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8952620
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic CRL1 neddylation reaction set.
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8955241
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic CRL (CAND1) reaction set.
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8955289
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic CRL (COMMD/CAND1) reaction set.
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8956040
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic CRL deneddylation (COP9) reaction set.
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-8956200
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic CRL1 (DCUN1D3) reaction set.
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983140
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic ubiquitination reaction set (transfer of Ub from E2 to substrate).
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983147
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic ubiquitination reaction set (release of E3 from substrate).
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983156
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic ubiquitination reaction set (polyubiquitination of substrate).
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-983157
  qualifier: located_in
  review:
    summary: Reactome pathway-level cytosol annotation from the generic ubiquitination reaction set (interaction of E3 with substrate and E2-Ub).
    action: KEEP_AS_NON_CORE
    reason: Plausible but redundant generic-pathway localization; not FBXL8-specific.
    supported_by:
    - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
      supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
references:
- id: PMID:19159283
  title: 'Array MAPPIT: high-throughput interactome analysis in mammalian cells.'
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput MAPPIT interactome; FBXL8 GOA IPI WITH/FROM is SKP1 (P63208), consistent with SCF assembly.
- id: PMID:25416956
  title: A proteome-scale map of the human interactome network.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput interactome; FBXL8 IPI WITH/FROM is SKP1 (P63208).
- id: PMID:26496610
  title: A human interactome in three quantitative dimensions organized by stoichiometries
    and abundances.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Quantitative interactome; FBXL8 IPI WITH/FROM is SKP1 (P63208).
- id: PMID:27705803
  title: A High-Density Map for Navigating the Human Polycomb Complexome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: AP-MS complexome map; FBXL8 IPI WITH/FROM is SKP1 (P63208), reflecting SCF assembly rather than a Polycomb-specific function.
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Binary interactome reference map; FBXL8 IPI partners include SKP1 (P63208), ALAS1 (P13196) and PICK1 (Q9NRD5). The latter two are candidate substrates/partners but not experimentally validated as FBXL8 substrates.
- id: PMID:33234069
  title: 'The FBXL family of F-box proteins: variations on a theme.'
  findings:
  - statement: FBXL-family F-box proteins serve as substrate-recognition subunits of SCF E3 ubiquitin ligases, recruiting SKP1/CUL1 via the F-box for substrate ubiquitination and proteasomal degradation.
    reference_section_type: LITERATURE_REVIEW
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Family-level review (full text available) supporting the general FBXL SCF substrate-receptor model; no FBXL8-specific functional data found.
- id: PMID:33961781
  title: Dual proteome-scale networks reveal cell-specific remodeling of the human
    interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: BioPlex interactome; FBXL8 IPI WITH/FROM is SKP1 (P63208).
- id: PMID:40205054
  title: Multimodal cell maps as a foundation for structural and functional genomics.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Multimodal cell-map interactome; FBXL8 IPI WITH/FROM is SKP1 (P63208).
- id: file:human/FBXL8/FBXL8-deep-research-falcon.md
  title: Falcon deep research report for human FBXL8
  findings:
  - statement: FBXL8 is the substrate-recognition adaptor of an SCF E3 ubiquitin ligase whose net biological effect is strongly context-dependent rather than uniformly oncogenic or tumor-suppressive.
    supporting_text: "FBXL8 is an SCF-type E3 ubiquitin ligase substrate receptor (F-box + LRR protein) that determines substrate specificity and promotes ubiquitination of distinct targets in a context-dependent manner, influencing proteasomal degradation or other ubiquitin-mediated outcomes"
  - statement: SCF-FBXL8 binds p53, increases its ubiquitination and accelerates its turnover in colorectal cancer; both the F-box and the C-terminal region are required.
    supporting_text: "FBXL8 physically associates with p53 (co-immunoprecipitation), increases p53 ubiquitination, and accelerates p53 turnover in cycloheximide chase experiments; **both the F-box and LRR regions are required** for this effect, because ΔFbox and ΔLRR constructs fail to influence p53 stability like full-length FBXL8"
  - statement: SCF-FBXL8 recognizes Thr283-phosphorylated cyclin D3 and drives its proteasomal degradation, suppressing lymphoma growth.
    supporting_text: "FBXL8 (as SCF-FBXL8) recognizes **Thr-283 phosphorylated cyclin D3**, polyubiquitylates it, and drives proteasomal degradation."
  - statement: In post-myocardial-infarction cardiac fibroblasts FBXL8 promotes ubiquitin-proteasome degradation of Snail1, reducing RhoA activation and myofibroblast differentiation.
    supporting_text: "In post-myocardial infarction (MI) remodeling, FBXL8 interacts with **Snail1** and promotes its **ubiquitin–proteasome degradation**, with a defined interaction requirement (FBXL8 ΔC3 domain requirement). This down-modulates downstream **RhoA activation** and dampens myofibroblast differentiation"
  - statement: FBXL8 recognizes and ubiquitylates unphosphorylated c-MYC, a distinct pool from that controlled by FBXW7, and is reported to be largely cytoplasmic.
    supporting_text: "FBXL8 recognizes and ubiquitylates **unphosphorylated c-MYC**, distinct from the canonical phospho-degron recognition by FBXW7. Concurrent loss of FBXL8 and FBXW7 additively elevates c-MYC, consistent with regulation of distinct c-MYC pools."
  reference_review:
    relevance: HIGH
    correctness: UNVERIFIED
    review_notes: Falcon (Edison Scientific) deep research synthesis. Reports multiple experimentally-supported FBXL8 substrates (p53, cyclin D3/CCND3 phospho-Thr283, Snail1, unphosphorylated c-MYC, candidate CCND2/IRF5) from primary literature (Yao 2023, Yoshida 2021, Li 2024, Bajpai 2022, Chang 2020) citing DOIs rather than PMIDs; these primary papers are not in the local cache so the citations are not independently PubMed-verified here. Treated as leads that substantially upgrade the previously "poorly characterized" framing into a well-characterized context-dependent SCF substrate receptor.
- id: Reactome:R-HSA-8952618
  title: AcM-UBE2M transfers NEDD8 to CRL1 E3 ubiquitin ligase complex
  findings: []
- id: Reactome:R-HSA-8952620
  title: NEDD8:AcM-UBE2M binds CRL1 E3 ubiquitin ligase complex
  findings: []
- id: Reactome:R-HSA-8955241
  title: CAND1 binds cytosolic CRL E3 ubiquitin ligases
  findings: []
- id: Reactome:R-HSA-8955289
  title: COMMDs displace CAND1 from cytosolic CRL E3 ubiquitin ligase complexes
  findings: []
- id: Reactome:R-HSA-8956040
  title: COP9 signalosome deneddylates cytosolic CRL E3 ubiquitin ligase complexes
  findings: []
- id: Reactome:R-HSA-8956200
  title: MyrG-DCUN1D3 binds CRL1 E3 ubiquitin ligase complex
  findings: []
- id: Reactome:R-HSA-983140
  title: Transfer of Ub from E2 to substrate and release of E2
  findings: []
- id: Reactome:R-HSA-983147
  title: Release of E3 from polyubiquitinated substrate
  findings: []
- id: Reactome:R-HSA-983156
  title: Polyubiquitination of substrate
  findings: []
- id: Reactome:R-HSA-983157
  title: Interaction of E3 with substrate and E2-Ub complex
  findings: []
core_functions:
- description: Substrate-recognition subunit of an SCF (SKP1-CUL1-F-box) E3 ubiquitin ligase complex; uses its F-box domain to assemble with SKP1/CUL1/RBX1 and its C-terminal substrate-binding region to recruit substrates for ubiquitination and proteasomal degradation. Despite the gene name it lacks canonical leucine-rich repeats, but the C-terminal region is functionally required for substrate turnover.
  molecular_function:
    id: GO:1990756
    label: ubiquitin-like ligase-substrate adaptor activity
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: file:human/FBXL8/FBXL8-uniprot.txt
    supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex.
  - reference_id: file:human/FBXL8/FBXL8-deep-research-falcon.md
    supporting_text: "FBXL8 physically associates with p53 (co-immunoprecipitation), increases p53 ubiquitination, and accelerates p53 turnover in cycloheximide chase experiments; **both the F-box and LRR regions are required** for this effect, because ΔFbox and ΔLRR constructs fail to influence p53 stability like full-length FBXL8"
  directly_involved_in:
  - id: GO:0031146
    label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process
proposed_new_terms: []
suggested_questions:
- question: What determines the strikingly context-dependent outcomes of FBXL8 activity (oncogenic in colorectal/breast cancer versus tumor-suppressive in lymphoma and anti-fibrotic in heart)? Is it substrate availability, post-translational state of substrates, or tissue-specific cofactors?
- question: Are the binary-interactome partners ALAS1 and PICK1 genuine FBXL8 substrates, distinct from the validated p53/CCND3/Snail1/c-MYC set, and under what conditions are they degraded?
- question: What is the basis of FBXL8 substrate recognition given that it lacks canonical leucine-rich repeats yet its C-terminal region is required for substrate engagement?
suggested_experiments:
- description: Perform affinity purification-mass spectrometry of tagged FBXL8 (stabilized with proteasome and neddylation inhibitors) across multiple cell types (colorectal, lymphoma, cardiac fibroblast) to map the tissue-dependent substrate repertoire relative to an F-box-deletion mutant unable to assemble into SCF.
- description: Reconstitute SCF-FBXL8 in vitro and test direct ubiquitination of validated and candidate substrates (p53, phospho-Thr283 cyclin D3, Snail1, unphosphorylated c-MYC, ALAS1, PICK1), mapping ubiquitin-chain linkage type and the C-terminal residues required for substrate binding.