id: B7FXQ8 gene_symbol: HSP20A product_type: PROTEIN aliases: - PHATRDRAFT_35158 status: COMPLETE taxon: id: NCBITaxon:556484 label: Phaeodactylum tricornutum CCAP 1055/1 description: >- HSP20A (PHATRDRAFT_35158) is a 163-residue small heat shock protein (sHSP) of the marine diatom Phaeodactylum tricornutum. It belongs to the HSP20/alpha-crystallin family (PANTHER PTHR11527) and has the canonical sHSP architecture: an N-terminal region rich in aromatic residues, a complete alpha-crystallin domain (residues 47-155) and a short C-terminal extension carrying the I-X-I oligomerization motif (IAI, residues 150-152). By homology with characterised sHSPs it is expected to form dynamic oligomers and act as an ATP-independent holdase. In this role it would bind non-native proteins during heat and other proteotoxic stress and keep them from aggregating, so that ATP-dependent chaperones can later refold them. It is not expected to refold clients itself. In an alpha-crystallin-domain tree it groups with a Thalassiosira pseudonana sHSP of PANTHER subfamily SF175, inside a diatom lineage that contains no experimentally characterised protein. Its activity, expression, oligomeric state and subcellular location have not been measured. No signal peptide or targeting presequence is annotated, which points to a cytosolic protein, but this has not been tested. existing_annotations: - term: id: GO:0009408 label: response to heat evidence_type: ISS original_reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md review: summary: >- Proposed from the PTHR11527 family-root PAINT IBD. B7FXQ8 has no GO annotations because P. tricornutum is not a PANTHER genome. action: NEW reason: >- PAINT placed response to heat at PTN000163021, the root of PTHR11527 (species LUCA), so every placed family member inherits it. That includes the diatom PTHR11527:SF175 sHSPs that group with B7FXQ8 in the ACD tree. B7FXQ8 is a full-length family member with a complete alpha-crystallin domain. sHSPs take part in the heat response by holding heat-denatured proteins, so the gene product does part of the work rather than only being induced. No heat-induction or heat-protection data exist for B7FXQ8 itself. supported_by: - reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md supporting_text: >- The PTHR11527 tree root is PTN000163021 (species LUCA). Its PAINT IBDs are protein folding (GO:0006457), response to heat (GO:0009408), response to salt stress (GO:0009651), response to hydrogen peroxide (GO:0042542) and protein complex oligomerization (GO:0051259). - reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md supporting_text: >- In the NJ tree, B7FXQ8 groups with the T. pseudonana SF175 protein B8CD87. - term: id: GO:0051259 label: protein complex oligomerization evidence_type: ISS original_reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md review: summary: >- Proposed from the PTHR11527 family-root PAINT IBD and the conserved C-terminal I-X-I assembly motif of B7FXQ8. action: NEW reason: >- Oligomer assembly is a conserved property of sHSPs. It was placed by PAINT at the PTHR11527 root and is directly demonstrated for archaeal Hsp16.5 (PMID:29983375). B7FXQ8 keeps the I-X-I motif (IAI 150-152) that cross-links neighbouring alpha-crystallin dimers. The protein assembles itself, so it carries out the process. supported_by: - reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md supporting_text: >- The C-terminal I-X-I motif, which sHSPs use for oligomer assembly, is present in the target as IAI at residues 150-152. - reference_id: PMID:29983375 supporting_text: >- Small heat-shock proteins (sHSPs) are molecular chaperones that bind partially and globally unfolded states of their client proteins. core_functions: - description: >- Predicted ATP-independent holdase chaperone activity. The family-level evidence is that sHSPs bind partially or globally unfolded client proteins and prevent their aggregation. The activity is carried by oligomeric assemblies built from alpha-crystallin-domain dimers that the I-X-I motif cross-links, and B7FXQ8 has both the complete domain and the motif. GO has no suitable term. Obsolete GO:0051082 covered binding only. GO:0044183 protein folding chaperone requires assisting folding. GO:0140309 keeps a carrier definition that requires escort to an acceptor. The activity is therefore recorded as a proposed term, following the repository convention for sHSPs such as Arabidopsis HSP17.7 and yeast HSP26. The only process terms linked here are the ones PAINT placed at the PTHR11527 root node PTN000163021, from which every member descends: response to heat and protein complex oligomerization. No location is asserted because none has been measured. proposed_molecular_function: holdase chaperone activity directly_involved_in: - id: GO:0009408 label: response to heat - id: GO:0051259 label: protein complex oligomerization supported_by: - reference_id: UniProt:B7FXQ8 supporting_text: >- Belongs to the small heat shock protein (HSP20) family. {ECO:0000256|PROSITE-ProRule:PRU00285, ECO:0000256|RuleBase:RU003616}. - reference_id: PMID:29983375 supporting_text: >- Small heat-shock proteins (sHSPs) are molecular chaperones that bind partially and globally unfolded states of their client proteins. - reference_id: PMID:11576425 supporting_text: The chaperone activity of At-HSP17.6A was demonstrated in vitro. - reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md supporting_text: >- The PTHR11527 tree root is PTN000163021 (species LUCA). Its PAINT IBDs are protein folding (GO:0006457), response to heat (GO:0009408), response to salt stress (GO:0009651), response to hydrogen peroxide (GO:0042542) and protein complex oligomerization (GO:0051259). - reference_id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md supporting_text: >- The C-terminal I-X-I motif, which sHSPs use for oligomer assembly, is present in the target as IAI at residues 150-152. proposed_new_terms: - proposed_name: holdase chaperone activity proposed_definition: >- Binding to an unfolded or misfolded protein to prevent its aggregation without actively catalyzing refolding. The holdase maintains the client protein in a soluble, folding-competent state. justification: >- sHSPs, including PTHR11527 members, act as ATP-independent holdases. They bind non-native clients and prevent aggregation but do not refold them. Obsolete GO:0051082 captured binding only. GO:0044183 requires assisting folding. GO:0140309 (relabelled 'unfolded protein holdase activity') keeps a carrier definition requiring escort to an acceptor molecule or location. Same proposal as for Arabidopsis HSP17.7 and yeast HSP26; see go-ontology#30552. proposed_parent: id: GO:0003674 label: molecular_function supported_by: - reference_id: PMID:29983375 supporting_text: >- Small heat-shock proteins (sHSPs) are molecular chaperones that bind partially and globally unfolded states of their client proteins. references: - id: PMID:18923393 title: The Phaeodactylum genome reveals the evolutionary history of diatom genomes. findings: - statement: >- The genome sequence of P. tricornutum strain CCAP 1055/1, from which HSP20A (PHATRDRAFT_35158) was predicted. reference_review: relevance: LOW correctness: VERIFIED review_notes: >- Genome paper cited in the UniProt entry as the source of the gene model. It provides no functional data on HSP20A. - id: PMID:29983375 title: Engineering of a Polydisperse Small Heat-Shock Protein Reveals Conserved Motifs of Oligomer Plasticity. findings: - statement: >- sHSPs bind partially and globally unfolded client proteins. The archaeal Hsp16.5 forms ordered 24-subunit oligomers, and engineering it showed conserved motifs that govern oligomer plasticity. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Family-level evidence for sHSP client binding and oligomerization. Hsp16.5 is archaeal and has no PANTHER assignment, so this paper supports general sHSP biology only, not a B7FXQ8-specific transfer. - id: PMID:11576425 title: At-HSP17.6A, encoding a small heat-shock protein in Arabidopsis, can enhance osmotolerance upon overexpression. findings: - statement: >- In vitro chaperone activity of a PTHR11527 member, Arabidopsis cytosolic class II sHSP HSP17.6A. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Shows chaperone activity in a characterised PTHR11527 member that shares the family root node with B7FXQ8. It is a family-level relative, not an ortholog, so the osmotolerance phenotype is not transferred. - id: file:PHATC/B7FXQ8/B7FXQ8-deep-research-falcon.md title: Deep research report on HSP20A (B7FXQ8) in P. tricornutum findings: - statement: >- Comprehensive literature synthesis on sHSP family biology and P. tricornutum thermal stress response systems. reference_review: relevance: LOW correctness: UNVERIFIED review_notes: >- Retained as background. In the 2026-10-21 re-review it is no longer used as support. It states that no direct experimental data exist for HSP20A, and its chaperone claims are generic family biology attributed to secondary reviews. Its P. tricornutum HSF statements concern transcription factors, not this protein. - id: file:PHATC/B7FXQ8/B7FXQ8-hypotheses/core-function-1-go-0051082/openscientist.md title: OpenScientist hypothesis investigation - unfolded protein binding as core function of HSP20A findings: - statement: >- GO:0051082 (unfolded protein binding) is officially obsolete. Recommended replacement is GO:0044183 (protein folding chaperone), following precedent of 25 reviewed sHSPs in Swiss-Prot. B7FXQ8 has zero GO annotations in any database; only one of seven P. tricornutum sHSPs has any GO annotation. - statement: >- All seven independent domain databases (CDD, InterPro, Pfam, PROSITE, PANTHER, SMART, Gene3D) classify B7FXQ8 as an sHSP. The IXI motif (IAI at position 150) is present, supporting oligomeric assembly capability. reference_review: relevance: LOW correctness: DISPUTED review_notes: >- Retained. The IXI motif finding was re-checked (IAI at 150-152, see B7FXQ8-bioinformatics/RESULTS.md). Its recommendation to use GO:0044183 is not followed. GO:0044183 requires assisting folding, while sHSPs are holdases that do not refold clients, so the repository's proposed holdase term is used instead. - id: file:PHATC/B7FXQ8/B7FXQ8-bioinformatics/RESULTS.md title: B7FXQ8 (HSP20A) phylogenetic placement among small heat shock proteins findings: - statement: >- B7FXQ8 is in PTHR11527 at family level only, and P. tricornutum is not a PANTHER genome. In an ACD neighbour-joining tree it groups with T. pseudonana B8CD87 (PTHR11527:SF175). No characterised protein falls inside that diatom lineage. The PAINT family root PTN000163021 carries protein folding, response to heat, response to salt stress, response to H2O2 and protein complex oligomerization. suggested_questions: - question: >- Is HSP20A induced by heat or other proteotoxic stress in P. tricornutum, and is its promoter bound by any of the expanded set of P. tricornutum HSF transcription factors? - question: >- Is HSP20A cytosolic, or is any fraction targeted to the plastid or mitochondrion despite lacking a recognisable presequence? - question: >- Should the PAINT root IBDs for PTHR11527 (response to salt stress, response to hydrogen peroxide) be considered family-wide, given that they rest on plant phenotypes? B7FXQ8 would inherit them if P. tricornutum were added to PANTHER. - question: >- What oligomeric state does HSP20A adopt, and does heat shift it toward the smaller species that are typically the active holdase form? suggested_experiments: - hypothesis: HSP20A transcript and protein levels rise after heat shock in P. tricornutum. description: >- Time-course RT-qPCR and immunoblot (or tagged-line) measurements after a temperature shift (e.g. 20 to 30 degrees C), compared with the other P. tricornutum sHSP paralogs. - hypothesis: Recombinant HSP20A suppresses thermal aggregation of model clients. description: >- Light-scattering aggregation assays with citrate synthase or luciferase. Follow with refolding assays in the presence of HSP70/HSP40 to test holdase-to-foldase handoff. Include an IXI-motif mutant (IAI150-152 to GAG). - hypothesis: HSP20A forms heat-responsive oligomers. description: >- Size-exclusion chromatography with multi-angle light scattering and native mass spectrometry at permissive and heat-shock temperatures. - hypothesis: Loss of HSP20A reduces thermotolerance. description: >- CRISPR-Cas9 knockout of HSP20A, alone and combined with its closest paralogs, followed by growth and survival measurements after acute and chronic heat stress.