Zebrafish alpha-crystallin B chain b (cryabb, also called cryab2, hspb5b or alphaB2-crystallin) is a small heat shock protein (HSP20 family) with an alpha-crystallin domain. It is one of two alphaB-crystallin paralogs produced by the teleost genome duplication; cryaba became lens-restricted, whereas cryabb kept the mammalian CRYAB-like pattern of predominant lens expression plus constitutive expression in heart, brain, skeletal muscle and liver (PMID:16420472). Recombinant cryabb is an ATP-independent holdase chaperone that suppresses chemically induced aggregation of model substrates, with greater activity than human CRYAB at 25-30 degrees C (PMID:16420472). It is a minor lens component (about 0.16% of lens protein) whose lens expression rises after 10 days post-fertilization (PMID:38705506). Its transcription responds to oxidative and glucocorticoid stress and is strongly elevated in Nrf2-deficient heart and brain (PMID:37577747). Loss of cryabb causes embryonic lens defects and stress-potentiated cardiac edema in null mutants (PMID:37577747) and myofibrillar degeneration with heart failure in morphants (PMID:25866181), consistent with human CRYAB as a myofibrillar myopathy, cardiomyopathy and cataract gene.
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. This is mechanistically distinct from foldase activity (GO:0044183) and from carrier-holdase activity (GO:0140309).
Justification: cryabb: cryabb is the broadly expressed zebrafish alphaB-crystallin paralog that retained the widespread protective function of mammalian CRYAB after gene duplication (PMID:16420472). Obsolete GO:0051082 captured binding only; GO:0044183 requires assisting folding, and GO:0140309 (relabelled 'unfolded protein holdase activity') keeps a carrier-specific definition requiring escort to an acceptor molecule or location, which is not demonstrated here. See go-ontology#30552.
Parent term: molecular_function
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0043066 negative regulation of apoptotic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT IBA from the vertebrate alpha-crystallin/HSPB1 node (PTN002264124), seeded by mouse, rat and human CRYAA/CRYAB/HSPB1 anti-apoptotic annotations. No zebrafish study tests apoptosis for cryabb; the only cryabb-specific data are aggregation-suppression assays (PMID:16420472) and stress-responsive transcription (PMID:37577747). Reason: cryabb sits inside the vertebrate CRYAB clade that carries the anti-apoptotic function, and it retained the broadly expressed, protective role of mammalian CRYAB rather than the lens-restricted role of cryaba (PMID:16420472), so the node placement transfers plausibly. Apoptosis suppression is, however, a downstream consequence of holdase activity that has not been measured in zebrafish, so it is retained as non-core rather than as a core function. Supporting Evidence: PMID:16420472 zebrafish alphaB2 maintained the widespread protective role also found in mammalian alphaB-crystallin, while zebrafish alphaB1 adopted a more restricted, nonchaperone role in the lens. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from the sHSP ancestral node (PTN000897708); the zebrafish gene itself is in the WITH/FROM list. sHSPs are soluble, ATP-independent cytosolic chaperones, and cryabb is recovered as a soluble lens protein (0.16% of total lens protein, PMID:16420472). UniProt carries no SUBCELLULAR LOCATION line and ZFIN records ND for cellular component, so there is no direct zebrafish localization image. Reason: The cytoplasm is where an sHSP holdase acts on its aggregation-prone clients, and the node placement is sound for the whole family (invertebrate and vertebrate members alike). Recovery of cryabb in the soluble lens proteome (PMID:16420472) is consistent with a cytoplasmic protein. Accepted as the location of the core activity; a direct immunolocalization in zebrafish tissue would still be valuable. Supporting Evidence: PMID:16420472 2D gel electrophoresis indicated that alphaB2-crystallin makes up approximately 0.16% of total zebrafish lens protein. file:DANRE/cryabb/cryabb-deep-research-falcon.md crystallins are described as **soluble cytoplasmic** proteins in vertebrate optical tissues, supporting a primary **intracellular/cytosolic** localization |
| GO:0005634 nucleus | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA from the sHSP node (PTN000897708) seeded by mammalian HSPB1/CRYAA/CRYAB nuclear-localization data; nuclear entry of mammalian sHSPs is stress-dependent. No zebrafish evidence addresses nuclear cryabb. Reason: Stress-dependent nuclear translocation is documented for mammalian sHSPs and cryabb is inside the clade from which it was inferred, so the transfer is not wrong; but it is a conditional, secondary location with no zebrafish support, and the core holdase activity is cytoplasmic. Kept as non-core. |
| GO:0009408 response to heat | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from the sHSP node (PTN000897708), with the zebrafish gene in WITH/FROM. cryabb (hspb5b) is a heat shock protein family member whose recombinant protein suppresses aggregation across 25-40 degrees C (PMID:16420472). Zebrafish heat-shock profiling found hspb5b expressed in lens and only modestly heat-inducible: it is not among the sHSPs induced more than five-fold (PMID:17888590). It is transcriptionally induced by oxidative and glucocorticoid stress (PMID:37577747). Reason: The node placement is sound: the sHSP family arose as heat-inducible aggregation suppressors and cryabb retained the general protective role of mammalian CRYAB (PMID:16420472). Zebrafish-specific evidence shows only modest heat induction (PMID:17888590) but clear stress-responsive transcription (PMID:37577747) and temperature-dependent chaperone protection (PMID:16420472), which is what the term claims. Accepted as the process context of the core holdase activity. Supporting Evidence: PMID:16420472 At 25 degrees C and 30 degrees C, zebrafish alphaB2 showed greater chaperone-like activity than human alphaB-crystallin, and at 35 degrees C and 40 degrees C, the human protein provided greater protection against aggregation. PMID:17888590 In response to a one-hour heat shock at 37Β°C, we found the expression of several sHSPs, hspb1, hspb4, hspb8, hspb9 , and hspb11 , to be upregulated more than five-fold at one or more stages of development PMID:37577747 Our data suggest that cryabb functions as a stressβresponse gene regulated by both glucocorticoid stress and oxidative stress. |
| GO:0042026 protein refolding | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA from the Eumetazoa sHSP node (PANTHER:PTN000897708; Drosophila sHSP donors). The term (protein refolding) describes assisted restoration of native structure; alpha-crystallins are ATP-independent holdases that prevent aggregation of unfolding proteins but do not refold them. The cryabb assay measured only suppression of chemically induced aggregation (PMID:16420472). Reason: The node inherits chaperone involvement correctly, but the biological-process term GO:0042026 (protein refolding) overstates an active foldase step performed by ATP-dependent HSP70/HSP100 machinery. Although GO:0042026 is a biological process term, sHSP holdase sequestration occurs during stress to prevent irreversible aggregation independently of whether the client's downstream fate is HSP70-mediated refolding or proteasomal/autophagic degradation, and sHSPs are neither subunits nor cofactors of the ATP-dependent HSP70/J-domain/NEF refolding reaction once substrate is released, so attributing the downstream protein refolding process (GO:0042026) to the upstream stress holdase conflates distinct proteostasis steps (go-ontology#30552). cryabb was assayed solely for prevention of aggregation of alpha-lactalbumin and lysozyme (PMID:16420472), the ATP-independent holdase readout (for which the molecular-function row GO:0140309 is modified to the proposed holdase chaperone activity NTR below). Marked as over-annotated for consistency with zebrafish paralogs DANRE/cryaa and DANRE/cryaba as well as human/HSPB1, human/HSPB2, human/CRYAB, human/HSPB6, and the C. elegans hsp-16 paralogs. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000897708 Β· sHSP / alpha-crystallin ancestral node (PTHR45620) SUPPORTS SOURCE BUT NOT TARGET Descent from this sHSP node correctly transfers chaperone involvement, but GO:0042026 (protein refolding) overstates a foldase activity that cryabb lacks; it is an ATP-independent holdase preventing aggregation without refolding. FB:FBgn0001226 Β· Drosophila small heat-shock protein donor SUPPORTS SOURCE BUT NOT TARGET Representative Drosophila sHSP donor on the node; holdase-vs-foldase distinction not captured by the term. Supporting Evidence: PMID:16420472 The chaperone-like activity of purified recombinant alphaB2 protein was assayed by measuring its ability to prevent the chemically induced aggregation of alpha-lactalbumin and lysozyme. PMID:16420472 At 25 degrees C and 30 degrees C, zebrafish alphaB2 showed greater chaperone-like activity than human alphaB-crystallin, and at 35 degrees C and 40 degrees C, the human protein provided greater protection against aggregation. |
| GO:0005198 structural molecule activity | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: ARBA statistical prediction of generic structural molecule activity, derived from the lens-crystallin keyword. cryabb is only 0.16% of zebrafish lens protein (PMID:16420472), far below a bulk refractive crystallin, and its measured activity is chaperone-like aggregation suppression. Reason: The generic parent adds nothing beyond the InterPro-derived GO:0005212 child and is not what the protein has been shown to do; cryabb is a minor lens component whose defining activity is holdase chaperoning (PMID:16420472). Not wrong enough to remove, but over-broad and not a core function. Supporting Evidence: PMID:16420472 2D gel electrophoresis indicated that alphaB2-crystallin makes up approximately 0.16% of total zebrafish lens protein. |
| GO:0005737 cytoplasm | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of cytoplasmic location, matching the IBA row for the same term. Reason: Same claim as the accepted IBA row: sHSP holdases act in the cytosol and cryabb is recovered as a soluble lens protein (PMID:16420472). Accepted for consistency; no independent evidence beyond the IBA. Supporting Evidence: PMID:16420472 2D gel electrophoresis indicated that alphaB2-crystallin makes up approximately 0.16% of total zebrafish lens protein. |
| GO:0043066 negative regulation of apoptotic process | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA prediction of negative regulation of apoptosis, matching the IBA row for the same term. Reason: Same claim as the IBA row and graded the same way: plausible for the broadly expressed CRYAB-like paralog (PMID:16420472) but untested in zebrafish and downstream of the holdase activity. Kept as non-core. Supporting Evidence: PMID:16420472 zebrafish alphaB2 maintained the widespread protective role also found in mammalian alphaB-crystallin, while zebrafish alphaB1 adopted a more restricted, nonchaperone role in the lens. |
| GO:0036438 maintenance of lens transparency | IMP PMID:38705506 Loss of Ξ±Ba-crystallin, but not Ξ±A-crystallin, increases age... | KEEP AS NON CORE | Summary: ZFIN IMP from the aged cryabb null line (PMID:38705506). The abstract reports that cryaba, not cryaa, loss increases age-related cataract and that cryabb lens expression becomes prominent only after 10 dpf; the cryabb mutant result is in the full text (not cached). Embryonic cryabb knockouts do show lens defects (PMID:37577747). Reason: A lens holdase does the work of keeping lens proteins soluble, so participation is direct and the term is appropriate for an alpha-crystallin; the curator read the full text and is deferred to. It is graded non-core because the abstract identifies cryaba as the paralog that drives age-related lens protection while cryabb is the broadly expressed, minor lens component (PMID:38705506, PMID:16420472). Supporting Evidence: PMID:38705506 zebrafish express one lens-specific Ξ±A-crystallin gene (cryaa), they express two Ξ±B-crystallin genes, with one evolving lens specificity (cryaba) and the other retaining the broad expression of its mammalian ortholog (cryabb). PMID:38705506 expression of cryaba and cryabb becoming more prominent after 10 dpf. These data show that cryaa is the primary Ξ±-crystallin during early lens development, while the protective role for cryaba becomes more important during lens aging. PMID:37577747 Previously, we showed that knockout of either Ξ±B-crystallin causes lens defects in zebrafish embryos (Mishra et al., 2018; Zou et al., 2015). |
| GO:0007519 skeletal muscle tissue development | IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene... | KEEP AS NON CORE | Summary: ZFIN IMP from morpholino knockdown in a screen of human myofibrillar myopathy genes (PMID:25866181; abstract-only cache, cryabb not named in the abstract). Knockdown of the MFM genes compromised skeletal muscle function through myofibrillar degeneration. Reason: cryabb is constitutively expressed in skeletal muscle (PMID:16420472), so a muscle phenotype is expected, and the curator saw the full text; but the morphant readout is degeneration of already-formed myofibrils, a maintenance defect. Tissue development is the downstream phenotype, not the step the chaperone performs, so the row is kept as non-core with the direct work captured under myofibril assembly. Supporting Evidence: PMID:25866181 targeted ablation of MFM genes in zebrafish led to compromised skeletal muscle function mostly due to myofibrillar degeneration as well as severe heart failure. PMID:16420472 RT-PCR showed that alphaB2-crystallin is expressed predominantly in lens but, reminiscent of mammalian alphaB-crystallin, also has lower constitutive expression in heart, brain, skeletal muscle and liver. |
| GO:0007626 locomotory behavior | IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene... | KEEP AS NON CORE | Summary: ZFIN IMP from the same morpholino screen (PMID:25866181): impaired swimming accompanies the muscle degeneration. Reason: Locomotor impairment is a whole-organism consequence of myofibrillar degeneration in the morphants, not a process in which a cytosolic holdase does any of the work. Kept as non-core; not removed because the observation is real and the curator read the full text. Supporting Evidence: PMID:25866181 targeted ablation of MFM genes in zebrafish led to compromised skeletal muscle function mostly due to myofibrillar degeneration as well as severe heart failure. |
| GO:0030239 myofibril assembly | IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene... | ACCEPT | Summary: ZFIN IMP from morpholino knockdown (PMID:25866181): loss of cryabb causes myofibrillar degeneration. Mammalian CRYAB is a myofibrillar myopathy gene that stabilises sarcomeric clients (desmin, titin), and cryabb is the paralog that retained constitutive muscle expression and chaperone activity (PMID:16420472). Reason: This is the process in which the holdase activity itself does work: sHSP binding of sarcomeric proteins keeps them assembly-competent, and the morphant phenotype (myofibrillar degeneration, PMID:25866181) is the expected readout of losing that. The abstract-only cache does not name cryabb, so the curator reading of the full text is deferred to; the biology is consistent with cryabb having heart and skeletal-muscle expression (PMID:16420472). Morpholino-only evidence is a caveat noted in suggested_experiments. Supporting Evidence: PMID:25866181 targeted ablation of MFM genes in zebrafish led to compromised skeletal muscle function mostly due to myofibrillar degeneration as well as severe heart failure. PMID:16420472 RT-PCR showed that alphaB2-crystallin is expressed predominantly in lens but, reminiscent of mammalian alphaB-crystallin, also has lower constitutive expression in heart, brain, skeletal muscle and liver. |
| GO:0060047 heart contraction | IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene... | KEEP AS NON CORE | Summary: ZFIN IMP from the morpholino screen (PMID:25866181), where MFM-gene knockdowns produced severe heart failure. Independently, cryabb null zebrafish show embryonic cardiac edema whose penetrance rises under glucocorticoid stress (PMID:37577747). Reason: Two independent zebrafish models (morphant, PMID:25866181; CRISPR null, PMID:37577747) link cryabb loss to cardiac failure, consistent with human CRYAB cardiomyopathy. But heart contraction is the organ-level output; the chaperone does its work on cardiomyocyte proteostasis, not in the contractile cycle. Kept as non-core rather than accepted as a core function. Supporting Evidence: PMID:25866181 targeted ablation of MFM genes in zebrafish led to compromised skeletal muscle function mostly due to myofibrillar degeneration as well as severe heart failure. PMID:37577747 In addition to lens defects, the loss of Ξ±B-crystallin function is associated with a cardiac phenotype that presents as embryonic cardiac edema (Mishra et al., 2018). |
| GO:0005575 cellular_component | ND GO_REF:0000015 | ACCEPT | Summary: ZFIN ND placeholder stating that no experimental cellular-component data exist for cryabb; the cytoplasm and nucleus rows are inferred (IBA/IEA), not observed. Reason: Accurate: no zebrafish localization experiment has been published for cryabb. Kept until an immunolocalization or tagged-protein study appears. The gene's deep-research record likewise finds no zebrafish cryabb subcellular localization study. Supporting Evidence: file:DANRE/cryabb/cryabb-deep-research-falcon.md Direct zebrafish cryabb subcellular localization experiments were not present in the extracted evidence. |
| GO:0005212 structural constituent of eye lens | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO mapping of the alpha-crystallin N-terminal domain (IPR003090) to structural constituent of eye lens. cryabb is expressed predominantly in lens (PMID:16420472) and its lens expression rises after 10 dpf (PMID:38705506), but it is a minor lens protein (0.16%) and the lens-specialised paralog is cryaba. Reason: cryabb is a genuine lens protein and null mutants show lens defects (PMID:37577747), so the term is not wrong; but the mapping fires on every alpha-crystallin-domain protein regardless of abundance, and for cryabb the lens role is secondary to the broadly expressed chaperone function retained from mammalian CRYAB (PMID:16420472). Kept as non-core. Supporting Evidence: PMID:16420472 RT-PCR showed that alphaB2-crystallin is expressed predominantly in lens but, reminiscent of mammalian alphaB-crystallin, also has lower constitutive expression in heart, brain, skeletal muscle and liver. PMID:16420472 2D gel electrophoresis indicated that alphaB2-crystallin makes up approximately 0.16% of total zebrafish lens protein. PMID:38705506 expression of cryaba and cryabb becoming more prominent after 10 dpf. These data show that cryaa is the primary Ξ±-crystallin during early lens development, while the protective role for cryaba becomes more important during lens aging. |
| GO:0005886 plasma membrane | IEA GO_REF:0000117 | REMOVE | Summary: ARBA statistical prediction of plasma membrane location. cryabb has no signal peptide, transmembrane segment or lipid anchor, UniProt carries no SUBCELLULAR LOCATION comment, ZFIN records ND for cellular component, and no zebrafish or ortholog evidence in the cached literature places the protein at the plasma membrane. Reason: Nothing supports this location for cryabb: the protein is a soluble cytosolic sHSP recovered from the soluble lens proteome (PMID:16420472), and the ARBA rule that generated the row is a co-occurrence prediction rather than a localization observation. Removal does not deny transient membrane association reported for mammalian sHSPs under stress; it records that there is no evidence for this zebrafish protein. Supporting Evidence: PMID:16420472 2D gel electrophoresis indicated that alphaB2-crystallin makes up approximately 0.16% of total zebrafish lens protein. |
| GO:0140309 unfolded protein holdase activity | IDA PMID:16420472 Gene duplication and separation of functions in alphaB-cryst... | MODIFY | Summary: ZFIN IDA migrated from obsolete GO:0051082 (unfolded protein binding). PMID:16420472 measured the ability of recombinant cryabb to prevent chemically induced aggregation of alpha-lactalbumin and lysozyme at 25-40 degrees C, a classic holdase assay. GO:0140309 is labelled unfolded protein holdase activity but is defined as a carrier activity that escorts the unfolded protein to an acceptor molecule or location, which was not tested and is not what crystallins do. Reason: The experiment is sound and directly on the zebrafish protein, so the activity should be kept; the problem is the term. GO:0140309 retains its carrier definition (go-ontology#30552), so the migration overstates what was measured. The replacement is the proposed holdase chaperone activity NTR under proposed_new_terms (aggregation suppression without refolding or delivery), the same replacement carried by zebrafish paralogs cryaa and cryaba on GO:0140309; when GO creates such a term this row should move to it. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:16420472 The chaperone-like activity of purified recombinant alphaB2 protein was assayed by measuring its ability to prevent the chemically induced aggregation of alpha-lactalbumin and lysozyme. PMID:16420472 At 25 degrees C and 30 degrees C, zebrafish alphaB2 showed greater chaperone-like activity than human alphaB-crystallin, and at 35 degrees C and 40 degrees C, the human protein provided greater protection against aggregation. PMID:16420472 zebrafish alphaB2 maintained the widespread protective role also found in mammalian alphaB-crystallin, while zebrafish alphaB1 adopted a more restricted, nonchaperone role in the lens. |
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Download this section (compressed HTML)Q: Does cryabb bind sarcomeric clients (desmin, titin, actin) in zebrafish muscle, as mammalian CRYAB does, or is the myofibrillar phenotype an indirect proteostasis effect?
Q: Where is cryabb protein located in zebrafish cells (cytosol, Z-disc, nucleus under stress)? No zebrafish localization experiment exists; ZFIN records ND for cellular component.
Experiment: Compare skeletal and cardiac muscle ultrastructure and sarcomeric protein solubility in the CRISPR cryabb null line (PMID:37577747) with the morphant phenotype (PMID:25866181), and co-immunoprecipitate cryabb from heart lysate under heat or oxidative stress to identify client proteins.
Hypothesis: cryabb acts as a holdase for sarcomeric proteins in zebrafish muscle.
Type: genetic and biochemical
Experiment: Immunostain or knock-in-tag cryabb in larval muscle and lens with and without heat shock to obtain direct localization data replacing the ND cellular-component annotation.
Hypothesis: cryabb is a cytosolic protein that redistributes to the nucleus or Z-disc under stress.
Type: imaging
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