cryaba

Proposed New Ontology Terms

holdase chaperone activity

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: cryaba: cryaba retains some holdase chaperone activity, though reduced compared to zebrafish cryaa and mammalian CRYAB (PMID:15692462, 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:

PMID:15692462 : The chaperone-like activities of the two zebrafish alpha-crystallins were highly divergent, with alphaA-crystallin showing much greater activity than alphaB-crystallin.

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0043066 negative regulation of apoptotic process	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: IBA annotation based on phylogenetic inference from mammalian alpha-crystallins (CRYAA, CRYAB, HSPB1) which have documented anti-apoptotic roles. The anti-apoptotic function of alpha-crystallins is well-established for mammalian orthologs. While not directly demonstrated for zebrafish cryaba, the phylogenetic inference is reasonable. However, this is not a core molecular function of cryaba but rather a downstream biological process. Reason: Anti-apoptotic activity is a recognized function of the sHSP family but represents a downstream biological process rather than a core molecular function. The IBA inference from mammalian CRYAB orthologs is phylogenetically sound. Retained as non-core since the primary function is structural role in the lens and holdase activity.
GO:0005737 cytoplasm	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for cytoplasmic localization, inferred phylogenetically from multiple sHSP orthologs across fly, worm, mouse, rat, human, and zebrafish. Consistent with the known biology of alpha-crystallins as cytoplasmic proteins in lens fiber cells. Reason: Cytoplasmic localization is well-established for alpha-crystallins. cryaba is a cytoplasmic protein in lens fiber cells. The IBA inference is consistent with established biology and IEA annotations.
GO:0005634 nucleus	IBA GO_REF:0000033	KEEP AS NON CORE	Summary: IBA annotation for nuclear localization based on phylogenetic inference from mammalian sHSPs that translocate to the nucleus under stress conditions. Nuclear localization is not the primary site of action for alpha-crystallins. Reason: Nuclear localization has been reported for some mammalian sHSP orthologs. The IBA inference is phylogenetically supported but represents a secondary or stress-dependent localization. Retained as non-core.
GO:0009408 response to heat	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for heat stress response, inferred phylogenetically from multiple sHSP orthologs. cryaba belongs to the sHSP/HSP20 family and retains some chaperone-like activity (PMID:16420472), consistent with a role in heat stress response. Reason: cryaba is a member of the sHSP family which is fundamentally involved in heat stress response. The IBA inference from multiple orthologs is phylogenetically well-supported. Though cryaba has reduced chaperone activity compared to mammalian CRYAB, it retains measurable activity (PMID:16420472). Falcon deep research reinforces the holdase mechanism underlying the stress response. Supporting Evidence: file:DANRE/cryaba/cryaba-deep-research-falcon.md they bind partially unfolded or destabilized proteins to inhibit aggregation, maintaining proteostasis under basal conditions and especially during stress.
GO:0042026 protein refolding	IBA GO_REF:0000033	MODIFY	Summary: IBA annotation for protein refolding, inferred primarily from Drosophila sHSP orthologs. Alpha-crystallins function as holdases rather than foldases -- they prevent aggregation of denaturing proteins but do not actively refold them. cryaba has reduced but measurable chaperone-like (holdase) activity (PMID:15692462, PMID:16420472). The protein refolding term is inaccurate for a holdase. Reason: Alpha-crystallins are holdase chaperones that prevent aggregation of unfolded proteins but do not catalyze refolding. GO:0042026 implies active refolding activity, which is inaccurate for cryaba. GO:0140309 (unfolded protein carrier activity) is not appropriate because it is carrier-specific (per go-ontology#30552). The replacement is the proposed holdase chaperone activity NTR (see proposed_new_terms). Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN000897708 · sHSP / alpha-crystallin ancestral node (PTHR45620) SUPPORTS TRANSFER Descent from this sHSP node correctly transfers chaperone involvement, but GO:0042026 (protein refolding) overstates a foldase activity that cryaba lacks; it is an ATP-independent holdase preventing aggregation without refolding. FB:FBgn0001226 · Drosophila small heat-shock protein donor SUPPORTS TRANSFER Representative Drosophila sHSP donor seeding the node; holdase-vs-foldase distinction not captured by the term. Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:15692462 The chaperone-like activities of the two zebrafish alpha-crystallins were highly divergent, with alphaA-crystallin showing much greater activity than alphaB-crystallin.
GO:0051082 unfolded protein binding	IBA GO_REF:0000033	MODIFY	Summary: IBA annotation for unfolded protein binding based on phylogenetic inference from multiple alpha-crystallin and sHSP orthologs. GO:0051082 is now formally obsolete. cryaba has been demonstrated to have some chaperone-like activity, particularly at lower temperatures (PMID:16420472). The holdase function should be captured by GO:0140309. Reason: GO:0051082 is now formally obsolete. The holdase activity of cryaba, though reduced compared to mammalian CRYAB, has been demonstrated by in vitro chaperone assays (PMID:15692462, PMID:16420472). GO:0140309 (unfolded protein carrier activity) is not appropriate because it is carrier-specific (per go-ontology#30552). The replacement is the proposed holdase chaperone activity NTR (see proposed_new_terms). Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN000897708 · sHSP / alpha-crystallin ancestral node (PTHR45620) SUPPORTS TRANSFER The node is well supported for unfolded-protein binding across sHSP orthologs, so the transfer itself is sound. The issue is term scoping: GO:0051082 is proposed for obsoletion, so the term (not the propagation) needs updating. UniProtKB:P02511 · Human alphaB-crystallin (CRYAB) SUPPORTS TRANSFER Experimentally characterized alphaB-crystallin ortholog donor supporting holdase / unfolded-protein binding. Proposed replacements: holdase chaperone 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. file:DANRE/cryaba/cryaba-deep-research-falcon.md The same review summarizes that cryaba has lower baseline chaperone activity than cryabb, consistent with functional divergence after duplication.
GO:0005212 structural constituent of eye lens	IEA GO_REF:0000120	ACCEPT	Summary: IEA annotation based on InterPro domain match (IPR003090 Alpha-crystallin_N) and UniProt keyword (KW-0273 Eye lens protein). Alpha-crystallins are major structural proteins of the vertebrate lens. cryaba is predominantly expressed in the lens (PMID:16420472) and its loss increases age-related cataract (PMID:38705506). Reason: Structural role in the eye lens is a well-established core function of alpha-crystallins. cryaba is predominantly lens-expressed and its loss leads to increased age-related cataract (PMID:38705506). The IEA inference is correct and supported by direct experimental data. Supporting Evidence: PMID:38705506 we found that the loss of the alphaBa-crystallin gene cryaba led to an increase in lens opacity compared to cryaa null fish at 24 months of age.
GO:0005737 cytoplasm	IEA GO_REF:0000117	ACCEPT	Summary: IEA annotation for cytoplasmic localization based on ARBA machine learning models. Consistent with the IBA annotation for the same term and the established biology of alpha-crystallins as cytoplasmic proteins. Reason: Cytoplasmic localization is well-established. This IEA annotation is consistent with the IBA annotation. Acceptable as automated confirmation.
GO:0046872 metal ion binding	IEA GO_REF:0000043	MODIFY	Summary: IEA annotation based on UniProt keyword mapping (KW-0479 Metal-binding). The UniProt entry annotates zinc-binding residues at positions 101, 103, and 108 based on PIRSR evidence (PIRSR036514-1). While the annotation to GO:0046872 (metal ion binding) is technically correct, it is very general. The more specific term GO:0008270 (zinc ion binding) would be more informative given the specific zinc-binding sites annotated. Reason: The annotation is too general. UniProt annotates specific zinc-binding residues at positions 101, 103, and 108, indicating zinc ion binding rather than generic metal ion binding. A more specific term would be more informative. Proposed replacements: zinc ion binding
GO:0036438 maintenance of lens transparency	IMP PMID:38705506 Loss of αBa-crystallin, but not αA-crystallin, increases age...	ACCEPT	Summary: IMP annotation for maintenance of lens transparency based on Posner et al. 2024 (PMID:38705506). The study used individual mutant zebrafish lines for all three alpha-crystallin genes and found that loss of cryaba led to an increase in lens opacity compared to cryaa null fish at 24 months of age. This is the first study to show that cryaba plays a leading role in preventing age-related cataract in zebrafish. Reason: Strong experimental evidence demonstrating that cryaba loss increases age-related cataract in zebrafish (PMID:38705506). This is a core biological process for cryaba, directly linked to its structural and chaperone roles in the lens. Supporting Evidence: PMID:38705506 we found that the loss of the alphaBa-crystallin gene cryaba led to an increase in lens opacity compared to cryaa null fish at 24 months of age. PMID:38705506 Our finding that the lens-specific zebrafish alphaBa-crystallin plays the leading role in preventing age-related cataract adds a new twist to our understanding of vertebrate lens evolution. file:DANRE/cryaba/cryaba-deep-research-falcon.md A zebrafish crystallin review summarizes that in hsf4−/− zebrafish lenses, cryaba expression decreases and lysosomal pH is increased, consistent with cryaba acting downstream of lens stress/proteostasis regulation to maintain lysosomal function required for organelle degradation in differentiating fibers.
GO:0007519 skeletal muscle tissue development	IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene...	KEEP AS NON CORE	Summary: IMP annotation for skeletal muscle tissue development based on Buhrdel et al. 2015 (PMID:25866181). The study used morpholino-mediated knockdown of MFM (myofibrillar myopathy) disease genes in zebrafish, including cryaba (which is the zebrafish ortholog of human CRYAB, a known MFM gene). Knockdown led to compromised skeletal muscle function due to myofibrillar degeneration. This reflects a role in muscle maintenance rather than a core lens function. Reason: The morpholino knockdown evidence (PMID:25866181) supports a role in skeletal muscle tissue development/maintenance, consistent with the broader sHSP family role in muscle. However, unlike mammalian CRYAB which is broadly expressed in muscle, zebrafish cryaba is predominantly lens-specific (PMID:16420472), making this a secondary function. Falcon deep research notes that CRISPR null cryaba lines showed milder phenotypes than morpholino knockdowns and explicitly disagree with earlier high-penetrance morpholino reports, so this morpholino-derived muscle phenotype should be interpreted with caution. Retained as non-core. 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. file:DANRE/cryaba/cryaba-deep-research-falcon.md one CRISPR study reported no significant early lens defects for cryaba−/− (and cryabb−/−) and suggested low early lens expression could explain mild phenotypes; it also emphasizes disagreement with earlier high‑penetrance reports.
GO:0007626 locomotory behavior	IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene...	KEEP AS NON CORE	Summary: IMP annotation for locomotory behavior based on Buhrdel et al. 2015 (PMID:25866181). Morpholino knockdown of cryaba led to compromised skeletal muscle function which would affect locomotion. This is a downstream consequence of the muscle defect rather than a direct role in locomotory behavior. Reason: The locomotory behavior phenotype from cryaba knockdown (PMID:25866181) is a downstream consequence of myofibrillar degeneration rather than a direct role in locomotion. This is a non-core function for a predominantly lens-specific protein.
GO:0030239 myofibril assembly	IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene...	KEEP AS NON CORE	Summary: IMP annotation for myofibril assembly based on Buhrdel et al. 2015 (PMID:25866181). Morpholino knockdown of cryaba led to myofibrillar degeneration. This is consistent with the known role of mammalian CRYAB in maintaining myofibrillar integrity, but zebrafish cryaba is predominantly lens-specific rather than muscle-expressed. Reason: The myofibrillar degeneration phenotype from cryaba knockdown (PMID:25866181) supports a role in myofibril assembly/maintenance. However, zebrafish cryaba is predominantly lens-specific (PMID:16420472), making this a secondary function. The muscle-associated function may be more attributable to cryabb which retains broader expression. Retained as non-core.
GO:0060047 heart contraction	IMP PMID:25866181 In vivo characterization of human myofibrillar myopathy gene...	KEEP AS NON CORE	Summary: IMP annotation for heart contraction based on Buhrdel et al. 2015 (PMID:25866181). Morpholino knockdown of MFM genes including cryaba led to severe heart failure in zebrafish. This reflects the broader role of sHSPs in muscle maintenance. Reason: The heart failure phenotype from cryaba knockdown (PMID:25866181) supports involvement in cardiac function. However, zebrafish cryaba is predominantly lens-specific (PMID:16420472), making cardiac function a secondary role. Falcon deep research adds independent genetic support: Park et al. 2023 report that cryaba loss contributes to stress-induced heart edema that is enhanced by nrf2 deficiency, indicating the cardiac role is stress-dependent rather than a core constitutive function. Retained as non-core. Supporting Evidence: file:DANRE/cryaba/cryaba-deep-research-falcon.md Park et al. (2023) report that cryaba loss‑of‑function contributes to stress‑induced heart edema, and that compromised nrf2 function increases penetrance of the cardiac phenotype (including in cryaba−/−; nrf2 double mutants).
GO:0005886 plasma membrane	IDA PMID:18406404 Lengsin expression and function during zebrafish lens format...	ACCEPT	Summary: IDA annotation for plasma membrane localization based on Cassidy-Hanley et al. 2008 (PMID:18406404). The study on lengsin expression in zebrafish lens showed that alphaB1-crystallin (cryaba) localizes around the entire fiber cell membrane, producing a honeycomb appearance in cross-sectional profiles of adult lens tissue. This membrane-associated localization pattern is distinct from the cytoplasmic localization typical of sHSPs and may reflect a lens-specific structural role. Reason: Direct experimental evidence from immunofluorescence in zebrafish lens (PMID:18406404) demonstrates that cryaba localizes to the plasma membrane of lens fiber cells. This is a valid localization annotation, though the membrane association may reflect a lens-specific structural role rather than a general property of the protein. This is consistent with the structural constituent of eye lens function.
GO:0051082 unfolded protein binding	IDA PMID:15692462 Zebrafish alpha-crystallins: protein structure and chaperone...	MODIFY	Summary: IDA annotation based on Dahlman et al. 2005 (PMID:15692462), which compared chaperone-like activity of zebrafish and mammalian alpha-crystallins. Zebrafish alphaB-crystallin (cryaba) showed reduced chaperone-like activity compared to mammalian CRYAB and to zebrafish cryaa. GO:0051082 is now formally obsolete. The holdase function demonstrated is not captured by the carrier-specific GO:0140309; a holdase chaperone activity NTR is proposed instead. Reason: GO:0051082 is now formally obsolete. The chaperone-like activity assays in PMID:15692462 demonstrate reduced but measurable holdase function for cryaba. GO:0140309 does not fit: despite its 'unfolded protein holdase activity' label its definition still requires escort to an acceptor molecule or location, which crystallins do not do (issue #2222, go-ontology#30552). The replacement is the proposed 'holdase chaperone activity' NTR (see proposed_new_terms). Proposed replacements: holdase chaperone activity Supporting Evidence: PMID:15692462 The chaperone-like activities of the two zebrafish alpha-crystallins were highly divergent, with alphaA-crystallin showing much greater activity than alphaB-crystallin. PMID:15692462 The reduced chaperone-like function of zebrafish alphaB-crystallin and its lack of extralenticular expression indicates that it plays a different physiological role from its mammalian ortholog.