CHIB (P29023; ChitB / seed chitinase B; gene synonym CTB1; maize gene model GRMZM2G005633) is a maize (Zea mays) endochitinase (EC 3.2.1.14) of glycoside hydrolase family 19 (GH19). GH19 chitinases are endo-acting enzymes that hydrolyze the beta-1,4 glycosidic linkages of chitin - a linear polymer of N-acetylglucosamine (GlcNAc) that is the principal structural polysaccharide of fungal cell walls and arthropod exoskeletons - producing chitooligosaccharides via a single-step inverting mechanism. The protein is synthesized as a precursor with an N-terminal signal peptide (residues 1-33) and is secreted; the mature 248-residue chain (residues 34-281) carries an N-terminal hevein-type chitin-binding domain (Chitin-binding type-1, residues 34-68), a short Gly-rich hinge (residues 69-78), and a C-terminal GH19 catalytic domain (residues 79-281, proton-donor active site at Glu145). The chitin-binding module enhances activity on insoluble chitin and contributes to antifungal effectiveness. CHIB is a classic pathogenesis-related (PR-3) basic chitinase: it was first purified from maize seed and characterized as an antifungal protein that defends against chitin-containing fungal pathogens (Huynh et al. 1992), and its transcript is induced ~10-fold by mechanical wounding and fall-armyworm (Spodoptera frugiperda) herbivory, placing it within plant defense responses. Its core molecular function is therefore endochitinase / chitinase activity coupled to chitin binding; its core biological process is chitin catabolism serving antifungal (and likely anti-herbivore) defense in the extracellular/apoplastic compartment and in seed. Maize chitinase B is reported to be somewhat less active than maize chitinase A, and its activity is countered in planta by fungal polyglycine hydrolases that cleave its Gly-rich hinge to disrupt chitin binding.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004568 chitinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Chitinase activity assigned by combined automated IEA methods (ARBA/InterPro). This is the core molecular function of CHIB, a GH19 endochitinase (EC 3.2.1.14). Reason: Correct and central to the gene's function. CHIB belongs to glycoside hydrolase family 19 and is annotated EC 3.2.1.14 (endochitinase); GH19 chitinases hydrolyze the beta-1,4 linkages of chitin. The activity is consistent with the UniProt catalytic-activity statement (random endo-hydrolysis of N-acetyl-beta-D-glucosaminide (1->4)-beta-linkages in chitin and chitodextrins). This is a core function and is appropriately specific. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md GH19 chitinases are generally described as **endo-acting** enzymes that hydrolyze file:MAIZE/CHIB/CHIB-deep-research-falcon.md a key structural component of fungal cell walls and arthropod exoskeletons. |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: Extracellular localization assigned from the UniProtKB Subcellular Location vocabulary (Secreted). Consistent with the predicted signal peptide and secretory deployment of CHIB. Reason: CHIB carries an N-terminal signal peptide (residues 1-33) and is annotated "Secreted" in UniProt, consistent with an apoplastic/extracellular pathogenesis-related chitinase that encounters chitin-containing microbes and chitinous insect structures at the cell surface. The localization is appropriate and supported by the secretory architecture. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md consistent with secretion and extracellular/apoplastic deployment file:MAIZE/CHIB/CHIB-deep-research-falcon.md Likely secreted / apoplastic or extracellular, consistent with signal peptide and seed/pathogenesis-related chitinase annotation |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Carbohydrate metabolic process assigned by InterPro2GO (IPR016283, Glyco_hydro_19). This is a very broad parent of the gene's true process, chitin catabolism. Reason: Not wrong - chitin is a carbohydrate (polysaccharide) and its hydrolysis is carbohydrate metabolism - but "carbohydrate metabolic process" is a high-level grouping term that conveys little once the specific process "chitin catabolic process" (GO:0006032, also present in the current GOA) is annotated. The precise function is endo-hydrolysis of chitin. Retaining the vague parent adds no information beyond the specific term, so it is best treated as an over-annotation. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md its primary biochemical function is file:MAIZE/CHIB/CHIB-deep-research-falcon.md producing chitooligosaccharides. |
| GO:0006032 chitin catabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: Chitin catabolic process assigned by InterPro2GO (IPR000726, Glyco_hydro_19_cat). This is the precise biological process for a GH19 endochitinase and is the core process of CHIB. Reason: Correct and at the right level of specificity. CHIB hydrolyzes the beta-1,4 linkages of chitin, a beta-(1->4)-linked N-acetyl-D-glucosamine polysaccharide, breaking it down into chitooligosaccharides - precisely "chitin catabolic process". This is the core biological process and is more informative than the generic parents "carbohydrate metabolic process" and "polysaccharide catabolic process". Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md its primary biochemical function is file:MAIZE/CHIB/CHIB-deep-research-falcon.md producing chitooligosaccharides. file:MAIZE/CHIB/CHIB-deep-research-falcon.md GH19 chitinases are generally described as **endo-acting** enzymes that hydrolyze |
| GO:0008061 chitin binding | IEA GO_REF:0000002 | ACCEPT | Summary: Chitin binding assigned by InterPro2GO (IPR001002/IPR018371/IPR036861, chitin-binding type-1 / hevein domain). CHIB contains an N-terminal chitin-binding domain. Reason: Correct and supported by the protein architecture. CHIB has an N-terminal hevein-type chitin-binding domain (Chitin-binding type-1, residues 34-68); the CAZy CBM18 / Pfam Chitin_bind_1 assignments confirm the module. The chitin-binding domain enhances activity on insoluble chitin and contributes to antifungal effectiveness. This molecular function is a genuine, specific accessory activity supporting substrate engagement. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md typically includes an N-terminal file:MAIZE/CHIB/CHIB-deep-research-falcon.md improves performance against insoluble chitinous substrates and contributes to defense effectiveness. |
| GO:0008843 endochitinase activity | IEA GO_REF:0000003 | ACCEPT | Summary: Endochitinase activity assigned by EC2GO mapping from EC 3.2.1.14. This is the most precise molecular-function term for CHIB and represents its core catalytic activity. Reason: Correct and the most informative MF term for the gene. CHIB is named "Endochitinase B" with EC 3.2.1.14, and the UniProt catalytic-activity statement describes random endo-hydrolysis of chitin and chitodextrins - i.e. an endo-acting (rather than exo-acting) chitinase. The term is preferred over the more generic "chitinase activity" (GO:0004568) because it specifies the endo mode of cleavage. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md GH19 chitinases are generally described as **endo-acting** enzymes that hydrolyze |
| GO:0016998 cell wall macromolecule catabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Cell wall macromolecule catabolic process assigned by InterPro2GO (IPR000726). This captures the biologically relevant target of CHIB: degradation of chitin in fungal cell walls. Reason: Reasonable and biologically meaningful: chitin is a structural component of fungal cell walls, and a secreted plant endochitinase degrades fungal-cell-wall chitin as the basis of its antifungal action. The term is correct but is a process-level framing of the same activity already captured by "chitin catabolic process" (GO:0006032), which is the core, more direct term. Retain this as a non-core descriptor of the host-defense context (degradation of the microbial cell-wall macromolecule) rather than as the primary process. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md a key structural component of fungal cell walls and arthropod exoskeletons. file:MAIZE/CHIB/CHIB-deep-research-falcon.md biologically relevant form in fungal cell walls. |
| GO:0005576 extracellular region | ISS GO_REF:0000024 | ACCEPT | Summary: Extracellular localization transferred by curator ISS from the ortholog UniProtKB:P29022 (maize endochitinase A). Duplicates the IEA extracellular-region annotation with stronger, orthology-based evidence. Reason: Correct and consistent with the secretory architecture of CHIB (signal peptide, "Secreted" subcellular location) and with the IEA extracellular-region annotation. ISS transfer from the closely related maize endochitinase A (P29022) is appropriate given the shared GH19 chitinase family, signal peptide and secreted-defense-protein role. Duplicate annotations with different evidence codes are acceptable. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md Likely secreted / apoplastic or extracellular, consistent with signal peptide and seed/pathogenesis-related chitinase annotation |
| GO:0008843 endochitinase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Endochitinase activity transferred by curator ISS from the ortholog UniProtKB:P29022 (maize endochitinase A). Duplicates the EC-based IEA endochitinase annotation. Reason: Correct core molecular function, here supported by orthology-based curator transfer from maize endochitinase A (P29022), the entry from which CHIB's EC 3.2.1.14 / catalytic-activity statement was itself transferred by similarity. Consistent with the EC2GO IEA endochitinase annotation and with GH19 family assignment. Duplicate annotations with different evidence codes are acceptable. Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md GH19 chitinases are generally described as **endo-acting** enzymes that hydrolyze |
| GO:0006952 defense response | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt keyword "Plant defense"; snapshot-only, removed in the current GOA release. CHIB is a bona fide antifungal pathogenesis-related (PR-3) chitinase, so the defense role is genuinely correct - but the bare "defense response" parent is over-broad relative to the gene's specific antifungal action. Reason: The essence is correct: CHIB was first purified and characterized as an antifungal protein from maize seed and is annotated by UniProt as functioning in "defense against chitin-containing fungal pathogens", and its transcript is induced ~10-fold by wounding and fall-armyworm herbivory. However, "defense response" (GO:0006952) is a high-level grouping term, and the antifungal defense operates THROUGH the gene's chitinase enzymatic activity (secreted GH19 endochitinase degrading fungal-cell-wall chitin). Once the core molecular function (chitinase / endochitinase activity, chitin binding) and the chitin/cell-wall-macromolecule catabolic processes are annotated, the bare "defense response" parent adds little. The genuine biology is better captured by the more specific child term "defense response to fungus" (GO:0050832), proposed below as a NEW annotation. GOA's removal of the generic keyword-derived term is therefore acceptable; the defense role should be retained at the more precise antifungal level. Proposed replacements: defense response to fungus Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md A maize primary study investigating insect herbivory identified file:MAIZE/CHIB/CHIB-deep-research-falcon.md This supports CHIB’s placement within plant defense signaling frameworks in which physical damage and herbivore-associated cues activate defense gene expression. |
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000043 | MODIFY | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt keywords "Polysaccharide degradation" / "Chitin degradation"; snapshot-only, removed in the current GOA release. The term is correct (chitin is a polysaccharide and CHIB catabolizes it) but is a generic parent of the precise process, chitin catabolic process. Reason: Chitin is a linear polysaccharide of beta-(1->4)-linked N-acetyl-D-glucosamine, and CHIB hydrolyzes it - so "polysaccharide catabolic process" (GO:0000272) is not wrong, just under-specific. The exact process is "chitin catabolic process" (GO:0006032), which is the direct child describing breakdown of chitin and which is ALREADY present and retained in the current GOA (InterPro2GO, IPR000726). The annotation should therefore be MODIFIED to GO:0006032; the keyword-derived generic parent can be dropped because the specific child is already annotated. No correct biology is lost by removing the generic SPKW term. Proposed replacements: chitin catabolic process Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md its primary biochemical function is file:MAIZE/CHIB/CHIB-deep-research-falcon.md producing chitooligosaccharides. file:MAIZE/CHIB/CHIB-deep-research-falcon.md GH19 chitinases are generally described as **endo-acting** enzymes that hydrolyze |
| GO:0050832 defense response to fungus | IEA GO_REF:0000043 | NEW | Summary: CHIB is an antifungal pathogenesis-related chitinase whose biological role is defense against chitin-containing fungal pathogens. This specific child of "defense response" is the accurate process term and is proposed to replace the retired generic "defense response" SPKW annotation. Reason: CHIB ("Seed chitinase B") was first purified and characterized as an antifungal protein from maize seed (Huynh et al. 1992), and UniProt annotates its FUNCTION as "Defense against chitin-containing fungal pathogens"; the host-defense mechanism is enzymatic degradation of fungal-cell-wall chitin by the secreted GH19 endochitinase, an action that fungi counter via polyglycine hydrolases that cleave the chitinase hinge. The biologically precise process term is therefore "defense response to fungus" (GO:0050832), which is more informative than the bare "defense response" parent and captures the antifungal specificity supported by the chitinase molecular function. (Note: the inducible-defense evidence also includes a wounding/herbivory component; the antifungal role is the best-supported and most specific defense process.) Supporting Evidence: file:MAIZE/CHIB/CHIB-deep-research-falcon.md a key structural component of fungal cell walls and arthropod exoskeletons. file:MAIZE/CHIB/CHIB-deep-research-falcon.md improves performance against insoluble chitinous substrates and contributes to defense effectiveness. |
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Download this section (compressed HTML)Q: Does the purified maize CHIB protein have direct antifungal activity against plant-pathogenic fungi (e.g. growth inhibition / hyphal-tip lysis), and how does its potency compare with maize chitinase A, which is reported to be more active?
Q: What are CHIB's biochemical parameters (Km, pH/temperature optima, soluble vs colloidal/ insoluble chitin preference) and is the N-terminal chitin-binding domain required for activity on insoluble chitin, as suggested for related maize GH19 chitinases?
Q: Is CHIB's antifungal role compromised in planta by fungal polyglycine hydrolases that cleave its Gly-rich hinge, and does this hinge cleavage measurably reduce chitin binding and antifungal efficacy during infection?
Experiment: Express and purify recombinant mature CHIB and assay endochitinase activity on soluble (4-MU-chitooligosaccharide) and insoluble/colloidal chitin substrates, determining kinetic constants and pH/temperature optima; include a chitin-binding-domain-deletion variant.
Hypothesis: CHIB is an active endo-acting GH19 chitinase whose N-terminal chitin-binding domain is required for efficient hydrolysis of insoluble chitin.
Type: in vitro enzyme kinetics
Experiment: Test purified CHIB for antifungal activity against chitin-containing maize pathogens (e.g. Aspergillus, Fusarium) by radial-growth-inhibition and hyphal-lysis assays, and compare to maize chitinase A.
Hypothesis: CHIB inhibits fungal growth by degrading hyphal-tip cell-wall chitin, consistent with its classification as an antifungal pathogenesis-related chitinase.
Type: antifungal bioassay
Experiment: Generate maize CHIB loss-of-function and overexpression lines and challenge with fungal pathogens and fall-armyworm larvae, scoring disease/herbivory resistance and quantifying in-planta chitinase activity and CHIB hinge cleavage by fungal polyglycine hydrolases.
Hypothesis: CHIB contributes to quantitative antifungal and anti-herbivore defense, and its efficacy is modulated by fungal hinge-cleaving polyglycine hydrolases.
Type: reverse-genetics and pathogen/herbivore challenge
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