NprE (bacillolysin) is a secreted zinc metalloendopeptidase of the peptidase M4 (thermolysin-like) family. It is one of the two dominant extracellular proteases in stationary-phase B. subtilis culture supernatants, along with AprE (subtilisin). NprE is synthesized as a preproenzyme with an N-terminal Sec signal peptide (residues 1-27), a large propeptide (residues 28-221) that assists folding and transiently inhibits activity, and a mature catalytic domain (residues 222-521). The active site contains the conserved HExxH zinc-binding motif characteristic of M4 family proteases. The enzyme binds one catalytic Zn2+ ion and four structural Ca2+ ions that provide stability. NprE expression is tightly regulated and restricted to post-exponential/ stationary phase through the integrated action of repressors CodY, ScoC, and AbrB, and activator DegU~P. Together with AprE, deletion of nprE accounts for approximately 95% reduction in bulk extracellular protease activity, indicating its major contribution to extracellular proteolysis for nutrient scavenging during stationary phase.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006508 proteolysis | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation of proteolysis based on phylogenetic inference from PANTHER (PTN002138658) and thermolysin (UniProtKB:P14756). NprE is a well-characterized metalloendopeptidase that catalyzes hydrolysis of peptide bonds, directly contributing to proteolysis in the extracellular milieu. Reason: This is a core function of NprE. The enzyme is a secreted zinc metalloprotease that hydrolyzes peptide bonds, directly performing proteolysis. The phylogenetic inference from thermolysin (a closely related M4 family member) is appropriate. Multiple reviews confirm NprE as one of the two dominant extracellular proteases in B. subtilis stationary phase cultures. Supporting Evidence: file:BACSU/nprE/nprE-deep-research-falcon.md NprE is a secreted zinc metallopeptidase of the peptidase M4 family (thermolysin-like proteases) and is one of the two dominant extracellular proteases in stationary phase culture supernatants (the other is AprE/subtilisin) |
| GO:0004222 metalloendopeptidase activity | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation from InterPro domain mapping (IPR001570, IPR013856, IPR023612). NprE contains the Peptidase_M4 domain and related domains characteristic of thermolysin-like metalloendopeptidases. Reason: This is the most specific and accurate molecular function term for NprE. The enzyme is a zinc-dependent metalloendopeptidase that hydrolyzes internal peptide bonds using a catalytic mechanism involving Zn2+-activated water. The HExxH motif characteristic of M4 metalloproteases is present, and the catalytic mechanism is well-established for this enzyme family. Supporting Evidence: file:BACSU/nprE/nprE-deep-research-falcon.md Thermolysin-like M4 metalloproteases are "Glu-zincins," using the conserved HExxH motif to bind Zn2+ (His-His) and a downstream Glu as the third zinc ligand; catalysis proceeds via activation of a zinc-bound water/hydroxide for peptide bond hydrolysis |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation from UniProtKB subcellular location vocabulary mapping. UniProt records NprE as secreted (ECO:0000250). The protein has an N-terminal signal peptide for Sec-dependent export. Reason: This is a core localization for NprE. The enzyme is secreted via the Sec pathway and functions in the extracellular milieu. The signal peptide (residues 1-27) directs export, and the mature protease accumulates in stationary-phase culture supernatants. Supporting Evidence: file:BACSU/nprE/nprE-deep-research-falcon.md Mature NprE is extracellular/secreted, accumulating in stationary-phase culture supernatants UniProt:P68736 SUBCELLULAR LOCATION: Secreted |
| GO:0006508 proteolysis | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation from combined automated methods (InterPro:IPR023612 Peptidase_M4 and UniProtKB-KW:KW-0645 Protease). This is a duplicate of the IBA annotation for proteolysis. Reason: Duplicate annotation with different evidence source is acceptable. The IEA annotation reinforces the IBA annotation and is based on valid domain and keyword mappings. Both evidence lines correctly identify proteolysis as a core biological process for this enzyme. Supporting Evidence: file:BACSU/nprE/nprE-deep-research-falcon.md Deletion of nprE and aprE together reduces culture supernatant protease activity by ~95% in stationary phase |
| GO:0008233 peptidase activity | IEA GO_REF:0000043 | ACCEPT | Summary: IEA annotation from UniProtKB keyword mapping (KW-0645 Protease). GO:0008233 peptidase activity is a parent term of GO:0004222 metalloendopeptidase activity. Reason: While this is a more general term than GO:0004222, it is not incorrect. The IEA annotation is based on a valid keyword mapping. Since the more specific term GO:0004222 (metalloendopeptidase activity) is also annotated, this broader term is acceptable as it provides a different evidence lineage and could be useful for hierarchical queries. Supporting Evidence: UniProt:P68736 RecName: Full=Bacillolysin; EC=3.4.24.28 |
| GO:0008237 metallopeptidase activity | IEA GO_REF:0000043 | ACCEPT | Summary: IEA annotation from UniProtKB keyword mapping (KW-0482 Metalloprotease). GO:0008237 metallopeptidase activity is a parent term of GO:0004222 metalloendopeptidase activity. Reason: This term accurately describes NprE as a metal-dependent peptidase. While more general than GO:0004222, it correctly captures the metallopeptidase nature of the enzyme. The zinc dependence of the catalytic mechanism is well-established. Supporting Evidence: file:BACSU/nprE/nprE-deep-research-falcon.md Activity is abolished by chelators (e.g., EDTA), and Ca2+ enhances stability/activity |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: IEA annotation from UniProtKB keyword mapping (KW-0378 Hydrolase). GO:0016787 hydrolase activity is a high-level parent term in the GO hierarchy. Reason: While technically correct (metalloendopeptidases are hydrolases that use water as a nucleophile), this term is too general to be informative. The more specific terms GO:0004222 (metalloendopeptidase activity) and GO:0008237 (metallopeptidase activity) are already annotated and provide much more useful functional information. Retaining this very broad term adds little value. Supporting Evidence: file:BACSU/nprE/nprE-deep-research-falcon.md catalysis proceeds via activation of a zinc-bound water/hydroxide for peptide bond hydrolysis |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | MODIFY | Summary: IEA annotation from UniProtKB keyword mapping (KW-0479 Metal-binding). NprE binds one catalytic Zn2+ ion and four structural Ca2+ ions. Reason: While NprE does bind metal ions, this generic term does not capture the specific and functionally important metal binding. The enzyme specifically binds zinc (essential for catalysis) and calcium (essential for structural stability). More specific terms GO:0008270 (zinc ion binding) and GO:0005509 (calcium ion binding) would be more informative and accurately represent the documented metal binding properties. Proposed replacements: zinc ion binding calcium ion binding Supporting Evidence: UniProt:P68736 Binds 4 Ca(2+) ions per subunit UniProt:P68736 Binds 1 zinc ion per subunit file:BACSU/nprE/nprE-deep-research-falcon.md Ca2+ ions stabilize structure and thermal resistance |
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