CelM is a putative aminopeptidase belonging to the M42 peptidase family, originally misannotated as an endoglucanase (hence the "cel" nomenclature). Despite its misleading name, CelM has been experimentally characterized as a metalloaminopeptidase with leucine aminopeptidase (LAP) activity, requiring cobalt ions (Co2+) as a cofactor. The protein forms an active dodecameric complex and preferentially cleaves nonpolar aliphatic L-amino acid residues from the N-terminus of peptide substrates, with L-leucine-pNA being the optimal substrate. CelM shows no significant cellulase activity despite earlier erroneous reports. The protein does not contain a dockerin domain and is therefore not a component of the cellulosome complex. Its biological role may involve protein turnover or peptide processing in the cytoplasm of A. thermocellus.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004177 aminopeptidase activity | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is correct and supported by experimental evidence from Dutoit et al. (2012) PMID:23226342, who demonstrated that CelM has leucine aminopeptidase (LAP) activity using L-leucine-pNA as substrate. The protein preferentially hydrolyzes nonpolar aliphatic L-amino acid-pNA substrates at the N-terminus. Reason: CelM has been experimentally demonstrated to function as an aminopeptidase. Although the annotation is derived from keyword mapping (IEA), it accurately reflects the experimentally determined function of the protein. This is a core molecular function of the enzyme. Supporting Evidence: UniProt:P55742 Belongs to the peptidase M42 family. |
| GO:0006508 proteolysis | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is correct as CelM participates in proteolytic degradation through its aminopeptidase activity. The protein hydrolyzes peptide bonds at the N-terminus of peptides. Reason: Aminopeptidases are proteolytic enzymes that participate in proteolysis by cleaving N-terminal amino acid residues from peptides. This is the appropriate biological process term for the enzymatic activity of CelM. Supporting Evidence: UniProt:P55742 Belongs to the peptidase M42 family. |
| GO:0008233 peptidase activity | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is correct but represents a parent term of the more specific aminopeptidase activity annotation. CelM is a peptidase of the M42 metallopeptidase family. Reason: This is a valid annotation as CelM is indeed a peptidase. However, the more specific term GO:0004177 (aminopeptidase activity) or GO:0070006 (metalloaminopeptidase activity) is more informative. This general term is acceptable as an IEA annotation alongside the more specific annotations. Supporting Evidence: UniProt:P55742 Belongs to the peptidase M42 family. |
| GO:0008237 metallopeptidase activity | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is correct. CelM is an M42 family metallopeptidase that requires divalent metal cations (specifically Co2+ for optimal activity) for catalysis. The metal ions are essential for the enzymatic mechanism. Reason: CelM is a metallopeptidase that requires metal ions for activity. EDTA chelation inhibits activity, confirming the metal-dependence of the enzyme. The annotation correctly reflects the catalytic mechanism of the enzyme. Supporting Evidence: UniProt:P55742 Binds 2 divalent metal cations per subunit. |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is correct but very general. CelM is a hydrolase that catalyzes the hydrolysis of peptide bonds. This is a parent term of the more specific peptidase and aminopeptidase activity terms. Reason: While correct, this is the most general term in the hierarchy. The more specific annotations (aminopeptidase activity, metallopeptidase activity) are more informative. This annotation is acceptable as part of the IEA annotation set derived from keyword mapping, but provides limited functional insight beyond the more specific terms. Supporting Evidence: UniProt:P55742 Belongs to the peptidase M42 family. |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation is correct. CelM binds divalent metal cations which are essential for its catalytic activity. UniProt reports binding of 2 divalent metal cations per subunit at defined binding sites. Reason: CelM requires divalent metal cations for activity. UniProt documents metal binding sites at residues 65, 168, 199, 221, and 307. While cobalt provides optimal activity in vitro, the in vivo metal cofactor may differ. The general metal ion binding term is appropriate given this uncertainty. Supporting Evidence: UniProt:P55742 Binds 2 divalent metal cations per subunit. |
| GO:0070006 metalloaminopeptidase activity | IDA PMID:23226342 Functional characterization of two M42 aminopeptidases erron... | NEW | Summary: This is the most appropriate and specific molecular function term for CelM. The protein is an M42 family metalloaminopeptidase that cleaves N-terminal amino acids using a metal-dependent catalytic mechanism. Reason: This term should be added as a new annotation based on the experimental characterization by Dutoit et al. (2012). CelM is a metalloaminopeptidase that requires metal ions for catalysis and cleaves N-terminal amino acids from peptide substrates. This is more specific than both aminopeptidase activity and metallopeptidase activity. Supporting Evidence: UniProt:P55742 Belongs to the peptidase M42 family. |
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Download this section (compressed HTML)Q: What is the physiological metal cofactor of CelM in vivo? While cobalt provides optimal activity in vitro, the in vivo cofactor may be different.
Q: What is the biological role of CelM in A. thermocellus? Is it involved in protein turnover, processing of cellulosomal components, or another cellular function?
Q: Does CelM have any role related to the cellulosome, or is it entirely a cytoplasmic housekeeping enzyme?
Experiment: Determine the native metal cofactor of CelM through metal content analysis of purified enzyme from A. thermocellus.
Experiment: Construct celM deletion mutant to assess phenotypic effects and identify physiological role.
Experiment: Identify physiological substrates through proteomics or peptide library screening.
Experiment: Determine if CelM is cytoplasmic or secreted, and if it has any association with cellulosome components.
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