HbpA (Hydrophobic protein A) is a small, non-catalytic accessory scaffoldin from Clostridium cellulovorans. It contains a single cohesin domain for recruiting dockerin-bearing enzymes and an SLH-like (surface layer homology) module for cell surface attachment. Located within the cbpA/cellulosomal gene cluster (Clocel_2820), HbpA functions as an accessory component of the cellulosome system, contributing to enzyme recruitment and cell surface anchoring. Unlike the primary scaffoldin CbpA, HbpA has only one cohesin and appears to play a supplementary organizational role.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is incorrect. HbpA is a non-catalytic scaffoldin-like protein that organizes enzymes but does not itself catalyze polysaccharide catabolism. Like CbpA, it should not be annotated with catalytic process terms. Reason: HbpA has no catalytic domain. It is a structural/organizational protein that recruits enzymes via its cohesin domain but does not directly participate in polysaccharide breakdown. Supporting Evidence: file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md Acts as a small scaffoldin/accessory scaffoldin that can recruit dockerin-bearing enzymes |
| GO:0030246 carbohydrate binding | IEA GO_REF:0000002 | UNDECIDED | Summary: This annotation is questionable. While UniProt shows CBM2/CBM3 superfamily hits, the curated literature emphasizes the cohesin and SLH-like modules rather than a validated carbohydrate-binding module. The CBM signal may reflect distant homology. Reason: The CBM annotation is inconsistent with experimental literature which describes HbpA as having cohesin and SLH-like domains. Until direct carbohydrate binding is validated, this annotation should be treated with caution. Supporting Evidence: file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md A CBM2/3 superfamily signal exists in UniProt but has not been emphasized experimentally for HbpA |
| GO:0043263 cellulosome | TAS file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md | NEW | Summary: HbpA is a cellulosome-associated accessory scaffoldin encoded within the cbpA/cellulosomal gene cluster. Reason: HbpA is part of the cellulosome system with its cohesin domain recruiting dockerin-bearing enzymes into the complex. Supporting Evidence: file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md HbpA (UniProt Q9RGE7), a hydrophobic protein of Clostridium cellulovorans, is a small, cellulosome-associated scaffoldin-like component |
| GO:0005576 extracellular region | TAS file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md | NEW | Summary: HbpA is secreted and functions extracellularly, associated with the cell surface via its SLH-like module. Reason: HbpA is secreted to the extracellular space where it associates with the cell surface and participates in cellulosome organization. Supporting Evidence: file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md Secreted, extracellular; SLH/S-layer-like modules mediate association with the cell surface |
| GO:0044575 cellulosome assembly | TAS file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md | NEW | Summary: HbpA contributes to cellulosome assembly by recruiting dockerin-bearing enzymes via its cohesin domain. Reason: As an accessory scaffoldin, HbpA's primary function is organizing the cellulosome by binding dockerin-containing catalytic subunits. Supporting Evidence: file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md Accessory scaffoldin role—recruiting dockerin-bearing glycoside hydrolases via its cohesin |
| GO:1990308 type-I dockerin domain binding | TAS file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md | NEW | Summary: HbpA contains a cohesin domain that binds type I dockerins on cellulolytic enzymes, enabling cellulosome assembly. Reason: The cohesin domain mediates Ca2+-dependent binding to dockerin-bearing enzymes, which is the primary molecular function of HbpA. Supporting Evidence: file:CLOCL/Q9RGE7/Q9RGE7-deep-research-falcon.md Cohesin–dockerin binding is Ca2+-dependent and highly specific |
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Download this section (compressed HTML)Q: What is the binding specificity of HbpA's cohesin domain for different dockerins?
Q: Does HbpA have validated carbohydrate-binding function or is the CBM annotation an artifact?
Experiment: Direct binding assays to test whether HbpA binds cellulose or other carbohydrates through its putative CBM domain.
Hypothesis: The CBM superfamily annotation may reflect distant structural homology rather than functional carbohydrate binding activity.
Experiment: Gene knockout studies of hbpA to assess its contribution to cellulosome assembly and cellulolytic activity.
Hypothesis: HbpA may provide supplementary enzyme recruitment capacity that enhances cellulosome function under certain conditions.
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