CipA is the primary cellulosomal scaffoldin protein in Acetivibrio thermocellus (formerly Clostridium thermocellum). It is a large (1853 aa) non-catalytic structural protein that serves as the central organizing scaffold of the cellulosome - a supramolecular multi-enzyme complex that efficiently degrades plant cell wall polysaccharides. CipA contains nine type I cohesin domains that bind dockerin-bearing catalytic enzymes (cellulases, hemicellulases), a central CBM3a carbohydrate-binding module that targets the complex to crystalline cellulose, and a C-terminal type II dockerin (with associated X module) that anchors the cellulosome to cell-surface proteins bearing type II cohesins (SdbA, OlpB, Orf2p). CipA does NOT possess any catalytic/enzymatic activity - its function is purely structural and organizational, bringing multiple enzymes into proximity for synergistic cellulose degradation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: CipA is involved in polysaccharide catabolism but only indirectly through its scaffolding role. CipA itself has no catalytic activity; it organizes dockerin-bearing enzymes that perform the actual hydrolysis. The annotation captures participation in the process but overstates direct involvement. Reason: While CipA is essential for efficient polysaccharide degradation by the cellulosome, it is not directly involved in the catabolic process itself. CipA is a non-catalytic structural organizer that assembles multiple cellulases and hemicellulases into a multienzyme complex. The scaffoldin promotes enzyme synergy but does not catalyze any reactions. This annotation is acceptable as a secondary/non-core function reflecting its organizational role in the process. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CipA is a non-catalytic structural organizer that assembles multiple cellulases and hemicellulases into a multienzyme complex through high-affinity type I cohesin-dockerin interactions |
| GO:0004553 hydrolase activity, hydrolyzing O-glycosyl compounds | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is INCORRECT. CipA has no hydrolase activity whatsoever. It is a non-catalytic scaffoldin protein that organizes enzymes but does not itself possess any enzymatic function. This appears to be an erroneous transfer from the dockerin domain (IPR002105), which is also present in catalytic cellulosomal enzymes. Reason: CipA is explicitly described in the literature as a non-catalytic scaffolding glycoprotein (UniProt CC-FUNCTION). The UniProt function annotation states that CipA acts as a scaffolding protein in the cellulosome and promotes binding of cellulose to the catalytic domains of the cellulolytic enzymes - note it promotes binding TO catalytic domains, it does not have catalytic activity itself. The deep research confirms CipA is a non-catalytic structural organizer (cipA-deep-research-falcon.md). The InterPro domain IPR002105 (Dockerin_1_rpt) is present in both scaffoldins and catalytic enzymes; the hydrolase activity annotation was incorrectly transferred because dockerins are commonly found in hydrolases, but the scaffoldin CipA only uses its dockerin (type II) for anchoring to cell-surface proteins, not for catalysis. This is a clear over-annotation error. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CipA is a non-catalytic structural organizer that assembles multiple cellulases and hemicellulases into a multienzyme complex |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: This annotation is correct. CipA is a secreted protein that functions in the extracellular space, either cell-surface anchored or as part of cell-free cellulosomes. Reason: UniProt explicitly states that CipA is secreted and remains at the cell surface. The deep research confirms that CipA-enabled cellulosomes are predominantly cell surface-associated through anchoring scaffoldins bearing SLH domains. C. thermocellum also produces cell-free (diffusible) cellulosomes that operate at a distance from the bacterial surface. The protein has a signal peptide (residues 1-28) and functions extracellularly. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CipA-enabled cellulosomes are predominantly cell surface-associated through anchoring scaffoldins bearing SLH domains. C. thermocellum also produces cell-free (diffusible) cellulosomes that operate at a distance from the bacterial surface |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | MODIFY | Summary: This annotation is too general and misrepresents CipA's role. CipA does not metabolize carbohydrates; it organizes enzymes that do. This is an overly broad annotation derived from the CBM3 domain. Reason: CipA's CBM3a domain binds cellulose but does not metabolize it. The protein's role is structural - organizing the cellulosome for efficient carbohydrate degradation. A more appropriate annotation would be GO:0044575 (cellulosome assembly) which captures the actual biological process CipA participates in. The annotation to carbohydrate metabolic process incorrectly implies direct metabolic activity. Proposed replacements: cellulosome assembly Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CBM3a function: targets crystalline cellulose to increase local enzyme concentration and synergize hydrolysis; CBM3a is the prevalent cellulosomal CBM in C. thermocellum |
| GO:0030245 cellulose catabolic process | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: Similar to GO:0000272, this annotation captures CipA's indirect involvement in cellulose degradation through its scaffolding function but overstates direct participation in the catabolic process. Reason: CipA is essential for efficient cellulose catabolism via the cellulosome, but it does not itself degrade cellulose. Its CBM3a module targets the complex to cellulose substrate, and its cohesins organize the catalytic enzymes that perform hydrolysis. The annotation reflects involvement in the process but should be considered non-core since CipA's primary function is structural organization (cellulosome assembly) rather than direct catalysis. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CipA is essential for high-efficiency cellulose deconstruction, functioning at the interface of enzyme synergy, substrate targeting, and cell-surface attachment |
| GO:0030246 carbohydrate binding | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is correct but could be made more specific. CipA binds carbohydrates via its CBM3a domain, specifically binding crystalline cellulose. Reason: CipA contains a well-characterized CBM3a (carbohydrate-binding module family 3a) domain at positions 365-523. The deep research confirms that CBM3a targets crystalline cellulose to increase local enzyme concentration and synergize hydrolysis. CBM3a is the prevalent cellulosomal CBM in C. thermocellum (cipA-deep-research-falcon.md). The crystal structure of this domain has been solved (PDB: 1NBC at 1.75 A resolution). This is a core molecular function. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CBM3a function: targets crystalline cellulose to increase local enzyme concentration and synergize hydrolysis |
| GO:0030248 cellulose binding | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is correct and represents a core molecular function of CipA. The CBM3a domain specifically binds crystalline cellulose. Reason: CipA's CBM3a domain (residues 365-523) specifically binds crystalline cellulose, targeting the cellulosome to its substrate. This is experimentally verified through structural studies - the crystal structure of the CBM3a domain was solved at 1.75 A resolution (PDB: 1NBC; Tormo et al., EMBO J 1996). The deep research states that CBM3a function targets crystalline cellulose to increase local enzyme concentration and synergize hydrolysis (cipA-deep-research-falcon.md). This is a core function of CipA - without cellulose binding, the cellulosome cannot target its substrate. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CBM3a function: targets crystalline cellulose to increase local enzyme concentration and synergize hydrolysis |
| GO:0071555 cell wall organization | IEA GO_REF:0000043 | REMOVE | Summary: This annotation is misleading. CipA is involved in PLANT cell wall DEGRADATION (as a target substrate), not in bacterial cell wall organization. The UniProt keyword mapping appears to have conflated cell wall degradation with cell wall organization. Reason: CipA functions in the degradation of plant cell walls (cellulose, hemicellulose), not in organizing the bacterial cell wall. While CipA is anchored at the cell surface, its function is to organize the cellulosome for degrading extracellular plant material, not to organize the bacterium's own cell wall. The UniProt keywords include Cell wall biogenesis/degradation which likely triggered this annotation, but the context is substrate degradation, not endogenous cell wall organization. This is a semantic error in the automated annotation pipeline. Supporting Evidence: file:ACET2/cipA/cipA-deep-research-falcon.md CipA's modular architecture orchestrates assembly of numerous dockerin-bearing CAZymes and anchors the complex to the cell surface through SLH-bearing secondary scaffoldins |
| GO:0005515 protein binding | IPI PMID:14623971 Cellulosome assembly revealed by the crystal structure of th... | MODIFY | Summary: This annotation is experimentally supported but should be made more specific. PMID:14623971 reports the crystal structure of the cohesin-dockerin complex, demonstrating that CipA's cohesin domains bind dockerin domains on enzymes. Reason: The IPI annotation from PMID:14623971 is based on the crystal structure of the type I cohesin-dockerin complex from C. thermocellum (WITH: UniProtKB:P51584, XynY xylanase). The study demonstrates that the beta-sheet cohesin domain interacts predominantly with one of the helices of the dockerin (PMID:14623971). However, protein binding is too vague. CipA specifically binds type I dockerin domains through its nine type I cohesin modules. The annotation should be changed to GO:1990308 (type-I dockerin domain binding) which precisely describes this molecular function. Proposed replacements: type-I dockerin domain binding Supporting Evidence: PMID:14623971 The data show that the beta-sheet cohesin domain interacts predominantly with one of the helices of the dockerin. |
| GO:0005515 protein binding | IPI PMID:16384918 Mechanism of bacterial cell-surface attachment revealed by t... | MODIFY | Summary: This annotation documents CipA's C-terminal type II dockerin binding to type II cohesins on cell-surface anchoring proteins. The interaction demonstrated is between CipA's X-DocII module and a type II cohesin from Cthe_1307. Reason: PMID:16384918 reports the structure of the type II cohesin-dockerin complex involving CipA's C-terminal X-dockerin module (WITH: UniProtKB:A3DF10, Cthe_1307). The study describes an ultra-high-affinity complex between type II Doc together with its neighboring X module from the cellulosome scaffold of Clostridium thermocellum and a type II Coh module associated with the bacterial cell surface. The annotation should be more specific: GO:1990312 (type-II cohesin domain binding) captures that CipA's dockerin binds to type II cohesins. Proposed replacements: type-II cohesin domain binding Supporting Evidence: PMID:16384918 Here, we report the structure of an ultra-high-affinity (K(a) = 1.44 x 10(10) M(-1)) complex between type II Doc, together with its neighboring X module from the cellulosome scaffold of Clostridium thermocellum, and a type II Coh module associated with the bacterial cell surface. |
| GO:0005515 protein binding | IPI PMID:17360613 Evidence for a dual binding mode of dockerin modules to cohe... | MODIFY | Summary: This annotation provides additional evidence for CipA's cohesin-dockerin interactions, demonstrating a dual binding mode where dockerins can interact with cohesins in two orientations. Reason: PMID:17360613 provides structural evidence for the dual binding mode of type I dockerins to type I cohesins. The study shows that both repeats could interact with cohesins by a common mechanism and that dockerins can bind in a 180-degree rotated orientation (PMID:17360613). This further characterizes the type I cohesin-dockerin interaction. As with PMID:14623971, the annotation should be made more specific to GO:1990308 (type-I dockerin domain binding). Proposed replacements: type-I dockerin domain binding Supporting Evidence: PMID:17360613 The dual binding mode is predicted to impart significant plasticity into the orientation of the catalytic subunits within this supramolecular assembly |
| GO:0043263 cellulosome | IDA PMID:14623971 Cellulosome assembly revealed by the crystal structure of th... | NEW | Summary: CipA is the primary structural component of the cellulosome. This cellular component annotation is essential and missing from the current annotation set. Reason: CipA is THE primary scaffoldin of the C. thermocellum cellulosome - it IS the cellulosome scaffold. UniProt names it Cellulosomal-scaffolding protein A and states it acts as a scaffolding protein in the cellulosome. The deep research confirms that cipA encodes the primary cellulosomal scaffoldin in Clostridium thermocellum (cipA-deep-research-falcon.md). This CC annotation is critical for understanding CipA's identity and should be added with IDA evidence based on direct characterization studies. Supporting Evidence: PMID:14623971 This megadalton catalytic machine organizes an enzymatic consortium on a multifaceted molecular scaffold |
| GO:0044575 cellulosome assembly | IDA PMID:14623971 Cellulosome assembly revealed by the crystal structure of th... | NEW | Summary: CipA's primary biological process is assembling the cellulosome by recruiting dockerin-bearing enzymes. This is a core function that should be annotated. Reason: CipA's nine type I cohesin domains bind dockerin-bearing enzymes, directly assembling the cellulosome complex. PMID:14623971 reveals the structure of the cohesin-dockerin complex from Clostridium thermocellum showing how cellulosome assembly occurs. The deep research states that CipA is a non-catalytic structural organizer that assembles multiple cellulases and hemicellulases into a multienzyme complex through high-affinity type I cohesin-dockerin interactions (cipA-deep-research-falcon.md). This is CipA's primary biological process. Supporting Evidence: PMID:14623971 This megadalton catalytic machine organizes an enzymatic consortium on a multifaceted molecular scaffold whose "cohesin" domains interact with corresponding "dockerin" domains of the enzymes. |
| GO:0005198 structural molecule activity | IDA PMID:14623971 Cellulosome assembly revealed by the crystal structure of th... | NEW | Summary: CipA provides the structural framework of the cellulosome. This molecular function annotation captures its non-catalytic scaffolding role. Reason: CipA contributes to the structural integrity of the cellulosome complex. It is explicitly described as a non-catalytic scaffolding glycoprotein that provides the structural backbone for enzyme assembly. Unlike catalytic cellulosomal components, CipA's molecular function is purely structural - organizing and maintaining the multi-enzyme complex. This annotation appropriately captures that CipA's molecular function is structural organization rather than catalysis. Supporting Evidence: PMID:14623971 This megadalton catalytic machine organizes an enzymatic consortium on a multifaceted molecular scaffold |
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Download this section (compressed HTML)Q: Are there conditions under which CipA might directly participate in carbohydrate modification or processing beyond its scaffolding role?
Experiment: Comprehensive interactome analysis of CipA cohesin domains to quantify the full repertoire of dockerin-bearing proteins recruited to the cellulosome in vivo.
Hypothesis: While the cohesin-dockerin interaction is well-characterized structurally, the complete set of CipA interaction partners in vivo remains incompletely defined.
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