CipB (Cellulosomal-scaffolding protein B) is a secondary scaffoldin protein in the Acetivibrio thermocellum cellulosome complex. Unlike the primary scaffoldin CipA, CipB is NOT catalytically active and does not possess hydrolase activity. It functions as a structural/organizing protein containing three cohesin domains that bind dockerin domains of catalytic enzymes, one CBM3 (carbohydrate-binding module family 3) domain that binds cellulose, and a C-terminal type II dockerin domain that binds to type II cohesins on anchoring scaffoldins for cell-surface attachment. CipB contributes to higher-order cellulosome assembly and organization, connecting catalytic components to the cell surface anchoring network. It is secreted and functions extracellularly as part of the cell-attached polycellulosome complex.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000120 | REMOVE | Summary: CipB is a non-catalytic scaffoldin protein that does not directly participate in polysaccharide catabolism. While the cellulosome complex as a whole degrades polysaccharides, CipB itself lacks any catalytic domains and functions purely as a structural organizing protein. The annotation appears to be inferred from the presence of cohesin and dockerin domains, which are associated with the cellulosome but do not confer catalytic activity. According to the deep research, CipB has "a large secreted scaffold-like component that contributes to higher-order assembly via its type II dockerin rather than functioning as a catalytic enzyme" (Bras 2012). Reason: CipB does not directly catalyze polysaccharide breakdown. It is a non-catalytic scaffoldin that organizes catalytic enzymes within the cellulosome complex. The annotation incorrectly attributes a catalytic process to a purely structural protein. UniProt explicitly states CipB "Acts as a scaffolding protein in the cellulosome" with no mention of catalytic activity. Supporting Evidence: file:ACET2/Q01866/Q01866-deep-research-falcon.md CipB is a second large, non-catalytic, scaffold-like component that contributes to higher-order assembly via its type II dockerin rather than functioning as a catalytic enzyme |
| GO:0004553 hydrolase activity, hydrolyzing O-glycosyl compounds | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is incorrect. CipB does NOT have hydrolase activity. The annotation was propagated via InterPro from the dockerin domain (IPR002105), but dockerin domains are protein-protein interaction modules, not catalytic domains. CipB is explicitly described in UniProt as a scaffolding protein that "promotes binding of cellulose to the catalytic domains of the cellulolytic enzymes" - meaning it organizes enzymes that have hydrolase activity, but CipB itself lacks catalytic function. The deep research confirms CipB "is a large secreted scaffold-like component that contributes to higher-order assembly via its type II dockerin rather than functioning as a catalytic enzyme" (Bras 2012). Reason: CipB is a non-catalytic scaffoldin protein. The IEA annotation from InterPro IPR002105 (Dockerin_1_rpt) incorrectly associates dockerin domains with hydrolase activity. Dockerin domains are protein-protein interaction modules that bind cohesin domains, enabling assembly of the cellulosome complex. CipB contains no glycosyl hydrolase catalytic domains. This is a clear case of over-annotation from automated pipelines that conflate domain presence with catalytic function. |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: This annotation is correct. CipB is secreted and functions extracellularly. UniProt subcellular location explicitly states "Secreted. Note=Remains at cell surface." The protein contains an N-terminal signal peptide consistent with secretion (Bras 2012). CipB becomes part of cell-attached polycellulosomes by binding cell-surface anchoring scaffoldins via its type II dockerin. Reason: CipB is clearly documented as a secreted, extracellular protein that functions at the cell surface as part of the cellulosome complex. Supporting Evidence: UniProt:Q01866 SUBCELLULAR LOCATION: Secreted. Note=Remains at cell surface. |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | MODIFY | Summary: This is an overly broad annotation. While CipB is a component of the cellulosome which functions in carbohydrate metabolism, CipB itself does not directly participate in metabolic reactions. It is a structural scaffoldin that organizes catalytic enzymes. The annotation is derived from InterPro domains IPR001956 (CBM3) and IPR036966 (CBM3_sf). The CBM3 domain enables cellulose binding, not metabolic activity. A more appropriate BP annotation would be cellulosome assembly (GO:0044575). Reason: The annotation is too broad and implies direct metabolic activity. CipB's role is structural/organizational - it assembles and organizes catalytic enzymes that perform carbohydrate metabolism. The protein's function is better captured by cellulosome assembly. Proposed replacements: cellulosome assembly |
| GO:0030245 cellulose catabolic process | IEA GO_REF:0000043 | REMOVE | Summary: Similar to the polysaccharide catabolic process annotation, this incorrectly attributes a catalytic process to a non-catalytic scaffoldin protein. CipB does not directly catabolize cellulose - it organizes the catalytic enzymes that do. The annotation was derived from UniProt keyword KW-0136 (Cellulose degradation), which may have been applied broadly to cellulosome components without distinguishing catalytic from structural subunits. Reason: CipB does not catalyze cellulose breakdown. It is a scaffoldin protein that organizes cellulolytic enzymes within the cellulosome. The "involved_in" relationship to a catabolic process is inappropriate for a non-catalytic structural protein. |
| GO:0030246 carbohydrate binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: This annotation is broadly correct but could be more specific. CipB contains a CBM3 (carbohydrate-binding module family 3) domain that specifically binds cellulose. The more specific term GO:0030248 (cellulose binding) is already annotated and is more informative. This general carbohydrate binding annotation adds little value given the more specific annotation exists. Reason: While technically correct due to the CBM3 domain, this is redundant with the more specific cellulose binding annotation (GO:0030248). The CBM3 domain specifically binds cellulose, not carbohydrates in general. Keeping as non-core since it is not incorrect but is less informative than the specific term. |
| GO:0030248 cellulose binding | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is correct. CipB contains a well-characterized CBM3 (carbohydrate-binding module family 3) domain at positions 277-435 that specifically binds cellulose. This domain enables CipB to anchor the cellulosome complex to its cellulose substrate. UniProt explicitly annotates this CBM3 domain, and the deep research confirms "InterPro/CBM3-superfamily annotations are consistent with scaffoldin-related architecture" (Bras 2012). Reason: CipB unambiguously contains a CBM3 domain that confers cellulose binding activity. This is a core molecular function of the scaffoldin protein. Supporting Evidence: UniProt:Q01866 DOMAIN 277..435 /note="CBM3" /evidence="ECO:0000255|PROSITE-ProRule:PRU00513" |
| GO:0071555 cell wall organization | IEA GO_REF:0000043 | REMOVE | Summary: This annotation is problematic. CipB functions in cellulose degradation of PLANT cell walls (the substrate), not in organization of the bacterium's own cell wall. The annotation was derived from UniProt keyword KW-0961 (Cell wall biogenesis/degradation), but this keyword conflates degradation of external substrates with biogenesis of the organism's own cell wall. CipB is not involved in A. thermocellus cell wall organization - it is part of an extracellular complex that degrades plant biomass. Reason: CipB is a cellulosome component that degrades external plant cell wall material, not a protein involved in organizing the bacterium's own cell wall. This annotation confuses substrate degradation with self-organization. The GO term "cell wall organization" refers to organization of the organism's own cell wall, not degradation of external cell wall substrates. |
| GO:0043263 cellulosome | IDA PMID:1490597 Identification of the cellulose-binding domain of the cellul... | NEW | Summary: CipB is a documented component of the cellulosome complex. The cellulosome is defined as "An extracellular multi-enzyme complex containing up to 11 different enzymes aligned on a non-catalytic scaffolding glycoprotein." CipB is one such non-catalytic scaffolding protein. UniProt describes CipB as "Cellulosomal-scaffolding protein B" and states it "Acts as a scaffolding protein in the cellulosome." Reason: CipB is a core structural component of the cellulosome. This CC annotation accurately captures its location within this multienzyme complex. Supporting Evidence: UniProt:Q01866 RecName: Full=Cellulosomal-scaffolding protein B PMID:1490597 Identification of the cellulose-binding domain of the cellulosome subunit S1 from Clostridium thermocellum YS |
| GO:0044575 cellulosome assembly | IDA PMID:1490597 Identification of the cellulose-binding domain of the cellul... | NEW | Summary: CipB plays a direct role in cellulosome assembly through its cohesin-dockerin interactions. Its three cohesin domains bind dockerin domains of catalytic enzymes, and its C-terminal type II dockerin binds type II cohesins on anchoring scaffoldins. The deep research states CipB "contributes to higher-order assembly via its type II dockerin" (Bras 2012). The term definition "The assembly of a cellulosome, a macromolecular multi-enzyme complex in bacteria that facilitates the breakdown of cellulase, hemicellulase and pectin in the plant cell wall" precisely describes CipB's biological role. Reason: This is the most appropriate BP annotation for CipB. As a scaffoldin protein, its primary biological process function is assembling and organizing the cellulosome complex. Supporting Evidence: UniProt:Q01866 Acts as a scaffolding protein in the cellulosome. It promotes binding of cellulose to the catalytic domains of the cellulolytic enzymes |
| GO:1990308 type-I dockerin domain binding | ISS UniProt:Q01866 | NEW | Summary: CipB contains three cohesin domains (positions 1-80, 94-240, 462-607) that bind type-I dockerin domains present on catalytic cellulosome enzymes. UniProt states the cohesin domains "bind to the dockerin domain born by the catalytic components of the cellulosome." This molecular function enables CipB to organize cellulolytic enzymes within the cellulosome complex. Reason: The cohesin domains of CipB specifically bind type-I dockerin domains on catalytic enzymes. This is a core molecular function that enables CipB's scaffolding role. Supporting Evidence: UniProt:Q01866 The cohesin domains bind to the dockerin domain born by the catalytic components of the cellulosome. |
| GO:1990312 type-II cohesin domain binding | IDA PDB:2VT9 | NEW | Summary: CipB contains a C-terminal type II dockerin domain (positions 704-771) that binds type II cohesins on anchoring scaffoldins (ScaC, ScaD, ScaE). The deep research describes structural evidence: "a 1.98 A crystal structure of ScaC2 cohesin bound to CipB X-Doc (PDB 2VT9) identifies key specificity residues and Ca2+ ions in the complex" (Bras 2012). This interaction anchors CipB-containing cellulosome subcomplexes to the cell surface. Reason: The type II dockerin domain of CipB binds type II cohesin domains on anchoring scaffoldins. This is a well-characterized molecular function with structural evidence from crystallography. Supporting Evidence: PDB:2VT9 ScaC2-CipB X-Doc complex solved to 1.98 A; structure contains Ca2+ ions; ITC and mutagenesis support high-affinity, Ca2+-dependent type II binding |
| GO:0005198 structural molecule activity | ISS UniProt:Q01866 | NEW | Summary: CipB functions as a structural molecule that contributes to the integrity of the cellulosome complex. It does not have catalytic activity but instead provides a scaffold that organizes catalytic enzymes. The GO definition "The action of a molecule that contributes to the structural integrity of a complex" precisely describes CipB's molecular function. Reason: As a non-catalytic scaffoldin, CipB's primary molecular function is structural - it provides a scaffold for cellulosome assembly and organization. This is an appropriate parent term for the more specific cohesin/dockerin binding activities. Supporting Evidence: UniProt:Q01866 Acts as a scaffolding protein in the cellulosome. |
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Download this section (compressed HTML)Q: What is the relative abundance and stoichiometry of CipB vs CipA in the native cellulosome under different growth conditions?
Q: Does CipB have specific enzyme recruitment preferences compared to CipA, or are the cohesin domains functionally equivalent?
Q: What is the functional significance of the internal repeat region with ~19 copies of a ~41 aa cysteine-containing motif?
Experiment: Quantitative proteomics of cellulosome composition under different carbon sources to understand CipB's regulatory role
Hypothesis: CipB abundance varies with substrate complexity, with higher levels on mixed cellulose/hemicellulose substrates
Experiment: Structural studies of CipB cohesin domains bound to various dockerin-containing enzymes to assess specificity
Hypothesis: CipB cohesin domains may have distinct binding preferences compared to CipA cohesins
Experiment: Deletion/complementation studies to assess CipB's contribution to cellulose degradation efficiency
Hypothesis: Loss of CipB reduces cellulosome efficiency on complex substrates but may be partially compensated by CipA
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