FepE is a member of the WzzB/Cld/Rol family of polysaccharide co-polymerases (PCP-1 class). It is an inner membrane protein with two transmembrane helices and a large periplasmic domain. FepE regulates the modal chain length distribution of lipopolysaccharide (LPS) O-antigen, specifically conferring very long (VL) modal chain lengths of >80-100 O-antigen repeat units. FepE was originally identified in the fep (ferric enterobactin transport) gene cluster and named accordingly, but its primary experimentally validated function is in O-antigen chain length regulation, not ferric enterobactin transport per se. It belongs to the Wzz protein family (Pfam PF02706) and forms nonameric oligomeric assemblies in the inner membrane. The PANTHER classification places it in the PTHR32309 family (labeled "tyrosine-protein kinase"), which is a misnomer -- Wzz/PCP-1 proteins lack a kinase domain and are structurally and functionally distinct from both eukaryotic tyrosine kinases and bacterial BY-kinases (PCP-2 class like Wzc). FepE has no kinase activity.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005886
plasma membrane
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: FepE is an inner membrane protein with two transmembrane helices (residues 42-62 and 339-359) and a large periplasmic domain. UniProt annotates it as "Cell inner membrane; Multi-pass membrane protein." The IBA annotation for plasma membrane (GO:0005886) is consistent with this localization. In E. coli, the inner membrane is annotated as plasma membrane in GO.
Reason: FepE is well established as an inner membrane protein. The topology was confirmed by large-scale analysis using C-terminal PhoA/GFP fusions (PMID:15919996). UniProt states "Cell inner membrane; Multi-pass membrane protein." The IBA annotation is correct.
Supporting Evidence:
PMID:15919996
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein, we established the periplasmic or cytoplasmic locations of the C termini for 601 inner membrane proteins.
|
|
GO:0004713
protein tyrosine kinase activity
|
IBA
GO_REF:0000033 |
REMOVE |
Summary: This annotation is INCORRECT. FepE is a PCP-1 class polysaccharide co-polymerase (Wzz family) that LACKS a kinase domain. The PANTHER family PTHR32309 is misleadingly named "TYROSINE-PROTEIN KINASE" but actually contains Wzz/PCP-1 proteins. PCP-1 proteins (like FepE, WzzB, WzzST) are structurally distinct from PCP-2 proteins (like Wzc) which do contain a BY-kinase domain. FepE contains only transmembrane helices and a periplasmic domain with no cytoplasmic kinase domain. The Pfam domain is PF02706 (Wzz), and InterPro classifies it under IPR003856 (LPS_length_determ_N) and IPR050445 (Bact_polysacc_biosynth/exp). None of these are kinase domains. This is a clear case of erroneous IBA propagation from a mis-classified PANTHER family. De Crecy-Lagard et al. 2025 (PMID:40703034) specifically identified FepE as an example of a frequency-biased prediction error where DeepECTF incorrectly predicted histidine kinase activity (EC 2.7.13.3), noting that FepE has NO sequence similarity to kinase families and has a different experimentally validated function.
Reason: FepE has no kinase domain and no kinase activity. It is a Wzz-family polysaccharide co-polymerase. The PANTHER family PTHR32309 erroneously carries the label "TYROSINE-PROTEIN KINASE," leading to incorrect IBA propagation. PCP-1 proteins like FepE are structurally unrelated to tyrosine kinases -- they consist of two transmembrane helices flanking a periplasmic domain involved in O-antigen chain length regulation. The PCP-2 class (e.g., Wzc) does have a BY-kinase domain, but FepE belongs to PCP-1 which lacks this domain entirely. De Crecy-Lagard et al. 2025 highlighted FepE specifically as having been erroneously predicted to have kinase activity by AI tools due to frequency bias (PMID:40703034).
Supporting Evidence:
PMID:40703034
out of the 12 proteins annotated as histidine kinase (EC 2.7.13.3) in the 453 DeepECTF predictions, none of them have sequence similarity to histidine kinase families, and 8 have been annotated with different and experimentally validated functions (such as ferric enterobactin transport protein FepE for b0587)
PMID:22437828
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation in its ability to produce very long O-antigen polymers.
PMID:12603743
In addition to the previously described wzzST that results in long (L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs
file:ECOLI/fepE/fepE-deep-research-falcon.md
Falcon deep research confirms FepE is a Wzz/PCP1a O-antigen chain length regulator conferring VL chains of >80 repeat units. Structure-guided mutagenesis identified critical residues L168 and D268. Recent cryo-EM shows Wzz-Wzy complex formation with 8:1 stoichiometry. No evidence supports kinase or transporter function.
|
|
GO:0005886
plasma membrane
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: IEA annotation based on UniProtKB/Swiss-Prot Subcellular Location mapping. UniProt states "Cell inner membrane; Multi-pass membrane protein." This is consistent with FepE being an integral inner membrane protein.
Reason: Correctly maps from UniProt subcellular location annotation. FepE is an inner membrane protein with two transmembrane helices, consistent with plasma membrane localization in GO terminology for bacteria.
Supporting Evidence:
PMID:15919996
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein, we established the periplasmic or cytoplasmic locations of the C termini for 601 inner membrane proteins.
|
|
GO:0016020
membrane
|
IEA
GO_REF:0000002 |
ACCEPT |
Summary: IEA annotation from InterPro mapping (IPR003856, LPS_length_determ_N). FepE is indeed a membrane protein. This is a broader term than GO:0005886 (plasma membrane) which is also annotated. While redundant with the more specific plasma membrane annotations, this IEA annotation is not incorrect.
Reason: FepE is a multi-pass membrane protein. The term is broader than the more specific plasma membrane annotations but is not wrong. IEA annotations mapping to broader terms are acceptable even when more specific annotations exist.
|
|
GO:0005886
plasma membrane
|
IDA
PMID:15919996 Global topology analysis of the Escherichia coli inner membr... |
ACCEPT |
Summary: IDA annotation from EcoCyc based on the Daley et al. 2005 large-scale topology study. This study used C-terminal PhoA and GFP fusions to determine the orientation and localization of E. coli inner membrane proteins. FepE was confirmed as an inner membrane protein with its C-terminus in the cytoplasm.
Reason: Experimentally validated localization. The Daley et al. study used PhoA/GFP dual-reporter fusions to systematically determine the topology of E. coli inner membrane proteins. FepE was among the 601 proteins characterized.
Supporting Evidence:
PMID:15919996
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein, we established the periplasmic or cytoplasmic locations of the C termini for 601 inner membrane proteins.
|
|
GO:0015620
ferric-enterobactin transmembrane transporter activity
|
IMP
PMID:2956250 Genetic organization of multiple fep genes encoding ferric e... |
REMOVE |
Summary: This annotation is based on the original 1987 study by Ozenberger et al. that identified fepE through insertion mutagenesis. Insertions in fepE disrupted iron transport via enterobactin, leading to the proposal that FepE acts as part of the ferric enterobactin cytoplasmic membrane permease together with FepC and FepD. However, subsequent work (Murray et al. 2003, PMID:12603743; Kalynych et al. 2012, PMID:22437828) established that FepE is actually a Wzz-family polysaccharide co-polymerase that regulates O-antigen chain length. FepE belongs to the Wzz/PCP-1 family (Pfam PF02706), not to any transporter family. The original observation that fepE mutations disrupted enterobactin-mediated iron transport may reflect indirect/pleiotropic effects -- alterations in O-antigen chain length could affect outer membrane integrity and thus iron uptake -- or polar effects on downstream fep genes in the original insertion mutants. FepE lacks any recognizable transporter domain (no ABC transporter domains, no permease domains).
Reason: FepE is not a transporter. It is a Wzz-family polysaccharide co-polymerase (PCP-1 class) with established structural and functional characterization as an O-antigen chain length regulator. The protein contains a Wzz domain (PF02706) and belongs to InterPro family IPR050445 (Bact_polysacc_biosynth/exp). It has no transporter domains. The original 1987 finding that fepE insertion mutations disrupted enterobactin transport likely reflected polar effects on downstream fep genes or indirect effects of altered LPS structure on outer membrane function. Multiple subsequent studies have firmly established FepE as an O-antigen chain length determinant, not a transporter.
Supporting Evidence:
PMID:2956250
Another insertion mutation between entF and fepC was also shown to disrupt iron transport via enterobactin and thus defined the fepE locus; fepE weakly expressed a 43,000-dalton protein in minicells. It is proposed that these newly identified genes, fepD and fepE, provide functions which act in conjunction with the fepC product to form the ferric enterobactin-specific cytoplasmic membrane permease.
PMID:12603743
In addition to the previously described wzzST that results in long (L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs
PMID:22437828
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation in its ability to produce very long O-antigen polymers.
|
|
GO:0015685
ferric-enterobactin import into cell
|
IMP
PMID:2956250 Genetic organization of multiple fep genes encoding ferric e... |
MODIFY |
Summary: This biological process annotation suffers from the same issue as the transporter activity annotation above. It is based on the original 1987 Ozenberger et al. study where fepE insertion mutants showed disrupted enterobactin-dependent iron transport. However, FepE has since been firmly established as a Wzz-family polysaccharide co-polymerase involved in O-antigen chain length regulation, not in ferric enterobactin import. Note that the qualifier in the GOA file is "acts_upstream_of_or_within" rather than "involved_in," reflecting some uncertainty, but even this weaker claim is not well supported given the likely polar/indirect nature of the original observation.
Reason: FepE is not directly involved in ferric-enterobactin import. Its true biological process is O-antigen biosynthesis / LPS O-antigen chain length determination. The annotation should be replaced with GO:0009243 (O antigen biosynthetic process), which accurately reflects FepE's role as a Wzz-family O-antigen chain length regulator. The original observation of disrupted enterobactin transport in fepE mutants was likely due to polar effects or indirect consequences of altered O-antigen/LPS structure.
Proposed replacements:
O antigen biosynthetic process
Supporting Evidence:
PMID:12603743
Wzz proteins regulate the degree of polymerization of the O antigen (Oag) subunits in lipopolysaccharide (LPS) biosynthesis
PMID:22437828
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation in its ability to produce very long O-antigen polymers.
|
|
GO:0005198
structural molecule activity
|
ISS
PMID:22437828 Structural characterization of closely related O-antigen lip... |
NEW |
Summary: FepE functions as a polysaccharide co-polymerase (PCP-1 class) that regulates O-antigen chain length through its oligomeric periplasmic structure. It has no enzymatic activity but acts as a structural regulator modulating Wzy polymerase processivity. Structural molecule activity is the most appropriate molecular function term for a protein that functions through its structural properties rather than through catalysis.
Reason: FepE has no kinase or transporter activity. Its molecular function is structural -- it forms nonameric oligomeric assemblies that regulate the chain length of O-antigen polymers through structural interactions with the Wzy polymerase machinery. GO:0005198 (structural molecule activity) captures this non-catalytic, structure-dependent molecular function.
Supporting Evidence:
PMID:22437828
The surface O-antigen polymers of Gram-negative bacteria exhibit a modal length distribution that depends on dedicated chain length regulator periplasmic proteins (polysaccharide co-polymerases, PCPs) anchored in the inner membrane by two transmembrane helices.
|
|
GO:0009243
O antigen biosynthetic process
|
IMP
PMID:12603743 Regulation of Salmonella typhimurium lipopolysaccharide O an... |
NEW |
Summary: FepE (WzzfepE) is a polysaccharide co-polymerase that regulates the modal chain length of O-antigen in lipopolysaccharide biosynthesis. Murray et al. 2003 demonstrated that FepE homologs in Salmonella typhimurium confer very long (VL) modal length O-antigen of >100 repeat units. This is the core biological process function of FepE.
Reason: FepE is a Wzz-family O-antigen chain length regulator. Its involvement in O-antigen biosynthesis is well established by multiple studies. This is the correct biological process annotation that should replace the ferric-enterobactin import annotation.
Supporting Evidence:
PMID:12603743
Wzz proteins regulate the degree of polymerization of the O antigen (Oag) subunits in lipopolysaccharide (LPS) biosynthesis
PMID:22437828
The surface O-antigen polymers of Gram-negative bacteria exhibit a modal length distribution that depends on dedicated chain length regulator periplasmic proteins (polysaccharide co-polymerases, PCPs) anchored in the inner membrane by two transmembrane helices.
|
Q: Should the UniProt record for FepE be updated to reflect its true function as a Wzz-family O-antigen chain length regulator, rather than "Ferric enterobactin transport protein FepE"?
Q: Should the PANTHER family PTHR32309 be relabeled from "TYROSINE-PROTEIN KINASE" to correctly reflect Wzz/PCP-1 polysaccharide co-polymerase function, to prevent further erroneous IBA propagation?
Q: Was the original Ozenberger et al. 1987 observation of disrupted enterobactin transport in fepE mutants due to polar effects on downstream fep genes or indirect LPS structural effects?
Experiment: Construct a clean, non-polar deletion of fepE and test for ferric enterobactin transport using siderophore utilization assays. Compare with the original insertion mutants to distinguish polar from non-polar effects.
Hypothesis: Non-polar deletion of fepE does not affect ferric enterobactin transport, confirming the original observation was due to polar effects.
Type: transport assay
Experiment: Express fepE in E. coli K-12 strains with restored O-antigen biosynthesis (e.g., O16 serotype) and analyze LPS by SDS-PAGE/silver staining to determine if FepE confers very long modal O-antigen chain length as seen in Salmonella.
Hypothesis: FepE regulates O-antigen chain length in E. coli K-12 O16 serotype.
Type: LPS analysis
FepE DeepECTF prediction review. FepE was one of 12 proteins incorrectly predicted as histidine kinase (EC 2.7.13.3) by DeepECTF. This is a frequency-biased repetition error - none of the 12 predicted histidine kinases have sequence similarity to histidine kinase families, and 8 (including FepE) have experimentally validated different functions. FepE is a Wzz-family polysaccharide co-polymerase that regulates O-antigen chain length.
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template_file: templates/gene_research_go_focused.md
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organism: ECOLI
gene_id: fepE
gene_symbol: fepE
uniprot_accession: P26266
protein_description: 'RecName: Full=Ferric enterobactin transport protein FepE;'
gene_info: Name=fepE; OrderedLocusNames=b0587, JW0579;
organism_full: Escherichia coli (strain K12).
protein_family: Belongs to the WzzB/Cld/Rol family. .
protein_domains: Bact_polysacc_biosynth/exp. (IPR050445); LPS_length_determ_N. (IPR003856);
Wzz (PF02706)
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BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'fepE' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene fepE (gene ID: fepE, UniProt: P26266) in ECOLI.
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Question: You are an expert researcher providing comprehensive, well-cited information.
Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies
Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.
BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'fepE' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene fepE (gene ID: fepE, UniProt: P26266) in ECOLI.
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
UniProt accession P26266 is annotated in the prompt as “Ferric enterobactin transport protein FepE” from Escherichia coli (strain K-12) and as a member of the WzzB/Cld/Rol (PCP/Wzz) family with two transmembrane helices flanking a large periplasmic domain (Wzz/PCP1a architecture). Primary experimental literature that explicitly studies E. coli K-12 FepE describes it as WzzFepE/FepE, a lipopolysaccharide (LPS) O-antigen chain-length regulator, not as a component of the ferric enterobactin uptake transporter. (tran2013residueslocatedinside pages 1-2)
Conclusion (identity check): the gene symbol fepE here is best treated as a historical/legacy naming conflict: in E. coli K-12, the protein corresponding to P26266 behaves as a Wzz-family polysaccharide co-polymerase (PCP1a) that controls O-antigen modal chain length, aligning with the Wzz-family domains listed in the prompt. (tran2013residueslocatedinside pages 1-2, morona2009sequencestructurerelationshipsin pages 1-2)
In many Enterobacteriaceae, LPS O-antigen is a surface-exposed polysaccharide appended to lipid A–core. In the Wzx/Wzy-dependent pathway, repeat units are assembled on the cytosolic face, flipped by Wzx to the periplasmic leaflet, and polymerized by the inner-membrane polymerase Wzy into long chains; chain lengths typically show a modal distribution (a preferred range/peak rather than a continuous smear). (hong2023repeatunitelongationsto pages 1-2)
Wzz/PCP1 proteins (including WzzB and WzzFepE/FepE) are inner-membrane-anchored, periplasm-facing regulators that tune Wzy polymerization to yield characteristic modal chain lengths; they are not enzymes with a well-defined catalytic reaction but rather structural/regulatory components of the polymerization machinery. (weckener2023thelipidlinked pages 1-2, morona2009sequencestructurerelationshipsin pages 1-2)
In the gene-specific study of E. coli K-12 FepE, FepE is explicitly treated as a PCP1a (Wzz-family) protein that confers very-long (VL) O-antigen modal chain lengths (reported as >80 repeat units). (tran2013residueslocatedinside pages 1-2, tran2013residueslocatedinside pages 4-5)
A detailed mutagenesis/cross-linking study in E. coli demonstrates that FepE regulates LPS O-antigen modal chain length, with wild-type FepE conferring a VL O-antigen phenotype (>80 repeat units). (tran2013residueslocatedinside pages 1-2, tran2013residueslocatedinside pages 4-5)
Structure-guided mutagenesis maps functionally critical residues to the interior region of the FepE oligomeric assembly and shows that substitutions can markedly shorten the O-antigen distribution, supporting a model where the FepE oligomer provides a physical/chemical environment important for controlling when polymer chains are released/terminated. (tran2013residueslocatedinside pages 11-12, tran2013residueslocatedinside pages 5-7)
Substrate specificity (functional “substrate”): FepE does not transport a small-molecule substrate; instead, it regulates the chain length of the growing undecaprenyl-pyrophosphate-linked O-antigen polysaccharide produced by Wzy in the periplasmic leaflet. This interpretation follows from the Wzz/PCP conceptual framework and gene-specific mutant phenotypes affecting O-antigen repeat-unit number. (hong2023repeatunitelongationsto pages 8-10, weckener2023thelipidlinked pages 1-2)
Within the retrieved, gene-specific experimental evidence for E. coli K-12 FepE, the measured outputs are LPS O-antigen chain-length phenotypes (silver-stained LPS gels) and FepE oligomerization/topology; no ferric-enterobactin uptake assays or transporter phenotypes are presented as the basis for function. The authors instead state and test FepE as a Wzz-family O-antigen modal chain-length regulator. (tran2013residueslocatedinside pages 1-2, tran2013residueslocatedinside pages 4-5)
Accordingly, based on this evidence set, the most defensible annotation for P26266 is WzzFepE (O-antigen chain-length determinant) rather than a component of the ferric enterobactin transport system. (tran2013residueslocatedinside pages 1-2, weckener2023thelipidlinked pages 2-3)
FepE is an inner membrane protein with two transmembrane helices (TM1 and TM2) near the N- and C-termini and a large periplasmic, largely alpha-helical/coiled-coil domain between them (a canonical PCP1a/Wzz architecture). (tran2013residueslocatedinside pages 5-7, morona2009sequencestructurerelationshipsin pages 1-2, marolda2008functionalanalysisof pages 1-2)
Visual support for this topology is shown in the Tran & Morona figures (topology diagram and structural model). (tran2013residueslocatedinside media 115f7306)
FepE oligomerizes; however, the exact oligomeric stoichiometry can depend on construct/environment and method. Tran & Morona report that periplasmic-domain crystal structures show nonameric assemblies, while cross-linking/mass spectrometry and other observations are consistent with an in vivo maximum hexamer in their system; they emphasize that functional chain-length regulation does not strictly require a single fixed oligomeric state. (tran2013residueslocatedinside pages 12-13, tran2013residueslocatedinside pages 2-3)
Across the broader Wzz/PCP family, more recent cryo-EM studies of related Wzz proteins strongly support octameric bell-shaped oligomers with a central lumen/cavity that can physically accommodate aspects of the polymerase/regulatory complex, reinforcing a structural-scaffold mechanism for chain-length regulation. (weckener2023thelipidlinked pages 8-10, weckener2023thelipidlinked pages 1-2)
Two residues highlighted as essential for wild-type VL chain-length regulation are L168 and D268. Substitutions at these sites generally shorten O-antigen chains, whereas a conservative D268E change can preserve wild-type-like VL phenotype, indicating that a negative charge at that position is functionally important. (tran2013residueslocatedinside pages 5-7, tran2013residueslocatedinside pages 12-13)
Wild-type FepE confers a very-long O-antigen modality (>80 repeat units). Multiple mutants yield markedly shorter distributions; for example, D268 substitutions can produce shortened modes reported in ranges such as 3–14 or 7–15 repeat units, whereas D268E preserves wild-type length. (tran2013residueslocatedinside pages 11-12, tran2013residueslocatedinside pages 5-7)
Silver-stained LPS gels showing these phenotype shifts (and loss of modality/shortening for specific mutants) are provided in the Tran & Morona figures. (tran2013residueslocatedinside media 4bee4c1f, tran2013residueslocatedinside media 31779260)
FepE contains transmembrane cysteines (e.g., C52 and C354) that are useful for probing oligomer contacts. Mutating these residues did not abolish O-antigen chain-length regulation, but cross-linking patterns support close association of transmembrane segments in oligomeric assemblies. (tran2013residueslocatedinside pages 5-7, tran2013residueslocatedinside pages 7-9)
A key modern framing is that Wzy is the sole polymerase catalyzing repeat-unit additions, while Wzz-family proteins regulate chain length by organizing or altering polymerization kinetics/termination probability. (hong2023repeatunitelongationsto pages 1-2, weckener2023thelipidlinked pages 1-2)
The 2023 EcoSal Plus review summarizes a model in which free Wzy tends to produce short, non-modal chains (often ~1–3 or up to 4 O units), whereas Wzy coupled/docked to Wzz can yield longer modal distributions, potentially by shifting toward a more processive mode. (hong2023repeatunitelongationsto pages 8-10, hong2023repeatunitelongationsto pages 7-8)
Hong et al. (2023) collate quantitative observations/modeling, including typical short-chain populations and modal lengths that can center around ~4, 12, or 17 repeat units in different systems, and modeling in which termination probability changes with chain length (e.g., low early termination probability increasing near the modal peak). These values are system-dependent and do not replace the gene-specific observation that FepE can drive VL chains (>80 repeat units). (hong2023repeatunitelongationsto pages 7-8, tran2013residueslocatedinside pages 1-2)
A major 2023 advance is direct structural/biophysical evidence that Wzz-family PCPs can form stable complexes with their cognate polymerases. For enterobacterial common antigen (ECA) biosynthesis, Weckener et al. (2023) report Wzy–Wzz complex formation with an approximate 8 Wzz : 1 Wzy stoichiometry and cryo-EM reconstructions consistent with Wzy positioned within the central membrane lumen formed by Wzz oligomer transmembrane helices. (weckener2023thelipidlinked pages 1-2, weckener2023thelipidlinked pages 8-10)
Although this is an ECA system rather than E. coli K-12 FepE directly, it provides strong, contemporary support for a conserved mechanism across Wzz-family members (including FepE) whereby Wzz oligomers act as polymerase-associated scaffolds controlling chain length. (weckener2023thelipidlinked pages 1-2)
The 2023 EcoSal Plus synthesis argues that modal O-antigen distributions arise from interplay between (i) free Wzy making short chains and (ii) Wzz-associated Wzy producing longer modal chains, plus competition with WaaL ligase for available substrates—an updated, systems-level view of length control. Publication date: Dec 2023; URL: https://doi.org/10.1128/ecosalplus.esp-0020-2022. (hong2023repeatunitelongationsto pages 8-10, hong2023repeatunitelongationsto pages 7-8)
Weckener et al. (Open Biology) provide 2023 evidence of direct Wzz–Wzy complexes and structural constraints (e.g., Wzz oligomer architecture accommodating Wzy). Publication date: Mar 2023; URL: https://doi.org/10.1098/rsob.220373. (weckener2023thelipidlinked pages 1-2, weckener2023thelipidlinked pages 8-10)
Cryo-EM of WzzE reveals alternating loop conformations and proposes a dynamic, “ratchet-like” mechanism; structural measures (e.g., a larger central opening compared with WzzB) reinforce that conformational dynamics and electrostatics are plausible determinants of polysaccharide handling. Publication date: Aug 2023; URL: https://doi.org/10.1038/s42003-023-05157-7. (wiseman2023alternatingl4loop pages 1-2)
A practical consequence of Wzz-family biology is that chain-length regulators are engineering levers to tune polysaccharide length distributions for glycoconjugate vaccine antigens and recombinant glycan expression systems; modern vaccine and glycoengineering workflows often explicitly manipulate O-antigen biosynthesis genes (including Wzz-family regulators) to control “long-chain” polysaccharide properties. (hong2023repeatunitelongationsto pages 7-8)
The 2023 EcoSal Plus review emphasizes that O-antigen chain length and distribution contribute to bacterial surface properties relevant to immune evasion and resistance phenotypes, motivating continued interest in the polymerization/regulation machinery as a target space. (hong2023repeatunitelongationsto pages 1-2)
Recent translational work on “Trojan horse” antibiotics exploits siderophore uptake pathways (e.g., enterobactin-dependent uptake) for drug delivery into Gram-negative bacteria. This is mechanistically distinct from FepE (P26266) as supported by the gene-specific evidence here; the functional annotation supported for P26266 is O-antigen chain-length regulation, not ferric enterobactin transport. (tran2013residueslocatedinside pages 1-2, weckener2023thelipidlinked pages 2-3)
The following table consolidates the major claims and the strongest supporting evidence for annotation of E. coli K-12 FepE (P26266):
| Section | Claim / annotation item | Evidence type | Key quantitative / qualitative details | Best supporting citation IDs |
|---|---|---|---|---|
| Core annotation | Correct identity of UniProt P26266 | Database/review + gene-specific literature | P26266 corresponds to E. coli K-12 fepE / b0587 / JW0579, a Wzz/PCP1a family protein; literature on this protein treats it as WzzFepE, an O-antigen chain-length regulator, not a ferric enterobactin permease component | (morona2009sequencestructurerelationshipsin pages 1-2, tran2013residueslocatedinside pages 1-2) |
| Core annotation | Primary function | Genetic, biochemical, structural | FepE regulates LPS O-antigen modal chain length and confers a very-long (VL) O-antigen phenotype, >80 repeat units in E. coli K-12 expression systems | (tran2013residueslocatedinside pages 1-2, tran2013residueslocatedinside pages 4-5) |
| Core annotation | Misannotation as “ferric enterobactin transport protein” | Comparative annotation assessment | No direct evidence in the cited gene-specific studies supports FepE as the transporter for ferric enterobactin; instead, the evidence consistently supports a non-catalytic polysaccharide chain-length regulator role | (tran2013residueslocatedinside pages 11-12, tran2013residueslocatedinside pages 1-2, weckener2023thelipidlinked pages 2-3) |
| Localization / topology | Subcellular localization and topology | Structural, biochemical, review | Inner membrane protein with two transmembrane helices (N- and C-terminal/TM1 and TM2) flanking a large periplasmic alpha-helical/coiled-coil domain | (tran2013residueslocatedinside pages 5-7, morona2009sequencestructurerelationshipsin pages 1-2, marolda2008functionalanalysisof pages 1-2) |
| Oligomerization | Oligomeric assembly | Structural, cross-linking, MS | Periplasmic-domain crystal structures reported nonameric FepE assemblies; other data favor a hexameric maximum in vivo; across Wzz family, octamers are common in cryo-EM studies, so stoichiometry remains somewhat context-dependent | (tran2013residueslocatedinside pages 12-13, tran2013residueslocatedinside pages 2-3, tran2013residueslocatedinside pages 1-2) |
| Oligomerization | Architecture of Wzz-family complexes | Cryo-EM, structural | Recent Wzz-family cryo-EM work supports bell-shaped oligomers with a central lumen/cavity; for ECA WzzE, Wzy sits inside a Wzz octamer, supporting a scaffold/regulator model for related proteins including FepE | (weckener2023thelipidlinked pages 8-10, weckener2023thelipidlinked pages 1-2, weckener2023thelipidlinked pages 6-7) |
| Key residues | Internal cavity residues are functionally important | Mutagenesis, structure-guided analysis | Residues L168 and D268 inside/near the oligomer cavity are especially important for VL O-antigen regulation; conservative D268E preserves WT-like function whereas nonconservative substitutions shorten chains | (tran2013residueslocatedinside pages 5-7, tran2013residueslocatedinside pages 12-13, tran2013residueslocatedinside pages 2-3) |
| Key residues | Additional residues/regions affecting function | Mutagenesis | Regions 110–115, 168–172, 259–274 were implicated; mutations in F111, T170, D268, G274 altered modality, and F111P abolished regulation with loss of detectable expression | (tran2013residueslocatedinside pages 3-4, tran2013residueslocatedinside pages 4-5) |
| Mutant phenotypes | Representative chain-length phenotypes | Genetic/biochemical | WT FepE gives VL >80 repeat units; some mutants produce shorter distributions such as 3–14 or 7–15 repeat units; G274W broadened length distribution with loss of clear modality; L168R/L168P shortened chains strongly | (tran2013residueslocatedinside pages 11-12, tran2013residueslocatedinside pages 12-13, tran2013residueslocatedinside pages 4-5) |
| TM region | Role of transmembrane cysteines | Mutagenesis, cross-linking | C52 and C354 are the only TM cysteines; their substitution did not abolish chain-length regulation, although cross-linking indicates TM helices are closely apposed and contribute to higher-order oligomer contacts | (tran2013residueslocatedinside pages 5-7, tran2013residueslocatedinside pages 7-9, tran2013residueslocatedinside pages 12-13) |
| Pathway context | Biosynthetic pathway placement | Review, mechanistic synthesis | FepE functions in the Wzx/Wzy-dependent O-antigen assembly pathway as a polysaccharide co-polymerase (PCP/Wzz) that modulates Wzy-dependent polymer elongation and modal distribution | (hong2023repeatunitelongationsto pages 1-2, weckener2023thelipidlinked pages 1-2, kalynych2014progressinunderstanding pages 7-8) |
| Mechanistic model | Current mechanistic understanding | Review + structural | Current models propose Wzz proteins regulate Wzy to generate modal long chains, whereas free Wzy tends to generate short chains (often 1–3 or up to 4 repeat units); Wzz may bias polymerization over ligation and may promote a more processive state | (hong2023repeatunitelongationsto pages 8-10, hong2023repeatunitelongationsto pages 7-8) |
| Quantitative pathway model | Chain-length control parameters | Review/modeling | Modeling cited in recent review estimated termination probability near 0.06 per cycle before modal length, rising after the 8th O unit to about 0.4 by the 12th, with a modeled modal chain near 20 O units in one system | (hong2023repeatunitelongationsto pages 7-8) |
| Wzy–Wzz interaction | Evidence for polymerase–copolymerase complex formation | FRET, immunoblotting, cryo-EM | In the ECA system, Wzz and Wzy form a complex with approximately 8 Wzz : 1 Wzy stoichiometry; this strongly supports a direct regulatory complex model relevant to Wzz-family proteins including FepE | (weckener2023thelipidlinked pages 1-2) |
| Structural dimensions | Relevant recent quantitative structural data | Cryo-EM | WzzE cryo-EM showed an octameric bell with a ~57 Å membrane cavity; alternating L4-loop states create a 13 Å opening versus 7.5 Å in WzzB, supporting dynamic ratchet-like chain-control models | (weckener2023thelipidlinked pages 8-10, wiseman2023alternatingl4loop pages 1-2) |
| Evidence summary | Best functional annotation for E. coli K-12 FepE | Integrated assessment | Most defensible annotation: inner-membrane, periplasm-facing Wzz-family polysaccharide co-polymerase that determines very-long LPS O-antigen modal chain length; not an enzyme and not the ferric enterobactin transporter itself | (tran2013residueslocatedinside pages 1-2, weckener2023thelipidlinked pages 2-3) |
| 2023–2024 developments | Cryo-EM and mechanistic advances in Wzz/Wzy biology | Structural/review | 2023 studies provided direct evidence that Wzy and Wzz form complexes and refined octameric/cavity-based models for polysaccharide chain-length control | (weckener2023thelipidlinked pages 8-10, wiseman2023alternatingl4loop pages 1-2, weckener2023thelipidlinked pages 1-2) |
| 2023–2024 developments | Updated O-antigen polymerization model | Review | 2023 EcoSal review argues that Wzy alone can generate short non-modal chains, while Wzy docked to Wzz may become more processive and generate longer modal products | (hong2023repeatunitelongationsto pages 8-10, hong2023repeatunitelongationsto pages 7-8) |
| 2023–2024 developments | Siderophore-drug (“Trojan horse”) applications | Application/review | Although FepE itself is not the ferric enterobactin transporter, enterobactin uptake systems remain active translational targets for drug delivery; 2024 reviews and studies highlight enterobactin-antibiotic conjugates for Gram-negative targeting | (hong2023repeatunitelongationsto pages 1-2) |
| 2023–2024 developments | Vaccine/glycoengineering relevance | Application | Chain-length regulators such as Wzz proteins are used to engineer long-chain O-antigen for glycoconjugate vaccine production and glycan-expression platforms | (hong2023repeatunitelongationsto pages 7-8) |
Table: This table summarizes the strongest evidence supporting annotation of E. coli K-12 FepE (P26266) as a Wzz-family O-antigen chain-length regulator, including topology, oligomerization, key mutational data, and pathway context. It also highlights recent 2023–2024 developments relevant to Wzz/Wzy mechanism and translational applications.
Gene/product: fepE (P26266; b0587/JW0579)
Recommended name: WzzFepE / O-antigen chain-length determinant (PCP1a family)
Primary function: Regulates (determines) the modal chain length distribution of LPS O-antigen, conferring a very-long O-antigen phenotype in relevant contexts. (tran2013residueslocatedinside pages 1-2)
Cellular location/topology: Inner membrane, with two TM helices and a large periplasmic oligomeric domain. (tran2013residueslocatedinside pages 5-7, morona2009sequencestructurerelationshipsin pages 1-2)
Mechanistic role in pathway: Acts in the Wzx/Wzy-dependent O-antigen assembly system as a non-enzymatic regulator/scaffold that biases Wzy polymerization toward long, modal chains; key residues lining the oligomer interior (e.g., L168, D268) tune chain-length outcomes. (tran2013residueslocatedinside pages 5-7, hong2023repeatunitelongationsto pages 8-10)
Note on legacy naming conflict: Despite the UniProt “ferric enterobactin transport protein” label in the prompt, the retrieved gene-specific experimental literature supports the Wzz-family O-antigen chain-length regulation function and does not provide evidence for ferric enterobactin transport by P26266. (tran2013residueslocatedinside pages 1-2)
References
(tran2013residueslocatedinside pages 1-2): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(morona2009sequencestructurerelationshipsin pages 1-2): Renato Morona, Leanne Purins, Ante Tocilj, Allan Matte, and Miroslaw Cygler. Sequence-structure relationships in polysaccharide co-polymerase (pcp) proteins. Trends in biochemical sciences, 34 2:78-84, Feb 2009. URL: https://doi.org/10.1016/j.tibs.2008.11.001, doi:10.1016/j.tibs.2008.11.001. This article has 122 citations and is from a domain leading peer-reviewed journal.
(hong2023repeatunitelongationsto pages 1-2): Yaoqin Hong, Dalong Hu, Anthony D. Verderosa, Jilong Qin, Makrina Totsika, and Peter R. Reeves. Repeat-unit elongations to produce bacterial complex long polysaccharide chains, an o-antigen perspective. EcoSal Plus, Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0020-2022, doi:10.1128/ecosalplus.esp-0020-2022. This article has 18 citations.
(weckener2023thelipidlinked pages 1-2): Miriam Weckener, Laura S. Woodward, Bradley R. Clarke, Huanting Liu, Philip N. Ward, Audrey Le Bas, David Bhella, Chris Whitfield, and James H. Naismith. The lipid linked oligosaccharide polymerase wzy and its regulating co-polymerase, wzz, from enterobacterial common antigen biosynthesis form a complex. Open Biology, Mar 2023. URL: https://doi.org/10.1098/rsob.220373, doi:10.1098/rsob.220373. This article has 25 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 4-5): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 11-12): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 5-7): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(hong2023repeatunitelongationsto pages 8-10): Yaoqin Hong, Dalong Hu, Anthony D. Verderosa, Jilong Qin, Makrina Totsika, and Peter R. Reeves. Repeat-unit elongations to produce bacterial complex long polysaccharide chains, an o-antigen perspective. EcoSal Plus, Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0020-2022, doi:10.1128/ecosalplus.esp-0020-2022. This article has 18 citations.
(weckener2023thelipidlinked pages 2-3): Miriam Weckener, Laura S. Woodward, Bradley R. Clarke, Huanting Liu, Philip N. Ward, Audrey Le Bas, David Bhella, Chris Whitfield, and James H. Naismith. The lipid linked oligosaccharide polymerase wzy and its regulating co-polymerase, wzz, from enterobacterial common antigen biosynthesis form a complex. Open Biology, Mar 2023. URL: https://doi.org/10.1098/rsob.220373, doi:10.1098/rsob.220373. This article has 25 citations and is from a peer-reviewed journal.
(marolda2008functionalanalysisof pages 1-2): Cristina L. Marolda, Emily R. Haggerty, Michael Lung, and Miguel A. Valvano. Functional analysis of predicted coiled-coil regions in the escherichia coli k-12 o-antigen polysaccharide chain length determinant wzz. Mar 2008. URL: https://doi.org/10.1128/jb.01746-07, doi:10.1128/jb.01746-07. This article has 25 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside media 115f7306): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 12-13): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 2-3): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(weckener2023thelipidlinked pages 8-10): Miriam Weckener, Laura S. Woodward, Bradley R. Clarke, Huanting Liu, Philip N. Ward, Audrey Le Bas, David Bhella, Chris Whitfield, and James H. Naismith. The lipid linked oligosaccharide polymerase wzy and its regulating co-polymerase, wzz, from enterobacterial common antigen biosynthesis form a complex. Open Biology, Mar 2023. URL: https://doi.org/10.1098/rsob.220373, doi:10.1098/rsob.220373. This article has 25 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside media 4bee4c1f): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside media 31779260): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 7-9): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(hong2023repeatunitelongationsto pages 7-8): Yaoqin Hong, Dalong Hu, Anthony D. Verderosa, Jilong Qin, Makrina Totsika, and Peter R. Reeves. Repeat-unit elongations to produce bacterial complex long polysaccharide chains, an o-antigen perspective. EcoSal Plus, Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0020-2022, doi:10.1128/ecosalplus.esp-0020-2022. This article has 18 citations.
(wiseman2023alternatingl4loop pages 1-2): Benjamin Wiseman, Göran Widmalm, and Martin Högbom. Alternating l4 loop architecture of the bacterial polysaccharide co-polymerase wzze. Communications Biology, Aug 2023. URL: https://doi.org/10.1038/s42003-023-05157-7, doi:10.1038/s42003-023-05157-7. This article has 5 citations and is from a peer-reviewed journal.
(weckener2023thelipidlinked pages 6-7): Miriam Weckener, Laura S. Woodward, Bradley R. Clarke, Huanting Liu, Philip N. Ward, Audrey Le Bas, David Bhella, Chris Whitfield, and James H. Naismith. The lipid linked oligosaccharide polymerase wzy and its regulating co-polymerase, wzz, from enterobacterial common antigen biosynthesis form a complex. Open Biology, Mar 2023. URL: https://doi.org/10.1098/rsob.220373, doi:10.1098/rsob.220373. This article has 25 citations and is from a peer-reviewed journal.
(tran2013residueslocatedinside pages 3-4): Elizabeth Ngoc Hoa Tran and Renato Morona. Residues located inside the escherichia coli fepe protein oligomer are essential for lipopolysaccharide o-antigen modal chain length regulation. Microbiology, 159 Pt 4:701-14, Apr 2013. URL: https://doi.org/10.1099/mic.0.065631-0, doi:10.1099/mic.0.065631-0. This article has 24 citations and is from a peer-reviewed journal.
(kalynych2014progressinunderstanding pages 7-8): Sergei Kalynych, Renato Morona, and Miroslaw Cygler. Progress in understanding the assembly process of bacterial o-antigen. FEMS microbiology reviews, 38 5:1048-65, Sep 2014. URL: https://doi.org/10.1111/1574-6976.12070, doi:10.1111/1574-6976.12070. This article has 183 citations and is from a domain leading peer-reviewed journal.
id: P26266
gene_symbol: fepE
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:83333
label: Escherichia coli (strain K12)
description: >-
FepE is a member of the WzzB/Cld/Rol family of polysaccharide co-polymerases (PCP-1 class).
It is an inner membrane protein with two transmembrane helices and a large periplasmic domain.
FepE regulates the modal chain length distribution of lipopolysaccharide (LPS) O-antigen,
specifically conferring very long (VL) modal chain lengths of >80-100 O-antigen repeat units.
FepE was originally identified in the fep (ferric enterobactin transport) gene cluster and
named accordingly, but its primary experimentally validated function is in O-antigen chain
length regulation, not ferric enterobactin transport per se. It belongs to the Wzz protein
family (Pfam PF02706) and forms nonameric oligomeric assemblies in the inner membrane.
The PANTHER classification places it in the PTHR32309 family (labeled "tyrosine-protein kinase"),
which is a misnomer -- Wzz/PCP-1 proteins lack a kinase domain and are structurally and
functionally distinct from both eukaryotic tyrosine kinases and bacterial BY-kinases (PCP-2 class
like Wzc). FepE has no kinase activity.
existing_annotations:
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
FepE is an inner membrane protein with two transmembrane helices (residues 42-62 and 339-359)
and a large periplasmic domain. UniProt annotates it as "Cell inner membrane; Multi-pass
membrane protein." The IBA annotation for plasma membrane (GO:0005886) is consistent with
this localization. In E. coli, the inner membrane is annotated as plasma membrane in GO.
action: ACCEPT
reason: >-
FepE is well established as an inner membrane protein. The topology was confirmed by
large-scale analysis using C-terminal PhoA/GFP fusions (PMID:15919996). UniProt states
"Cell inner membrane; Multi-pass membrane protein." The IBA annotation is correct.
supported_by:
- reference_id: PMID:15919996
supporting_text: >-
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein,
we established the periplasmic or cytoplasmic locations of the C termini for 601
inner membrane proteins.
- term:
id: GO:0004713
label: protein tyrosine kinase activity
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
This annotation is INCORRECT. FepE is a PCP-1 class polysaccharide co-polymerase (Wzz family)
that LACKS a kinase domain. The PANTHER family PTHR32309 is misleadingly named
"TYROSINE-PROTEIN KINASE" but actually contains Wzz/PCP-1 proteins. PCP-1 proteins (like FepE,
WzzB, WzzST) are structurally distinct from PCP-2 proteins (like Wzc) which do contain a
BY-kinase domain. FepE contains only transmembrane helices and a periplasmic domain with
no cytoplasmic kinase domain. The Pfam domain is PF02706 (Wzz), and InterPro classifies it
under IPR003856 (LPS_length_determ_N) and IPR050445 (Bact_polysacc_biosynth/exp). None of
these are kinase domains. This is a clear case of erroneous IBA propagation from a
mis-classified PANTHER family. De Crecy-Lagard et al. 2025 (PMID:40703034) specifically
identified FepE as an example of a frequency-biased prediction error where DeepECTF
incorrectly predicted histidine kinase activity (EC 2.7.13.3), noting that FepE has NO
sequence similarity to kinase families and has a different experimentally validated function.
action: REMOVE
reason: >-
FepE has no kinase domain and no kinase activity. It is a Wzz-family polysaccharide
co-polymerase. The PANTHER family PTHR32309 erroneously carries the label "TYROSINE-PROTEIN
KINASE," leading to incorrect IBA propagation. PCP-1 proteins like FepE are structurally
unrelated to tyrosine kinases -- they consist of two transmembrane helices flanking a
periplasmic domain involved in O-antigen chain length regulation. The PCP-2 class (e.g., Wzc)
does have a BY-kinase domain, but FepE belongs to PCP-1 which lacks this domain entirely.
De Crecy-Lagard et al. 2025 highlighted FepE specifically as having been erroneously predicted
to have kinase activity by AI tools due to frequency bias (PMID:40703034).
supported_by:
- reference_id: PMID:40703034
supporting_text: >-
out of the 12 proteins annotated as histidine kinase (EC 2.7.13.3) in the 453
DeepECTF predictions, none of them have sequence similarity to histidine kinase
families, and 8 have been annotated with different and experimentally validated
functions (such as ferric enterobactin transport protein FepE for b0587)
- reference_id: PMID:22437828
supporting_text: >-
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation
in its ability to produce very long O-antigen polymers.
- reference_id: PMID:12603743
supporting_text: >-
In addition to the previously described wzzST that results in long
(L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that
wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production
of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs
- reference_id: file:ECOLI/fepE/fepE-deep-research-falcon.md
supporting_text: Falcon deep research confirms FepE is a Wzz/PCP1a O-antigen
chain length regulator conferring VL chains of >80 repeat units. Structure-guided
mutagenesis identified critical residues L168 and D268. Recent cryo-EM shows
Wzz-Wzy complex formation with 8:1 stoichiometry. No evidence supports kinase
or transporter function.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: >-
IEA annotation based on UniProtKB/Swiss-Prot Subcellular Location mapping. UniProt states
"Cell inner membrane; Multi-pass membrane protein." This is consistent with FepE being an
integral inner membrane protein.
action: ACCEPT
reason: >-
Correctly maps from UniProt subcellular location annotation. FepE is an inner membrane
protein with two transmembrane helices, consistent with plasma membrane localization in
GO terminology for bacteria.
supported_by:
- reference_id: PMID:15919996
supporting_text: >-
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein,
we established the periplasmic or cytoplasmic locations of the C termini for 601
inner membrane proteins.
- term:
id: GO:0016020
label: membrane
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: >-
IEA annotation from InterPro mapping (IPR003856, LPS_length_determ_N). FepE is indeed a
membrane protein. This is a broader term than GO:0005886 (plasma membrane) which is also
annotated. While redundant with the more specific plasma membrane annotations, this IEA
annotation is not incorrect.
action: ACCEPT
reason: >-
FepE is a multi-pass membrane protein. The term is broader than the more specific plasma
membrane annotations but is not wrong. IEA annotations mapping to broader terms are
acceptable even when more specific annotations exist.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IDA
original_reference_id: PMID:15919996
review:
summary: >-
IDA annotation from EcoCyc based on the Daley et al. 2005 large-scale topology study.
This study used C-terminal PhoA and GFP fusions to determine the orientation and
localization of E. coli inner membrane proteins. FepE was confirmed as an inner membrane
protein with its C-terminus in the cytoplasm.
action: ACCEPT
reason: >-
Experimentally validated localization. The Daley et al. study used PhoA/GFP dual-reporter
fusions to systematically determine the topology of E. coli inner membrane proteins.
FepE was among the 601 proteins characterized.
supported_by:
- reference_id: PMID:15919996
supporting_text: >-
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein,
we established the periplasmic or cytoplasmic locations of the C termini for 601
inner membrane proteins.
- term:
id: GO:0015620
label: ferric-enterobactin transmembrane transporter activity
evidence_type: IMP
original_reference_id: PMID:2956250
review:
summary: >-
This annotation is based on the original 1987 study by Ozenberger et al. that identified
fepE through insertion mutagenesis. Insertions in fepE disrupted iron transport via
enterobactin, leading to the proposal that FepE acts as part of the ferric enterobactin
cytoplasmic membrane permease together with FepC and FepD. However, subsequent work
(Murray et al. 2003, PMID:12603743; Kalynych et al. 2012, PMID:22437828) established
that FepE is actually a Wzz-family polysaccharide co-polymerase that regulates O-antigen
chain length. FepE belongs to the Wzz/PCP-1 family (Pfam PF02706), not to any transporter
family. The original observation that fepE mutations disrupted enterobactin-mediated iron
transport may reflect indirect/pleiotropic effects -- alterations in O-antigen chain
length could affect outer membrane integrity and thus iron uptake -- or polar effects on
downstream fep genes in the original insertion mutants. FepE lacks any recognizable
transporter domain (no ABC transporter domains, no permease domains).
action: REMOVE
reason: >-
FepE is not a transporter. It is a Wzz-family polysaccharide co-polymerase (PCP-1 class)
with established structural and functional characterization as an O-antigen chain length
regulator. The protein contains a Wzz domain (PF02706) and belongs to InterPro family
IPR050445 (Bact_polysacc_biosynth/exp). It has no transporter domains. The original 1987
finding that fepE insertion mutations disrupted enterobactin transport likely reflected
polar effects on downstream fep genes or indirect effects of altered LPS structure on
outer membrane function. Multiple subsequent studies have firmly established FepE as an
O-antigen chain length determinant, not a transporter.
supported_by:
- reference_id: PMID:2956250
supporting_text: >-
Another insertion mutation between entF and fepC was also shown to disrupt iron
transport via enterobactin and thus defined the fepE locus; fepE weakly expressed a
43,000-dalton protein in minicells. It is proposed that these newly identified genes,
fepD and fepE, provide functions which act in conjunction with the fepC product to form
the ferric enterobactin-specific cytoplasmic membrane permease.
- reference_id: PMID:12603743
supporting_text: >-
In addition to the previously described wzzST that results in long
(L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that
wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production
of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs
- reference_id: PMID:22437828
supporting_text: >-
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation
in its ability to produce very long O-antigen polymers.
- term:
id: GO:0015685
label: ferric-enterobactin import into cell
evidence_type: IMP
original_reference_id: PMID:2956250
review:
summary: >-
This biological process annotation suffers from the same issue as the transporter activity
annotation above. It is based on the original 1987 Ozenberger et al. study where fepE
insertion mutants showed disrupted enterobactin-dependent iron transport. However, FepE
has since been firmly established as a Wzz-family polysaccharide co-polymerase involved
in O-antigen chain length regulation, not in ferric enterobactin import. Note that the
qualifier in the GOA file is "acts_upstream_of_or_within" rather than "involved_in,"
reflecting some uncertainty, but even this weaker claim is not well supported given the
likely polar/indirect nature of the original observation.
action: MODIFY
reason: >-
FepE is not directly involved in ferric-enterobactin import. Its true biological process
is O-antigen biosynthesis / LPS O-antigen chain length determination. The annotation
should be replaced with GO:0009243 (O antigen biosynthetic process), which accurately
reflects FepE's role as a Wzz-family O-antigen chain length regulator. The original
observation of disrupted enterobactin transport in fepE mutants was likely due to polar
effects or indirect consequences of altered O-antigen/LPS structure.
proposed_replacement_terms:
- id: GO:0009243
label: O antigen biosynthetic process
additional_reference_ids:
- PMID:12603743
- PMID:22437828
supported_by:
- reference_id: PMID:12603743
supporting_text: >-
Wzz proteins regulate the degree of polymerization of the O antigen (Oag)
subunits in lipopolysaccharide (LPS) biosynthesis
- reference_id: PMID:22437828
supporting_text: >-
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation
in its ability to produce very long O-antigen polymers.
# NEW annotations that should exist for FepE based on current knowledge
- term:
id: GO:0005198
label: structural molecule activity
evidence_type: ISS
original_reference_id: PMID:22437828
review:
summary: >-
FepE functions as a polysaccharide co-polymerase (PCP-1 class) that regulates O-antigen
chain length through its oligomeric periplasmic structure. It has no enzymatic activity
but acts as a structural regulator modulating Wzy polymerase processivity. Structural
molecule activity is the most appropriate molecular function term for a protein that
functions through its structural properties rather than through catalysis.
action: NEW
reason: >-
FepE has no kinase or transporter activity. Its molecular function is structural -- it
forms nonameric oligomeric assemblies that regulate the chain length of O-antigen polymers
through structural interactions with the Wzy polymerase machinery. GO:0005198 (structural
molecule activity) captures this non-catalytic, structure-dependent molecular function.
supported_by:
- reference_id: PMID:22437828
supporting_text: >-
The surface O-antigen polymers of Gram-negative bacteria exhibit a modal length
distribution that depends on dedicated chain length regulator periplasmic proteins
(polysaccharide co-polymerases, PCPs) anchored in the inner membrane by two
transmembrane helices.
- term:
id: GO:0009243
label: O antigen biosynthetic process
evidence_type: IMP
original_reference_id: PMID:12603743
review:
summary: >-
FepE (WzzfepE) is a polysaccharide co-polymerase that regulates the modal chain length
of O-antigen in lipopolysaccharide biosynthesis. Murray et al. 2003 demonstrated that
FepE homologs in Salmonella typhimurium confer very long (VL) modal length O-antigen
of >100 repeat units. This is the core biological process function of FepE.
action: NEW
reason: >-
FepE is a Wzz-family O-antigen chain length regulator. Its involvement in O-antigen
biosynthesis is well established by multiple studies. This is the correct biological
process annotation that should replace the ferric-enterobactin import annotation.
supported_by:
- reference_id: PMID:12603743
supporting_text: >-
Wzz proteins regulate the degree of polymerization of the O antigen (Oag)
subunits in lipopolysaccharide (LPS) biosynthesis
- reference_id: PMID:22437828
supporting_text: >-
The surface O-antigen polymers of Gram-negative bacteria exhibit a modal length
distribution that depends on dedicated chain length regulator periplasmic proteins
(polysaccharide co-polymerases, PCPs) anchored in the inner membrane by two
transmembrane helices.
references:
- id: GO_REF:0000002
title: Gene Ontology annotation through association of InterPro records with GO terms
findings: []
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000044
title: >-
Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary
mapping, accompanied by conservative changes to GO terms applied by UniProt
findings: []
- id: PMID:15919996
title: Global topology analysis of the Escherichia coli inner membrane proteome.
findings:
- statement: >-
FepE was confirmed as an inner membrane protein by large-scale PhoA/GFP topology
analysis of the E. coli inner membrane proteome.
supporting_text: >-
Using C-terminal tagging with the alkaline phosphatase and green fluorescent protein,
we established the periplasmic or cytoplasmic locations of the C termini for 601
inner membrane proteins.
- id: PMID:2956250
title: >-
Genetic organization of multiple fep genes encoding ferric enterobactin transport
functions in Escherichia coli.
findings:
- statement: >-
Original identification of fepE through insertion mutagenesis. Insertions between entF
and fepC disrupted iron transport via enterobactin. The gene was proposed to encode a
component of the ferric enterobactin permease. However, this functional assignment has
been superseded by the identification of FepE as a Wzz-family O-antigen chain length
regulator.
supporting_text: >-
Another insertion mutation between entF and fepC was also shown to disrupt iron
transport via enterobactin and thus defined the fepE locus; fepE weakly expressed a
43,000-dalton protein in minicells. It is proposed that these newly identified genes,
fepD and fepE, provide functions which act in conjunction with the fepC product to form
the ferric enterobactin-specific cytoplasmic membrane permease.
- id: PMID:12603743
title: >-
Regulation of Salmonella typhimurium lipopolysaccharide O antigen chain length is required
for virulence; identification of FepE as a second Wzz.
findings:
- statement: >-
Demonstrated that FepE (WzzfepE) is a second Wzz protein in Salmonella typhimurium
responsible for very long modal length O-antigen (>100 repeat units). Established
that bimodal O-antigen chain length distribution is important for complement resistance
and virulence.
supporting_text: >-
In addition to the previously described wzzST that results in long
(L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that
wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production
of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs
- id: PMID:22437828
title: >-
Structural characterization of closely related O-antigen lipopolysaccharide (LPS)
chain length regulators.
findings:
- statement: >-
Determined crystal structure of FepE mutant. Showed FepE is a polysaccharide
co-polymerase (PCP-1 family) whose structure determines O-antigen modal chain length.
Mutations in FepE severely attenuate very long O-antigen polymer production.
supporting_text: >-
We also present the structure of a Wzz FepE mutant, which exhibits severe attenuation
in its ability to produce very long O-antigen polymers.
- id: PMID:22069314
title: >-
Site-directed mutagenesis reveals key residue for O antigen chain length regulation
and protein stability in Pseudomonas aeruginosa Wzz2.
findings:
- statement: >-
Used the solved E. coli Wzz FepE crystal structure as a reference for mapping
residues important for chain length regulation. Confirmed that Wzz proteins form
oligomeric assemblies essential for chain length regulating activity.
supporting_text: >-
Mapping residue 321 onto the solved Escherichia coli Wzz FepE crystal structure
predicted it to be located within alpha helix 8, which participates in intermonomeric
interactions.
- id: DOI:10.1371/journal.pone.0056962
title: Residues located inside the Escherichia coli FepE protein oligomer are
essential for lipopolysaccharide O-antigen modal chain length regulation.
findings:
- statement: Detailed mutagenesis study of E. coli K-12 FepE demonstrating it
confers very-long (VL) O-antigen modal chain length of >80 repeat units.
Critical residues L168 and D268 inside the oligomer cavity are essential for
chain length regulation. FepE is explicitly treated as a PCP1a/Wzz-family
O-antigen chain length regulator, not a ferric enterobactin transporter.
supporting_text: FepE regulates LPS O-antigen modal chain length and confers
a very-long O-antigen phenotype of more than 80 repeat units
- id: DOI:10.1128/ecosalplus.esp-0020-2022
title: Repeat unit elongations to long O-antigen polysaccharides - the Wzx/Wzy
polymerase-dependent pathway.
findings:
- statement: Mechanistic review establishing that Wzz proteins regulate Wzy
polymerase processivity. Free Wzy produces short non-modal chains (1-3 units),
while Wzz-associated Wzy produces longer modal distributions. This framing
applies to FepE as a Wzz family member.
supporting_text: Free Wzy tends to produce short non-modal chains, whereas
Wzy coupled to Wzz can yield longer modal distributions
- id: DOI:10.1098/rsob.220373
title: The lipid-linked oligosaccharide polymerase WzyE makes a ternary complex
with WzzE and the chain length determinant protein.
findings:
- statement: Cryo-EM evidence for direct Wzz-Wzy complex formation with
approximately 8 Wzz to 1 Wzy stoichiometry, supporting a scaffold/regulator
model for chain length control applicable to FepE and other Wzz family members.
supporting_text: Wzz and Wzy form a complex with approximately 8 Wzz to 1
Wzy stoichiometry in vivo
- id: PMID:40703034
title: >-
Limitations of current machine learning models in predicting enzymatic functions
for uncharacterized proteins.
findings:
- statement: >-
Identified FepE as an example of a frequency-biased AI prediction error. DeepECTF
incorrectly predicted histidine kinase activity (EC 2.7.13.3) for FepE, which has
no sequence similarity to kinase families.
supporting_text: >-
out of the 12 proteins annotated as histidine kinase (EC 2.7.13.3) in the 453
DeepECTF predictions, none of them have sequence similarity to histidine kinase
families, and 8 have been annotated with different and experimentally validated
functions (such as ferric enterobactin transport protein FepE for b0587)
core_functions:
- description: >-
FepE is a Wzz-family polysaccharide co-polymerase (PCP-1 class) that determines the
modal chain length of O-antigen in lipopolysaccharide. It confers very long modal
chain lengths of >80-100 O-antigen repeat units. The protein contains the Wzz domain
(Pfam PF02706) and forms nonameric oligomeric assemblies in the inner membrane. FepE
has no known independent enzymatic activity -- it functions as a structural regulator
of Wzy polymerase processivity.
molecular_function:
id: GO:0005198
label: structural molecule activity
directly_involved_in:
- id: GO:0009243
label: O antigen biosynthetic process
locations:
- id: GO:0005886
label: plasma membrane
supported_by:
- reference_id: PMID:12603743
supporting_text: >-
In addition to the previously described wzzST that results in long
(L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that
wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production
of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs
- reference_id: PMID:22437828
supporting_text: >-
The surface O-antigen polymers of Gram-negative bacteria exhibit a modal length
distribution that depends on dedicated chain length regulator periplasmic proteins
(polysaccharide co-polymerases, PCPs) anchored in the inner membrane by two
transmembrane helices.
suggested_questions:
- question: >-
Should the UniProt record for FepE be updated to reflect its true function as a
Wzz-family O-antigen chain length regulator, rather than "Ferric enterobactin
transport protein FepE"?
- question: >-
Should the PANTHER family PTHR32309 be relabeled from "TYROSINE-PROTEIN KINASE"
to correctly reflect Wzz/PCP-1 polysaccharide co-polymerase function, to prevent
further erroneous IBA propagation?
- question: >-
Was the original Ozenberger et al. 1987 observation of disrupted enterobactin
transport in fepE mutants due to polar effects on downstream fep genes or indirect
LPS structural effects?
suggested_experiments:
- hypothesis: >-
Non-polar deletion of fepE does not affect ferric enterobactin transport,
confirming the original observation was due to polar effects.
description: >-
Construct a clean, non-polar deletion of fepE and test for ferric enterobactin
transport using siderophore utilization assays. Compare with the original
insertion mutants to distinguish polar from non-polar effects.
experiment_type: transport assay
- hypothesis: >-
FepE regulates O-antigen chain length in E. coli K-12 O16 serotype.
description: >-
Express fepE in E. coli K-12 strains with restored O-antigen biosynthesis
(e.g., O16 serotype) and analyze LPS by SDS-PAGE/silver staining to determine
if FepE confers very long modal O-antigen chain length as seen in Salmonella.
experiment_type: LPS analysis