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 oligomeric assemblies in the inner membrane. Its supported molecular role is non-catalytic polysaccharide chain-length control through the periplasmic Wzz/PCP domain, rather than kinase or transporter activity.
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.
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.
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).
Propagation Review
Root cause:PROPAGATION BAD
Failure modes:WRONG ORTHOLOG OR PARALOGFUNCTIONAL DIVERGENCEROLE CONFLATION
Sources checked:
PANTHER:PTN000794252 · PANTHER mixed PCP/BY-kinase source node SUPPORTS SOURCE BUT NOT TARGET
GOA traces the GO:0004713 IBA to a mixed PANTHER source node that includes true BY-kinases and non-catalytic Wzz/PCP chain-length regulators; the kinase term should not transfer to FepE.
UniProtKB:P38134 · E. coli Etk SUPPORTS SOURCE BUT NOT TARGET
Etk is a true bacterial tyrosine kinase source, but FepE lacks the cytoplasmic ATP-binding kinase module and tyrosine cluster.
UniProtKB:P76387 · E. coli Wzc SUPPORTS SOURCE BUT NOT TARGET
Wzc supports BY-kinase activity in the family, but FepE retains only the Wzz/PCP chain-length-regulator architecture.
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)
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
Core Functions
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.
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
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?
Suggested Experiments
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
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
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.
Review rationale: Repetition/frequency bias error. FepE was one of 12 proteins incorrectly annotated as histidine kinase (EC 2.7.13.3) by DeepECTF. None of these 12 have sequence similarity to histidine kinase families, and 8 have been annotated with different experimentally validated functions. E. coli has 29 actual histidine kinases, making this a high-frequency label in the training data. The model appears to default to this common EC number when it cannot identify informative sequence features. FepE is actually a Wzz-family polysaccharide co-polymerase (PCP-1 class) that determines very-long modal O-antigen chain length. The OpenScientist kinase audit independently confirmed that FepE has the periplasmic Wzz domain rather than the cytoplasmic machinery required for a bacterial tyrosine kinase, strengthening the conclusion that no kinase domain is present. FepE functions through structural oligomerization, not catalysis.
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:40703034: "This type of error may be due to inherent limitations in how AI methods operate...the model is expected to make frequency-dependent predictions that reflect the training data, as demonstrated with histidine kinases"
file:ECOLI/fepE/fepE-hypotheses/function-hypothesis-go-0004713/openscientist.md: "FepE's activity, process, and location are all inconsistent with GO:0004713: no ATP-binding catalytic domain (activity), no phosphotransfer role in its pathway (process), and its bulk faces the periplasm while the tiny cytoplasmic tail cannot host a kinase"