id: P18004 gene_symbol: traC product_type: PROTEIN status: DRAFT taxon: id: NCBITaxon:83333 label: Escherichia coli K-12 description: >- TraC is an 875-residue F-plasmid protein required to assemble mature F-pilin subunits into the extended F pilus, the filament that establishes contact with a recipient cell at the start of conjugation. It is not an integral membrane protein: expressed alone it is cytoplasmic, and it partitions with the membrane only in the presence of the other transfer-region proteins, so its association with the inner membrane is peripheral and depends on its partners. A mutant that leaves pilin processing intact but abolishes the extended pilus places TraC at the assembly step rather than at pilin maturation, and the same mutant transfers DNA at greatly reduced frequency, linking the protein to conjugative transfer as well as to pilus biogenesis. By position in the transfer region and by broad similarity it corresponds to the VirB4-family ATPase of other type IV secretion systems, but no nucleotide binding or hydrolysis has been demonstrated for the F protein itself. reference_proteome_assessments: - reference_proteomes: - UP000000625 status: NO_SUPPORTED_COUNTERPART reviewed_on: '2026-10-19' reason: Retain P18004 as the primary Swiss-Prot accession. No significant reference-proteome hit was returned for TraC. This reviewed protein belongs to the F-plasmid conjugation system, so accessory-replicon coverage is a relevant explanation to investigate rather than replacing it with a chromosomal protein. No supported alternative is asserted. evidence: - type: PROTEOME_SEARCH description: MMseqs2 searched 4,403 sequences across UP000000625 at E <= 1e-5 and sensitivity 7.5, returning 0 hits. The search table and candidate FASTA are empty; no alignments were generated. The heuristic search does not establish genomic absence or unique locus identity. supported_by: - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: 'target_sequence_count: 4403' - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: 'candidate_count: 0' - type: PROTEOME_MEMBERSHIP description: The primary accession is outside every searched exact-taxon reference proteome. supported_by: - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: 'primary_in_any_reference_proteome: false' - type: RECORD_HISTORY description: 'Sampled UniSave observations: 11-NOV-2015, entry version 89.: no Proteomes cross-reference; 02-DEC-2020, entry version 105.: no Proteomes cross-reference; 27-NOV-2024, entry version 119.: no Proteomes cross-reference. These dated observations do not establish uninterrupted membership/absence, the exact change date, or UniProt''s motivation.' supported_by: - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: |- entry_date: 11-NOV-2015, entry version 89. proteomes: [] - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: |- entry_date: 02-DEC-2020, entry version 105. proteomes: [] - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: |- entry_date: 27-NOV-2024, entry version 119. proteomes: [] - type: CURATION_CONTEXT description: The primary UniProt entry records plasmid origin. Reference-proteome membership at species or strain level does not ensure coverage of every accessory plasmid. supported_by: - reference_id: file:ECOLI/traC/traC-record-evidence.yaml supporting_text: 'geneEncodingType: Plasmid' references: - id: file:ECOLI/traC/traC-record-evidence.yaml title: Compact archived record and sequence evidence for P18004 - 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:2265751 title: Nucleotide sequence of the F plasmid gene, traC, and identification of its product. findings: - statement: >- Establishes the traC coding region as 875 residues, matching the length of the UniProt entry, and states the pilus-assembly requirement. supporting_text: The traC gene of the F plasmid tra operon is required for the assembly of mature F-pilin subunits into extended F pili. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Cached record is abstract-only. The reported 875-residue coding region matches P18004 exactly, which independently confirms this paper is about this protein. - id: PMID:2885308 title: A traC mutant that retains sensitivity to f1 bacteriophage but lacks F pili. findings: - statement: >- The traC1044 mutant pairs with recipients but transfers conjugal DNA at greatly reduced frequency, linking TraC to conjugative transfer by mutant phenotype. supporting_text: Mutant cells were able to pair with recipient cells during bacterial conjugation, but transfer of conjugal DNA occurred at a greatly reduced frequency. - statement: >- Mature pilin accumulates normally in the mutant, so TraC acts at pilus assembly and not at pilin processing. supporting_text: Membranes of hosts carrying the F' mutation contained a full complement of mature F-pilin subunits, so the product of traC is presumably required for pilus assembly but not for pilin processing. - statement: >- Complementation with a traC-bearing plasmid restores piliation and conjugation efficiency, confirming the gene assignment. supporting_text: When a plasmid carrying traC was introduced into hosts harboring the F' mutation, phage sensitivity, the ability to elaborate F pili, and conjugation efficiency were restored. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- The primary genetic evidence for both processes proposed in this review. Cached record is abstract-only. - id: PMID:1350587 title: Localization of TraC, a protein involved in assembly of the F conjugative pilus. findings: - statement: >- TraC is cytoplasmic when expressed alone and membrane-associated when expressed from F, so its inner-membrane association is peripheral and partner-dependent rather than via its own anchor. supporting_text: These data suggest that TraC is normally associated with the membrane through interactions with other proteins specified by the tra region. - statement: TraC is not part of the pilus tip. supporting_text: TraC does not appear to be part of the tip of the F pilus, as neither anti-TraC antibodies nor purified TraC had any effect on the infection of F-containing bacteria by the filamentous bacteriophage f1. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Supports the peripheral-membrane qualifier on the location annotation. Cached record is abstract-only. existing_annotations: - term: id: GO:0005886 label: plasma membrane evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in supporting_entities: - UniProtKB-SubCell:SL-0037 review: summary: >- Correct, and independently supported by experiment rather than only by the subcellular-location mapping. TraC fractionates with the membrane when expressed from F in the presence of the other transfer proteins. The association is peripheral and partner-dependent: expressed on its own the protein is cytoplasmic, which is why UniProt qualifies the location as peripheral membrane protein. action: ACCEPT reason: >- The term is right and no more specific plasma-membrane child would be honest, since TraC has no transmembrane segment of its own. The conditionality of the association is a property worth recording in prose but is not expressible by swapping the term. - term: id: GO:0009297 label: pilus assembly evidence_type: IMP original_reference_id: PMID:2885308 qualifier: involved_in review: summary: >- The traC1044 mutant abolishes the extended F pilus while leaving a full complement of mature pilin subunits in the membrane, which places TraC at the assembly step specifically. Complementation restores piliation. action: NEW reason: >- This is the protein's best-established role and GOA carries no biological process annotation for it at all, so the term fills a real gap rather than duplicating coverage. The mutant evidence distinguishes assembly from pilin processing, so the specific term is warranted rather than a vague parent. TraC performs part of the assembly work rather than merely being required for it, since mature subunits are present and only their polymerisation fails. - term: id: GO:0009291 label: unidirectional conjugation evidence_type: IMP original_reference_id: PMID:2885308 qualifier: involved_in review: summary: >- The same mutant pairs with recipient cells but transfers conjugal DNA at greatly reduced frequency, and complementation restores conjugation efficiency. action: NEW reason: >- Proposed because the transfer defect is separable from the pairing step in this mutant, so the phenotype is informative about transfer rather than only about pilus loss. This is the same term and evidence class that GOA already carries for the F coupling protein TraD, so the annotation is consistent with how the sibling transfer proteins are treated. core_functions: - description: >- Assembly factor for the F conjugative pilus. TraC acts after pilin maturation to polymerise mature F-pilin subunits into the extended filament, working as a peripheral inner-membrane component recruited by the other transfer-region proteins rather than as an integral membrane protein or a component of the pilus tip. Loss of TraC leaves pilin intact but abolishes the pilus and sharply reduces conjugative DNA transfer. directly_involved_in: - id: GO:0009297 label: pilus assembly - id: GO:0009291 label: unidirectional conjugation locations: - id: GO:0005886 label: plasma membrane supported_by: - reference_id: PMID:2265751 supporting_text: The traC gene of the F plasmid tra operon is required for the assembly of mature F-pilin subunits into extended F pili. - reference_id: PMID:2885308 supporting_text: Membranes of hosts carrying the F' mutation contained a full complement of mature F-pilin subunits, so the product of traC is presumably required for pilus assembly but not for pilin processing. - reference_id: PMID:1350587 supporting_text: These data suggest that TraC is normally associated with the membrane through interactions with other proteins specified by the tra region. knowledge_gaps: - gap_statement: >- No molecular function is known for F TraC. It is conventionally described as the VirB4-family ATPase of the F system, but the F protein itself has never been assayed for nucleotide binding or hydrolysis, and GOA carries no molecular function annotation for it. The loss-of-function phenotype is sharp and reproducible while the activity producing it is unidentified. boundary: >- Assert the two processes that mutant phenotype supports and no molecular function at all, rather than importing an ATPase term from VirB4 by family inference. gap_kind: - BIOLOGY dark_aspect: MF_DARK significance: >- The assignment propagates: a VirB4-family ATPase role for TraC is assumed in reviews and in pathway modelling of the F system. PANTHER's sequence classification does not even place P18004 with VirB4, putting it in the unnamed family PTHR38467 while VirB4 falls in PTHR30121, so the family inference is weaker than its ubiquity suggests. suggested_questions: - question: >- Has F TraC ever been shown to bind or hydrolyse ATP? The protein is routinely called the VirB4-family ATPase of the F system, but the F literature found here covers sequencing, the traC1044 genetics and localisation only, and soluble TraC has been purified, so the experiment appears feasible and may simply never have been reported. - question: >- Is the PANTHER placement of P18004 outside the VirB4 family a real evolutionary signal or a classification artefact? PANTHER puts TraC in the unnamed family PTHR38467 rather than with VirB4, which if genuine would weaken the standard family-based functional inference. suggested_experiments: - description: >- Assay purified F TraC directly for ATP binding and hydrolysis. Soluble TraC has already been purified from inclusion bodies, so the material is obtainable. Test both soluble and membrane-derived preparations, since those gave opposite answers for the pKM101 VirB4 homologue, and a result either way would close the molecular-function gap recorded here. - description: >- Test whether the traC1044 substitution, which weakens the TraC-membrane interaction, separates pilus assembly from DNA transfer. Quantifying piliation and transfer frequency across an allelic series would establish whether the transfer defect is wholly a consequence of pilus loss or a partly independent contribution.