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.
The preferred accession for this review is P18004. These relationships are manually assessed.
Reviewed 2026-10-19 · Searched: UP000000625
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.
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.
file:ECOLI/traC/traC-record-evidence.yaml
target_sequence_count: 4403
file:ECOLI/traC/traC-record-evidence.yaml
candidate_count: 0
The primary accession is outside every searched exact-taxon reference proteome.
file:ECOLI/traC/traC-record-evidence.yaml
primary_in_any_reference_proteome: false
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.
file:ECOLI/traC/traC-record-evidence.yaml
entry_date: 11-NOV-2015, entry version 89. proteomes: []
file:ECOLI/traC/traC-record-evidence.yaml
entry_date: 02-DEC-2020, entry version 105. proteomes: []
file:ECOLI/traC/traC-record-evidence.yaml
entry_date: 27-NOV-2024, entry version 119. proteomes: []
The primary UniProt entry records plasmid origin. Reference-proteome membership at species or strain level does not ensure coverage of every accessory plasmid.
file:ECOLI/traC/traC-record-evidence.yaml
geneEncodingType: Plasmid
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | 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. 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. |
| GO:0009297 pilus assembly | IMP PMID:2885308 A traC mutant that retains sensitivity to f1 bacteriophage b... | NEW | 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. 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. |
| GO:0009291 unidirectional conjugation | IMP PMID:2885308 A traC mutant that retains sensitivity to f1 bacteriophage b... | NEW | Summary: The same mutant pairs with recipient cells but transfers conjugal DNA at greatly reduced frequency, and complementation restores conjugation efficiency. 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. |
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Download this section (compressed HTML)Q: 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.
Q: 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.
Experiment: 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.
Experiment: 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.
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