Major contractile tail sheath protein gp18 of bacteriophage T4 that polymerizes into 23 hexameric rings (138 subunits total) forming the outer helical sheath around the rigid central tail tube (gp19). Upon host cell attachment and baseplate conformational switching, gp18 subunits undergo a concerted rigid-body rearrangement that contracts the sheath to less than half its extended length, mechanically driving the central tail tube through the host bacterial outer membrane to create a channel for viral genomic DNA ejection.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0046718 symbiont entry into host cell | IEA GO_REF:0000043 | MODIFY | Reason: While gp18 is essential for phage genome delivery into the host bacterium, symbiont entry into host cell is overly general; Bacteriophage T4 delivers its genome via contractile tail sheath contraction without whole-virion internalization, which is specifically captured by GO:0099000 (symbiont genome ejection through host cell envelope, contractile tail mechanism). Proposed replacements: symbiont genome ejection through host cell envelope, contractile tail mechanism Supporting Evidence: PMID:19229296 The contraction of the sheath drives the tail tube through the outer membrane, creating a channel for the viral genome delivery. file:BPT4/18/18-uniprot.txt During infection, contraction of the sheath drives the central tube through the host outer membrane, creating a channel for DNA ejection from the capsid into the host cell. |
| GO:0098015 virus tail | IEA GO_REF:0000043 | MODIFY | Reason: gp18 is specifically the structural subunit of the contractile tail sheath rather than an unspecified tail component; the more specific child term GO:0098027 (virus tail, sheath) is already annotated and directly supported by structural evidence. Proposed replacements: virus tail, sheath Supporting Evidence: PMID:19229296 The sheath consists of 138 copies of the tail sheath protein, gene product (gp) 18, which surrounds the central non-contractile tail tube. |
| GO:0098027 virus tail, sheath | IEA GO_REF:0000043 | ACCEPT | Reason: gp18 is the sole structural protein that forms the 23 hexameric rings of the bacteriophage T4 contractile tail sheath. Supporting Evidence: file:BPT4/18/18-uniprot.txt A total of 138 copies of gp18 arranged into 23 hexameric rings constitutes the sheath. file:BPT4/18/18-deep-research-falcon.md The tail sheath is the outer contractile cylinder that wraps around a central, relatively rigid tail tube; gp18 is the major structural subunit that polymerizes to form this contractile sheath. |
| GO:0099000 symbiont genome ejection through host cell envelope, contractile tail mechanism | IEA GO_REF:0000043 | ACCEPT | Reason: Contraction of the gp18 tail sheath provides the mechanical force that drives the rigid gp19 tail tube across the host outer membrane to enable viral DNA ejection into the host cell. Supporting Evidence: PMID:19229296 The contraction of the sheath drives the tail tube through the outer membrane, creating a channel for the viral genome delivery. file:BPT4/18/18-uniprot.txt During infection, contraction of the sheath drives the central tube through the host outer membrane, creating a channel for DNA ejection from the capsid into the host cell. |
| GO:0098027 virus tail, sheath | IDA PMID:19229296 The tail sheath structure of bacteriophage T4: a molecular m... | ACCEPT | Reason: X-ray crystallography and cryo-electron microscopy reconstructions directly demonstrate that 138 copies of gp18 form the contractile tail sheath surrounding the central tail tube. Supporting Evidence: PMID:19229296 The sheath consists of 138 copies of the tail sheath protein, gene product (gp) 18, which surrounds the central non-contractile tail tube. |
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Download this section (compressed HTML)Q: What are the precise atomic contacts between the C-terminal Domain IV of gp18 and the gp19 tail tube subunits that maintain the high-energy extended conformation prior to baseplate triggering?
Q: Does the bound purine nucleotide historically reported in gp18 preparations play any allosteric or stabilizing role during sheath polymerization onto the tail tube template?
Experiment: Determine high-resolution cryo-EM structures of partially contracted T4 tail intermediates trapped by targeted disulfide cross-links between adjacent gp18 rings to resolve the ring-by-ring propagation wave of sheath contraction.
Experiment: Perform hydrogen-deuterium exchange mass spectrometry (HDX-MS) on monomeric gp18, extended tail sheaths, and contracted sheaths to map the inter-domain hinge regions and beta-sheet augmentation interfaces that lock the contracted state.
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