Baseplate wedge protein gp25 is a 132-residue structural protein (6 copies per virion) located at the inner rim of the bacteriophage T4 baseplate adjacent to the central tail tube. Added as the final subunit in the sequential assembly of the six baseplate wedges, gp25 associates with gp53 and the (gp6)2-gp7 heterotrimeric module and adopts a fold homologous to the C-terminal polymerization domain of the contractile tail sheath protein gp18. By accepting beta-strand arms from the first ring of gp18 subunits to nucleate tail sheath polymerization in the extended state and rotating with the (gp6)2-gp7 module during the dome-to-star transition, gp25 couples host attachment to contractile sheath triggering.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0098015 virus tail | IEA GO_REF:0000043 | MODIFY | Summary: gp25 is a component of the bacteriophage T4 tail, but this term is overly general compared to its specific localization within the viral tail baseplate. Reason: Cryo-EM and X-ray crystallographic structures of the bacteriophage T4 tail resolve six copies of gp25 specifically at the inner rim of the tail baseplate wedges where the baseplate joins the contractile tail sheath. Modifying GO:0098015 (virus tail) to its more specific child term GO:0098025 (virus tail, baseplate), which is already asserted on this gene, provides accurate localization. Proposed replacements: virus tail, baseplate Supporting Evidence: file:BPT4/25/25-uniprot.txt Baseplate protein that is located next to the tail tube (inner baseplate). Involved in sheath assembly. The gp25-(gp6)2-gp7 module is involved in sheath contraction (PubMed:27193680). file:BPT4/25/25-deep-research-falcon.md The strongest convergent conclusion from structural biology and assembly models is that **T4 gp25 is a structural baseplate wedge protein that also acts as the “sheath initiator” (nucleator)** for polymerization of the contractile tail sheath protein **gp18** in the extended (pre-infection) conformation. |
| GO:0098025 virus tail, baseplate | IEA GO_REF:0000043 | ACCEPT | Summary: Keyword-based annotation accurately places gp25 in the viral tail baseplate. Reason: Supported by atomic-resolution cryo-EM reconstructions of the T4 baseplate (PMID:27193680) showing six copies of gp25 forming the inner baseplate ring at the sheath-baseplate interface. Supporting Evidence: PMID:27193680 Here we present the atomic structure of the approximately 6-megadalton bacteriophage T4 baseplate in its pre- and post-host attachment states and explain the events that lead to sheath contraction in atomic detail. file:BPT4/25/25-uniprot.txt Note=Present in 6 copies in the baseplate. file:BPT4/25/25-deep-research-falcon.md The strongest convergent conclusion from structural biology and assembly models is that **T4 gp25 is a structural baseplate wedge protein that also acts as the “sheath initiator” (nucleator)** for polymerization of the contractile tail sheath protein **gp18** in the extended (pre-infection) conformation. |
| GO:0098025 virus tail, baseplate | IDA PMID:27193680 Structure of the T4 baseplate and its function in triggering... | ACCEPT | Summary: Atomic structures of the T4 baseplate resolve six gp25 subunits at the inner rim of the baseplate wedges in complex with gp53 and the (gp6)2-gp7 heterotrimeric module. Reason: Direct structural determination of the pre- and post-attachment T4 baseplate confirms that gp25 is an integral subunit of the inner baseplate wedge that anchors and initiates the contractile tail sheath. Supporting Evidence: PMID:27193680 Here we present the atomic structure of the approximately 6-megadalton bacteriophage T4 baseplate in its pre- and post-host attachment states and explain the events that lead to sheath contraction in atomic detail. file:BPT4/25/25-uniprot.txt Homodimer. Interacts with gp53 and with the (gp6)2-gp7 heterotrimeric molecule; The gp25-(gp6)2-gp7 module is involved in sheath contraction (PubMed:27193680). file:BPT4/25/25-deep-research-falcon.md - “Six gp25 molecules must accommodate twelve long arms” from the first layer of sheath subunits, implying gp25 provides a scaffold that accepts donor-strand/exchange-like arms during early sheath assembly. |
| GO:0039660 structural constituent of virion | IDA PMID:27193680 Structure of the T4 baseplate and its function in triggering... | NEW | Summary: Six copies of gp25 form the inner baseplate wedge ring that structurally links the baseplate to the first annulus of the contractile gp18 tail sheath in the bacteriophage T4 virion. Reason: Atomic structures of the T4 baseplate (PMID:27193680) demonstrate that gp25 is an integral structural subunit of the virion inner baseplate wedge and forms a conserved handshake beta-sheet interface with the gp18 sheath. Supporting Evidence: PMID:27193680 Here we present the atomic structure of the approximately 6-megadalton bacteriophage T4 baseplate in its pre- and post-host attachment states and explain the events that lead to sheath contraction in atomic detail. file:BPT4/25/25-uniprot.txt Baseplate protein that is located next to the tail tube (inner baseplate). Involved in sheath assembly. The gp25-(gp6)2-gp7 module is involved in sheath contraction (PubMed:27193680). |
| GO:0098003 viral tail assembly | IDA PMID:27193680 Structure of the T4 baseplate and its function in triggering... | NEW | Summary: gp25 is essential for viral tail assembly, acting both to complete baseplate wedge assembly and as the structural initiator that nucleates polymerization of the extended gp18 contractile tail sheath. Reason: Biochemical assembly studies and atomic cryo-EM reconstructions (PMID:27193680) establish that gp25 is directly required for tail sheath assembly onto the baseplate-tail tube complex by accepting the N- and C-terminal beta-strand arms of the first gp18 sheath ring. Supporting Evidence: file:BPT4/25/25-uniprot.txt Baseplate protein that is located next to the tail tube (inner baseplate). Involved in sheath assembly. The gp25-(gp6)2-gp7 module is involved in sheath contraction (PubMed:27193680). file:BPT4/25/25-deep-research-falcon.md The strongest convergent conclusion from structural biology and assembly models is that **T4 gp25 is a structural baseplate wedge protein that also acts as the “sheath initiator” (nucleator)** for polymerization of the contractile tail sheath protein **gp18** in the extended (pre-infection) conformation. |
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Download this section (compressed HTML)Q: Why does purified gp25 form a homodimer in solution whereas six monomeric copies of gp25 incorporate symmetrically into the six baseplate wedges?
Q: How do the beta-sheet augmentation interfaces formed by gp25 and gp6 with the N- and C-terminal arms of the first ring of gp18 selectively stabilize the high-energy extended sheath conformation over the lower-energy contracted conformation during assembly?
Experiment: Perform structure-guided mutagenesis of the gp25 beta-sheet edge residues that form the four-stranded handshake sheet with the N- and C-terminal arms of gp18 and assay in vitro sheath polymerization onto assembled baseplates-tail tubes.
Hypothesis: Disrupting the beta-strand augmentation surface on gp25 will abolish nucleation of the extended gp18 sheath on the baseplate while leavinggp18's intrinsic ability to form contracted polysheaths in solution intact.
Experiment: Characterize the solution-to-wedge monomerization transition of gp25 upon binding to pre-formed gp10-gp7-gp8-gp6-gp53 wedges using native mass spectrometry and analytical ultracentrifugation.
Hypothesis: Binding to gp53 and the (gp6)2-gp7 trifurcation interface competes directly with the gp25 homodimerization surface, ensuring stoichiometric recruitment of a single gp25 monomer per baseplate wedge.
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