gp36 (221 amino acids, ~23.3 kDa) is the structural subunit that forms a parallel homotrimer comprising the proximal segment ("upper shin") of the distal half-fiber of the bacteriophage T4 long tail fiber (LTF). During sequential assembly of the distal half-fiber, the gp36 homotrimer associates with the N-terminal globular domain of the gp37 distal shaft/adhesin trimer, stabilizing its proximal rod region, and subsequently recruits a monomeric gp35 hinge/knee cap to form the mature gp35-gp36-gp37 distal half-fiber before joining the gp34 proximal half-fiber. Within the intact virion, gp36 serves as the rigid structural link between the gp35 knee hinge and the gp37 receptor-binding shaft that transmits host recognition signals from the fiber tip toward the baseplate.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019062 virion attachment to host cell | IEA GO_REF:0000043 | MODIFY | Summary: gp36 is a structural subunit of the long tail fiber required for host attachment, but this general UniProt-keyword term should be refined to the specific descendant GO:0098671 (adhesion receptor-mediated virion attachment to host cell). Reason: Although gp36 does not directly bind host LPS or OmpC (which is mediated by the distal tip of gp37), the gp36 homotrimer is an essential structural segment of the distal long tail fiber required to assemble a functional receptor-binding fiber and transmit the attachment signal to the baseplate. Because the more specific descendant term GO:0098671 (adhesion receptor-mediated virion attachment to host cell) is already present on this record, this broader parent term is modified to GO:0098671. Proposed replacements: adhesion receptor-mediated virion attachment to host cell Supporting Evidence: file:BPT4/36/36-deep-research-falcon.md Multiple authoritative T4 morphogenesis and structural reviews explicitly identify **gp36** as a **trimeric structural component of the T4 long tail fiber (LTF)**, positioned in the **distal/hinge region** and present at **3 copies per fiber** (6 fibers per virion). |
| GO:0046718 symbiont entry into host cell | IEA GO_REF:0000043 | MODIFY | Summary: Broad automated mapping from the 'Viral attachment to host cell' keyword; gp36 functions specifically in long tail fiber assembly and receptor-mediated attachment rather than genome entry. Reason: gp36 is an external long tail fiber structural shaft subunit and does not cross the host envelope or directly mediate viral genome penetration. Its role in the entry pathway is restricted to assembling the long tail fiber apparatus that mediates reversible host receptor attachment, which is better captured by GO:0098671 (adhesion receptor-mediated virion attachment to host cell). Proposed replacements: adhesion receptor-mediated virion attachment to host cell Supporting Evidence: file:BPT4/36/36-deep-research-falcon.md Multiple authoritative T4 morphogenesis and structural reviews explicitly identify **gp36** as a **trimeric structural component of the T4 long tail fiber (LTF)**, positioned in the **distal/hinge region** and present at **3 copies per fiber** (6 fibers per virion). |
| GO:0098015 virus tail | IEA GO_REF:0000043 | MODIFY | Summary: gp36 localizes specifically to the long tail fiber, so the broader parent term virus tail should be refined to GO:0098024 (virus tail, fiber). Reason: gp36 is a constituent of the distal half of the long tail fiber rather than the tail tube or sheath. Because the more specific descendant term GO:0098024 (virus tail, fiber) is already annotated to this gene, GO:0098015 is modified to GO:0098024. Proposed replacements: virus tail, fiber Supporting Evidence: file:BPT4/36/36-uniprot.txt Structural component of the distal-half of the long-tail |
| GO:0098024 virus tail, fiber | IEA GO_REF:0000043 | ACCEPT | Summary: gp36 is an integral homotrimeric subunit of the distal half of the bacteriophage T4 long tail fiber. Reason: Biochemical, genetic, and electron-microscopic analyses establish that three copies of gp36 form the ~14-16 nm proximal rod ("upper shin") of each distal long tail fiber half-fiber. GO:0098024 accurately represents its core localization in the mature virion. Supporting Evidence: file:BPT4/36/36-uniprot.txt Structural component of the distal-half of the long-tail file:BPT4/36/36-deep-research-falcon.md Multiple authoritative T4 morphogenesis and structural reviews explicitly identify **gp36** as a **trimeric structural component of the T4 long tail fiber (LTF)**, positioned in the **distal/hinge region** and present at **3 copies per fiber** (6 fibers per virion). |
| GO:0098671 adhesion receptor-mediated virion attachment to host cell | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: gp36 contributes structurally to the long tail fiber required for receptor-mediated attachment, although direct host receptor binding is performed by the distal tip of gp37. Reason: While the distal tip of gp37 directly recognizes host surface LPS and OmpC, the gp36 homotrimer is an essential structural segment of the distal half-fiber required for assembling a functional receptor-binding fiber and transmitting the attachment signal through the gp35 knee hinge to the baseplate. Because gp36's primary role is structural and assembly-related rather than direct receptor recognition, GO:0098671 is retained as non-core. Supporting Evidence: file:BPT4/36/36-deep-research-falcon.md Multiple authoritative T4 morphogenesis and structural reviews explicitly identify **gp36** as a **trimeric structural component of the T4 long tail fiber (LTF)**, positioned in the **distal/hinge region** and present at **3 copies per fiber** (6 fibers per virion). |
| GO:0039660 structural constituent of virion | IEA file:BPT4/36/36-uniprot.txt | NEW | Summary: gp36 is an essential homotrimeric structural protein of the mature T4 virion long tail fiber (18 copies per virion across six fibers). Reason: Existing GOA annotations for gp36 lack a molecular function term. Because gp36 forms the structural "upper shin" rod of the distal half of the long tail fiber in the mature virion, GO:0039660 (structural constituent of virion) is proposed as its core molecular function. Supporting Evidence: file:BPT4/36/36-uniprot.txt Structural component of the distal-half of the long-tail file:BPT4/36/36-deep-research-falcon.md Multiple authoritative T4 morphogenesis and structural reviews explicitly identify **gp36** as a **trimeric structural component of the T4 long tail fiber (LTF)**, positioned in the **distal/hinge region** and present at **3 copies per fiber** (6 fibers per virion). |
| GO:0098004 virus tail fiber assembly | IEA file:BPT4/36/36-deep-research-falcon.md | NEW | Summary: Binding of the gp36 homotrimer to the N-terminal domain of the gp37 trimer is an obligate intermediate step in distal long tail fiber assembly that creates the docking site for gp35. Reason: In the sequential distal long tail fiber assembly pathway, gp36 binds the N-terminal region of trimerized gp37 to form the 14S gp36-gp37 intermediate, which is strictly required before gp35 can bind and before the distal half-fiber can join the gp34 proximal half-fiber. Adding GO:0098004 (virus tail fiber assembly) captures this essential morphogenetic function. Supporting Evidence: file:BPT4/36/36-deep-research-falcon.md Multiple authoritative T4 morphogenesis and structural reviews explicitly identify **gp36** as a **trimeric structural component of the T4 long tail fiber (LTF)**, positioned in the **distal/hinge region** and present at **3 copies per fiber** (6 fibers per virion). |
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Download this section (compressed HTML)Q: What is the atomic structure of the gp36 homotrimer and its asymmetric interface with the monomeric gp35 hinge protein and the N-terminal collar domain of the gp37 trimer?
Q: How does association of gp36 with the N-terminal region of gp37 induce or stabilize the conformational state required for gp35 binding during distal half-fiber assembly?
Experiment: Determine the high-resolution structure of a reconstituted gp35-gp36-gp37(N-terminal domain) complex using single-particle cryo-EM or X-ray crystallography to resolve the trimer-to-trimer (gp37-gp36) and trimer-to-monomer (gp36-gp35) interfaces.
Experiment: Perform hydrogen-deuterium exchange mass spectrometry (HDX-MS) on isolated gp37 N-terminal trimers before and after gp36 binding to map the conformational changes that license subsequent gp35 recruitment.
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