| Evidence type | Molecular function | Biological process step | Subcellular/virion localization | Key structural/assembly details (stoichiometry, dimensions, helical parameters, domains) | Quantitative data | Key references (with year, DOI URL) |
|---|---|---|---|---|---|---|
| **Identity verification / database-to-literature mapping** (direct T4 gp18 in papers; **UniProt P13332 mapping assumed from user-provided accession because retrieved papers do not explicitly cite P13332**) | Structural tail sheath protein (gp18; gene product 18) of bacteriophage T4 | Tail morphogenesis and infection apparatus function | Outer contractile sheath surrounding the gp19 tail tube in the mature virion | gp18 is described as the T4 tail sheath protein; wild-type protein is 659 aa and forms the sheath around the tail tube; literature consistently refers to “gene product 18/gp18” as the sheath subunit, but no retrieved paper explicitly maps this to UniProt P13332 | 659 aa; ~71 kDa; sheath contains 138 subunits arranged as 23 hexameric rings / six-start helix | Aksyuk et al., 2009, https://doi.org/10.1038/emboj.2009.36; Kostyuchenko et al., 2005, https://doi.org/10.1038/nsmb975; Fokine et al., 2013, https://doi.org/10.1016/j.jmb.2013.02.012 (pqac-00000003, pqac-00000000, pqac-00000001) |
| **Direct T4 gp18** | Contractile sheath subunit that converts stored metastable/elastic energy into mechanical work to drive tail tube penetration | Infection initiation after host recognition and baseplate activation | External sheath wrapped around the central non-contractile tail tube; bottom ring contacts the baseplate, top ring interfaces with tail terminator/neck region | Contraction is triggered by receptor engagement via baseplate rearrangement; gp18 subunits move largely as rigid bodies and slide relative to one another rather than refolding extensively | Sheath shortens from ~925 Å to ~420 Å; diameter expands from ~240 Å to ~330 Å; ring rise changes from ~40.6 Å to ~16.4 Å; twist changes from ~17.2° to ~32.9° | Kostyuchenko et al., 2005, https://doi.org/10.1038/nsmb975; Aksyuk et al., 2009, https://doi.org/10.1038/emboj.2009.36 (pqac-00000015, pqac-00000011) |
| **Direct T4 gp18** | Major structural component of the tail sheath; scaffold for a six-start helical contractile machine | Tail assembly/morphogenesis | Surrounds gp19 tail tube along the tail shaft | 138 gp18 molecules arranged into 23 stacked hexameric rings; equivalently described as a six-start helix; assembly occurs around the preformed tail tube, with tube length set by tape-measure protein gp29 and termination involving gp3/gp15 | 23 rings × 6 subunits = 138 subunits; successive rings rotated by 17.2° and translated by 40.6 Å in the extended state | Fokine et al., 2013, https://doi.org/10.1016/j.jmb.2013.02.012; Arisaka et al., 2016, https://doi.org/10.1007/s12551-016-0230-x; Zinke et al., 2022, https://doi.org/10.1016/j.jbc.2021.101472 (pqac-00000009, pqac-00000012, pqac-00000014) |
| **Direct T4 gp18** | Self-assembling sheath protein capable of polymerizing into contracted-like tubular polysheaths in the absence of the full virion | Sheath self-assembly/polymerization | Normally virion tail sheath; experimentally can form free polysheaths in vitro/in vivo | Wild-type gp18 and truncation constructs self-assemble into tubular polysheaths with helical parameters resembling the contracted sheath; indicates intrinsic polymerization program of the sheath protein | Polysheaths resemble contracted-state geometry; even substantial C-terminal truncations still allow tubular assembly, though with altered helical parameters | Aksyuk et al., 2009, https://doi.org/10.1038/emboj.2009.36; Arisaka et al., 2016, https://doi.org/10.1007/s12551-016-0230-x (pqac-00000011, pqac-00000004) |
| **Direct T4 gp18** | Multi-domain structural protein mediating sheath assembly, sheath-tube interaction, and baseplate-linked triggering | Structural stabilization and contraction coupling during tail assembly/infection | Domains are arranged within the sheath subunit; termini oriented toward the sheath interior; protease-resistant core surface exposed | gp18M/gp18PR structural studies define domains I–III with an unresolved C-terminal domain IV; domain I likely participates in baseplate/retracting tail-fiber interactions; C-terminal domain interacts with the tail tube and terminal ring contacts gp15 | Domain I: residues 98–188; Domain II: residues 88–97 and 189–345; Domain III: residues 20–87 and 346–510; unresolved domain IV ~510–659 | Aksyuk et al., 2009, https://doi.org/10.1038/emboj.2009.36; Fokine et al., 2013, https://doi.org/10.1016/j.jmb.2013.02.012 (pqac-00000008, pqac-00000009) |
| **Direct T4 gp18** | Cooperative polymerizing sheath component in tail biogenesis | Ordered tail assembly before sheath termination/head attachment | External tail sheath assembled on tube-baseplate intermediate | gp18 polymerizes cooperatively around the growing tail tube soon after tube polymerization starts; nucleation is difficult and intermediate-length sheaths are uncommon; gp18 assembles before sheath terminator gp15 | ~138 copies per virion; contraction is exothermic by microcalorimetry; heat or urea can induce contraction experimentally | Arisaka et al., 2016, https://doi.org/10.1007/s12551-016-0230-x (pqac-00000012, pqac-00000004) |
| **Direct T4 gp18 + modeling inference from direct T4 geometry** | Mechanical energy storage/release element of a nanoscale injector | Force generation for piercing the host envelope and supporting DNA delivery | Contractile sheath of the T4 injection machinery | Modeling treats the sheath as six interacting helical protein strands/elastic rods coupled to tail tube and capsid; informed by direct T4 structural parameters | Prior estimates cited in modeling work: ~3400 kcal/mol gp18 for urea-induced contraction, ~6000 kcal/mol gp18 for heat-induced contraction; lower-bound cell-rupture force estimate ~103 pN | Maghsoodi et al., 2017, https://doi.org/10.1016/j.bpj.2017.05.029; Maghsoodi et al., 2016, https://doi.org/10.1115/1.4033554 (pqac-00000021, pqac-00000022) |
| **Homolog/inference from broader sheath-family analyses** | Conserved contractile sheath fold that mediates interaction with tail tube and assembly across myophages/CISs | Evolution of contractile injection systems; comparative functional inference for gp18 family | Contractile sheaths of myophages and related systems | Comparative structural analyses indicate a conserved sheath core shared with other phage sheaths and contractile injection systems; added domains likely modulate stability/host interactions | Structural conservation observed across modeled sheath proteins; T4 gp18 used as a reference in evolutionary analyses | Evseev et al., 2022, https://doi.org/10.3390/v14061148; Aksyuk et al., 2011, https://doi.org/10.1016/j.str.2011.09.012 (pqac-00000002, pqac-00000012) |
| **Homolog/application inference anchored to T4 components** | Inspiration/template for engineered bio-nanomachines and delivery systems; sheath-derived nanotube/scaffold concept | Nanotechnology, intracellular delivery, and phage engineering applications | Engineered derivatives of phage tail components rather than native virion localization | Reviews of T4-derived bio-nanomachines emphasize modular use of tail components; gp18 specifically noted in nano-assembled sheath tubes, while other T4 tail parts (e.g., gp5 needle) have been engineered for cargo delivery | No direct clinical implementation for gp18 itself reported here; application evidence is primarily preclinical/conceptual engineering | Inaba & Ueno, 2018, https://doi.org/10.1007/s12551-017-0336-9; Iglesias et al., 2024, https://doi.org/10.1038/s42003-024-06985-x (pqac-00000026, pqac-00000016) |
| **Recent 2023–2024 context (homolog/inference, not direct gp18-specific experiments)** | Contractile sheath proteins remain central to therapeutic phage structural atlases and engineering-ready phage characterization | Structural annotation of therapeutic/engineering-relevant myophages | Contractile tails of therapeutic or chassis phages | 2024 cryo-EM atlases of therapeutic myophages and 2024 intact-tail structures of myophages reinforce conservation of T4-like sheath architecture/mechanistic principles and support rational engineering | Examples include ~500 Å resolved tail fibers in therapeutic phage Pa193 and ~1045 Å contractile tail in cyanophage A-1(L) | Iglesias et al., 2024, https://doi.org/10.1038/s42003-024-06985-x; Yu et al., 2024, https://doi.org/10.1038/s41467-024-47006-z (pqac-00000016, pqac-00000019) |


*Table: This table summarizes experimentally supported functional annotation for Enterobacteria phage T4 gene product 18 (gp18), the tail sheath protein, and clearly distinguishes direct T4 evidence from homolog-based inference. It also flags that mapping to UniProt P13332 is assumed from the user-provided accession because the retrieved papers identify gp18 directly but do not explicitly cite the UniProt entry.*