| Feature | Evidence summary | Key citations | Primary sources with URL and publication date |
|---|---|---|---|
| Identity / target verification | The literature consistently identifies T4 gene product 20 (gp20) as the portal protein of *Enterobacteria phage T4*, matching UniProt P13334 and distinguishing it from unrelated uses of “gp20” or gene “20” in other phages/organisms. It forms the unique portal vertex of the T4 head. | (pqac-00000000, pqac-00000003, pqac-00000004) | Sun et al., 2015, cryo-EM structure of bacteriophage T4 portal protein assembly, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, *Viruses*, https://doi.org/10.3390/v15020527, Feb 2023 |
| Localization | gp20 localizes at the unique special vertex of the prolate T4 capsid, where it connects the head to the packaging motor during assembly and later to neck/tail proteins in the mature virion. The clip domain is exposed outside the capsid, whereas stem/wing/crown regions project into the head lumen and channel. | (pqac-00000006, pqac-00000009, pqac-00000015) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023 |
| Oligomeric state | gp20 assembles predominantly as a dodecamer. Cryo-EM particle analysis found ~90% 12-mers in the initial sample and ~95% 12-mers after repurification, with minor 11-mer/13-mer species. | (pqac-00000000, pqac-00000023) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015 |
| Domain architecture | Each gp20 subunit contains crown, wing, stem, and clip domains, with a tunnel loop projecting into the channel. The clip domain is the main external interaction platform; wing/stem/crown help shape the portal channel and contact capsid and DNA. | (pqac-00000006, pqac-00000001, pqac-00000024) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023 |
| Key interactions | gp20 interacts with gp23 capsid protein at the portal vertex, docks the pentameric packaging ATPase gp17 via the clip domain, and later engages gp13 and gp14 to seal the packaged head and connect to the tail. Specific gp20 clip residues such as N291, M292, R295, and K296 are implicated in motor/neck contacts. | (pqac-00000005, pqac-00000008, pqac-00000011, pqac-00000013) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023; Sun et al., 2025 preprint, https://doi.org/10.21203/rs.3.rs-6853951/v1, Jun 2025 |
| Role in head assembly | gp20 is not a passive pore; it nucleates/initiates head assembly and helps recruit or organize co-polymerization of gp23 capsomers and scaffolding components. Structural work suggests portal conformation and portal-capsid contacts help trigger and propagate capsid expansion during maturation. | (pqac-00000011, pqac-00000015, pqac-00000010) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023 |
| Role in DNA packaging | gp20 forms the central conduit for DNA entry into the head and serves as the docking platform for five gp17 ATPase subunits. DNA contacts are concentrated at three loop regions in the channel, and basic residues near the entrance are required for efficient packaging initiation. | (pqac-00000005, pqac-00000014, pqac-00000020) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023 |
| Gating / valve evidence | Structural and mutational data support a valve-like role for gp20: inner clip, channel, and tunnel loops can constrict the channel, likely preventing DNA backsliding and regulating genome flow during packaging/ejection. Tunnel-loop deletion disrupted proper oligomerization and blocked completion of full-length genome packaging. | (pqac-00000008, pqac-00000014, pqac-00000002) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023 |
| Neck sealing / post-packaging retention | After headful packaging, gp20 clip/stem surfaces are remodeled to bind gp13 and gp14, sealing the portal-neck connector before tail completion. A 2025 preprint further proposes a double “genome-gate” involving gp14 plus host Hfq, with gp20 conformational changes exposing binding surfaces for secure retention of the pressurized genome. | (pqac-00000010, pqac-00000013) | Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023; Sun et al., 2025 preprint, https://doi.org/10.21203/rs.3.rs-6853951/v1, Jun 2025 |
| Quantitative measurements | Reported values include: gp20 monomer length 524 aa; portal complex ~660 kDa; overall portal length ~120 Å; channel narrows from ~44 Å to ~28 Å; portal-capsid contact area ~819 Ų per monomer; adjacent subunit interface ~4,100 Ų; T4 packages ~171 kb DNA into a 120 × 86 nm head, with packaging speeds up to ~2000 bp/s and ~70% head volume increase upon expansion. | (pqac-00000019, pqac-00000020, pqac-00000016, pqac-00000023) | Sun et al., 2015, https://doi.org/10.2210/pdb3ja7/pdb, Jul 2015; Rao et al., 2023, https://doi.org/10.3390/v15020527, Feb 2023 |
| Mutational / structure-function evidence | Structure-function analysis identified stalk/channel loop regions as important for DNA translocation, packaging completion, and later neck/tail attachment, supporting direct mechanistic roles for gp20 rather than a merely structural role. This bridges pre-atomic modeling and later near-atomic cryo-EM. | (pqac-00000017, pqac-00000021) | Padilla-Sanchez et al., 2014, *Journal of Molecular Biology*, https://doi.org/10.1016/j.jmb.2013.10.011, Mar 2014 |
| Current understanding / expert synthesis | Recent expert review concludes that gp20 is a multifunctional portal-connector coordinating symmetry mismatch, assembly initiation, motor docking, DNA translocation, and post-packaging sealing. The 2023 synthesis emphasizes structural morphing at the portal vertex as central to capsid expansion and genome handling. | (pqac-00000003, pqac-00000004, pqac-00000018) | Rao et al., 2023, *Viruses*, https://doi.org/10.3390/v15020527, Feb 2023 |


*Table: This table summarizes the core functional annotation evidence for Enterobacteria phage T4 gp20, the portal protein encoded by gene 20. It highlights identity verification, localization, structure, assembly and packaging roles, gating evidence, and key quantitative measurements with source links and dates.*