| Feature | Evidence summary | Key quantitative values | Key sources with year+URL |
|---|---|---|---|
| Identity | gp23 in Enterobacteria phage T4 corresponds to the major capsid (head) protein; the mature virion contains the processed form gp23*. It is the principal protein forming the hexagonal capsid lattice of the T4 head. (pqac-00000006, pqac-00000009) | Mature shell contains ~930 gp23* subunits arranged as 155 hexameric capsomers | Rao & Black 2010 — https://doi.org/10.1186/1743-422x-7-356; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Primary function | gp23/gp23* is a structural protein, not an enzyme: its primary role is to build the protective capsid shell that encloses the T4 dsDNA genome and withstands high internal pressure after DNA packaging. (pqac-00000011, pqac-00000017) | Capsid contains ~171 kbp dsDNA; internal pressure ~25 atm | Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Structural fold/domains | Structural modeling and cryo-EM indicate gp23* has an HK97-like major capsid protein fold, with T4-specific additions including an insertion (I) domain and N-fist that contribute to intra- and intercapsomer stabilization. (pqac-00000008, pqac-00000011) | Unique features include ~60-residue I domain and ~25-residue N-fist | Rao & Black 2010 — https://doi.org/10.1186/1743-422x-7-356; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Assembly state | gp23* assembles as hexamers/capsomers that form the prolate head lattice; gp24* forms pentamers at 11 vertices, while a gp20 dodecamer forms the unique portal vertex. In gp24 bypass mutants, gp23 can also occupy vertex positions. (pqac-00000006, pqac-00000008, pqac-00000011) | 155 gp23* hexamers; 11 gp24* pentamers; 1 gp20 portal dodecamer | Black & Rao 2012 — https://doi.org/10.1016/B978-0-12-394621-8.00018-2; Rao & Black 2010 — https://doi.org/10.1186/1743-422x-7-356; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Maturation processing | gp23 is synthesized as a prohead protein and proteolytically processed during maturation to gp23* by removal of an N-terminal peptide; this cleavage accompanies large conformational changes and capsid expansion. (pqac-00000009, pqac-00000010, pqac-00000011) | 65 N-terminal residues removed; prohead-to-head inner volume increase ~70% | Mesyanzhinov et al. 2004 — https://doi.org/10.1007/pl00021751; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Protease/chaperones | The phage-encoded protease gp21 mediates maturation cleavage, and productive gp23 folding requires host GroEL together with the phage cochaperonin gp31; GroES cannot substitute for gp31 in gp23 folding. (pqac-00000005, pqac-00000006, pqac-00000010) | gp21 also processes many prohead proteins; gp23 folding specifically depends on GroEL+gp31 | Black & Rao 2012 — https://doi.org/10.1016/B978-0-12-394621-8.00018-2; Rao & Black 2010 — https://doi.org/10.1186/1743-422x-7-356; Mesyanzhinov et al. 2004 — https://doi.org/10.1007/pl00021751 |
| Virion localization | gp23* localizes to the outer shell of the T4 head/capsid, forming the hexagonal lattice over most of the prolate virion head. Hoc binds at the centers of gp23* hexamers and Soc binds between adjacent gp23* subunits/hexamers on the exterior. (pqac-00000000, pqac-00000012, pqac-00000013) | 1 Hoc per gp23* hexamer; Soc forms trimers at junctions of three gp23* hexamers | Zhu et al. 2023 — https://doi.org/10.1038/s41467-023-38364-1; Leiman et al. 2003 — https://doi.org/10.1007/s00018-003-3072-1 |
| Key interaction partners | Major structural partners are gp24* (vertex protein), gp20 (portal vertex), Soc (capsid clamp/reinforcement protein), and Hoc (outer fiber-like decoration protein). Soc bridges adjacent gp23* subunits and reinforces the pressurized shell; Hoc projects outward from gp23* hexamer centers. (pqac-00000000, pqac-00000007, pqac-00000011) | Soc ~870 copies/capsid; Hoc ~155 copies/capsid | Zhu et al. 2023 — https://doi.org/10.1038/s41467-023-38364-1; Yap & Rossmann 2014 — https://doi.org/10.2217/fmb.14.91; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Quantitative stoichiometry | Recent and classic structural summaries converge on a shell made predominantly of gp23* with fixed numbers of gp24*, gp20, Hoc, and Soc. Minor variation exists in review summaries, but the consensus mature capsid is ~930 gp23*, 55 gp24*, 12 gp20, 155 Hoc, and 870 Soc. (pqac-00000000, pqac-00000006, pqac-00000010) | gp23* ~930; gp24* ~55; gp20 12; Hoc ~155; Soc ~870 | Zhu et al. 2023 — https://doi.org/10.1038/s41467-023-38364-1; Black & Rao 2012 — https://doi.org/10.1016/B978-0-12-394621-8.00018-2; Mesyanzhinov et al. 2004 — https://doi.org/10.1007/pl00021751 |
| Capsid geometry/dimensions | The T4 head is a prolate icosahedron with triangulation parameters Tend = 13 laevo and Tmid = 20. Across studies it is consistently reported as about 120 nm long and 86 nm wide, with the shell built from gp23* hexamers and gp24* pentamers. (pqac-00000006, pqac-00000009, pqac-00000017) | ~120 × 86 nm; Tend = 13 laevo; Tmid = 20 | Black & Rao 2012 — https://doi.org/10.1016/B978-0-12-394621-8.00018-2; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |
| Relevant 2023-2024 developments/applications | Recent work has leveraged the gp23*-based T4 capsid as a programmable nanomaterial: engineered artificial viral vectors used the 120 × 86 nm shell, Soc/Hoc display sites, and ATP-driven refilling to deliver DNAs, proteins, RNAs, and RNPs into human cells. 2023 structural reviews also emphasized cryo-EM-resolved unexpanded/expanded gp23* states and ~70% volume gain as key design parameters. (pqac-00000015, pqac-00000016, pqac-00000018) | Capsid payload capacity ~171 kbp DNA; WT capsid net charge 6,829 negative charges; 9DE mutant 15,199 negative charges | Zhu et al. 2023 — https://doi.org/10.1038/s41467-023-38364-1; Rao et al. 2023 — https://doi.org/10.3390/v15020527 |


*Table: This table summarizes the functional annotation of bacteriophage T4 major capsid protein gp23/gp23*, including identity, structural role, maturation, localization, stoichiometry, and recent applications. It is useful as a concise evidence-backed reference for interpreting UniProt P04535 in the context of T4 head assembly and engineering.*