| Aspect | Specific details | Key citations (pqac ids) | Primary sources (paper title, year, DOI URL) |
|---|---|---|---|
| Molecular function | T4 gp19 is the major tail tube protein (tail tube structural subunit) that polymerizes into stacked hexameric rings to form the hollow inner tube of the contractile tail; this tube serves as the conduit for dsDNA delivery during infection rather than catalyzing a chemical reaction. | (pqac-00000018, pqac-00000017) | Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472; Structural studies of the phage G capsid and helical tail sheath using cryo-EM, 2021, https://doi.org/10.25394/pgs.15156780.v1 |
| Biological process | gp19 functions in T4 tail morphogenesis: tube assembly begins at the baseplate, grows around the tape measure protein, is capped by a terminator, and then acts as the template/scaffold for contractile sheath assembly. | (pqac-00000016, pqac-00000018, pqac-00000019) | Tail Structure and Dynamics, 2021, https://doi.org/10.1016/b978-0-12-809633-8.20965-5; Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472 |
| Cellular component / localization | gp19 is a virion structural protein located in the phage tail tube, external to the host prior to adsorption and then positioned as the injection conduit spanning from the baseplate toward the host envelope during infection. Its lumen encloses/aligns with the internal tape measure protein before genome release. | (pqac-00000016, pqac-00000018) | Tail Structure and Dynamics, 2021, https://doi.org/10.1016/b978-0-12-809633-8.20965-5; Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472 |
| Key interactions | gp19 assembles around gp29 (tape measure protein) in the lumen; polymerization is nucleated by baseplate-associated initiators gp48 and gp54; the proximal end is capped by hexameric terminator gp3; the completed gp19 tube is encased by and templates assembly of sheath protein gp18. | (pqac-00000016, pqac-00000018, pqac-00000017) | Tail Structure and Dynamics, 2021, https://doi.org/10.1016/b978-0-12-809633-8.20965-5; Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472; Structural studies of the phage G capsid and helical tail sheath using cryo-EM, 2021, https://doi.org/10.25394/pgs.15156780.v1 |
| Quantitative structural data | T4 gp19 forms 24 stacked hexameric rings (144 subunits total) in the mature tail tube; monomer mass is ~18.5 kDa; the central channel is reported at ~40 Å diameter. | (pqac-00000018, pqac-00000011) | Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472; Structural studies of the phage G capsid and helical tail sheath using cryo-EM, 2021, https://doi.org/10.25394/pgs.15156780.v1 |
| Quantitative structural data | Review-level structural comparison reports T4 gp19 hexameric rings as ~40.2 Å thick with ~17.9° rotation between rings; the tube lumen is lined by a 24-stranded β-barrel with negative electrostatic potential favorable for DNA conduction. | (pqac-00000012, pqac-00000010) | Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472; Tall tails: cryo-electron microscopy of phage tail DNA ejection conduits, 2022, https://doi.org/10.1042/bst20210799 |
| Structural definition | The T4 gp19 fold is a conserved tail tube protein architecture centered on a β-sandwich core with an α-helix and extended hairpin/loop elements; assembled subunits form a continuous lumen-lining β-barrel. T4 gp19 is explicitly represented by PDB 5W5F in comparative structural reviews. | (pqac-00000010, pqac-00000013, pqac-00000015) | Tall tails: cryo-electron microscopy of phage tail DNA ejection conduits, 2022, https://doi.org/10.1042/bst20210799; Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472 |
| Quantitative structural data | General contractile-tail context: polymerized tail tube proteins form straight tubes with ~4 nm internal diameter; during contraction, ~40 nm of inner tube can become exposed, enough to traverse a ~30 nm periplasm. These values are not exclusive to T4 gp19 but are used in T4-focused tail tube reviews. | (pqac-00000004) | Tall tails: cryo-electron microscopy of phage tail DNA ejection conduits, 2022, https://doi.org/10.1042/bst20210799 |
| Evidence type | Evidence is primarily structural and morphogenetic: cryo-EM/X-ray-derived models of T4-like tail tube proteins, comparative structural reviews, and assembly-pathway analyses support assignment of gp19 as a non-enzymatic virion tube/scaffold protein. | (pqac-00000010, pqac-00000018, pqac-00000016) | Tall tails: cryo-electron microscopy of phage tail DNA ejection conduits, 2022, https://doi.org/10.1042/bst20210799; Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472; Tail Structure and Dynamics, 2021, https://doi.org/10.1016/b978-0-12-809633-8.20965-5 |
| Recent developments (comparative context, 2023) | A 2023 structure of Vibrio phage XM1 reports a tail tube protein with a β-sandwich fold similar to phage T4 gp19, reinforcing that T4 gp19 represents a conserved contractile-injection-system tube architecture across myophages. | (pqac-00000007) | Structure of Vibrio Phage XM1, a Simple Contractile DNA Injection Machine, 2023, https://doi.org/10.3390/v15081673 |
| Recent developments (comparative context, 2023) | The therapeutic Pseudomonas phage E217 structural atlas resolved the extended/contracted tail and baseplate, showing a rigid tube surrounded by sheath and a baseplate cap protein (gp36) structurally similar to the tail tube protein; this provides recent high-resolution context for T4-like contractile tail tube design in biomedically relevant phages. | (pqac-00000005, pqac-00000006) | High-resolution cryo-EM structure of the Pseudomonas bacteriophage E217, 2023, https://doi.org/10.1038/s41467-023-39756-z |
| Recent developments (comparative context, 2024) | The 2024 Pa193 cryo-EM study, on a therapeutic contractile-tailed phage, resolved the tail tube tertiary structure as a β-sandwich with N-terminal extensions and quantified helical tail parameters; although not T4 gp19 itself, it extends the modern structural framework for annotation of T4-like tube proteins in phage therapy candidates. | (pqac-00000009) | Cryo-EM analysis of Pseudomonas phage Pa193 structural components, 2024, https://doi.org/10.1038/s42003-024-06985-x |
| Recent developments (comparative context, 2024-2025 domain usage) | Recent environmental and omics studies continue to identify the PF06841/Phage_T4_gp19 domain as a marker of T4-like tail tube proteins in viral and bacterial predatory systems, supporting family-level annotation of gp19 homologs, though these studies do not experimentally redefine T4 gp19 function. | (pqac-00000000) | A hunting ground for predatory bacteria at the Zhenbei seamount in the South China Sea, 2025, https://doi.org/10.1093/ismeco/ycaf042 |
| Annotation confidence | High confidence for primary function: all retrieved T4-specific and comparative sources consistently support gp19 as a structural tail tube protein in the T4-like viruses Gp19 family, with no credible alternative enzymatic or receptor-binding primary role identified. | (pqac-00000018, pqac-00000016, pqac-00000010) | Major tail proteins of bacteriophages of the order Caudovirales, 2022, https://doi.org/10.1016/j.jbc.2021.101472; Tail Structure and Dynamics, 2021, https://doi.org/10.1016/b978-0-12-809633-8.20965-5; Tall tails: cryo-electron microscopy of phage tail DNA ejection conduits, 2022, https://doi.org/10.1042/bst20210799 |


*Table: This table summarizes the functional annotation of bacteriophage T4 gp19 (UniProt P13333), including its role in tail tube assembly, localization, interactions, and quantitative structural features. It also adds 2023-2024 comparative structural studies to show how recent work supports and extends the annotation of T4-like gp19 proteins.*