| Aspect | What is known / current understanding | Key evidence & quantitative details | Key sources (with year) |
|---|---|---|---|
| Identity mapping to UniProt | Target is correctly identified as Enterobacteria phage T4 long-tail fiber protein gp37, the distal receptor-recognizing component of the long tail fiber; Bartual et al. explicitly link T4 gp37 to UniProt P03744. | PNAS structure paper states T4 gp37 corresponds to UniProt code **P03744** and solves the receptor-binding tip structure of gp37(785–1026); reviews independently describe gp37 as the distal “foot”/tip of the LTF. | Bartual et al., 2010; Leiman et al., 2010 (pqac-00000000, pqac-00000022, pqac-00000009) |
| Protein size / oligomerization | gp37 is a large structural tail-fiber protein that forms a **homotrimer** and constitutes most of the distal half-fiber. | Size is consistently reported as **1026 aa**, about **109 kDa** per subunit; three gp37 chains per long tail fiber; LTF stoichiometry overall is **gp34:gp35:gp36:gp37 = 3:1:3:3**. | Mourosi et al., 2022; Leiman et al., 2010 (pqac-00000008, pqac-00000009, pqac-00000013) |
| Virion localization / stoichiometry | gp37 is located in the **distal half** of each of the six T4 long tail fibers, distal to gp36 and farthest from the baseplate, where it forms the receptor-binding tip. | T4 has **6 LTFs** anchored around the baseplate; mature infective particles often carry LTFs folded back against the sheath/capsid; in cryo-ET, D10–D11/gp37 tip density contacts the capsid in the retracted state. | Hu et al., 2015; Leiman et al., 2010; Arisaka et al., 2016 (pqac-00000014, pqac-00000009, pqac-00000015) |
| Domain architecture / residue ranges | The best-resolved region is the C-terminal receptor-binding segment, organized into **collar, needle/stem, and head/tip** domains. The overall tip framework is conserved, while the distal head is more variable and likely determines specificity. | Crystal structure of gp37 tip (**PDB 2XGF**) covers roughly **residues 811–1026**. Collar: **811–860 and 1016–1026**; needle/stem: **881–933 and 960–1008/1009**; compact head/tip: **934–959 / 932–959**. Receptor-binding region proposed around **907–996**. | Bartual et al., 2010; Mourosi et al., 2022 (pqac-00000016, pqac-00000018, pqac-00000019, pqac-00000020) |
| Receptor specificity | gp37 mediates **primary, reversible adsorption** to host receptors and is a major determinant of T4 host range. Canonical receptors are **rough LPS** on E. coli B and **OmpC** on E. coli K-12. | Reviews and structural work identify the distal C-terminus/tip as the receptor-binding domain. OmpC residues **P177** and **F182** are key for interaction; LPS recognition depends on terminal glucose features in rough LPS. | Bartual et al., 2010; Leiman et al., 2010; Mourosi et al., 2022 (pqac-00000000, pqac-00000009, pqac-00000028, pqac-00000050, pqac-00000051) |
| Binding mechanics / biophysics | Binding is **weak, multivalent, and dynamic**, enabling a “touch-and-search” or surface “walking” mechanism rather than immediate irreversible locking. Excessively tight binding can reduce infectivity. | AFM-measured interaction forces for T4 tip with host vs non-host LPS were about **70 ± 29 pN** vs **46 ± 13 pN**; some gp37 tip mutants (**I933A, N937A, G938A**) increased OmpC binding **2–3-fold** yet abolished infectivity, implying that lateral mobility and correct binding dynamics matter. | Mourosi et al., 2022 (pqac-00000010, pqac-00000027, pqac-00000049, pqac-00000050, pqac-00000060) |
| Structural stabilization features | The distal tip is unusually rigid and stable, likely because of **multiple Fe-bound His-X-His motifs** running along the needle axis. | Seven iron sites are coordinated by histidine doublets including **His-883/885, 915/917, 929/931, 966/968, 980/982, 989/991, 998/1000**; the needle is ~**150 Å** long. Aromatic/basic residues such as **W936, Y949, Y953, K945, R954** are candidate receptor-contact residues. | Bartual et al., 2010 (pqac-00000016, pqac-00000017, pqac-00000018) |
| Assembly / chaperones | Correct gp37 folding and trimerization require phage-encoded chaperones, especially **gp57A** and **gp38**; gp37 assembles with gp36 to form the distal half-fiber. | Reviews state gp57A is required for correct trimerization of gp34 and gp37, while gp38 is specifically required for gp37 folding/assembly. Co-expression of **gp37 + gp57A + gp38** yields soluble folded gp37; some C-terminal extensions can bypass gp38 dependence. | Leiman et al., 2010; Mourosi et al., 2022; Arisaka et al., 2016 (pqac-00000008, pqac-00000009, pqac-00000012, pqac-00000015, pqac-00000065) |
| Role in infection triggering | gp37 does not just bind receptors; its engagement helps transmit a **mechanical signal** from the LTF to the baseplate, promoting short-tail-fiber deployment, irreversible attachment, sheath contraction, and DNA injection. | Productive infection usually requires **≥3 LTFs** engaged/oriented toward the cell. LTFs cycle between retracted and extended states; receptor recognition by gp37-containing fibers contributes to baseplate transition from metastable to activated states. | Leiman et al., 2010; Hu et al., 2015; Mourosi et al., 2022 (pqac-00000009, pqac-00000027, pqac-00000029, pqac-00000031, pqac-00000032) |
| Recent developments (2023–2024) | Recent work emphasizes gp37 and gp37-like distal tips as **engineering targets** and as archetypes for understanding T4-like myovirus host recognition. | 2024 Chemical Reviews summarizes T4 gp37 distal-tip mutant libraries and identifies adsorption-critical positions including gp37 **937** and **942**; selected mutants adsorbed to alternative receptors such as **E. coli O157 OmpC** and **E. coli K12 LPS**. 2023 CkP1 study shows a T4-like/S16-like LTF binds LPS with **nanomolar affinity** and recognizes all tested C. koseri strains. | Peng et al., 2024; Oliveira et al., 2023 (pqac-00000035, pqac-00000047, pqac-00000053, pqac-00000055) |
| Applications: engineering & biosensors | gp37/gp37-like LTF tips are used or proposed as modules for **host-range reprogramming**, **phage therapy optimization**, and **bacterial detection**. For diagnostics, binding alone can be useful even when productive infection is not. | 2024 reviews note T4 gp37 as an engineered RBP targeting **E. coli/OmpC** and emphasize that **binding ≠ infectivity** for therapeutic redesign. Recombinant gp37 has been produced with a two-chaperone system; isolated tail fibers can serve as capture reagents. General phage-RBP biosensor literature cited in 2024 reports tail-fiber-based detection down to **10^2 CFU/mL** and reporter-phage formats as low as **33 CFU/mL**, though these LODs are not T4-gp37-specific. | Peng et al., 2024; Parker & Nugen, 2024; Oliveira et al., 2023 (pqac-00000039, pqac-00000041, pqac-00000043, pqac-00000046, pqac-00000055) |


*Table: This table condenses the main functional, structural, mechanistic, and application-focused findings for Enterobacteria phage T4 gp37 (UniProt P03744). It is designed as a quick reference linking current understanding to specific evidence and recent sources.*