| Feature | Summary statement | Key supporting citations with DOI URLs and publication years |
|---|---|---|
| Verified identity | UniProt P06808 matches **Enterobacteria phage T4 gene t**, also called **rV**, encoding **holin T**, the T4 lysis protein; multiple T4-focused studies explicitly state that **rV is allelic to t** and that T is the holin controlling lysis timing. (pqac-00000001, pqac-00000004, pqac-00000006) | Tran et al., 2005, *J Bacteriol*, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Ramanculov & Young, 2001, *Mol Microbiol*, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Moussa et al., 2014, *J Bacteriol*, https://doi.org/10.1128/jb.01548-14 |
| Primary function (T4-specific) | Holin T is the **inner-membrane timer/trigger** for host lysis: it accumulates in the membrane and then suddenly triggers to permeabilize the cytoplasmic membrane, allowing **endolysin E** access to the periplasmic peptidoglycan and thereby terminating infection. (pqac-00000001, pqac-00000004, pqac-00000006) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14 |
| Mechanistic class | T is an atypical **class III holin / bitopic holin** with **one N-terminal transmembrane domain** and a **large periplasmic C-terminal domain**; compared with canonical multi-TM holins, much of its regulatory behavior is mediated by this unusually large periplasmic region. (pqac-00000001, pqac-00000003, pqac-00000006) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Young, 2002; Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14 |
| Localization/topology | Experimental work supports **N-in/C-out topology**: the **N terminus is cytoplasmic**, residues around **35-55 form the single TMD**, and the bulk **C-terminal ~163 aa region is periplasmic**. T therefore acts at the **host inner membrane** while sensing/regulation occurs largely through its periplasmic domain. (pqac-00000001, pqac-00000003, pqac-00000004, pqac-00000006) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Young, 2002; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14 |
| Key domains | Functionally important regions include the **N-terminal cytoplasmic segment**, the **single TMD**, and the **C-terminal periplasmic domain (TCTD)**. Genetic dissection recovered lysis-defective mutations in all three regions, indicating that membrane triggering and regulation both require distributed determinants. (pqac-00000005, pqac-00000006) | Young, 2002; Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14 |
| Role of the periplasmic domain | The large periplasmic domain acts as a major **regulatory/timing domain** and as the **binding site for antiholin-mediated lysis inhibition**. Deleting or supplying this domain in trans strongly alters lysis timing/LIN behavior, while many “clock” mutations map there. (pqac-00000000, pqac-00000005, pqac-00000007) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Young, 2002 |
| LIN regulation (T4-specific) | During **lysis inhibition (LIN)**, secondary infection delays lysis for extended periods by preventing T from triggering. The central T4-specific model is that **RI antiholin**, activated/stabilized during superinfection, binds the **periplasmic domain of T** and blocks hole formation; membrane depolarization can subvert this block. (pqac-00000000, pqac-00000001, pqac-00000004, pqac-00000005) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Young, 2002 |
| RI antiholin interaction | The **periplasmic domains of T and RI** are necessary/sufficient for authentic LIN signaling. RI-T complex formation was supported by **co-immunoprecipitation/crosslinking**, and TCTD fragments expressed in trans can **abolish LIN**, consistent with competitive sequestration of RI. (pqac-00000000, pqac-00000004, pqac-00000007) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x |
| RIII contribution | **RIII** is a second antiholin regulator in T4 lysis inhibition, acting from the **cytoplasmic side**; available evidence indicates it contributes to **LIN stability** but does **not replace RI**, whose periplasmic interaction with T is the core inhibitory mechanism. (pqac-00000003, pqac-00000005) | Young, 2002; Chen & Young, 2016 background summarized in evidence stream (not used for table claims beyond contextual mention in gathered evidence) |
| Structural model (2020) | Structural analysis supports a model in which **RI and T form a heterotetrameric complex** early in infection; membrane depolarization and/or signal-dependent rearrangements disrupt the complex, freeing T to oligomerize. The soluble C-terminal domain of T shows a **GAF-like fold**, offering a structural framework for how LIN-regulated conformational changes may control triggering. (pqac-00000002, pqac-00000013) | Krieger et al., 2020, *J Mol Biol*, https://doi.org/10.1016/j.jmb.2020.06.013 |
| Oligomerization and triggering | T accumulates in the membrane and transitions from monomer/dimer states to **higher-order SDS-resistant oligomers** during triggering; this supports the view that T hole formation is an oligomerization-dependent switch rather than gradual leakage. (pqac-00000004, pqac-00000005) | Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Young, 2002 |
| Key interacting partners | The most direct T4-relevant partners are **endolysin E** (downstream effector released after T triggering), **RI antiholin** (periplasmic inhibitor binding TCTD), and **RIII** (cytoplasmic LIN stabilizer/modulator). (pqac-00000001, pqac-00000004, pqac-00000005) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Young, 2002 |
| Key phenotypes of mutation | **t null mutants** show an **absolute lysis defect** with intracellular accumulation of virions and lysozyme; **rV/t missense mutants** are often **LIN defective** and form classic **rapid-lysis (“r”) plaques**. Some mutations allow lysis but impair LIN specifically, showing that lethal triggering and LIN sensitivity are genetically separable. (pqac-00000003, pqac-00000005, pqac-00000006) | Young, 2002; Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14 |
| Quantitative timings/data (T4-specific) | In ordinary T4 infection, lysis is reported at about **~25 min**; under LIN, the vegetative cycle can be extended **for several hours**. In heterologous λ-based tests, **energy poisons** can trigger **T-mediated lysis at ~20 min**, compared with **~50 min** for λ S in the cited comparison. (pqac-00000003, pqac-00000004) | Young, 2002; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x |
| Quantitative structural/biophysical details | Holin lesions in general are described as **micrometer-scale holes** for canonical holins, often **1-3 per cell** in review-level discussion; more recent general holin modeling cites membrane lesions around **~300-350 nm** diameter, but this value is **general holin context, not T4-specific**. (pqac-00000006, pqac-00000009) | Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14; Mondal et al., 2024, https://doi.org/10.1021/acs.jpcb.4c03040 |
| Evidence type | Functional annotation for T4 gene t is supported by **genetics** (t/rV allelism; mutant phenotypes), **heterologous complementation**, **fusion/deletion topology studies**, **crosslinking/co-IP**, and **structural biology** of the RI-T soluble complex. (pqac-00000000, pqac-00000004, pqac-00000006, pqac-00000002) | Tran et al., 2005, https://doi.org/10.1128/jb.187.19.6631-6640.2005; Ramanculov & Young, 2001, https://doi.org/10.1046/j.1365-2958.2001.02491.x; Moussa et al., 2014, https://doi.org/10.1128/jb.01548-14; Krieger et al., 2020, https://doi.org/10.1016/j.jmb.2020.06.013 |
| 2023-2024 contextual developments (general, not T4-specific unless stated) | Recent work broadens holin biology beyond the classic holin-endolysin pair: 2024 studies describe **extra lysis factors acting at the holin level** (e.g., LambdaSo **Lcc6**), **holin-modulating proteins** that fine-tune lysis timing, and **predictive physicochemical models** linking holin hydrophobicity to lysis time; 2024 theoretical work on **T-even LIN** argues LIN can confer competitive advantage by increasing yield under high secondary adsorption. These developments are relevant context for interpreting T4 T as a regulated lysis-timing module. (pqac-00000008, pqac-00000009, pqac-00000011, pqac-00000012) | Kim et al., 2024, *BMC Microbiol*, https://doi.org/10.1186/s12866-024-03684-9; Mondal et al., 2024, *J Phys Chem B*, https://doi.org/10.1021/acs.jpcb.4c03040; Hvid & Mitarai, 2024, https://doi.org/10.1101/2024.02.07.579269; Thöneböhn et al., 2024, *J Bacteriol*, https://doi.org/10.1128/jb.00022-24 |


*Table: This table summarizes the experimentally supported functional annotation of Enterobacteria phage T4 gene t/rV (holin T, UniProt P06808), separating T4-specific findings from broader recent holin literature. It highlights function, topology, regulation by lysis inhibition, key interaction partners, phenotypes, and quantitative details with citation-linked sources.*