Holin T (gpT, also known as protein rV) is a 218-amino-acid single-pass type II membrane protein of bacteriophage T4 that functions as the canonical class III holin and master timer of host cell lysis. During late infection, holin T accumulates in the Escherichia coli inner (plasma) membrane with its N-terminus in the cytoplasm and a large globular C-terminal domain in the periplasm. At a genetically programmed time, it triggers to form micron-scale pores in the host inner membrane, collapsing the membrane potential and releasing the cytoplasmic endolysin (gpE) into the periplasm to degrade the peptidoglycan layer. Its periplasmic domain also serves as the direct binding partner for the periplasmic antiholin RI, which blocks holin oligomerization to establish lysis inhibition (LIN) upon superinfection.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016020 membrane | IEA GO_REF:0000104 | MODIFY | Summary: Holin T is an integral membrane protein, but localization specifically to the host bacterial inner (plasma) membrane is well established. Reason: The generic term membrane (GO:0016020) is too broad when holin T is specifically localized to the Escherichia coli inner membrane, represented in GO by host cell plasma membrane (GO:0020002). Proposed replacements: host cell plasma membrane Supporting Evidence: file:BPT4/t/t-uniprot.txt SUBCELLULAR LOCATION: Host cell inner membrane {ECO:0000255|HAMAP-Rule:MF_04104, ECO:0000305|PubMed:16166524}; Single-pass type II membrane protein file:BPT4/t/t-deep-research-falcon.md Gene **t** (synonym **rV**) of *Enterobacteria phage T4* encodes **holin T**, a 218-aa bitopic (single-pass) inner-membrane protein that acts as the **lysis timer/trigger** for phage release. |
| GO:0020002 host cell plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Holin T integrates into the Escherichia coli inner (cytoplasmic) membrane as a single-pass type II transmembrane protein. Reason: In Gram-negative bacteria, the inner (cytoplasmic) membrane corresponds to host cell plasma membrane (GO:0020002), which is the site where holin T accumulates and forms lytic pores. Supporting Evidence: file:BPT4/t/t-uniprot.txt SUBCELLULAR LOCATION: Host cell inner membrane {ECO:0000255|HAMAP-Rule:MF_04104, ECO:0000305|PubMed:16166524}; Single-pass type II membrane protein file:BPT4/t/t-deep-research-falcon.md Gene **t** (synonym **rV**) of *Enterobacteria phage T4* encodes **holin T**, a 218-aa bitopic (single-pass) inner-membrane protein that acts as the **lysis timer/trigger** for phage release. |
| GO:0031640 killing of cells of another organism | IEA GO_REF:0000043 | MODIFY | Summary: Holin T mediates host bacterial cell death specifically as part of the phage lysis pathway that releases progeny virions at the end of the infective cycle. Reason: Killing of cells of another organism (GO:0031640) is a broad parent term mapped from UniProt keywords; viral release from host cell by cytolysis (GO:0044659) is a more specific descendant term that accurately captures holin-mediated phage release. Proposed replacements: viral release from host cell by cytolysis Supporting Evidence: file:BPT4/t/t-uniprot.txt Accumulates harmlessly in the cytoplasmic membrane until it reaches a critical concentration that triggers the formation of micron-scale pores (holes) causing host cell membrane disruption and endolysin escape into the periplasmic space (PubMed:11255004). file:BPT4/t/t-deep-research-falcon.md Gene **t** (synonym **rV**) of *Enterobacteria phage T4* encodes **holin T**, a 218-aa bitopic (single-pass) inner-membrane protein that acts as the **lysis timer/trigger** for phage release. |
| GO:0044659 viral release from host cell by cytolysis | IEA GO_REF:0000002 | ACCEPT | Summary: Holin T is essential for scheduled host cell lysis and release of progeny T4 virions by permeabilizing the inner membrane and allowing endolysin access to the peptidoglycan layer. Reason: This term directly and specifically describes the core biological process of the T4 holin in cytolytic virion egress. Supporting Evidence: file:BPT4/t/t-uniprot.txt Accumulates harmlessly in the cytoplasmic membrane until it reaches a critical concentration that triggers the formation of micron-scale pores (holes) causing host cell membrane disruption and endolysin escape into the periplasmic space (PubMed:11255004). file:BPT4/t/t-deep-research-falcon.md Gene **t** (synonym **rV**) of *Enterobacteria phage T4* encodes **holin T**, a 218-aa bitopic (single-pass) inner-membrane protein that acts as the **lysis timer/trigger** for phage release. |
| GO:0140911 pore-forming activity | IEA GO_REF:0000104 | ACCEPT | Summary: Upon reaching a critical concentration in the host inner membrane, holin T oligomerizes to form micron-scale pores that permeabilize the membrane and release cytoplasmic endolysin into the periplasm. Reason: Pore-forming activity (GO:0140911) accurately represents the core molecular function of holin T in forming inner-membrane holes. Supporting Evidence: file:BPT4/t/t-uniprot.txt Accumulates harmlessly in the cytoplasmic membrane until it reaches a critical concentration that triggers the formation of micron-scale pores (holes) causing host cell membrane disruption and endolysin escape into the periplasmic space (PubMed:11255004). file:BPT4/t/t-deep-research-falcon.md Gene **t** (synonym **rV**) of *Enterobacteria phage T4* encodes **holin T**, a 218-aa bitopic (single-pass) inner-membrane protein that acts as the **lysis timer/trigger** for phage release. |
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Download this section (compressed HTML)Q: How does the transmembrane helix of holin T rearrange within the inner membrane once the periplasmic domain is released from antiholin RI inhibition or reaches its critical triggering threshold?
Q: Does the cytoplasmic N-terminal domain of holin T interact directly with the cytoplasmic antiholin RIII simultaneously with periplasmic RI binding to the C-terminal domain in vivo?
Experiment: Perform cryo-electron tomography and subtomogram averaging of triggered holin T micron-scale lesions in Escherichia coli inner membrane vesicles to resolve the architecture of the transmembrane pore wall.
Hypothesis: Holin T oligomerizes into higher-order two-dimensional rafts that nucleate a lipid-protein lined mega-pore upon collapse of the proton motive force.
Experiment: Use single-molecule fluorescence bleaching and FRET in live infected E. coli cells co-expressing fluorescently tagged holin T, periplasmic RI, and cytoplasmic RIII to quantify complex stoichiometry before and after secondary phage adsorption.
Hypothesis: Superinfection stabilizes a ternary RI-T-RIII complex across the inner membrane that locks holin T in a non-oligomerizing state.
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