Terminal DNA protecting protein gp2 (also known historically as gp64; 274 amino acids) is an internal head protein of bacteriophage T4 that binds noncovalently to the ends of the packaged linear double-stranded DNA genome at the portal junction and accompanies the viral DNA into the Escherichia coli cytoplasm upon infection. Once injected, gp2 caps the free linear dsDNA termini to shield them from exonucleolytic degradation by the host RecBCD (exonuclease V) complex, enabling viral transcription and DNA replication to proceed, and also contributes to head completion and head-to-tail joining during virion morphogenesis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0052031 symbiont-mediated perturbation of host defense response | IEA GO_REF:0000043 | MODIFY | Summary: gp2 protects injected T4 linear dsDNA ends from degradation by the host RecBCD exonuclease complex, which is more specifically captured by GO:0099016 (symbiont-mediated evasion of DNA end degradation by host). Reason: While protecting viral DNA from host exonuclease V is a form of host defense evasion, GO:0052031 is overly broad compared to the descendant term GO:0099016 (symbiont-mediated evasion of DNA end degradation by host) already present on this gene. Proposed replacements: symbiont-mediated evasion of DNA end degradation by host Supporting Evidence: file:BPT4/2/2-uniprot.txt Binds to the viral DNA ends and protects the viral DNA against recBCD mediated degradation. file:BPT4/2/2-deep-research-falcon.md Enterobacteria phage T4 **gene 2** encodes **gp2** (historically also called **gp64**), a small, highly basic virion-associated protein that **protects the linear T4 dsDNA termini from degradation by host exonuclease V (ExoV; RecBCD complex)** immediately upon injection into the host cytoplasm. |
| GO:0052170 symbiont-mediated suppression of host innate immune response | IEA GO_REF:0000043 | MODIFY | Summary: Derived from a broad UniProt keyword mapping; gp2 does not inhibit a host innate immune signaling pathway or enzyme directly, but instead physically caps injected phage DNA termini to prevent host RecBCD exonuclease degradation. Reason: gp2 acts as a steric cap on linear viral DNA ends rather than suppressing host innate immune signaling; the specific biological process is GO:0099016 (symbiont-mediated evasion of DNA end degradation by host). Proposed replacements: symbiont-mediated evasion of DNA end degradation by host Supporting Evidence: file:BPT4/2/2-uniprot.txt Binds to the viral DNA ends and protects the viral DNA against recBCD mediated degradation. |
| GO:0099016 symbiont-mediated evasion of DNA end degradation by host | IEA GO_REF:0000043 | ACCEPT | Summary: Core biological role of gp2, which binds to linear T4 dsDNA ends and protects the injected viral genome from degradation by host RecBCD (exonuclease V). Reason: Genetic and biochemical studies demonstrate that gene 2 mutant virions inject DNA that is rapidly degraded by host RecBCD exonuclease, whereas infectivity of gene 2 mutants is fully restored in recBCD-deficient E. coli hosts. Supporting Evidence: file:BPT4/2/2-uniprot.txt Binds to the viral DNA ends and protects the viral DNA against recBCD mediated degradation. file:BPT4/2/2-deep-research-falcon.md Enterobacteria phage T4 **gene 2** encodes **gp2** (historically also called **gp64**), a small, highly basic virion-associated protein that **protects the linear T4 dsDNA termini from degradation by host exonuclease V (ExoV; RecBCD complex)** immediately upon injection into the host cytoplasm. |
| GO:0003677 DNA binding | NAS file:BPT4/2/2-deep-research-falcon.md | NEW | Summary: gp2 binds directly and noncovalently to the blunt termini of linear T4 double-stranded DNA to cap and protect them from host RecBCD exonuclease. Reason: End-protection activity of gp2 depends on its direct noncovalent binding to viral genomic dsDNA ends inside the capsid and after DNA ejection into the host cell. Supporting Evidence: file:BPT4/2/2-uniprot.txt Binds to the viral DNA ends and protects the viral DNA against recBCD mediated degradation. file:BPT4/2/2-deep-research-falcon.md Enterobacteria phage T4 **gene 2** encodes **gp2** (historically also called **gp64**), a small, highly basic virion-associated protein that **protects the linear T4 dsDNA termini from degradation by host exonuclease V (ExoV; RecBCD complex)** immediately upon injection into the host cytoplasm. |
| GO:0019028 viral capsid | NAS file:BPT4/2/2-deep-research-falcon.md | NEW | Summary: gp2 is packaged inside the phage capsid at the portal/neck junction bound to the leading ends of the genomic dsDNA. Reason: gp2 is an internal head completion protein incorporated into the capsid and ejected with the viral DNA during infection. Supporting Evidence: file:BPT4/2/2-uniprot.txt Binds to the viral DNA ends and protects the viral DNA against recBCD mediated degradation. file:BPT4/2/2-deep-research-falcon.md Enterobacteria phage T4 **gene 2** encodes **gp2** (historically also called **gp64**), a small, highly basic virion-associated protein that **protects the linear T4 dsDNA termini from degradation by host exonuclease V (ExoV; RecBCD complex)** immediately upon injection into the host cytoplasm. |
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Download this section (compressed HTML)Q: Does gp2 remain stably bound to the linear T4 DNA termini inside the E. coli cytoplasm after genome ejection, or does it merely pilot the DNA through the portal/tail channel and dissociate once early recombination-dependent priming begins?
Q: What is the atomic structure and stoichiometry of gp2 bound to the terminal base pairs of glucosylated hydroxymethylcytosine T4 DNA within the portal channel?
Experiment: Determine the high-resolution asymmetric cryo-EM reconstruction of the DNA-filled T4 neck/portal vertex to resolve the position and stoichiometry of gp2 bound to the leading DNA terminus prior to ejection.
Experiment: Perform in vitro single-molecule DNA curtains with fluorescently labeled recombinant gp2 and purified E. coli RecBCD to measure how gp2 end-binding blocks RecBCD helicase/nuclease loading at blunt dsDNA ends.
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