Portal protein gp20 (524 amino acids) forms a homododecameric (12-subunit) ring complex that occupies the unique fivefold vertex of the bacteriophage T4 prolate icosahedral capsid. During early head morphogenesis, gp20 docks at the cytoplasmic face of the Escherichia coli inner membrane (via host YidC and phage helper gp40) to nucleate assembly of the prohead scaffolding core and major capsid shell; subsequently, its exposed clip domain docks the pentameric gp17 large terminase ATPase motor to translocate genomic dsDNA into the capsid through its central channel, and after motor release it binds the neck proteins (gp13/gp14) to form the conduit for genome ejection during infection.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019028 viral capsid | IEA GO_REF:0000120 | MODIFY | Summary: gp20 is located at the special fivefold vertex of the capsid, where it specifically forms the dodecameric viral portal complex (GO:0046798). Reason: GO:0019028 (viral capsid) is true but less specific than GO:0046798 (viral portal complex), which is already experimentally annotated to this gene. Proposed replacements: viral portal complex Supporting Evidence: PMID:26144253 The gp20 structure also verifies that the portal assembly is required for initiating head assembly, for attachment of the packaging motor, and for participation in DNA packaging. file:BPT4/20/20-uniprot.txt Forms the portal vertex of the capsid. |
| GO:0019072 viral genome packaging | IEA GO_REF:0000104 | ACCEPT | Summary: The dodecameric gp20 portal complex docks the pentameric gp17 terminase ATPase via its clip domain, forms the central translocation channel for dsDNA entry, and prevents DNA slippage via its internal tunnel loops. Reason: Cryo-EM structural and mutational analyses confirm that gp20 directly binds the gp17 packaging motor and actively participates in translocating and retaining full-length genomic dsDNA inside the capsid. Supporting Evidence: PMID:26144253 The gp20 structure also verifies that the portal assembly is required for initiating head assembly, for attachment of the packaging motor, and for participation in DNA packaging. file:BPT4/20/20-deep-research-falcon.md The target protein is **gene product 20 (gp20)** of **Enterobacteria phage T4**, which is experimentally characterized as the **portal protein** forming the unique portal vertex of the T4 capsid (head). |
| GO:0019076 viral release from host cell | IEA GO_REF:0000104 | REMOVE | Summary: Over-annotation derived from a broad UniProt keyword; gp20 is the capsid portal vertex protein involved in head assembly, DNA packaging, and genome ejection rather than host cell lysis or viral release. Reason: gp20 is a structural portal protein of the capsid vertex and has no role in holin/endolysin/spanin-mediated host cell lysis or virion release from the bacterial cell. Supporting Evidence: file:BPT4/20/20-uniprot.txt Forms the portal vertex of the capsid. |
| GO:0020002 host cell plasma membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Nascent gp20 associates with the cytoplasmic face of the E. coli inner (plasma) membrane at YidC (together with phage gp40) to initiate prohead core and shell assembly before detaching into the cytoplasm. Reason: Host inner membrane localization is an authentic, transient morphogenetic stage during prohead nucleation prior to detachment of the maturing prohead for DNA packaging. Supporting Evidence: PMID:22855489 With a cross-linking approach, we show here that gp20 and gp20s are escorted by the chaperones DnaK, trigger factor, and GroEL and dock on the membrane at the membrane protein YidC. |
| GO:0020002 host cell plasma membrane | IDA PMID:22855489 Membrane interaction of the portal protein gp20 of bacteriop... | KEEP AS NON CORE | Summary: Cross-linking and membrane-binding assays in PMID:22855489 show that gp20 is escorted by DnaK, trigger factor, and GroEL to dock on the E. coli inner membrane at YidC during prohead initiation. Reason: Directly demonstrated transient localization at the host inner membrane where gp20 nucleates prohead assembly before the empty head detaches for packaging. Supporting Evidence: PMID:22855489 With a cross-linking approach, we show here that gp20 and gp20s are escorted by the chaperones DnaK, trigger factor, and GroEL and dock on the membrane at the membrane protein YidC. |
| GO:0046718 symbiont entry into host cell | IEA GO_REF:0000043 | MODIFY | Summary: Broad keyword-based annotation; during infection, the gp20 portal channel serves as the exit conduit for viral DNA during contractile-tail-mediated genome ejection (GO:0099000). Reason: GO:0046718 is overly general compared to GO:0099000 (symbiont genome ejection through host cell envelope, contractile tail mechanism), which is already present on this gene. Proposed replacements: symbiont genome ejection through host cell envelope, contractile tail mechanism Supporting Evidence: file:BPT4/20/20-uniprot.txt Forms the portal vertex of the capsid. |
| GO:0099000 symbiont genome ejection through host cell envelope, contractile tail mechanism | IEA GO_REF:0000120 | ACCEPT | Summary: The central channel of the dodecameric gp20 portal complex forms the head exit conduit through which packaged dsDNA is ejected into the tail tube and host cell upon tail sheath contraction. Reason: The gp20 portal complex connects the capsid interior to the neck and tail tube, forming the continuous channel for dsDNA exit during infection. Supporting Evidence: PMID:26144253 Tailed bacteriophages and herpesviruses translocate their genome into and out of their capsid (or ‘head') through a 12-fold symmetric portal protein assembly (or simply ‘portal'), located at a special pentameric vertex of an icosahedral or prolate-icosahedral head |
| GO:0046798 viral portal complex | IDA PMID:26144253 Cryo-EM structure of the bacteriophage T4 portal protein ass... | ACCEPT | Summary: 3.6 A cryo-EM structure in PMID:26144253 demonstrates that 12 copies of gp20 form the viral portal complex at the special fivefold vertex of the T4 capsid. Reason: Near-atomic cryo-EM reconstruction directly establishes that gp20 assembles into the homododecameric viral portal complex. Supporting Evidence: PMID:26144253 The T4 portal protein assembly is dodecameric and has a length of ∼120 Å (Fig. 1). file:BPT4/20/20-deep-research-falcon.md The target protein is **gene product 20 (gp20)** of **Enterobacteria phage T4**, which is experimentally characterized as the **portal protein** forming the unique portal vertex of the T4 capsid (head). |
| GO:0039660 structural constituent of virion | NAS file:BPT4/20/20-deep-research-falcon.md | NEW | Summary: gp20 is an essential structural polypeptide forming the dodecameric portal vertex of the T4 capsid. Reason: Twelve copies of gp20 form a permanent structural ring at the neck-connected vertex of the mature virion capsid. Supporting Evidence: file:BPT4/20/20-uniprot.txt Forms the portal vertex of the capsid. file:BPT4/20/20-deep-research-falcon.md The target protein is **gene product 20 (gp20)** of **Enterobacteria phage T4**, which is experimentally characterized as the **portal protein** forming the unique portal vertex of the T4 capsid (head). |
| GO:0019069 viral capsid assembly | NAS PMID:26144253 Cryo-EM structure of the bacteriophage T4 portal protein ass... | NEW | Summary: gp20 is required to initiate T4 prohead assembly at the host inner membrane, nucleating both the scaffolding core and the major capsid protein (gp23) lattice. Reason: Genetic and cryo-EM studies show that the membrane-docked gp20 portal vertex initiates copolymerization of the prohead core and gp23 shell. Supporting Evidence: PMID:26144253 The gp20 structure also verifies that the portal assembly is required for initiating head assembly, for attachment of the packaging motor, and for participation in DNA packaging. |
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Download this section (compressed HTML)Q: How do the N-terminal 73 residues of gp20, which are disordered in the mature cryo-EM structure, interact with phage helper gp40 and host insertase YidC at the inner membrane to nucleate the prohead scaffolding core?
Q: How is the conformational transition of the gp20 tunnel loop (residues 374-398) coupled to gp17 ATPase termination and subsequent gp13 neck binding to lock the packaged genome inside the head?
Experiment: Determine the cryo-EM structure of the membrane-bound gp20-gp40-YidC initiation complex in lipid nanodiscs to resolve the N-terminal membrane anchor of gp20 and the mechanism of prohead nucleation.
Experiment: Perform single-molecule fluorescence resonance energy transfer (smFRET) on fluorophore-labeled gp20 tunnel-loop variants during active gp17-driven DNA packaging to track real-time valve gating as internal capsid pressure builds.
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