Major coat protein G8P (pVIII), which polymerizes in ~2,700-2,750 copies to form the ~6 nm-wide right-handed helical capsid tube encapsulating the circular single-stranded DNA genome of bacteriophage M13. Synthesized as a 73-residue precursor (M13 procoat) with a 23-residue N-terminal signal peptide, procoat inserts into the Escherichia coli inner membrane as a hairpin loop via the host YidC membrane insertase and is cleaved by signal peptidase I (LepB) to yield the 50-residue mature single-pass transmembrane protein (residues 24-73). During ATP-driven viral extrusion through the inner-membrane pI/pXI assembly machine, mature pVIII subunits laterally enter the assembly complex and displace pV dimers along the extruding (+)ssDNA, with basic C-terminal lysine residues neutralizing the viral DNA phosphate backbone while gently curved alpha-helices pack in a C5-symmetric shingle-like array (~16 Angstrom axial rise and ~36 degree twist per pentameric layer).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019028 viral capsid | IEA GO_REF:0000043 | MODIFY | Summary: General capsid term superseded by the specific child term helical viral capsid (GO:0019029) already annotated to this gene. Reason: Mature pVIII self-assembles specifically into a filamentous helical viral capsid (GO:0019029) with C5 pentameric layer symmetry around the viral ssDNA core. Proposed replacements: helical viral capsid Supporting Evidence: file:BPM13/VIII/VIII-uniprot.txt Self assembles to form a helical capsid wrapping up the viral genomic DNA. The capsid is made of gently-curved alpha-helix rods and displays a filamentous structure with a length of 760-1950 nm and a width of 6 nm (By similarity). |
| GO:0019029 helical viral capsid | IEA GO_REF:0000043 | ACCEPT | Summary: Core cellular component of pVIII, which forms the ~2,700-subunit helical capsid tube of the mature M13 virion. Reason: Solid-state NMR (PDB: 2MJZ) and cryo-EM (PDB: 8IXJ, 8JWT) structures establish that ~2,700 copies of pVIII form a right-handed Class I filamentous helical viral capsid with a ~16 Angstrom rise and ~36 degree twist. Supporting Evidence: file:BPM13/VIII/VIII-uniprot.txt Self assembles to form a helical capsid wrapping up the viral genomic DNA. The capsid is made of gently-curved alpha-helix rods and displays a filamentous structure with a length of 760-1950 nm and a width of 6 nm (By similarity). |
| GO:0033644 host cell membrane | IEA GO_REF:0000044 | MODIFY | Summary: Broad host membrane localization that should be refined to host cell plasma membrane (GO:0020002), where mature pVIII is stored as a single-pass transmembrane protein prior to virion assembly. Reason: Following YidC-mediated insertion and signal peptide cleavage, mature pVIII accumulates specifically in the Escherichia coli inner (plasma) membrane before lateral incorporation into the assembling virion. Proposed replacements: host cell plasma membrane Supporting Evidence: file:BPM13/VIII/VIII-uniprot.txt Prior to assembly, the major capsid protein is found associated with the bacterial host inner membrane. The capsid is composed of approximately 2700 copies of the capsid protein G8P (Probable). |
| GO:0033644 host cell membrane | ISS GO_REF:0000024 | MODIFY | Summary: Broad host membrane localization transferred from f1/fd ortholog P69539; refine to host cell plasma membrane (GO:0020002). Reason: Prior to virion assembly, mature pVIII is a single-pass type I membrane protein spanning the bacterial host inner (plasma) membrane. Proposed replacements: host cell plasma membrane Supporting Evidence: file:BPM13/VIII/VIII-uniprot.txt Prior to assembly, the major capsid protein is found associated with the bacterial host inner membrane. The capsid is composed of approximately 2700 copies of the capsid protein G8P (Probable). |
| GO:0039660 structural constituent of virion | ISS file:BPM13/VIII/VIII-uniprot.txt | NEW | Summary: Major coat protein pVIII self-assembles in ~2,700 copies to form the filamentous helical capsid tube wrapping the M13 ssDNA genome. Reason: Mature pVIII is the principal structural protein of the M13 virion (~2,700 copies per particle), forming the C5-symmetric helical protein coat around the viral genomic DNA. Supporting Evidence: file:BPM13/VIII/VIII-uniprot.txt Self assembles to form a helical capsid wrapping up the viral genomic DNA. The capsid is made of gently-curved alpha-helix rods and displays a filamentous structure with a length of 760-1950 nm and a width of 6 nm (By similarity). |
| GO:0099045 viral extrusion | ISS file:BPM13/VIII/VIII-uniprot.txt | NEW | Summary: Inner-membrane-embedded mature pVIII subunits laterally incorporate into the growing filamentous capsid to coat the viral (+)ssDNA during extrusion from the bacterial host. Reason: Prior to assembly, mature pVIII accumulates in the host inner membrane and is directly transferred onto the extruding viral ssDNA genome during concomitant phage assembly and extrusion. Supporting Evidence: file:BPM13/VIII/VIII-uniprot.txt Prior to assembly, the major capsid protein is found associated with the bacterial host inner membrane. The capsid is composed of approximately 2700 copies of the capsid protein G8P (Probable). |
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Download this section (compressed HTML)Q: How does the L-shaped membrane-bound conformation of mature pVIII in the Escherichia coli inner membrane transition into the continuous, gently curved alpha-helix observed in the assembled virion capsid?
Q: How do the basic C-terminal lysine residues of pVIII accommodate variable nucleotide sequences and non-integer DNA-to-coat stoichiometries along the filamentous capsid lumen?
Experiment: Capture high-resolution cryo-EM structures of the pI/pXI inner-membrane assembly ring actively incorporating membrane-embedded pVIII subunits onto extruding ssDNA.
Experiment: Perform solid-state NMR and molecular dynamics simulations of M13 virions packaging synthetic ssDNA genomes of defined purine/pyrimidine composition to probe C-terminal lysine-phosphate interactions.
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