Enterobacteria phage M13 — Gene VIII (CAPSD / G8P / pVIII)

Overview

UniProt Accession: P69541 (CAPSD_BPM13)
Gene Symbol: VIII
Protein Product: Major coat protein G8P (pVIII, Coat protein B), synthesized as a 73 aa precursor ("M13 procoat") with a 23 aa N-terminal signal peptide cleaved by host leader peptidase LepB to yield the 50 aa mature coat protein (residues 24–73) (file:BPM13/VIII/VIII-uniprot.txt).

YidC-Dependent Inner-Membrane Insertion: Newly synthesized M13 procoat inserts into the Escherichia coli inner membrane (GO:0020002 host cell plasma membrane) as a hairpin loop via a SecYEG-independent, YidC membrane insertase-dependent pathway (file:BPM13/VIII/VIII-uniprot.txt). Following signal peptide cleavage, mature pVIII spans the inner membrane as a single α-helix with an acidic periplasmic N-terminus (residues 24–43), a hydrophobic transmembrane segment (residues 44–62), and a basic cytoplasmic C-terminus (residues 63–73) (file:BPM13/VIII/VIII-uniprot.txt).
Helical Capsid Tube Assembly: During ATP-driven viral extrusion through the pI/pXI inner-membrane assembly machine, ~2,700–2,750 copies of mature pVIII laterally enter the assembly complex and replace pV dimers along the extruding circular (+)ssDNA genome, with basic C-terminal lysine residues neutralizing the DNA phosphate backbone (file:BPM13/VIII/VIII-uniprot.txt, PMID:35746635). The gently curved α-helical pVIII subunits pack in a C5-symmetric, right-handed shingle-like helical array (Class I inovirus symmetry: ~16 Å axial rise and ~36° twist per pentameric layer, PDB: 2MJZ, 8IXJ, 8JWT) forming a flexible filament ~6 nm in diameter and ~900 nm in length capped by pVII/pIX at the distal end and pIII/pVI at the proximal end (file:BPM13/VIII/VIII-uniprot.txt).

Molecular Function: GO:0039660 (structural constituent of virion).
Cellular Component: GO:0019029 (helical viral capsid) and GO:0020002 (host cell plasma membrane).
Biological Process: GO:0019068 (virion assembly) and GO:0099045 (viral extrusion).