Portal protein functions akin to a DNA-sensor that couples genome-packaging to icosahedral capsid maturation

Nat Commun. 2017 Jan 30;8:14310. doi: 10.1038/ncomms14310.


Tailed bacteriophages and herpesviruses assemble infectious particles via an empty precursor capsid (or 'procapsid') built by multiple copies of coat and scaffolding protein and by one dodecameric portal protein. Genome packaging triggers rearrangement of the coat protein and release of scaffolding protein, resulting in dramatic procapsid lattice expansion. Here, we provide structural evidence that the portal protein of the bacteriophage P22 exists in two distinct dodecameric conformations: an asymmetric assembly in the procapsid (PC-portal) that is competent for high affinity binding to the large terminase packaging protein, and a symmetric ring in the mature virion (MV-portal) that has negligible affinity for the packaging motor. Modelling studies indicate the structure of PC-portal is incompatible with DNA coaxially spooled around the portal vertex, suggesting that newly packaged DNA triggers the switch from PC- to MV-conformation. Thus, we propose the signal for termination of 'Headful Packaging' is a DNA-dependent symmetrization of portal protein.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Bacteriophage P22 / physiology*
  • Bacteriophage P22 / ultrastructure
  • Capsid / physiology*
  • Capsid / ultrastructure
  • Capsid Proteins / chemistry*
  • Capsid Proteins / physiology
  • Capsid Proteins / ultrastructure
  • Crystallography, X-Ray
  • DNA Packaging / physiology
  • DNA, Viral / physiology*
  • DNA, Viral / ultrastructure
  • Endodeoxyribonucleases / metabolism
  • Genome, Viral / physiology
  • Microscopy, Electron
  • Molecular Docking Simulation
  • Protein Multimerization / physiology
  • Protein Structure, Quaternary / physiology
  • Virus Assembly / physiology*


  • Capsid Proteins
  • DNA, Viral
  • portal protein, bacteriophage P22
  • Endodeoxyribonucleases
  • terminase