Multiple functional roles of the accessory I-domain of bacteriophage P22 coat protein revealed by NMR structure and CryoEM modeling

Structure. 2014 Jun 10;22(6):830-41. doi: 10.1016/j.str.2014.04.003. Epub 2014 May 15.


Some capsid proteins built on the ubiquitous HK97-fold have accessory domains imparting specific functions. Bacteriophage P22 coat protein has a unique insertion domain (I-domain). Two prior I-domain models from subnanometer cryoelectron microscopy (cryoEM) reconstructions differed substantially. Therefore, the I-domain's nuclear magnetic resonance structure was determined and also used to improve cryoEM models of coat protein. The I-domain has an antiparallel six-stranded β-barrel fold, not previously observed in HK97-fold accessory domains. The D-loop, which is dynamic in the isolated I-domain and intact monomeric coat protein, forms stabilizing salt bridges between adjacent capsomers in procapsids. The S-loop is important for capsid size determination, likely through intrasubunit interactions. Ten of 18 coat protein temperature-sensitive-folding substitutions are in the I-domain, indicating its importance in folding and stability. Several are found on a positively charged face of the β-barrel that anchors the I-domain to a negatively charged surface of the coat protein HK97-core.

Publication types

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

MeSH terms

  • Bacteriophage P22 / chemistry*
  • Bacteriophage P22 / ultrastructure*
  • Capsid Proteins / chemistry*
  • Capsid Proteins / ultrastructure*
  • Cryoelectron Microscopy
  • Models, Molecular
  • Molecular Dynamics Simulation
  • Nuclear Magnetic Resonance, Biomolecular
  • Protein Structure, Quaternary
  • Protein Structure, Tertiary
  • Protein Subunits
  • Static Electricity


  • Capsid Proteins
  • Protein Subunits