Structure of the small outer capsid protein, Soc: a clamp for stabilizing capsids of T4-like phages

J Mol Biol. 2010 Jan 29;395(4):728-41. doi: 10.1016/j.jmb.2009.10.007. Epub 2009 Oct 14.


Many viruses need to stabilize their capsid structure against DNA pressure and for survival in hostile environments. The 9-kDa outer capsid protein (Soc) of bacteriophage T4, which stabilizes the virus, attaches to the capsid during the final stage of maturation. There are 870 Soc molecules that act as a "glue" between neighboring hexameric capsomers, forming a "cage" that stabilizes the T4 capsid against extremes of pH and temperature. Here we report a 1.9 A resolution crystal structure of Soc from the bacteriophage RB69, a close relative of T4. The RB69 crystal structure and a homology model of T4 Soc were fitted into the cryoelectron microscopy reconstruction of the T4 capsid. This established the region of Soc that interacts with the major capsid protein and suggested a mechanism, verified by extensive mutational and biochemical studies, for stabilization of the capsid in which the Soc trimers act as clamps between neighboring capsomers. The results demonstrate the factors involved in stabilizing not only the capsids of T4-like bacteriophages but also many other virus capsids.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Bacteriophage T4 / chemistry*
  • Bacteriophage T4 / genetics
  • Bacteriophage T4 / physiology
  • Bacteriophage T4 / ultrastructure
  • Binding Sites / genetics
  • Capsid Proteins / chemistry*
  • Capsid Proteins / genetics
  • Capsid Proteins / ultrastructure
  • Cryoelectron Microscopy
  • Crystallography, X-Ray
  • Evolution, Molecular
  • Models, Molecular
  • Molecular Sequence Data
  • Mutation
  • Protein Multimerization
  • Protein Stability
  • Protein Structure, Quaternary
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Sequence Homology, Amino Acid
  • Virus Assembly


  • Capsid Proteins
  • Recombinant Proteins
  • small outer capsid protein, bacteriophage T4

Associated data

  • PDB/3IG9
  • PDB/3IGE