Structure of p22 headful packaging nuclease

J Biol Chem. 2012 Aug 10;287(33):28196-205. doi: 10.1074/jbc.M112.349894. Epub 2012 Jun 19.


Packaging of viral genomes into preformed procapsids requires the controlled and synchronized activity of an ATPase and a genome-processing nuclease, both located in the large terminase (L-terminase) subunit. In this paper, we have characterized the structure and regulation of bacteriophage P22 L-terminase (gp2). Limited proteolysis reveals a bipartite organization consisting of an N-terminal ATPase core flexibly connected to a C-terminal nuclease domain. The 2.02 Å crystal structure of P22 headful nuclease obtained by in-drop proteolysis of full-length L-terminase (FL-L-terminase) reveals a central seven-stranded β-sheet core that harbors two magnesium ions. Modeling studies with DNA suggest that the two ions are poised for two-metal ion-dependent catalysis, but the nuclease DNA binding surface is sterically hindered by a loop-helix (L(1)-α(2)) motif, which is incompatible with catalysis. Accordingly, the isolated nuclease is completely inactive in vitro, whereas it exhibits endonucleolytic activity in the context of FL-L-terminase. Deleting the autoinhibitory L(1)-α(2) motif (or just the loop L(1)) restores nuclease activity to a level comparable with FL-L-terminase. Together, these results suggest that the activity of P22 headful nuclease is regulated by intramolecular cross-talk with the N-terminal ATPase domain. This cross-talk allows for precise and controlled cleavage of DNA that is essential for genome packaging.

Publication types

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

MeSH terms

  • Adenosine Triphosphatases / chemistry*
  • Adenosine Triphosphatases / metabolism
  • Amino Acid Motifs
  • Bacteriophage P22 / enzymology*
  • Crystallography, X-Ray
  • Deoxyribonucleases / chemistry*
  • Deoxyribonucleases / metabolism
  • Genome, Viral / physiology
  • Protein Structure, Tertiary
  • Viral Proteins / chemistry*
  • Viral Proteins / metabolism
  • Virus Assembly / physiology


  • Viral Proteins
  • Deoxyribonucleases
  • Adenosine Triphosphatases

Associated data

  • PDB/4DKW