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. Author manuscript; available in PMC: 2012 Oct 18.
Published in final edited form as: Adv Exp Med Biol. 2012;726:423–439. doi: 10.1007/978-1-4614-0980-9_19

Table 1.

Points of resemblance between phage capsids and herpesvirus capsids

  • Initial formation of a procapsid into which DNA is subsequently packaged

  • Packaging is accompanied by major structural changes in the procapsid (maturation)

  • Procapsid has an internal scaffolding protein that is expelled during maturation

  • Maturation is initiated by proteolytic processing of capsid protein and/or scaffolding protein (a)

  • In maturation, capsid protein subunits undergo large (~ 40°) rotations and other changes, re-assigning inter-subunit and inter-capsomer interactions

  • Procapsid is rounder; mature capsid is more polyhedral (flatter facets)

  • Procapsid has asymmetric hexamers that become symmetric on maturation

  • Capsid protein fold based on HK97 prototype, i.e. conserved core domain of ~ 300aa plus additional domains on a case-by-case basis (Fig. 3)

  • Capsid geometry is based on hexamer/pentamer packing on icosahedral or quasi-icosahedral (b) foldings of a hexagonal surface lattice with 14 nm periodicity (mature capsid)

  • One capsid vertex is occupied by a 12-fold ring - the portal protein, conduit for DNA

  • Portal proteins may also conform to a conserved core structure plus additional domains on a case-by-case basis (c)

  • DNA is packaged into procapsid from a replicating concatemer

  • DNA is packaged by viral terminase, an ATPase/translocase/endonuclease

  • DNA is densely bundled with ~ 25Å inter-duplex spacing; outer layers coaxially coiled (d); no histone-like proteins

(a)

Bioinformatics suggest that herpesvirus proteases (4 known structures) and some phage proteases have similar folds. Some phages (e.g. T7, P22) have no protease.

(b)

Term refers to non-spherical capsids, such as that of T4 which represents an icosahedron extended along a symmetry axis by insertion of additional rings of hexamers.