Assembly of an icosahedral virus must proceed with exquisite fidelity, and is a paradigm for the self-organization of multiple subunits required to form complex macromolecular structures. We investigate virus assembly using the bacteriophage P22. In phage P22, herpesvirus and many other dsDNA viruses, the initial product of the assembly reaction is a precursor capsid, known as the procapsid 1. Scaffolding protein directs proper assembly of coat protein, the major capsid protein, to form the procapsid 1. The procapsid undergoes expansion during DNA packaging to become the mature virion 2; 3; 4. P22 assembly is an excellent model system because these complex in vivo processes can be mimicked in vitro.

P22 procapsids can be induced to undergo the structural transformation that accompanies capsid maturation by heat or chemical treatment. We examined bacteriophage P22 morphogenesis by comparing three-dimensional structures of capsids expanded both in vitro by heat treatment and in vivo by DNA packaging 8. The structures were determined by electron cryo-microscopy to 24 Å.

Figure 1. The assembly pathway of phage P22 showing the proteins involved and the nucleation, elongation and maturation reactions.

The heat-expanded capsid has a structure that is virtually the same as the in vivo expanded capsid except that the pentons, normally present at the icosahedral five-fold positions, are released (Figure 2). The similarities of these two capsid structures suggest that the mechanism of heat expansion is similar to in vivo expansion; however, the loss of the pentons indicates that specific penton-hexon interactions are crucial to maintain the proper structure during expansion. The instability of pentons may be a common theme among dsDNA containing viruses because bacteriophage T4, adenovirus, and herpesvirus also release their pentons during chemical treatment 9; 10; 11, so all of these viruses will have developed a mechanism to prevent penton release during maturation.