Susanne I. Schmid and Patrick Hearing
The assembly of adenovirus particles proceeds through an ordered series of assembly events (reviewed in ref. 1). The assembly of the virus particles has been probed using viral temperature-conditional mutants blocked at different stages of assembly at the restrictive temperature and by pulse-chase kinetic analyses. The first recognizable viral assembly intermediate is a light intermediate particle (buoyant density of 1.315 g/cc in a CsCl equilibrium gradient). These particles contain the capsid structural components and no or very little viral DNA and associated core proteins. Additionally, light intermediate particles contain several proteins that exit the particle during maturation (50 kDa and 39 kDa polypeptides) and may represent the adenoviral equivalent of phage scaffolding proteins. The light intermediate particles mature into heavy intermediate particles (1.37 g/cc buoyant density) with the insertion of viral DNA. The 50 kDa and 39 kDa polypeptides are released from the particle during this maturation step. The heavy intermediate particles appear to lack core proteins, which enter the particle during the next maturation step with the formation of young virus particles (1.34 g/cc buoyant density). The issue of separate or combined entry of DNA and core proteins, however, remains controversial. As the final step in maturation, the virus-encoded and encapsidated proteinase performs numerous cleavages of multiple viral proteins to generate the mature, infectious virion (see chapter 8). Four minor virus-encoded proteins (IIIa, VI, VIII and IX) appear to either enhance the assembly of subviral components and/or stabilize viral protein-protein interactions, and hence particle integrity, once formed (see chapter 2). The salient conclusion from these analyses is that adenovirus virion assembly likely follows an ordered series of maturation events, with a capsid prohead assembled as the initial target for the DNA encapsidation process. In this way, the assembly of infectious adenovirus particles may follow the paradigm of prokaryotic phage assembly.