bOur understanding of the third domain of life, Archaea, has greatly increased since its establishment some 20 years ago. The increasing information on archaea has also brought their viruses into the limelight. Today, about 100 archaeal viruses are known, which is a low number compared to the numbers of characterized bacterial or eukaryotic viruses. Here, we have performed a comparative biological and structural study of seven pleomorphic viruses infecting extremely halophilic archaea. The pleomorphic nature of this novel virion type was established by sedimentation analysis and cryo-electron microscopy. These nonlytic viruses form virions characterized by a lipid vesicle enclosing the genome, without any nucleoproteins. The viral lipids are unselectively acquired from host cell membranes. The virions contain two to three major structural proteins, which either are embedded in the membrane or form spikes distributed randomly on the external membrane surface. Thus, the most important step during virion assembly is most likely the interaction of the membrane proteins with the genome. The interaction can be driven by single-stranded or double-stranded DNA, resulting in the virions having similar architectures but different genome types. Based on our comparative study, these viruses probably form a novel group, which we define as pleolipoviruses.

No abstract

Structural comparisons between bacteriophage PRD1 and adenovirus have revealed an evolutionary relationship that has contributed significantly to current ideas on virus phylogeny. However, the structural organization of the receptor-binding spike complex and how the different symmetry mismatches are mediated between the spike-complex proteins are not clear. We determined the architecture of the PRD1 spike complex by using electron microscopy and three-dimensional image reconstruction of a series of PRD1 mutants. We constructed an atomic model for the fulllength P5 spike protein by using comparative modeling. P5 was shown to be bound directly to the penton base protein P31. P5 and the receptor-binding protein P2 form two separate spikes, interacting with each other near the capsid shell. P5, with a tumor necrosis factor-like head domain, may have been responsible for host recognition before capture of the current receptor-binding protein P2.electron cryomicroscopy ͉ symmetry mismatch ͉ vertex ͉ virus

PRD1 is a tailless icosahedrally symmetric virus containing an internal lipid membrane beneath the protein capsid. Its linear dsDNA genome and covalently attached terminal proteins are delivered into the cell where replication occurs via a protein-primed mechanism. Extensive studies have been carried out to decipher the roles of the 37 viral proteins in PRD1 assembly, their association in virus particles and lately, especially the functioning of the unique packaging machinery that translocates the genome into the procapsid. These issues will be addressed in this chapter especially in the context of the structure of PRD1. We will also discuss the major challenges still to be addressed in PRD1 assembly.