Lipid-containing viruses: bacteriophage PRD1 assembly.

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.

Efficient DNA packaging of bacteriophage PRD1 requires the unique vertex protein P6.

The assembly of bacteriophage PRD1 proceeds via formation of empty procapsids containing an internal lipid membrane, into which the linear double-stranded DNA genome is subsequently packaged. The packaging ATPase P9 and other putative packaging proteins have been shown to be located at a unique vertex of the PRD1 capsid. Here, we describe the isolation and characterization of a suppressor-sensitive PRD1 mutant deficient in the unique vertex protein P6. Protein P6 was found to be an essential part of the PRD1 packaging machinery; its absence leads to greatly reduced packaging efficiency. Lack of P6 was not found to affect particle assembly, because in the P6-deficient mutant infection, wild-type (wt) amounts of particles were produced, although most were empty. P6 was determined not to be a specificity factor, as the few filled particles seen in the P6-deficient infection contained only PRD1-specific DNA. The presence of P6 was not necessary for retention of DNA in the capsid once packaging had occurred, and P6-deficient DNA-containing particles were found to be stable and infectious, albeit not as infectious as wt PRD1 virions. A packaging model for bacteriophage PRD1, based on previous results and those obtained in this study, is presented.