Abstract
Author SummaryThe life cycle of a virus involves serial coordination of viral molecular machines. These machines facilitate functions such as membrane fusion and genome delivery during infection, and capsid formation and genome packaging during replication and shedding. Icosahedral dsDNA viruses use one genome-translocation machine for both genome delivery and packaging. The genome-translocation machine of the membrane-containing bacterial virus PRD1 is composed of four packaging protein species at a unique vertex. Because these proteins do not follow the dominating icosahedral symmetry of the viral capsid, the structure of this vertex has remained elusive. In this study, we localize the unique vertex in the virus from raw electron cryo-microscopy images of the virus. We show that the genome-packaging complex of PRD1 replaces the regular 5-fold structure at the unique vertex and contains a transmembrane conduit as a genome translocation channel. We extend our structural studies to the procapsid—a precursor of the virus—and three packaging mutant particles, allowing us to localize all individual protein species in the complex. Based on these structures, we propose a model of the molecular mechanism of assembly and packaging in the life cycle of the PRD1 virus.
Highlights
The functional and structural knowledge of assembly principles of macromolecular complexes, in general, and viruses, in particular, have extended our understanding of viral capsid maturation and genome packaging processes
We localize the unique vertex in the virus from raw electron cryo-microscopy images of the virus
We show that the genome-packaging complex of PRD1 replaces the regular 5-fold structure at the unique vertex and contains a transmembrane conduit as a genome translocation channel
Summary
The functional and structural knowledge of assembly principles of macromolecular complexes, in general, and viruses, in particular, have extended our understanding of viral capsid maturation and genome packaging processes. The common mechanism for the genome encapsidation in icosahedral dsDNA viruses, including head-tailed phages, herpes, pox, and adenoviruses, involves a translocation of the viral DNA into a preformed procapsid by an ATP-driven reaction powered by the packaging complex localized at a single vertex [1]. This single vertex-portal complex operates in both genome delivery and packaging. A dodecameric connector at a 5-fold vertex provides a conduit for nucleic acid entry into the capsid [2,3,4,5] It is an assembly site for the transiently associated packaging NTPase powering DNA translocation [6]. The only structural evidence for the packaging components of a tailless icosahedral virus with a membrane comes from the crystal structure of the archaeal Sulfolobus icosahedral virus 2 (STIV2) packaging ATPase, which shows that these ATPases belong to the FtsK-HerA superfamily of P-loop ATPases, having both cellular and viral members [9,10]
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