Baculoviruses are ubiquitous insect viruses that are fundamental to medical research and biotechnology and are used as gene expression vectors in laboratories around the world. However, Baculoviridae is one of the few viral families for which the architecture of the capsid is unknown. In addition to the major nucleocapsid protein that forms the helical tube containing the genome, at least a dozen other proteins have been found associated with the capsid in the base and cap structures located at either end. Using cryo-electron microscopy, we have revealed the architecture of the prototypical AcMNPV baculovirus nucleocapsid including the tube, base, and cap components.
The ambiguous helical symmetry of the nucleocapsid tube has thwarted structure determination for years. To solve the symmetry parameters and reconstruct the nucleocapsid, we used a combination of subtomogram averaging, and helical and localised reconstruction. Our structure revealed a unique dimeric subunit with a cross-linked network of disulphide bonds that stabilise the capsid. Localised reconstruction of the base and cap complexes revealed a 126-mer hub at both ends composed of 6 different proteins. Additional proteins involved in assembly and intracellular trafficking decorate the base hub, and the cap has an elaborate structure resembling a viral DNA portal.
Almost all known eukaryotic viruses have been classified into six viral realms according to the conservation of hallmark genes. Baculoviruses belong to the class Naldaviricetes which has been proposed to have evolved from either the Varidnaviria realm (pox-like viruses) or the Duplodnaviria realm (herpes-like viruses). Our structure of the baculovirus capsid reveals a unique fold unlike the double jelly-roll fold and HK97-fold of pox-like and herpes-like viruses, and structure-based phylogeny analysis of other hallmark baculovirus genes did not identify viral homologues outside Naldaviricetes. We propose that baculoviruses are representatives of a seventh viral realm with a separate origin from other viruses.