Assembly of complex viruses exemplified by a halophilic euryarchaeal virus.
De Colibus L., Roine E., Walter TS., Ilca SL., Wang X., Wang N., Roseman AM., Bamford D., Huiskonen JT., Stuart DI.
Many of the largest known viruses belong to the PRD1-adeno structural lineage characterised by conserved pseudo-hexameric capsomers composed of three copies of a single major capsid protein (MCP). Here, by high-resolution cryo-EM analysis, we show that a class of archaeal viruses possess hetero-hexameric MCPs which mimic the PRD1-adeno lineage trimer. These hetero-hexamers are built from heterodimers and utilise a jigsaw-puzzle system of pegs and holes, and underlying minor capsid proteins, to assemble the capsid laterally from the 5-fold vertices. At these vertices proteins engage inwards with the internal membrane vesicle whilst 2-fold symmetric horn-like structures protrude outwards. The horns are assembled from repeated globular domains attached to a central spine, presumably facilitating multimeric attachment to the cell receptor. Such viruses may represent precursors of the main PRD1-adeno lineage, similarly engaging cell-receptors via 5-fold spikes and using minor proteins to define particle size.