Spindle-shaped archaeal viruses evolved from rod-shaped ancestors to package a larger genome

Spindle- or lemon-shaped viruses that infect archaea have been observed in diverse environments around the globe. Due to the highly pleomorphic nature of these virions, which frequently can be found with cylindrical tails emanating from the spindle-shaped body, structural studies of these remarkable capsids have been challenging. We have determined the atomic structure of the capsid of Sulfolobus monocaudavirus 1, a virus that infects hosts living in nearly boiling acid. We show that the hydrophilic protein previously thought to be the major capsid protein is not involved in capsid formation.