Particle morphology of medusavirus inside and outside the cells reveals a new maturation process of giant viruses
Medusavirus, a giant virus, is phylogenetically closer to eukaryotes than the other giant viruses and has been recently classified as an independent species. However, details of its morphology and maturation process in host cells remain unclear. Here, we investigated the particle morphology of medusavirus inside and outside infected cells using conventional transmission electron microscopy (C-TEM) and cryo-electron microscopy (cryo-EM). The C-TEM of amoeba infected with the medusavirus showed four types of particles: empty, DNA-full, and the corresponding intermediates. Time-dependent changes in the proportion and following intracellular localization of these particles suggested a new maturation process for the medusavirus. Empty particles and viral DNAs were produced independently in the cytoplasm and nucleus, respectively, and only empty particles located near the nucleus incorporated the viral DNA into the capsid. All four types of particles were also found outside the cells. The cryo-EM of these particles showed that the intact capsid structure, covered with three different types of spikes, was conserved among all particle types, although with minor size-related differences. The internal membrane exhibited a structural array similar to that of the capsid, interacted closely with the capsid, and displayed open membrane structures in the empty and empty-intermediate particles. This result suggests that the open structures in the internal membrane are used for an exchange of scaffold proteins and viral DNA during the maturation process. This new model of the maturation process of medusavirus provides insight into the structural and behavioral diversity of giant viruses. ImportanceGiant viruses exhibit diverse morphologies and maturation processes. In the present study, medusavirus showed four types of particle morphologies both inside and outside the infected cells, when propagated in the laboratory using amoeba culture. Time-course analysis of the medusavirus particles in the infected cells reveals a new maturation process. Empty viral particles and viral DNAs were produced independently in the cytoplasm and nucleus, and only the empty particles located near the nucleus incorporated the viral DNA. Consequently, many immature particles, along with mature virions, were released from the host cells. Except for showing a small change in size, the capsid structures were well preserved during the maturation process. The empty viral particles and corresponding intermediates showed open membrane structures, which are presumably used for exchanging scaffold proteins and viral DNAs.