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Abstract Medusavirus is a giant virus classified into an independent family of Mamonoviridae . Medusavirus-infected amoebae release immature particles in addition to the virions. These particles were suggested to exhibit the maturation process of this virus, but the structure of these capsids during maturation remains unknown. Here, we applied a block-based reconstruction method in cryo-electron microscopy (cryo-EM) single particle analysis to these viral capsids, extending these resolutions to 7-10 Å. The map revealed a novel network consisted of minor capsid proteins (mCPs), supporting major capsid proteins (MCPs). A predicted molecular model of the MCP fitted into the cryo-EM map clarified the boundaries between the MCP and the underlining mCPs and between the MCP and the outer spikes, identifying the molecular interactions between the MCP and these components. Several structural changes of mCPs were observed beneath the 5-fold vertices of immature particles, depending on the presence or absence of the underlying internal membrane. Furthermore, the lower part of penton proteins on the 5-fold vertices were also lost in the mature virions. These dynamic structural changes of mCPs exhibit important functions in the maturation process of medusavirus. Importance Structural changes in giant virus capsids during maturation have not been known. Medusavirus is a unique giant virus, where amoebae infected with the virus release immature particles in addition to mature virions. In this study, immature and mature medusavirus particles were investigated using cryo-electron microscopy, and firstly reported the structural changes in the viral capsid during maturation. In DNA empty particles, the conformation of the minor capsid proteins dynamically changed depending on the presence or absence of the underlying internal membranes. In DNA full particles, the lower part of the penton proteins was lost. These are the first report of the structural changes in the viral capsid during giant virus maturation.
Watanabe et al. (Thu,) studied this question.