Members of Marseilleviridae, a family of icosahedral giant viruses, have been identified worldwide in all types of environments. The virion shows a characteristic internal membrane extrusion at the five-fold vertices of the capsid, but its structural details need to be elucidated. We now report the 4.4 Å cryo-electron microscopy structure of the melbournevirus capsid by using a block-based reconstruction approach. Results: An atomic model of the major capsid protein (MCP) shows a unique cup structure on the trimer that accommodates additional proteins. A polyalanine model of the Penton base protein shows internally extended N- and C-terminals, which indirectly connect to the internal membrane extrusion. The Marseilleviruses share the same orientational organization of the MCPs as previously reported for other giant viruses, but the unique minor capsid protein components named Scaffold may be alternatively utilized to control the dimensions of the capsid during assembly as the tape measure protein.
Burton-Smith et al. (Thu,) studied this question.