A research group led by Project Professor Kazuyoshi Murata at the Exploratory Research Center on Life and Living Systems (ExCELLS) / National Institute for Physiological Sciences (NIPS), in collaboration with Senior Researcher Kenta Okamoto at Uppsala University and Professor Chantal Abergel at Aix-Marseille University, has successfully determined, for the first time in the world, the capsid (outer shell) structure of Melbournevirus—a member of the giant virus family—at a resolution of 4.4 Å using cryo-electron microscopy (cryo-EM).

Low-Res_Figure1_MelbV_3D_nolavel

Source: Kazuyoshi Murata

Figure 1. Melbournevirus is an icosahedral giant virus with a diameter of approximately 250 nm. It is covered by capsid proteins composed of MCP and mCP, and it contains a folded DNA genome with characteristic protrusions inside. (A) External view. Colors indicate the distance from the particle center: blue, 1050 Å; green, 1150 Å; red, 1250 Å. (B) Cross-sectional view. The scale bar represents 500 nm.

In this study, the researchers applied a “block-based reconstruction method” to the analysis of electron microscopy images, achieving a dramatic improvement in the resolution of the three-dimensional reconstruction (Figure 1).

This enabled them to elucidate the detailed arrangement of proteins constituting the massive capsid (250 nm in diameter) (Figure 2).

Low-Res_図2

Source: Kazuyoshi Murata

Figure 2. The 12 pentagonal vertices (two of which are shown in the diagram) are composed of protein components designated as Penton (black), PC-α (magenta), PC-β (green), and PC-γ (blue). The 20 triangular facets located between the vertices (two of which are shown in the diagram) consist of Cement (cyan), Lattice (vermillion), and Support (ultramarine) protein components, which are interconnected by Glue (purple) and Zipper (orange) protein components. On the inner side of the capsid, Scaffold (yellow) protein components are arranged along the junctions.

These findings contribute to understanding the fundamental principles by which large, uniform, and robust structures are formed from a limited set of proteins. In addition, the results are expected to advance our knowledge of viral evolution and infection mechanisms, and may also be applied to the design of inclusion compounds and drug delivery carriers.

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