Many viruses are covered with shells of proteins that have an icosahedral structure which are known for the pattern of a soccer ball. The Reoviridae viruses have a distinctive structure further covered with a multilayered shell (capsid) of a different pattern. Rotavirus that causes infant diarrhea is well known as an example. In the paper, the formation of multilayered capsids with different patterns in Rice Dwarf Virus (RDV), one of the Reoviridae viruses, was revealed by genetic engineering technology and phase-contrast cryo-electron microscopy.
   RDV is covered with icosahedral capsids of two different patterns called T = 1 (inner shell) and T = 13 (outer shell). In this study, we coupled green fluorescent protein (GFP) to the outer capsid protein using genetic recombination technology and used it to form virus particles. As a result, RDV particles with an incomplete shell was created by the sterically hinderance of GFP. A phase-contrast cryo-electron microscopy revealed that the outer capsid proteins of RDV bind only to a specific region of the inner capsid (on the three-rotational axes). It was found that the outer shell protein of RDV is firstly bound on the three-rotational axes of the inner shell, and other pouter capsid proteins are sequentially inserted on the beside of the initially bound proteins so as to combine the pieces of the puzzle.
   This result revealed how the reovirus particles with multilayered shells are formed for the first time. In addition, by using this result to efficiently inhibit particle formation, it is expected to be applicable to the prevention of infectious diseases and development of therapeutic drugs.

Figure: Phase-contrast cryo-EM image of RDVs assembled with GFP-bound outer capsid proteins (background) and its 3D structure (middle). 3D image of Matured RDV (lower right corner).

Collaborative Researcher

Yusuke Nakamichi, Naoyuki Miyazaki, Kenta Tsutsumi, Akifumi Higashiura, Hirotaka Narita, Atsushi Nakagawa (Osaka Univ.)
Kazuyoshi Murata (NIPS)

Funding

KAKENHI, AMED, Collaborative Study Program of the National Institute for Physiological Sciences,