Cryo-Electron Microscopy of Spiroplasma Virus SpV4

Spiroplasma, a wall-free prokaryote of the class Mollicutes, is host to a small, naked, single-stranded DNA, isometric virus. Spiroplasma virus SpV4 belongs to the Microviridae family, members of which are non-enveloped, have icosahedral capsids, release progeny through a lytic cycle, and contain circular DNA.Measurements obtained from negatively stained SpV4 particles revealed a nucleocapsid of 27nm in diameter (figure 1). The three-dimensional structure reported here, obtained from unstained particles suspended in a layer of vitreous ice (figure 2), is in agreement with these earlier results, suggesting a 27nm average distance through the nucleocapsid (figure 3). Unreported in earlier studies is the presence of a 6nm, mushroom-shaped protrusion (made up of a stalk, 2.3nm long and 1.3nm wide, and a globular bud of dimensions ≈4.0×4.0×3.7nm) stemming from an ≈1.5nm deep depression at each of the 3-fold icosahedral axes of the virion. A cross section through the longitudinal axis of one protuberance (figure 4) reveals a cylindrical dimple (≈1.0nm in diameter and 2.3nm deep), originating on the axis of the outer surface of the globular bud domain.