Visualization of Assembly Intermediates and Budding Vacuoles of Singapore Grouper Iridovirus in Grouper Embryonic Cells

Abstract

Iridovirid infection is associated with the catastrophic loss in aquaculture industry and the population decline of wild amphibians and reptiles, but none of the iridovirid life cycles have been well explored. Here, we report the detailed visualization of the life cycle of Singapore grouper iridovirus (SGIV) in grouper cells by cryo-electron microscopy (cryoEM) and tomography (ET). EM imaging revealed that SGIV viral particles have an outer capsid layer, and the interaction of this layer with cellular plasma membrane initiates viral entry. Subsequent viral replication leads to formation of a viral assembly site (VAS), where membranous structures emerge as precursors to recruit capsid proteins to form an intermediate, double-shell, crescent-shaped structure, which curves to form icosahedral capsids. Knockdown of the major capsid protein eliminates the formation of viral capsids. As capsid formation progresses, electron-dense materials known to be involved in DNA encapsidation accumulate within the capsid until it is fully occupied. Besides the well-known budding mechanism through the cell periphery, we demonstrate a novel budding process in which viral particles bud into a tubular-like structure within vacuoles. This budding process may denote a new strategy used by SGIV to disseminate viral particles into neighbor cells while evading host immune response.

Outbreaks of ulcerative disease associated with ranavirus infection in barcoo grunter, Scortum barcoo (McCulloch & Waite)

“…The VAS can be visualized in the cytoplasm following SGV infection in grouper (Liu et al . ). Herein, the ranavirus‐associated inclusion body should be microscopically distinguished from the infection of megalocytiviruses, as the latter case regularly exhibits cytomegalic cells containing large granular inclusion bodies that compress nuclei to the cell's periphery (Kurita & Nakajima ).…”

“…However, the underlying reason of eosinophilic appearance of these viral inclusion bodies is yet to be determined. According to literature, the inclusion bodies found in the affected tissues correspond to the 'virus assembly site' (VAS) which help facilitating viral morphogenesis and accumulation within host cell (Liu et al 2016). The VAS can be visualized in the cytoplasm following SGV infection in grouper (Liu et al 2016).…”

“…According to literature, the inclusion bodies found in the affected tissues correspond to the 'virus assembly site' (VAS) which help facilitating viral morphogenesis and accumulation within host cell (Liu et al 2016). The VAS can be visualized in the cytoplasm following SGV infection in grouper (Liu et al 2016). Herein, the ranavirus-associated inclusion body should be microscopically distinguished from the infection of megalocytiviruses, as the latter case regularly exhibits cytomegalic cells containing large granular inclusion bodies that compress nuclei to the cell's periphery (Kurita & Nakajima 2012).…”

Organelle dynamics and viral infections: at cross roads

“…Singapore grouper iridovirus (SGIV) infection on grouper embryonic cells (GECs) from the brown-spotted grouper Epinephelus tauvina results in the formation of a large intracellular vacuole for viral accumulation. Later, the virus recruits the host cytosolic membrane-bending proteins in order to induce tubulation of vacuolar membrane [161]. The individual vacuoles fuse together to form a large vacuole which in turn fuses with the cell membrane.…”

“…The individual vacuoles fuse together to form a large vacuole which in turn fuses with the cell membrane. These events aid in the release of virions [161].…”