In Vivo Assembly of an Archaeal Virus Studied with Whole-Cell Electron Cryotomography

Abstract

Summary We applied whole cell electron cryotomography to the archaeon Sulfolobus infected by Sulfolobus turreted icosahedral virus (STIV), which belongs to the PRD1-Adeno lineage of dsDNA viruses. STIV infection induced the formation of pyramid-like protrusions with sharply defined facets on the cell surface. They had a thicker cross-section than the cytoplasmic membrane and did not contain an exterior surface protein layer (S-layer). Intra-pyramidal bodies often occupied the volume of the pyramids. Mature virions, procapsids without genome cores, and partially assembled particles were identified, suggesting that the capsid and inner membrane co-assemble in the cytoplasm to form a procapsid. A two-class reconstruction using a maximum likelihood algorithm demonstrated that no dramatic capsid transformation occurred upon DNA packaging. Virions tended to form tightly packed clusters or quasi-crystalline arrays while procapsids mostly scattered outside or on the edges of the clusters. The study revealed vivid images of STIV assembly, maturation and particle distribution in cell.

Structure and assembly mechanism of virus-associated pyramids

“…Upon infection, VAPs of different sizes (~20 -150 nm) were observed in the cell membrane of S. islandicus and S. solfataricus (Bize et al 2009;Brumfield et al 2009;Quax et al 2011). As their geometry is always the same, this observation suggests that VAPs develop by the formation of a small heptameric tip that grows into a pyramid by the gradual expansion of their seven triangular facets (Fu et al 2010;Quax et al 2011;Daum et al 2014). …”

“…Towards the end of the infection cycle, when the virions inside the cell have matured, all VAPs open approximately at the same stage of infection, much like the petals of a flower ( Fig. 1) (Fu et al 2010;Quax et al 2011;Daum et al 2014). This results in apertures of~150 nm in diameter, through which the virions can leave the cell (Fig.…”

“…2009). This process is concomitant with the replication and production of approximately 50-150 new virions in the cytoplasm of the cell (Bize et al 2009;Brumfield et al 2009;Fu et al 2010). As these VAPs are hollow and not closed at their base, their interior is continuous with the cytoplasm of the infected cell.…”

“…1). After releasing the virions, the VAPs remain open, thus causing complete lysis of host cell (Bize et al 2009;Brumfield et al 2009;Fu et al 2010;Daum et al 2014).…”

Archaeal viruses—novel, diverse and enigmatic

“…The physiological induction of the SulfolobusSSV2/pSSVx helper-satellite virus system has been further characterised and novel mechanisms for transcriptional termination were unravelled and a positive regulator implicated in virus induction was characterised [78,79]. There have been newer developments in characterising the virion release mechanisms for STIV [80,81] and SIRV2 [82,83] and evidence was provided for a co-chaperone activity of two ATV virion proteins and it was proposed that they played a role in facilitating the extracellular tail development [84]. A putative protein receptor was also characterised for this virus in the membrane of S. solfataricus [85].…”