Sulfolobus Spindle-Shaped Virus 1 Contains Glycosylated Capsid Proteins, a Cellular Chromatin Protein, and Host-Derived Lipids

Abstract

Geothermal and hypersaline environments are rich in virus-like particles, among which spindle-shaped morphotypes dominate. Currently, viruses with spindle-or lemon-shaped virions are exclusive to Archaea and belong to two distinct viral families. The larger of the two families, the Fuselloviridae, comprises tail-less, spindle-shaped viruses, which infect hosts from phylogenetically distant archaeal lineages. Sulfolobus spindle-shaped virus 1 (SSV1) is the best known member of the family and was one of the first hyperthermophilic archaeal viruses to be isolated. SSV1 is an attractive model for understanding virus-host interactions in Archaea; however, the constituents and architecture of SSV1 particles remain only partially characterized. Here, we have conducted an extensive biochemical characterization of highly purified SSV1 virions and identified four virus-encoded structural proteins, VP1 to VP4, as well as one DNA-binding protein of cellular origin. The virion proteins VP1, VP3, and VP4 undergo posttranslational modification by glycosylation, seemingly at multiple sites. VP1 is also proteolytically processed. In addition to the viral DNA-binding protein VP2, we show that viral particles contain the Sulfolobus solfataricus chromatin protein Sso7d. Finally, we provide evidence indicating that SSV1 virions contain glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, resolving a long-standing debate on the presence of lipids within SSV1 virions. A comparison of the contents of lipids isolated from the virus and its host cell suggests that GDGTs are acquired by the virus in a selective manner from the host cytoplasmic membrane, likely during progeny egress. Viruses infecting extremophilic archaea have evolved to withstand very high temperatures, low or high pH, or near-saturating salt concentrations (1-5). Remarkably, most of these viruses do not seem to be evolutionarily related to viruses of bacteria or eukaryotes and display a considerable diversity of unique virion morphotypes (3, 4). Indeed, 11 novel viral families have been established by the International Committee for the Taxonomy of Viruses (ICTV) for the classification of archaeal viruses, emphasizing the uniqueness of rod-shaped, spindle-shaped, dropletshaped, or even bottle-shaped particles that have never been observed among viruses infecting bacteria or eukaryotes (3). Functional studies proved to be highly challenging due to the lack of similarity between the protein sequences and structures of archaeal viruses and those from other viruses and cellular organisms (6-10). Among the morphotypes that are exclusively associated with archaea, spindle-shaped viruses are particularly widespread (11) and have been isolated from highly different environments, including deep-sea hydrothermal vents (12-14), hypersaline environments (15-18), anoxic freshwaters (19), cold Antarctic lakes (20), terrestrial hot springs (21-23), and acidic mines (24).Recently, we refined the evolutionary relationships among spindle-shaped viruses by assessing the morph...

A Novel Type of Polyhedral Viruses Infecting Hyperthermophilic Archaea

“…Most likely, the lipids are principal components of the envelope of the virion core. Notably, similar selectivity toward particular lipid species has been observed in several other archaeal viruses (38)(39)(40).…”

Genetic Analysis of the Major Capsid Protein of the Archaeal Fusellovirus SSV1: Mutational Flexibility and Conformational Change

“…Thus, it is not surprising that a complete deletion of the vp1 gene and a transposon insertion in the middle of the ORF both failed to yield infectious virus [6]. The VP1 protein appears to be proteolytically cleaved at an internal glutamate residue that is conserved in all known fuselloviruses [6] (Figure 1) to produce the mature protein [8,9] (Figure 1). To investigate if the encoded N-terminus is required for infectivity, this region was deleted using LIPCR while leaving the conserved glutamate intact (Figure 1) and the construct electroporated into Sulfolobus.…”

“…In purified SSV1 particles, the N-terminal residue of the VP1 major capsid protein is a glutamate, indicating that the protein is made as a precursor protein and is then proteolytically cleaved by an unknown protease [7][8][9]. Proteolytic cleavage of virus structural proteins is very well-known; from the polyproteins of picornaviruses, flaviviruses and retroviruses [10] to the maturation of receptorbinding proteins in orthomyxoviruses [11].…”

Genetic Analysis of the Major Capsid Protein of the Archaeal Fusellovirus SSV1: Mutational Flexibility and Conformational Change<strong> </strong>

“…In purified SSV1 particles, the N-terminal residue of the VP1 major capsid protein is a glutamate, indicating that the protein is made as a precursor protein and is then proteolytically cleaved by an unknown protease [5,11,12]. Proteolytic cleavage of virus structural proteins is very well-known; from the polyproteins of picornaviruses, flaviviruses and retroviruses [16] to the maturation of receptorbinding proteins in orthomyxoviruses [17].…”

“…SSV1 virions seem to form by budding through the Sulfolobus membrane [10]. Purified SSV1 virions contain VP1 and VP3, hydrophobic capsid proteins, VP2, a DNA-binding protein, VP4 the putative tail protein and host-derived lipids [11,12]. Recently an approximately 32A resolution cryo-EM structure for SSV1 was determined and a novel fused-fullerene cone model for the structure was proposed [13].…”