A cryo-electron tomography (cryo-ET) reconstruction at ∼20 Å resolution revealed that His1, which morphologically resembles SSVs and infects the extremely halophilic euryarchaeon Haloarcula hispanica, has a variable capsid structure and a uniform tail hub surrounded by six tailspikes ( 13). A cryo-electron microscopy (cryo-EM) reconstruction at ∼32 Å resolution indicated that Sulfolobus spindle-shaped virus 1 (SSV1) has a sixfold symmetrical tail structure ( 12). Efforts have been made to elucidate the structural basis of the morphogenesis, stability, and host recognition and adsorption of fuselloviruses ( 8, 10, 11). An SSV virion consists of a spindle-shaped capsid shell that encapsidates a circular double-stranded DNA (dsDNA) genome and has a tail at one end for host attachment ( 9). Fuselloviruses are divided into two genera (i.e., Alphafusellovirus and Betafusellovirus), which are similar in capsid shell but differ distinctly in tail structure ( 8). Sulfolobus spindle-shaped viruses (SSVs) of the family Fuselloviridae, which infect hyperthermophilic archaea of Sulfolobales in hot springs, are among the most widespread and best characterized archaeal viruses ( 7). Among the morphologically unique archaeal viruses, spindle-shaped viruses are found in archaea inhabiting acidic hot springs, hypersaline, and marine environments ( 6). Archaeal viruses are far more diverse in shape and less well-studied than Bacteria and Eukarya viruses are ( 4, 5). Viruses infect hosts in all three domains of life ( 1– 3). The nozzle protein resembles the stem and clip domains of the portals of herpesviruses and bacteriophages, implying an evolutionary relationship among the archaeal, bacterial, and eukaryotic viruses. Molecules of glycerol dibiphytanyl glycerol tetraether lipid were observed in hydrophobic clefts between the tail and the capsid shell. The tailspike harbors a putative endo-mannanase domain, which shares structural similarity to a Bacteroides thetaiotaomicro endo-mannanase. Ultrathin sections of the SSV19-infected cells show that SSV19 virions adsorb to the host cell membrane through the tail after penetrating the S-layer.
Sliding between adjacent strands is responsible for the variation of a virion in size. The spindle-shaped capsid shell is formed by seven left-handed helical strands, constructed of the hydrophobic major capsid protein, emanating from the highly glycosylated tail assembly. Our near-atomic structure reveals an unusual sevenfold symmetrical virus tail consisting of the tailspike, nozzle, and adaptor proteins. Here, we present the structure of SSV19, a spindle-shaped virus infecting the hyperthermophilic archaeon Sulfolobus sp. However, efforts to obtain the high-resolution structure of a spindle-shaped virus have been unsuccessful. Archaeal viruses with a spindle-shaped virion are abundant and widespread in extremely diverse environments.
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