Quantitative dissociation of archaeal virus SH1 reveals distinct capsid proteins and a lipid core

Viruses infecting archaeal cells are less well understood than those infecting eukaryotic and bacterial cells. Here we study the distribution of the structural proteins between the capsid and the membrane of icosahedral SH1 virus, an archaeal virus infecting extreme halophilic Haloarcula hispanica cells. General features such as morphology, linear dsDNA genome and presence of lipids suggest that it may belong to the recently proposed PRD1-adenovirus lineage of viruses. To investigate this we have initiated structural studies of the virion. Quantitative dissociation of SH1 by 3 M urea or by lowering the salt concentration identified a number of soluble capsid-associated proteins (VP2, VP3, VP4, VP6, VP7 and VP9). These released proteins left behind a particle, or lipid core, containing two major proteins VP10 and VP12 and viral phospholipids. VP1 was released from the lipid core in low ionic strength conditions but not with 3 M urea. Approximately half of the protein VP5 stayed with the lipid core and the other half was released. Analysis of the soluble capsid-associated proteins by their sedimentation and hydrodynamic properties suggests that the most abundant proteins, putative capsomers VP4 and VP7, form an intricate pattern of protein complexes. We also observed large differences in the sizes of the complexes determined by the two different methods suggesting an elongated overall structure for most of the capsid-associated proteins or protein complexes. This work verifies that there is an internal membrane vesicle residing inside the complex icosahedral capsid that is akin to the overall structure of PRD1-like viruses.