Structural basis for assembly of vertical single β-barrel viruses

Abstract

The vertical double β-barrel major capsid protein (MCP) fold, fingerprint of the PRD1-adeno viral lineage, is widespread in many viruses infecting organisms across the three domains of life. The discovery of PRD1-like viruses with two MCPs challenged the known assembly principles. Here, we present the cryo-electron microscopy (cryo-EM) structures of the archaeal, halophilic, internal membrane-containing Haloarcula californiae icosahedral virus 1 (HCIV-1) and Haloarcula hispanica icosahedral virus 2 (HHIV-2) at 3.7 and 3.8 Å resolution, respectively. Our structures reveal proteins located beneath the morphologically distinct two- and three-tower capsomers and homopentameric membrane proteins at the vertices that orchestrate the positioning of pre-formed vertical single β-barrel MCP heterodimers. The cryo-EM based structures together with the proteomics data provide insights into the assembly mechanism of this type of viruses and into those with membrane-less double β-barrel MCPs.

Novel haloarchaeal viruses from Lake Retba infecting Haloferax and Halorubrum species

“…D) and protein quantities (Table ), but the purified particles had specific infectivity of ~2 × 10 9 pfu/mg of protein (Table ), which is 3–4 magnitudes lower than e.g. the specific infectivities of the purified virus samples of haloarchaeal tailed virus HSTV‐1 (~9 × 10 12 pfu/mg of protein) and icosahedral membrane‐containing virus HCIV‐1 (~1 × 10 12 pfu/mg of protein), of which have been analysed structurally (Pietilä et al ., ; Demina et al ., ; Santos‐Perez et al ., ). The negative staining and TEM of the purified HFTV1 particles revealed that some of particles had lost their genome explaining partly the loss of infectivity (Fig.…”

The cryo-EM structure of African swine fever virus unravels a unique architecture comprising two icosahedral protein capsids and two lipoprotein membranes

“…Juxtaposed to the p72 proteinaceous shell, the icosahedrally ordered membrane is apparent, with an overall thickness of ϳ34 Å (peak-to-peak distance in Fig. 1C) that is in line with EDITORS' PICK: Cryo-EM of African swine fever virus other membrane-containing viruses (32,38,39). Weak densities across the p72 shell and the outer leaflet of the membrane support the contact of these two architectural constituents (Fig.…”

“…3C) (36), and the high hydrophobicity score of the first 10 residues [relative to the first 40 assessed using ProtScale (45)] support a model in which these helices contact the underneath membrane as also seen for PRD1 MCP P3 (32). Further contacts might also occur at the 5-fold vertices between penton proteins and the apex of the inner membrane as observed in membrane-containing viruses, such as STIV and HCIV-1, which interact with the above penton (39,44). It has been proposed that the capsid assembly of CroV and related giant viruses may initiate at a 5-fold vertex to continuously proceed outward in a spiraling fashion (27), and the same pathway might be used by ASFV.…”

“…The orientation of these capsomers is equivalent in each trisymmetron, but their orientation across adjacent trisymmetrons is related by a 60º rotation (27). Viruses of the PRD1-adenovirus lineage characterized by possessing a double jelly-roll MCP and smaller in size, use the same pseudo-hexameric capsomer morphology to build the whole triangular facet of the icosahedral particle, leaving space at the vertices only for the five copies of the penton protein, usually with a jelly-roll fold (31,32,39). Also, vertical single ␤-barrel viruses have been found to generate a heterodimeric vertical double jelly-roll with a pseudo-hexagonal shape that recapitulates the close packing seen in vertical double jelly-roll viruses (39).…”

“…Viruses of the PRD1-adenovirus lineage characterized by possessing a double jelly-roll MCP and smaller in size, use the same pseudo-hexameric capsomer morphology to build the whole triangular facet of the icosahedral particle, leaving space at the vertices only for the five copies of the penton protein, usually with a jelly-roll fold (31,32,39). Also, vertical single ␤-barrel viruses have been found to generate a heterodimeric vertical double jelly-roll with a pseudo-hexagonal shape that recapitulates the close packing seen in vertical double jelly-roll viruses (39). Thus, the use of vertical jelly-roll structures forming capsomers with a pseudo-hexameric footprint offers a means to scale up the size of capsid, exploiting the internal symmetry of these basic building blocks.…”