Figure 1 Structure of arterivirus virions. (A) Cryo-EM of PRRSV particles (strain SD-23983) in vitreous ice. The bar represents 100 nm. The white arrow points to a particle with a rectangular core. Black arrows indicate protruding features thought to correspond to complexes of the minor envelope proteins. Inset, magnified (×2) view of a single, typical PRRSV particle with dimensions indicated. A presumed envelope spike complex is indicated, as is the striated appearance most likely corresponding to transmembrane domains. The dark area on the left is part of the carbon support film (from Spilman et al. (2009). J. Gen. Virol., 90, 527-535; with permission from the Journal of General Virology. (B) Schematic representation of the arterivirus particle. N-glycosylation sites are indicated by stars. (C) Structure of the core. (1) Cutaway view of one PRRSV virion. The envelope, shown in mesh representation, was peeled away to reveal the internal core. The core is shown as an isosurface, coloured by the radius from the centre of the particle (from red to blue). (2) The core has been cut open to show the internal structure and the characteristic central density (red-orange). (3) A 63 nm thick slab through the centre of one particle tomogram, with several copies of the crystal structure of the dimer of the C-terminal domain of N rendered at a comparable resolution to the tomogram and superimposed on the oblong densities in the core
(from Spilman et al. (2009). J. Gen. Virol., 90, 527-535; with permission from the Journal of General Virology).
Figure 2 Arterivirus genome organization and expression. The general genome organization is shown at the top of the figure with the ORFs represented. The proteins encoded by the ORFs are indicated, while ORF colors for virion-associated proteins match those used in Figure 1B. Other domains: L, 5′ leader sequence or 5′ UTR; 3′ UTR and a 3′ poly(A) tail (zigzag line); filled circle, ORF 1a and 1b ribosomal frameshift site. The grey boxes immediately below represent the regions where PRRSV, LDV, and SHFV contain major insertions compared to EAV. In the infected cell, the full-length genome uses discontinuous transcription during minus strand synthesis to eventually produce a nested set of subgenomic messages (sgRNA), shown below. The proteins produced are listed to the right of the respective RNA. The structural protein composition of SHFV – except for GP5, M, and N – is unknown at present.
Figure 3 (A) Schematic of the major envelope proteins. The GP5 glycoproteins of LDV and PRRSV (left) and EAV (right) are disulfide bonded to the M protein. One neutralization domain has been identified on LDV and PRRSV, and 4 (A-D) on EAV. Hypervariable regions surround the neutralization domains. N-glycan addition (star) varies among arteriviruses, but the N-glycan residues closest to the disulfide bond between GP5 and M (S-S) are mostly conserved. (B) Predicted conformation of the minor envelope proteins in eukaryotic membranes.
(Reproduced with modifications from Perlman et al. (2008). ASM Press, Washington, DC with permission.)
Figure 4 Overview of the replication cycle of the arterivirus prototype (EAV). The genome organization, including replicase cleavage sites (arrowheads; see also Figure 5), is shown at the top of the figure. Abbreviations: ER, endoplasmic reticulum; PM, plasma membrane; DMV, double membrane vesicle; NC, nucleocapsid.
(Modified from the Eighth ICTV Report.)
Figure 5 Overview of proteolytic processing of the EAV replicase polyproteins, with examined or potential similarities with PRRSV and LDV indicated. The domains for SHFV have not been described. Polyprotein cleavage sites are depicted with arrowheads matching the colour of the proteinase involved. Abbreviations: ZF, zinc-finger; 1 (PLP1), papain-like cysteine proteinase; 2 (PLP2), papain-like, cysteine proteinase; SP (also called Mpro), chymotrypsin-like serine proteinase; TM, transmembrane domain; RdRp, RNA-dependent RNA polymerase; Z, zinc binding; HEL, NTPase/helicase; U, nidoviral endonuclease specific for U (NendoU). In addition, several cysteine/histidine (C/H) recognized motifs, as well as the ribosomal frameshift site (RFS), are indicated below the figures. In EAV, PLP1α has become inactivated although its remnants were detected in the EAV nsp1 region. The large hypervariable region in PRRSV nsp2 that is characterized by insertions and deletions among different strains is shown by the hatched gray bar.
Figure 6 Phylogenetic analysis of the Arteriviridae was completed using the nucleotide sequences of SHFV (strain LVR 42-0; NC_003092), EAV (strain Bucyrus VBS; DQ846750), LDV-P (Plagemann strain; NC_001639), Type 1 PRRSV (strain Lelystad; M96262) and Type 2 PRRSV (strain VR-2332; U87392). Unrooted phylogenetic analysis of the full-length ORF1b replicase polyprotein. The tree was generated using the Dayhoff substitution model in PHYML, after alignment with the MUSCLE Alignment algorithm (default parameters, 8 iterations) in Geneious Pro 4.8.3 (Biomatters Ltd). Distance lengths represent substitutions per site. (K.S. Faaberg, unpublished data.)