The family Arteriviridae currently comprises a single genus, Arterivirus, which contains all member viruses: equine arteritis virus, porcine reproductive and respiratory syndrome virus, lactate dehydrogenase-elevating virus, simian hemorrhagic fever virus, and provisionally, wobbly possum disease virus, a novel nidovirus that was identified recently in Australian brushtail possums (Trichosurus vulpecula) in New Zealand (Table 25.1). It has been proposed that the family Arteriviridae be further subdivided taxonomically to accommodate the recently identified, highly divergent arteriviruses of African nonhuman primates and rodents. Five genera are included in this proposed classification, based on sequence and phylogenetic analysis of the open reading frame 1b. The family Roniviridae currently contains a group of related viruses causing disease in crustaceans that are members of a single genus, Okavirus.

Arterivirus virions are enveloped, spherical, and 45–60 nm in diameter, which is only about half the size of those of coronaviruses (Fig. 25.1; also see Fig. 24.1). In contrast to the nucleocapsids of coronaviruses and roniviruses, which are helical, arterivirus nucleocapsids are isometric, 25–35 nm in diameter. Whereas envelope glycoprotein spikes are prominent on coronaviruses and roniviruses, they are small and indistinct on arterivirus virions. The genome of arteriviruses consists of a single molecule of linear positive-sense, single-stranded RNA, approximately 12.7–15.7 kb in size that includes 9–12 open reading frames (Fig. 25.2). There are untranslated regions at the 5′ and 3′ ends of the genome (156–224 and 59–177 nt, respectively), and a 3′-poly(A) terminal sequence. Arterivirus virions include a single nucleocapsid protein (N) and seven envelope proteins, designated E, GP2, GP3, GP4, ORF5a protein, GP5, and M. Of these, three minor envelope proteins (GP2, GP3, and GP4) form a heterotrimer, and the nonglycosylated triple-membrane spanning integral membrane protein, M, and the large envelope glycoprotein, GP5, form a heterodimer. The major neutralization determinants of arteriviruses are expressed on GP5, although the M protein exerts a conformational influence on GP5. The structural proteins of simian hemorrhagic fever virus are not as well characterized as that of the other arteriviruses, and its genome includes four additional open reading frames that may represent reduplications of genes encoding minor structural viral proteins (the reduplicated proteins being designated as E′, GP2′, GP3′, and GP4′).

Ronivirus virions are bacilliform, 40–60 nm×150–200 nm, with rounded ends and prominent glycoprotein envelope spikes (Fig. 25.3; also see Fig. 24.1). The nucleocapsid has helical symmetry and is comprised of a coiled filament 16–30 nm in diameter. The nucleocapsid is surrounded by the envelope, which has diffuse projections (approximately 8 nm thick and 11 nm in length) extending from the surface. Like other nidoviruses, the ronivirus genome is a positive-sense, single strand RNA molecule of 26.2–26.6 kb that contains a 5′ cap structure, a 3′-poly(A) tail, and includes five open reading frames (Fig. 25.2; also see Fig. 24.2). Virions consist of at least three envelope proteins (gp116, gp64, and an N-terminal triple-membrane spanning protein of unknown function). The envelope proteins are cleavage products of a larger polyprotein precursor.

Similar to other nidoviruses, replication and transcription of arteriviruses are processed through different minus-strand intermediates: a full-length minus-strand RNA template (or antigenome) is used for replication, while subgenome-sized minus strands produced during a process of discontinuous RNA synthesis are used to synthesize subgenomic mRNAs (Fig. 25.4). The two large open reading frames at the 5′ end of the arterivirus genome encode two replicase polyproteins that are expressed directly from viral genomic RNA through a ribosomal frameshifting mechanism. These replicase polyproteins are co- and post-translationally modified by viral proteinases into at least 13 (equine arteritis virus) or 16 (porcine reproductive and respiratory syndrome virus) nonstructural proteins (nsps) that mediate replication. The genes that encode the viral structural proteins are overlapping, and located in the 3′ end of the genome; they are expressed from a nested set of 3′ coterminal subgenomic RNAs (Fig. 25.2). These subgenomic RNAs all include a common 5′ leader sequence derived from the 5′ untranslated region of viral genomic RNA, at least one unique open reading frame encoding one or more structural virion proteins, and a common 3′-poly(A) tail. The individual open reading frames that are included in these subgenomic mRNAs reflect overlapping reading frames contained in the 3′ end of the viral genome. It is believed that the subgenomic mRNAs are generated by discontinuous transcription that links noncontiguous portions of the viral genome, to produce negative-strand templates that are transcribed into positive-strand subgenomic mRNAs that are then translated into the individual virion proteins.

In arterivirus-infected cells, newly synthesized nonstructural proteins are incorporated into cellular organelles, particularly the endoplasmic reticulum, which results in membrane pairing and formation of double-membrane vesicles. Immunoelectron microscopic studies have confirmed that viral nonstructural proteins that are part of the replication/transcription complexes and nascent viral RNA are associated with these double-membrane vesicles suggesting that these vesicles carry the enzyme complex responsible for virus replication and subgenomic mRNA synthesis. Arterivirus replication occurs in the cytoplasm of infected cells, although nonstructural (nsp1) and structural (N) proteins selectively translocate to the nucleus. During the early stages of infection, nsp1 is primarily located in the nucleus while its perinuclear cytoplasmic localization becomes evident later in infection. The newly synthesized genome is encapsidated into the N protein to form the nucleocapsid, which becomes enveloped by budding through the endoplasmic reticulum–Golgi intermediate compartment that contains membranes that include the viral envelope proteins. Newly formed virions mature in the Golgi complex during their movement through the exocytic pathway and are ultimately released from infected cells (Table 25.2).

Table 25.2

Properties of Arteriviruses

Virions are spherical, 50–70 nm in diameter, with an isometric nucleocapsid and a closely adherent smooth-surfaced envelope with ring-like structures

The genome consists of a single molecule of linear, positive-sense, single-stranded RNA, 13–15 kb in size. Virion RNA has a 5′ cap and its 3′ end is polyadenylated; the genomic RNA is infectious

Replication takes place in the cytoplasm; the genome is transcribed to form a full-length negative-sense RNA, from which is transcribed a 3′ coterminal nested set of mRNAs; only the unique sequences at the 5′ end of each mRNA are translated

Virions are formed by budding into the endoplasmic reticulum, from where they are released by exocytosis

The overall strategy for replication of roniviruses is similar to that of arteriviruses, but the structural proteins are expressed from a nested set of just two subgenomic mRNAs that each encode several proteins. The subgenomic mRNAs also lack a common 5′ leader sequence derived from genomic viral RNA.