A new family of vertebrate viruses: Toroviridae

The proposed family Toroviridae is characterized by eveloped, peplomer-bearing particles containing an elongated tubular nucleocapsid with helical symmetry. The capsid may bend into an open torus, conferring a biconcave disk or kidney-shaped morphology to the virion (largest diameter 120-140 nm) or the capsid may be straight, resulting in a rod-shaped particle (35 X 170 nm). Morphogenesis is by budding of preformed nucleocapsids through membranes mainly of the Golgi system and of the rough endoplasmic reticulum. Berne virus, which is proposed as the family prototype, contains a single strand of infectious positive-sense RNA, Mr about 6.5 X 10(6), which is polyadenylated. The RNA is surrounded by the major nucleocapsid phosphoprotein (Mr about 20,000) which, in turn, is enveloped by a membrane containing one major protein (Mr 22,000) and a phosphoprotein (Mr 37,000). The viral peplomers, about 20 nm long, carry determinants for neutralization and hemagglutination; they are formed by a polydisperse N-glycosylated protein (Mr 75,000-100,000). Four major subgenomic polyadenylated RNAs have been identified in infected cells, with Mrs of 3.0, 0.71, 0.46 and 0.26 X 10(6). Torovirus replication is inhibited by actinomycin D, alpha-amanitin and pre-irradiation of the host cell with UV light. All toroviruses identified so far cause enteric infections and are probably transmitted by the fecal-oral route. Serologic relationships between equine, bovine and human toroviruses have been demonstrated.