Toroviridae: a proposed new family of enveloped RNA viruses.

The proposed family Toroviridae is characterized by enveloped, peplomer-bearing particles containing an elongated tubular nucleocapsid of helical symmetry. The capsid may be bent into an open torus, conferring a biconcave disk or kidney shape on the virion (largest diameter 120-140 nm), or straight, resulting in a rod-shaped particle (dimensions 35 X 170 nm). Morphogenesis occurs by the budding of preformed tubular nucleocapsids through membranes 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, of Mr about 7.0 X 10(6), which is polyadenylated. The RNA is surrounded by the major nucleocapsid phosphoprotein (about 20 kDa) which, in turn, is enveloped by a membrane containing a major 22 kDa protein and a 37 kDa phosphoprotein. The viral peplomers measuring about 20 nm in length, carry determinants for neutralization and haemagglutination; the peplomers are formed by an N-glycosylated protein in the 75 to 100 kDa range. Six (to seven) subgenomic polyadenylated RNAs have been identified in infected cells, with Mr values of 2.6, 1.2, (1.0), 0.55, 0.35, 0.27 and 0.22 X 10(6). Torovirus replication requires some synthetic activity of the host cell. All toroviruses identified so far cause enteric infections and are probably transmitted by the faecal-oral route. Serological relationships between the equine, bovine and human viruses have been demonstrated.