The Structure of Cowpea Mosaic Virus at 3.5 Å Resolution

Abstract

There are nine groups of icosahedral, positive stranded RNA plant viruses (Francki et al., 1985) established on the basis of immunological relationships, genomic size, the number of RNA molecules composing the genome and the stability of the capsid. Eight of these groups are remarkably similar in both structural and functional properties while one group, the comoviruses, is quite different from the others (Goldbach and vanKammen, 1985). Cowpea mosaic virus (CPMV), the type member of the comovirus group, has only two properties in common with any of the other plant virus groups. The genome of CPMV is bipartite and a small protein (VpG) is linked to the 5′ terminus of each RNA molecule (Bruening, 1977; Daubert et al., 1979). In all its other properties CPMV is unique among the plant viruses. The RNA molecules of CPMV are polyadenylated at their 3′ termini while the genomes of the other virus groups are not. CPMV proteins are generated by the processing of two polyproteins, produced by translation of single open reading frames on each RNA molecule. The proteases that cleave the polyproteins are virally encoded and are initially part of the translation product of the larger of the two RNA molecules. The other plant virus groups utilize subgenomic RNA molecules as the messengers for protein synthesis with no posttranslational processing. The CPMV genome codes for a total of eight proteins, three (including the capsid proteins) derived from the small RNA molecule, and five from the large RNA.