Virion Composition

Nodavirus particles are composed of a protein shell surrounding one molecule of each of the two genomic RNAs. Evidence for copackaging of the RNAs comes from the uniform sedimentation coefficient (135— 140S) and buoyant density (1.33-1.35 g ml-1) of the particles, measurements of their particle weight (about 9X 103 kDa) and total RNA content (about 16%), and the demonstration that the two viral RNAs can be crosslinked by UV irradiation of virus particles. Most compelling, however, is the linear relationship that exists between virus particle number and infectivity as determined by plaque assay of FHV. Since both viral RNAs are required for a productive infection, this result establishes that they are repackaged.

The protein shell is composed of 180 protomers arranged with icosahedral symmetry on a T = 3 surface lattice (i.e. the asymmetric structural unit contains three chemically identical protomers). In the alpha nodaviruses, each protomer is composed of one copy of the capsid proteins /? (about 40 kDa) and y (about 5 kDa), which are produced by a single proteolytic cleavage of the coat protein precursor, protein a (about 45 kDa). Cleavage occurs relatively slowly after virus assembly, so fresh virus preparations usually contain a small amount of uncleaved protein a. Since protein y is small and stains poorly, it has been overlooked in the characterization of some of the nodaviruses, but in BBV, FHV and NOV it is present in virions in equimolar amounts with protein

The amino acid sequences of the coat protein precursors (a) of several of the alpha and beta nodaviruses have been inferred from the nucleotide sequences of the corresponding RNA 2 segments. They contain about 400 amino acid residues and clearly represent members of the same evolutionary lineage. The protein a sequences of BBV and FHV are 87% identical to one another, and about 50% identical to those of NOV and BoV. In contrast, they share only about 10% sequence identity with the capsid proteins of the beta nodaviruses. All a protein sequences have very basic regions near their N-termini, where about one-third of the first 50 amino acid residues are lysines or arginines. For the alpha nodaviruses, cleavage of protein a to form proteins ft and y occurs near the carboxy terminus, between asparagine 363 and alanine 364 in the BBV and FHV proteins, and the same two residues are present at the homologous positions in the proteins of NOV and BoV, suggesting evolutionary conservation of the cleavage site. Beta nodavirus particles contain roughly equal amounts of two related capsid proteins of 40 and 42 kDa. It has not yet been determined whether these correspond to the precursor protein a and its larger cleavage product fi.

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