RNA-protein interactions in spherical viruses

The three-dimensional structure of many non-enveloped spherical RNA viruses has been determined in great detail, mainly using X-ray crystallography. Great insight in the structure of the protein capsid has been obtained, but much less information is available about the secondary and tertiary structure of the RNA in situ, due to a number of methodological problems. In this paper the current knowledge about RNA-protein interactions and the folding of the RNA is reviewed, with a special emphasis on the plant virus Turnip yellow mosaic virus. A major characteristic of many spherical RNA viruses appears to be the positioning of A-type double helical segments of 7-9 basepairs at icosahedral symmetry axes, probably interacting via its phosphates with basic amino acid residues of the coat protein in a sequence-independent manner. It is only in the case of the RNA bacteriophages that we know in atomic detail how an RNA hairpin interacts with the coat protein.