The barley stripe mosaic virus 58-Kilodalton beta b protein is a multifunctional RNA binding protein

The barley stripe mosaic virus (BSMV) beta b gene product is the major viral nonstructural protein synthesized during early stages of the infection cycle and is required for systemic movement of the virus. To examine the biochemical properties of beta b, a histidine tag was engineered at the amino terminus and the protein was purified from BSMV-infected barley tissue by metal affinity chromatography. The beta b protein bound ATPs in vitro, with a preference for ATP over dATP, and also exhibited ATPase activity. In addition, beta b bound RNA without detectable sequence specificity. However, binding was selective, as the beta b protein had a strong affinity for both single-stranded (ss) and double-stranded (ds) RNAs but not for tRNA or DNA substrates. Mutational analyses of beta b purified from Escherichia coli indicated that the protein has multiple RNA binding sites. These sites appear to contribute differently, because mutants that were altered in their binding affinities for ss and ds RNA substrates were recovered.