RNA-binding activities of cocksfoot mottle sobemovirus proteins

Cocksfoot mottle virus (CfMV) has a positive-sense ssRNA genome containing four open reading frames (ORFs). ORF1 encoded protein (P1) is the putative movement protein; the product of ORF2a (P2a) contains VPg and the motifs characteristic of serine proteases. P2b, encoded by ORF2b, is the putative RNA-dependent RNA polymerase. P3, the coat protein, is encoded by ORF3. CfMV P1, P2a, P2b, and P3, containing a six histidine tag at the amino terminus, were expressed in Escherichia coli, purified and their RNA-binding activities were analysed. The northwestern blot assay showed that His-tagged P1, P2a, P2b, and P3 were able to interact with ssRNA transcripts in a sequence-nonspecific manner. The filter-binding assay confirmed the ssRNA-binding capacity of recombinant P1, P2a, and P3. The RNA-binding activities of His-tagged P3 and native coat protein were similar. P1 and P2a binding to ssRNA decreased markedly by increasing NaCl concentrations. In contrast, P3 had the RNA-binding optimum at 100–200 mM NaCl. We discuss the possible amino acid motifs involved in the RNA-binding of CfMV proteins.