RNA-binding properties of the 63 kDa protein encoded by the triple gene block of poa semilatent hordeivirus

The 63 kDa `63K' movement protein encoded by the triple gene block of poa s emilatent virus (PSLV) comprises the C-terminal NTPase/helicase domain and the N-terminal extension domain, which contains two positively charged sequ ence motifs, A and B. In this study, the in vitro RNA-binding properties of PSLV 63K and its mutants were analysed. Membrane-immobilized 63K and N-63K (isolated N-terminal extension domain) bound RNA at high NaCl concentratio ns. In contrast, C-63K (isolated NTPase/helicase domain) was able to bind R NA only at NaCl concentrations of up to 50 mM. In gel-shift assays, C-63K b ound RNA to form complexes that were unable to enter an agarose gel, wherea s complexes formed by N-63K could enter the gel. Full-length 63K formed bot h types of complexes. Visualization of the RNA-protein complexes formed by 63K, N-63K and C-63K by atomic force microscopy demonstrated that each comp lex had a different shape. Collectively, these data indicate that 63K has t wo distinct RNA-binding activities associated with the NTPase/helicase doma in and the N-terminal extension domain. Mutations in either of the positive ly charged sequence motifs A and B had little effect on the RNA binding of the N-terminal extension domain, whereas mutations in both motifs together inhibited RNA binding. Hybrid viruses with mutations in motifs A and B were able to infect inoculated leaves of Nicotiana benthamiana plants, but were unable to move systemically to uninoculated leaves, suggesting that the RN A-binding activity of the N-terminal extension domain of PSLV 63K is associ ated with virus long-distance movement.