Protein-protein- and protein-RNA-binding properties of the movement protein and VP25 coat protein of Apple latent spherical virus

To elucidate the mechanism of Apple latent spherical virus (ALSV) movement, various properties of its cell-to-cell movement protein (MP) were analyzed. ELISA and blot overlay assays demonstrated that the MP bound specifically to ALSV virions and in particular to one of the three coat proteins (VP25) but not to the other two coat proteins (VP20 and VP24). Mutational analyses have revealed that the MP contains two domains with independent VP25-binding activity (amino acid residues 1-188 and 189-281). Furthermore, nucleotide-binding experiments showed that the MP and VP25 bound to single-stranded RNA (ssRNA) and ssDNA without any sequence specificity, but these two proteins did not bind to double-stranded RNA (dsRNA) and dsDNA. The MP contains three potentially independent single-stranded nucleic acid-binding domains between amino acid residues 95-188, 189-281 and 277-376. The MP demonstrated cooperative and VP25 demonstrated non-cooperative binding to ssRNA in gel-retardation analyses. The cooperative RNA binding of the MP became non-cooperative when MP and VP25 were tested together in competition binding experiments, even though a sufficient amount of the MP for fully cooperative RNA binding the MP was supplied. The roles of the MP and VP25 interactions and nucleic acid binding activities in ALSV movement are discussed.