Molecular Interactions Between a Plant Virus Movement Protein and RNA: Force Spectroscopy Investigation

RNA–protein interactions are fundamental for different aspects of molecular biology such as gene expression, assembly of biomolecular complexes or macromolecular transport. The 3a movement protein (MP) of a plant virus, Cucumber mosaic virus (CMV), forms ribonucleoprotein (RNP) complexes with viral RNA, capable of trafficking from cell-to-cell throughout the infected plant only in the presence of the CMV capsid protein (CP). However, deletion of the C-terminal 33 amino acid residues of the CMV MP (in the mutant designated 3aΔC33 MP) resulted in CP-independent cell-to-cell movement. The biological differences in the behaviour of CMV wild type (wt) 3a MP and 3aΔC33 MP could have been a consequence of differences in the RNA-binding properties of the two MPs detected previously using biochemical assays on ensembles of molecules. To investigate the physical mechanisms of MP–RNA interactions at a single molecule level, we applied atomic force microscopy to measure for the first time unbinding forces between these individual binding partners. Minimal unbinding forces determined for individual interaction of the CMV RNA molecule with the CMV wt or truncated MPs were estimated to be ∼45 pN and ∼90 pN, respectively, suggesting that the distinct differences in the strength of MP–RNA interactions for the wt MP and truncated MP are attributable to the molecular binding mechanism. We also demonstrated that molecules of both CMV 3a MP and 3aΔC33 MP were capable of self-interaction with minimal unbinding forces of ∼50 pN and ∼70 pN, respectively, providing a physical basis for the cooperative mechanism of the RNA binding. The significance of intermolecular force measurements for understanding the structural and functional aspects of viral RNP formation and trafficking is discussed.