The genome of Cucumber mosaic virus consists of three single-stranded RNA molecules, RNAs 1, 2 and 3. RNAs 1 and 2 encode the 1a and 2a proteins, respectively, which are necessary for replication of the viral genome and have been implicated in movement of the viral RNAs in plants. The 3a movement protein (MP), encoded by RNA 3, is essential for transferring the RNA genomes from infected cells to adjacent cells across the plasmodesmata. Far-Western analysis demonstrated that bacterially expressed 2a polymerase protein directly interacted with the MP. Interaction was confirmed in a yeast two-hybrid assay, and co-immunoprecipitation analysis showed that the MP interacted only with the 2a polymerase protein. A yeast three-hybrid assay showed that the 1a-2a protein interaction relevant for replicase complex formation was not affected by the MP. Although the MP has no affinity for the 1a protein, it interacted indirectly with the 1a protein via the 2a polymerase protein. These results suggest that the replicase complex may be involved in movement through its interaction with the MP.
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