Identification of amino acid sequences determining interaction between the cucumber mosaic virus-encoded 2a polymerase and 3a movement proteins.

The cucumber mosaic virus (CMV)-encoded 3a movement protein (MP) is indispensable for CMV movement in plants. We have previously shown that MP interacts directly with the CMV-encoded 2a polymerase protein in vitro. Here, we further dissected this interaction and determined the amino acid sequences that are responsible for the MP and 2a polymerase protein interaction. Both the N-terminal 21 amino acids and the central GDD motif of the 2a polymerase protein were important for interacting with the MP. Although each of the regions alone was sufficient for the interaction with MP, quantitative yeast two-hybrid analyses showed that they acted synergistically to enhance the binding affinity. The MP N-terminal 20 amino acids were sufficient for interacting with the 2a polymerase protein, and the serine residue at position 14 played a critical role in the interaction. Multiple sequence alignment showed that the 2a protein interacting regions and the serine at position 14 in the MP are highly conserved among subgroup I and II CMV isolates.