A Tobacco CBL-Interacting Protein Kinase Homo log Is Involved in Phosphorylation of the N-Terminal Domain of the Cucumber Mosaic Virus Polymerase 2a Protein

The replication and transcription of cucumber mosaic virus (CMV) are catalyzed by multi-protein complex RNA-dependent RNA polymerase (RdRp), which is composed of the viral-encoded la and 2a proteins with host factors. We have reported that the N-terminal region of the :polymerase 2a protein, composed of 126 amino acids, is required for interaction with the helicase la protein, and that the phosphorylation of the region abrogated interaction with the la protein, suggesting a mechanism of resistance in host plants against viral infection. Here, we found that three protein 2a kinases, of 60, 55, and 38 kDa, co-purified with the tobacco membrane fraction in an in-gel kinase assay. By yeast two-hybrid library screening using the N-terminal 126 amino acids of 2a as a bait, we identified CBL-interacting protein kinase 12 (NtCIPK12) corresponding to 55 kDa protein 2a kinase. The bacterially expressed protein kinase showed protein 2a kinase (t2aK) activity in vitro. We found that NtCIPK12 stabilized upon CMV infection at the post-translational level, and accumulated more heavily to the membrane than in the cytosol.