A TOBACCO PROTEIN-KINASE, NPK2, HAS A DOMAIN HOMOLOGOUS TO A DOMAIN FOUND IN ACTIVATORS OF MITOGEN-ACTIVATED PROTEIN-KINASES (MAPKKS)

A cDNA (cNPK2) that encodes a protein of 518 amino acids was isolated from a library prepared from poly(A)(+) RNAs of tobacco cells in suspe nsion culture. The N-terminal half of the predicted NPK2 protein is si milar in amino acid sequence to the catalytic domains of kinases that activate mitogen-activated protein kinases (designated here MAPKKs) fr om various animals and to those of yeast homologs of MAPKKs. The N-ter minal domain of NPK2 was produced as a fusion protein in Escherichia c oli, and the purified fusion protein was found to be capable of autoph osphorylation of threonine and serine residues. These results indicate that the N-terminal domain of NPK2 has activity of a serine/threonine protein kinase. Southern blot analysis showed that genomic DNAs from various plant species, including Arabidopsis thaliana and sweet potato , hybridized strongly with cNPK2, indicating that these plants also ha ve genes that are closely related to the gene for NPK2. The structural similarity between the catalytic domain of NPK2 and those of MAPKKs a nd their homologs suggests that tobacco NPK2 corresponds to MAPKKs of other organisms. Given the existence of plant homologs of an MAP kinas e and tobacco NPK1, which is structurally and functionally homologous to one of the activator kinases of yeast homologs of MAPKK (MAPKKKs), it seems likely that a signal transduction pathway mediated by a prote in kinase cascade that is analogous to the MAP kinase cascades propose d in yeasts and animals, is also conserved in plants.