Abstract
Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules in eukaryotes. In plants, MAPK cascades are known to function in responses to various biotic and abiotic stresses. Completion of the Arabidopsis genome-sequencing project has revealed the existence of 20 MAPKs, 10 MAPKKs and 60 MAPKKKs. Here we studied AtMKK3, one of the Arabidopsis MAPKKs, and analyzed which AtMPKs were substrate for AtMKK3. We found that AtMKK3 specifically activate AtMPK8 by in vitro activation assay. AtMPK8 was strongly activated in transgenic plant that over-expressed AtMKK3, suggesting their functional interaction in vivo. Yeast complementation analysis revealed that AtMPK8 function as MAPK. By using yeast two-hybrid system, AtMPK8 was found to interact with calmodulin. Calmodulin was shown to activate AtMPK8. AtMKK3 and AtMPK8 are structurally unique in plants and no significant homologues are found in other eukaryotes. We are analyzing function of AtMKK3-AtMPK8 cascade and a role of calumodulin in the cascade.
