SIMKK, a mitogen-activated protein kinase (MAPK) kinase, is a specific activator of the salt stress-induced MAPK, SIMK

In eukaryotes, mitogen-activated protein kinases (MAPKs) play key roles in the transmission of external signals, such as mitogens, hormones, and diffe rent stresses. MAPKs are activated by MARK kinases through phosphorylation of MAPKs at both the threonine and tyrosine residues of the conserved TXY a ctivation motif. In plants, several MAPKs are involved in signaling of horm ones, stresses, cell cycle, and developmental cues. Recently, we showed tha t salt stress-induced MARK (SIMK) is activated when alfalfa cells are expos ed to hyperosmotic conditions. Here, we report the isolation and characteri zation of the alfalfa MARK kinase SIMKK (SIMK kinase). SIMKK encodes an act ive protein kinase that interacts specifically with SIMK, but not with thre e other MAPKs, in the yeast two-hybrid system. Recombinant SIMKK specifical ly activates SIMK by phosphorylating both the threonine and tyrosine residu es in the activation loop of SIMK. SIMKK contains a putative MARK docking s ite at the N terminus that is conserved in mammalian MARK kinases, transcri ption factors, and phosphatases. Removal of the MARK docking site of SIMKK partially compromises but does not completely abolish interaction with SIMK , suggesting that other domains of SIMKK also are involved in MARK binding. In transient expression assays, SIMKK specifically activates SIMK but not two other MAPKs. Moreover, SIMKK enhances the salt-induced activation of SI MK. These data suggest that the salt-induced activation of SIMK is mediated by the dual-specificity protein kinase SIMKK.