SIMKK, a Mitogen-Activated Protein Kinase (MAPK) Kinase, Is a Specific Activator of the Salt Stress–Induced MAPK, SIMK

Abstract

In eukaryotes, mitogen-activated protein kinases (MAPKs) play key roles in the transmission of external signals, such as mitogens, hormones, and different stresses. MAPKs are activated by MAPK kinases through phosphorylation of MAPKs at both the threonine and tyrosine residues of the conserved TXY activation motif. In plants, several MAPKs are involved in signaling of hormones, stresses, cell cycle, and developmental cues. Recently, we showed that salt stressinduced MAPK (SIMK) is activated when alfalfa cells are exposed to hyperosmotic conditions. Here, we report the isolation and characterization of the alfalfa MAPK kinase SIMKK (SIMK kinase). SIMKK encodes an active protein kinase that interacts specifically with SIMK, but not with three other MAPKs, in the yeast two-hybrid system. Recombinant SIMKK specifically activates SIMK by phosphorylating both the threonine and tyrosine residues in the activation loop of SIMK. SIMKK contains a putative MAPK docking site at the N terminus that is conserved in mammalian MAPK kinases, transcription factors, and phosphatases. Removal of the MAPK docking site of SIMKK partially compromises but does not completely abolish interaction with SIMK, suggesting that other domains of SIMKK also are involved in MAPK binding. In transient expression assays, SIMKK specifically activates SIMK but not two other MAPKs. Moreover, SIMKK enhances the salt-induced activation of SIMK. These data suggest that the salt-induced activation of SIMK is mediated by the dual-specificity protein kinase SIMKK.

Responsive modes of Medicago sativa proline dehydrogenase genes during salt stress and recovery dictate free proline accumulation

“…Although alfalfa SIMK kinase (SIMKK) is considered as the specific activator of SIMK (Kiegerl et al 2000), direct evidence that link the SIMK downstream signaling with specific promoter activation or repression are still missing.…”

LcMKK, a novel group A mitogen-activated protein kinase kinase gene in Lycium chinense, confers dehydration and drought tolerance in transgenic tobacco via scavenging ROS and modulating expression of stress-responsive genes

“…LcMKK belongs to group A, which consists of several well-characterized MAPKK genes, including AtMKK2, NtSIMKK, and GhMKK1. It has been documented that MAPKK members in groups A are predominantly implicated in abiotic stress responses (Kiegerl et al 2000). Clustering of these genes in the same group suggested that they may have a similar or even the same biological functions in abiotic stress responses.…”

“…In Arabidopsis, AtMKK1 can activate AtMPK3 to transmit cold, salt, and drought signals (Mizoguchi et al 1998). In tobaccos, NtSIMKK can regulate salt stress signals by activating SIMK (Kiegerl et al 2000). NtMEK2 can activate WIPK and SIPK to transmit osmotic stress signals (Mikołajczyk et al 2000).…”

Activation of members of a MAPK module in β-glucan elicitor-mediated non-host resistance of soybean

“…In general, MAPKKs are reported to be dual-speciWc kinases that phosphorylate threonine and tyrosine residues in the T-X-Y motif. In most cases, Thr alone or Thr and Tyr were shown to harbor the transferred phosphate group coinciding with the activation of plant MAPKs (Huang et al 2000;Kiegerl et al 2000;Nühse et al 2000). Huang et al (2000) concluded that AtMEK1 may not be a dual-speciWcity kinase.…”