ATMPK4, an Arabidopsis homolog of mitogen-activated protein kinase, is activated in vitro by AtMEK1 through threonine phosphorylation.

摘要

The in vitro activation of an Arabidopsis thaliana homolog of MAP kinase, ATMPK4, is described. ATMPK4 was phosphorylated in vitro by an Arabidopsis MEK homolog, AtMEK1. This phosphorylation occurred principally on threonine (Thr) residues and resulted in elevated ATMPK4 kinase activity. A second Arabidopsis MEK isoform, ATMAP2Kalpha, failed to phosphorylate ATMPK4 in vitro. Tyr dephosphorylation by the Arabidopsis Tyr-specific phosphatase AtPTP1 resulted in an almost complete loss of ATMPK4 activity. Immunoprecipitates of Arabidopsis extracts with anti-ATMPK4 antibodies displayed myelin basic protein kinase activity that was sensitive to treatment with AtPTP1. These results demonstrate that a plant MEK can phosphorylate and activate MAPK, and that Tyr phosphorylation is critical for the catalytic activity of MAPK in plants. Surprisingly, in contrast to the animal enzymes, AtMEK1 may not be a dual-specificity kinase but, rather, the required Tyr phosphorylation on ATMPK4 may result from autophosphorylation.