Ca2+/Calmodulin Activates an MAP Kinase Through the Inhibition of a Protein Phosphatase (DsPTP1) in Arabidopsis

Mitogen-activated protein kinases (MPKs) play roles as critical signal components in the environmental stress responses and developmental processes in plants. Calcium ion (Ca2+) is one of the most essential ubiquitous intracellular second messengers involved in many signal transduction pathways in plants. It was previously known that MPKs are activated by the increasing Ca2+ concentration. However, the mechanism of how Ca2+ activates MPKs is not elucidated yet. In this study, we revealed that Ca2+ could activate MPK signaling pathway via inhibiting the activity of a dual-specificity protein phosphatase1 (DsPTP1) by Ca2+/calmodulin (CaM). We showed that DsPTP1 directly interacts with MPK6 in vitro and in vivo. DsPTP1 was able to inactivate the active MPK6 by dephosphorylation. Interestingly, the DsPTP1-mediated dephosphorylation of MPK6 was strongly inhibited by Ca2+/CaM. Moreover, this inhibition was caused by the binding of CaM to the calmodulin-binding domain II (CaMBDII) of DsPTP1. This study implies that Ca2+/CaM is involved in the activation of MPKs through the inhibition of DsPTP1.