Phosphorylation of the M3/6 dual-specificity phosphatase enhances the activation of JNK by arsenite

► The JNK-specific M3/6 phosphatase is phosphorylated in response to arsenite. ► Phosphorylation sites on M3/6 were mapped and mutated to analyze their role. ► No effect seen on M3/6 activity, specificity, JNK & JIP interactions, or stability. ► The M3/6 phosphorylation mutants delayed JNK activation in response to arsenite. ► JNK-induced phosphorylation of M3/6 temporally modulates the response to stress.