Fig. 10.

Regulation of MKKK18 activity and stability by ABI1 PP2C. Under standard conditions, active ABI1 protein phosphatase inhibits MKKK18 kinase activity and protein stability. ABA receptors and dephosphorylated MKKK18 are directed for degradation via the proteasome pathway. In the presence of ABA, ABI1 PP2C activity is restrained by binding of ABI1 to the ABA–PYL complex (ABA signaling is turned on). Active MKKK18 activates the downstream cascade. ABA inhibits the degradation of ABA receptors by limiting their polyubiquitination.