FIGURE 4.

Regulation of protein phosphatase 2A (PP2A) by CHIP in plant stress responses. CHIP interacts with and monoubiquitinates the A1 and A3 subunits of PP2A to stimulate its activity. PP2A has regulatory roles in abscisic acid (ABA) signaling and affects ABA responses such as stomatal closure through an association with the sucrose non-fermenting-1-related protein kinase 2- (SnRK2-) type kinases. SnRK2 is inactivated by the interacting 2C-type protein phosphatases (PP2C), and this action of PP2C is inhibited by PYR/PVL ABA receptors in the presence of ABA. PP2A also regulates signaling of other phytohormones such as jasmonic acid and ethylene and affects responses to other stresses such as wounding, cold, and salt stresses.