Science 335, 85–88 (2012)

Proc. Natl. Acad. Sci. USA 108, 21259–21264 (2011)

Credit: X. EDWARD ZHOU

Abscisic acid (ABA) is a hormone that regulates plant responses to environmental stressors. Formation of an ABA–ABA receptor complex inactivates type 2C protein phosphatases (PP2Cs) and leads to activation of Snf1-related kinases (SnRK2s) and engagement of ABA signaling pathways. A three-dimensional structure of a ternary complex of ABA–ABA receptor–PP2C previously allowed the identification of a 'gate-latch-lock' mechanism for binding the PP2C phosphatase in an inactive form. Now Soon et al. provide complementary structural insights into how PP2C phosphatases inactivate SnRK2 in the absence of ABA, and, in a related study, Ng et al. show how SnRK2s autoactivate when PP2C is inhibited by ABA-bound receptors. The authors solved two structures of SnRK2 and a structure of SnRK2.6 in complex with PP2C HAB1. The binding interface of the SnRK2.6 protein structurally mimics that seen in the ABA-receptor complex; SnRK2.6 presents its kinase activation loop as a gate mimic that inserts into the phosphatase active cleft, whereas HAB1 adopts a rigid geometry and inserts the lock into the kinase active site. The structural studies, along with additional biochemical experiments, established that HAB1 inactivates SnRK2.6 in two ways: by dephosphorylation of a critical serine in its activation loop and by disruption of its kinase domain at the binding interface. In addition to providing insight into ABA signaling, the authors hypothesized that inhibitory kinase-phosphatase interactions may serve as a general mechanism for regulation of other signaling pathways.