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Identification and mechanism of ABA receptor antagonism


The phytohormone abscisic acid (ABA) functions through a family of fourteen PYR/PYL receptors, which were identified by resistance to pyrabactin, a synthetic inhibitor of seed germination. ABA activates these receptors to inhibit type 2C protein phosphatases, such as ABI1, yet it remains unclear whether these receptors can be antagonized. Here we demonstrate that pyrabactin is an agonist of PYR1 and PYL1 but is unexpectedly an antagonist of PYL2. Crystal structures of the PYL2–pyrabactin and PYL1–pyrabactin–ABI1 complexes reveal the mechanism responsible for receptor-selective activation and inhibition, which enables us to design mutations that convert PYL1 to a pyrabactin-inhibited receptor and PYL2 to a pyrabactin-activated receptor and to identify new pyrabactin-based ABA receptor agonists. Together, our results establish a new concept of ABA receptor antagonism, illustrate its underlying mechanisms and provide a rational framework for discovering novel ABA receptor ligands.

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Figure 1: Pyrabactin functions both as selective ABA receptor agonist and antagonist.
Figure 2: Structure of the PYL1–pyrabactin–ABI1 complex.
Figure 3: PYL2–pyrabactin antagonist structure.
Figure 4: I137V converts PYL1 into a pyrabactin-inhibited receptor.
Figure 5: A93F converts PYL2 into a pyrabactin-activated receptor.
Figure 6: Identification of pyrabactin-based ABA-receptor agonists.

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We thank the staff of Life Sciences Collaborative Access Team for assistance in data collection at the beam lines of sector 21, which is in part funded by the Michigan Economic Development Corporation and the Michigan Technology Tri-Corridor. Use of the Advanced Photon Source was supported by the Office of Science of the US Department of Energy. This work was supported by the Jay and Betty Van Andel Foundation (H.E.X.) and the US National Institutes of Health (H.E.X. and J-K.Z.). L.-M.N. and F.-F.S. were supported by an overseas PhD scholarship from the National University of Singapore Graduate School for Integrative Sciences & Engineering.

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K.M., J.L., E.-L.Y., S.R.C., J.-K.Z. and H.E.X. conceived the project and designed research; K.M., Y.X., L.-M.N., X.E.Z., F.-F.S., V.C., K.M.S.-P., A.K., F.S.T. and H.E.X. performed research; K.M., Y.X., L.-M.N., X.E.Z., F.-F.S., V.C., K.M.S.-P., A.K., F.S.T., S.R.C., J.-K.Z. and H.E.X. analyzed data; K.M. and H.E.X. wrote the paper with contributions from all authors.

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Correspondence to Karsten Melcher or H Eric Xu.

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The authors declare no competing financial interests.

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Melcher, K., Xu, Y., Ng, LM. et al. Identification and mechanism of ABA receptor antagonism. Nat Struct Mol Biol 17, 1102–1108 (2010).

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