Featured Article - November 2010
Short description: There are 14 members in the ABA receptor family in Arabidopsis thaliana. How these receptors differ in their response to a synthetic, seed-specific ABA agonist is now explored in two independent papers, using a combination of crystallography, NMR and biochemical approaches.
When plants encounter environmental stress conditions, such as drought, cold or high salinity, they can't just pick up and leave to find greener pastures. Instead, plants mount adaptive responses to survive such conditions, including closing of the leaf stomata, stimulating root growth and maintaining seeds and buds in a dormant state. All these physiological responses are mediated by plant hormone abscisic acid (ABA). Due to its potential agricultural applications, ABA signaling has been an active area of investigation. In particular, it was established that inhibition of type 2 protein phosphatases (PP2C) takes place in response to ABA, but the identity of the ABA receptor(s) remained undefined despite intensive research, with a few false candidates put forth.
Then, in 2009, several independent groups published genetic, biochemical and structural evidence that a family of proteins called PYR (for “pyrabactin resistance”), PYL (“PYR-like proteins”) or RCARs (“regulatory component of ABA receptor”) were the receptors for ABA. This body of work was highlighted as one of the scientific breakthroughs of 2009. The research showed that, when bound to ABA, PYR/PYL proteins interact with and inhibit PP2C. More specifically, the presence of ABA within the ligand-binding pocket of PYR/PYL proteins induced a conformational change (closure of 2 loops flanking the ligand binding pocket). Contributing to this breakthrough was the use of pyrabactin, a synthetic ABA agonist specific for response in seeds.
Now the structural basis for pyrabactin selective effects on PYR/PYL proteins are studied in atomic detail. Brian Volkman, Sean Cutler, George Phillips and colleagues (ref 1, PSI CESG) use multiple structural and biochemical approaches to show that pyrabactin adopts different conformations within the ligand binding pockets from different PYR/PYL. Crystal structures reveal that pyrabactin binding to PYR1 leads to closing of the loops and recruitment of PP2C, similar to the effect of ABA binding. In contrast, PYL2 is less responsive to pyrabactin, and this is explained by NMR data indicating that pyrabactin binds to the ligand-binding pocket in multiple conformations, including non-productive orientations, with the net result of inefficient loop closure. Substitution of Ile100 and Ile62 residues in the ligand-binding pocket of PYR1 led to variants that allowed non-productive orientation of pyrabactin.
An independent report, led by Karsten Melcher and Eric Xu (ref 2), also examines the effect of pyrabactin on different PYR/PYL proteins. In addition to confirming that pyrabactin does not promote the recruitment of PP2C to PYL2, the authors actually find that pyrabactin can compete with and inhibit the binding of ABA to PYL2; thus, pyrabactin is a PYL2-selective antagonist. The authors present crystal structures of PYL1-pyrabactin-PP2C and PYL2-pyrabactin complexes, observing features similar to those described above. They also show that mutations in residues within the ligand binding pockets of PYL1 or PYL2 can determine the effect of pyrabactin. Finally, the structural information on pyrabactin binding is used to identify compounds with agonist action on ABA receptors by computational docking, followed by in vitro validation.
Both of these studies indicate that the variations in the ligand-binding pocket of PYR/PYL members have a physiological role, and suggest that the regulation of ABA responses may involve endogenous signals other than ABA, as agonists or antagonists. They also pave the way for biotechnological applications, such as controlling stress tolerance and improving crop yields.
F. C. Peterson, E. S. Burgie, S.-Y. Park, D. R. Jensen, J. J. Weiner et al. Structural basis for selective activation of ABA receptors.
Nat. Struct. Mol. Biol. 17, 1109-1113 (2010). doi:10.1038/nsmb.1898
K. Melcher, Y. Xu, L.-M. Ng, X. E. Zhou, F.-F. Soon et al. Identification and mechanism of ABA receptor antagonism.
Nat. Struct. Mol. Biol. 17, 1102-1108 (2010). doi:10.1038/nsmb.1887