Understanding the functional mechanism of pyrabactin will shed light on the development of novel ABA agonists that may have applicable potentials in agriculture. ABA is not a stable compound, and its costly synthesis further restricts its potential applications. Pyrabactin thus provides an alternative to the development of novel ABA-related compounds. In this paper, we performed biochemical and structural studies and report here the crystal structure of pyrabactin in complex with PYL1. Our study nicely illustrates how pyrabactin serves as an ABA agonist. Structural comparison with ABA-bound PYL1 revealed a general principle in the arrangement of function groups of the ligands.
Photo provided by Flickr
The analyses also explained why the position of the pyridyl nitrogen is essential for pyrabactin function (). If the nitrogen is located at any other sites on the pyridyl ring, it will be out of reach of the polar residues in PYL1 even in the presence of water molecules. In addition, an amine group on any other positions of the pyridyl ring would interfere with the hydrophobic module 2′, which might be unfavorable to the hydrophobic environment within the receptor (C and B). To corroborate the analyses, we synthesized a few unreported variants of pyrabactin and examined their function. Removal of the pyridyl nitrogen (PYB1) or the bromide (PYB2) severely crippled the effect of the compounds on PYL1, whereas replacement of the bromide with a methyl group (PYB3) retains its function (E). PYL1 responds to PYB3 with an IC50 of 2.6 ± 0.17 μ, comparable with that of pyrabactin. This observation indicates that a bulkier hydrophobic plate is required at hydrophobic module 1′ so as to attract the CL2 switch to bend over (D).
identified group of ABA receptor proteins of the PYRABACTIN ..
Photo provided by Flickr
Pyrabactin is a synthetic abscisic acid (ABA) agonist that selectively inhibits seed germination. The use of pyrabactin was pivotal in the identification of the PYR1/PYL/RCAR family (PYL) of proteins as the ABA receptor. Although they both act through PYL proteins, pyrabactin and ABA share no apparent chemical or structural similarity. It remains unclear how pyrabactin functions as an ABA agonist. Here, we report the crystal structure of pyrabactin in complex with PYL1 at 2.4 Å resolution. Structural and biochemical analyses revealed that recognition of pyrabactin by the pocket residues precedes the closure of switch loop CL2. Structural comparison between pyrabactin- and ABA-bound PYL1 reveals a general principle in the arrangements of function groups of the two distinct ligands. The study provides a framework for the development of novel ABA agonists that may have applicable potentials in agriculture.