The abscisic acid receptor PYR1 in complex with abscisic acid

Abstract

The plant hormone abscisic acid (ABA) has a central role in coordinating the adaptive response in situations of decreased water availability as well as the regulation of plant growth and development. Recently, a 14-member family of intracellular ABA receptors, named PYR/PYL/RCAR, has been identified. These proteins inhibit in an ABA-dependent manner the activity of a family of key negative regulators of the ABA signalling pathway: the group-A protein phosphatases type 2C (PP2Cs). Here we present the crystal structure of Arabidopsis thaliana PYR1, which consists of a dimer in which one of the subunits is bound to ABA. In the ligand-bound subunit, the loops surrounding the entry to the binding cavity fold over the ABA molecule, enclosing it inside, whereas in the empty subunit they form a channel leaving an open access to the cavity, indicating that conformational changes in these loops have a critical role in the stabilization of the hormone-receptor complex. By providing structural details on the ABA-binding pocket, this work paves the way for the development of new small molecules able to activate the plant stress response.

Structural insights into PYR/PYL/RCAR ABA receptors and PP2Cs

“…For instance, apoPYL1 [15], apoPYL2 [15,19], ABA-bound PYL1 [16] and ABA-bound PYL2 [15,19] structures have been resolved. In the case of PYR1, in the crystallographic asymmetric unit, there is one ABA-bound and one ABA-free subunit [17,18]. A high degree of structural similarity is evident from the superposition of the three structures (Fig.…”

“…2), however, recent works using the pyrabactin molecule reveal subtle differences among the receptor binding pockets with important functional consequences, since pyrabactin is an agonist of PYR1 and PYL1, whereas it is an antagonist of PYL2 [20,21,22]. Different experimental data indicate that these receptors exist as dimers in solution, however, the receptor-PP2C complexes have a 1:1 stoichiometry, implying that the receptor dimers have to dissociate before interacting with the PP2C [17,18,19].…”

“…The structure of the ABA-bound forms of PYR1, PYL1 and PYL2 reveals the nature of the interactions stabilizing the hormone into the receptor binding pocket [15,16,17,18,19]. The ABA sits in a deep cavity almost completely buried from the external medium (see figure 3).…”

“…The PYR/PYL/RCAR family of ABA receptors has witnessed a rapid explosion of structural data, since five independent groups have reported structural and functional mechanisms on ABA signaling [15,16,17,18,19], and even more recently, using the seed ABA-agonist pyrabactin (Fig. 1C), the structural basis for selective activation of certain receptors and the mechanism of antagonism in other members [20,21,22].…”

Protein kinase CK2α subunits constitutively activate ABA signaling in Arabidopsis

“…1,2 In Arabidopsis , binding of the PYR/PYL/RCAR receptor proteins to ABA triggers the formation of ternary complexes with group-A protein phosphatase 2Cs (PP2Cs) and derepresses the subclass III SnRK2 protein kinases such as SRK2D/SnRK2.2, SRK2E/OST1/SnRK2.6, SRK2I/SnRK2.3. 3–7 Then, the SnRK2s phosphorylate basic leucine zipper (bZIP) transcription factors such as ABRE-binding protein/ABRE-binding factors (AREB/ABFs), which then activate the expression of ABA-responsive genes. 8–13 ABRE-dependent gene expression plays a major role in ABA-responsive gene expression as ABA signal output under dehydration stress.…”