ABA receptor PYL1 (ABA-bound form)
Mouse-ear cress (Arabidopsis thaliana)
Plant hormone called abscisic acid (ABA) plays a key role in the adaptation to drought stress, the regulation of seed maturation and dormancy, and so on. ABA signaling is regulated by phosphorylation/dephosphorylation system (Figure 1). Under the non-stress conditions, a type-2C protein phosphatase (PP2C) inhibits the activation of the kinase SnRK2, which is caused by autophosphorylation. Thus, ABA signaling is turned off by PP2C. When plants are in contrast exposed to dry conditions, accumulated ABA in plant cells is received by ABA receptor. The ABA-bound receptor interacts with PP2C to inhibit a phosphatase activity. Thus, SnRK2 is activated by keeping the phosphorylation state and then phosphorylates the transcription factor AREB/ABF. The activated AREB/ABF promotes the transcription of stress-tolerance genes.
The crystal structure of the ABA receptor PYL1 bound with ABA and that of the complex formed by the further binding of the PP2C protein ABI1 unveiled the inhibitory mechanism of PP2C by ABA receptor in ABA-dependent manner.
The structure of PYL1 exhibit a helix-grip fold, which is common in START protein superfamily and is composed of three helices and a seven-stranded antiparallel β-sheet (Figure 2). START proteins possess a distinctive hydrophobic cavity to bind a hydrophobic compound. PYL1 also possesses a large internal cavity, wherein an ABA molecule is almost completely trapped (Figure 2). This structure suggests that the structural change of PYL1 is necessary to entry ABA into the cavity. The “β3-β4 (gate)” and “β5-β6 (latch)” loops of PYL1 seem to put a lid on the cavity by hydrophobic contacts with ABA. The conformational changes of the lid loops are observed in the superposed structure of ABA-bound PYL1 with apo-PYL1 (PDB ID: 3kay).
The structure of ABA-bound PYL1 and ABI1 complex revealed that these loops in the closed conformation provide the major surface for the interaction with ABI1 (Figure 3).
When ABA-bound PYL1 interacts with ABI1, Trp300 of ABI1 contacts the hydrophobic pocket between the β3-β4 and β5-β6 loops of PYL1 and forms a water-mediated hydrogen bond with the two loops and ABA (Figure 4). This interaction stabilize the complex formation, which makes the β3-β4 loop properly located into the active site of ABI1. Ser112 on β3-β4 loop forms a hydrogen bond with a catalytic residue Glu142 of ABI1. Thus, ABA-bound PYL1 is capable of competitively inhibiting the phosphatase activity of ABI1 and activating ABA signaling.
Structures of ABA-bound PYL1 and its complex with ABI1 have revealed the key mechanisms for regulation of ABA signaling. These findings would be useful for generation of the transgenic crops that tolerates environmental stress and development of compounds to render plants stress resistance.
Protein Data Bank (PDB)
author: Masaru Tanokura, Takuya Miyakawa