Molecule of the Month: Abscisic Acid Receptor

Unlike animals, plants are unable to move from place to place when faced with adverse conditions, such as heat, cold, and nutrient deficiencies. As a result, plants have evolved a wide variety of mechanisms to sense and respond to their environment. At a molecular level, plants employ sensor molecules that can turn on signaling cascades, triggering transcriptional and translational changes that ultimately enable the plant to react and adapt to stressors. Such adaptations often come at a cost, however, such as slower or decreased growth when compared to plants grown under more ideal conditions. When crops face environmental challenges, yields often suffer as a result. Understanding how plants cope with difficult environments may help researchers produce more stress-resilient crops that are able to maintain yields despite challenging conditions.

When faced with unusually dry conditions (drought), plants carry out a variety of actions that can help them survive. Measures include closing stomata, small openings on the surface of leaves that are important for efficient photosynthesis, but can also allow for loss of water. During a drought, plants also promote seed maturation and delay germination. Translation of proteins involved in stress resistance is also turned on, initiating additional protective mechanisms.

Many of these stress responses are mediated by a signaling pathway activated by abscisic acid (ABA), a small-molecule hormone synthesized by plant tissue under drought conditions. ABA binds to ABA receptors of the PYR/PYL/RCAR family. On the right, the ABA receptor PYL1 from Arabidopsis thaliana is shown bound to ABA, which is shown in salmon (PDB 3JRS). The binding site for ABA is a conserved pocket flanked by two loops, known as the gate and the latch. Binding of ABA "locks" the gate and latch. You can take a closer look at this binding mechanism in the "Exploring the Structure" section below.

After ABA binds to the ABA receptor, the ABA-ABA receptor complex can bind to a type 2C phosphatase (PP2C). Binding occludes the PP2C active site, preventing it from recognizing and binding the kinase SnRK2 (PDB 3JRQ, 3KB3). SnRK2 is then free to phosphorylate targets, eventually leading to the activation of drought tolerance mechanisms.

Interestingly, scientists have found that the interaction between PP2C and SnRK2 is very similar to the interaction between PP2C and the ABA receptor. While the overall structure of ABA-bound ABA receptors and SnRK2 proteins are very different, the way that they bind to PP2C match closely, with each providing a binding pocket for a conserved tryptophan from PP2C. As shown in the insets on the left, this tryptophan contacts the bound ABA molecule within the ABA receptor complex and is thought to act as an ABA sensor (PDB 3KB3). The same tryptophan contacts the activation loop of SnRK2 (shown in orange, PDB 3UJG), preventing phosphorylating of substrate proteins.

This is an example of molecular mimicry, where two different proteins - SnRK2 and the ABA-bound ABA receptor - appear nearly identical to PP2C, allowing it to readily swap between these partners. This mimicry is thought to be important due to the large numbers of SnRK2 and PP2C homologs encoded in plant genomes, requiring mechanisms to ensure that only the correct protein partners are activated.