Limits...
Regulatory mechanism of abscisic acid signaling

View Article: PubMed Central - PubMed

ABSTRACT

Abscisic acid (ABA) is a major phytohormone that mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Studies on ABA signaling have progressed rapidly since the recent discovery of PYR/PYL receptor proteins as soluble ABA receptors. In plant cells, the receptor receives ABA to inhibit the phosphatase activity of type 2C protein phosphatase (PP2C), which is the major negative regulator in ABA signaling. SNF1-related protein kinase 2 (SnRK2) is then released from negative regulation by PP2C, turning on ABA signals by the phosphorylation of downstream factors. Insights into the regulation of PYR/PYL receptor proteins is therefore required in order to control drought-stress tolerance in plants. This article reviews the regulatory mechanism of the ABA receptor by ABA and its selective agonist. Structural analyses of PYR/PYL receptors have clearly elucidated the mechanism of ABA perception of the receptor or the mechanism of interaction with PP2C that leads to inhibition of its phosphatase activity. Moreover, the structures of PYR/PYL receptors complexed with pyrabactin, a selective ABA agonist, have provided the structural basis of ABA agonism and antagonism.

No MeSH data available.


ABA-binding mode of the ABA receptor. (a) Overall structure of PYL1 complexed with ABA. Stick and ribbon diagrams show ABA and PYL1, respectively. An ABA molecule binds in an internal cavity, which is shown by a surface diagram. Red, blue and green surfaces represent polar, charged and hydrophobic residues, respectively. (b) Structural basis of (+)ABA stereoselectivity. Polar residues (blue) and K86 (green) form a hydrogen bond network mediated by water molecules (cyan). Hydrophobic residues (orange) are located around the 2,6,6-trimethyl-cyclohexene ring of ABA. Panels (a) and (b) were created using Protein Data Bank (PDB) coordinates of ABA-bound PYL1 (3JRS). (c, d) Chemical structures of ABA stereoisomers: (c) (+)ABA; (d) (−)ABA. An asterisk shows the position of asymmetric carbon.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC5036775&req=5

f2-7_123: ABA-binding mode of the ABA receptor. (a) Overall structure of PYL1 complexed with ABA. Stick and ribbon diagrams show ABA and PYL1, respectively. An ABA molecule binds in an internal cavity, which is shown by a surface diagram. Red, blue and green surfaces represent polar, charged and hydrophobic residues, respectively. (b) Structural basis of (+)ABA stereoselectivity. Polar residues (blue) and K86 (green) form a hydrogen bond network mediated by water molecules (cyan). Hydrophobic residues (orange) are located around the 2,6,6-trimethyl-cyclohexene ring of ABA. Panels (a) and (b) were created using Protein Data Bank (PDB) coordinates of ABA-bound PYL1 (3JRS). (c, d) Chemical structures of ABA stereoisomers: (c) (+)ABA; (d) (−)ABA. An asterisk shows the position of asymmetric carbon.

Mentions: Structural analyses of PYR/PYL receptors clearly elucidated the mechanism of ABA perception9–13. Figure 2a shows the structure of ABA-bound PYL1, a PYR/PYL family member in Arabidopsis. The receptor exhibits a helix-grip fold similar to those of the START protein superfamily, and binds an ABA molecule in its internal cavity. This structure defines the binding mode of ABA and explains its stereoselectivity (Fig. 2b). The carboxyl group of ABA forms an ion pair with K86 and a water-mediated hydrogen bond network with five other polar residues. This network is required for the binding of ABA to PYL1. All other residues in the cavity are hydrophobic and form van der Waals contacts with a 2,6,6-trimethyl-cyclohexene ring. PYR/PYL receptors show a higher affinity to the (+)stereoisomer of ABA than to the biologically less active (−)stereoisomer (Fig. 2c, d)9,10. The flipped dimethyl group in the (−)ABA would cause steric hindrance between the dimethyl group and the narrow pocket that accommodates the monomethyl group9. Thus, the arrangement of the hydrophobic residues surrounding the cyclohexene moiety defines the stereoselectivity of ABA isomers.


Regulatory mechanism of abscisic acid signaling
ABA-binding mode of the ABA receptor. (a) Overall structure of PYL1 complexed with ABA. Stick and ribbon diagrams show ABA and PYL1, respectively. An ABA molecule binds in an internal cavity, which is shown by a surface diagram. Red, blue and green surfaces represent polar, charged and hydrophobic residues, respectively. (b) Structural basis of (+)ABA stereoselectivity. Polar residues (blue) and K86 (green) form a hydrogen bond network mediated by water molecules (cyan). Hydrophobic residues (orange) are located around the 2,6,6-trimethyl-cyclohexene ring of ABA. Panels (a) and (b) were created using Protein Data Bank (PDB) coordinates of ABA-bound PYL1 (3JRS). (c, d) Chemical structures of ABA stereoisomers: (c) (+)ABA; (d) (−)ABA. An asterisk shows the position of asymmetric carbon.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC5036775&req=5

f2-7_123: ABA-binding mode of the ABA receptor. (a) Overall structure of PYL1 complexed with ABA. Stick and ribbon diagrams show ABA and PYL1, respectively. An ABA molecule binds in an internal cavity, which is shown by a surface diagram. Red, blue and green surfaces represent polar, charged and hydrophobic residues, respectively. (b) Structural basis of (+)ABA stereoselectivity. Polar residues (blue) and K86 (green) form a hydrogen bond network mediated by water molecules (cyan). Hydrophobic residues (orange) are located around the 2,6,6-trimethyl-cyclohexene ring of ABA. Panels (a) and (b) were created using Protein Data Bank (PDB) coordinates of ABA-bound PYL1 (3JRS). (c, d) Chemical structures of ABA stereoisomers: (c) (+)ABA; (d) (−)ABA. An asterisk shows the position of asymmetric carbon.
Mentions: Structural analyses of PYR/PYL receptors clearly elucidated the mechanism of ABA perception9–13. Figure 2a shows the structure of ABA-bound PYL1, a PYR/PYL family member in Arabidopsis. The receptor exhibits a helix-grip fold similar to those of the START protein superfamily, and binds an ABA molecule in its internal cavity. This structure defines the binding mode of ABA and explains its stereoselectivity (Fig. 2b). The carboxyl group of ABA forms an ion pair with K86 and a water-mediated hydrogen bond network with five other polar residues. This network is required for the binding of ABA to PYL1. All other residues in the cavity are hydrophobic and form van der Waals contacts with a 2,6,6-trimethyl-cyclohexene ring. PYR/PYL receptors show a higher affinity to the (+)stereoisomer of ABA than to the biologically less active (−)stereoisomer (Fig. 2c, d)9,10. The flipped dimethyl group in the (−)ABA would cause steric hindrance between the dimethyl group and the narrow pocket that accommodates the monomethyl group9. Thus, the arrangement of the hydrophobic residues surrounding the cyclohexene moiety defines the stereoselectivity of ABA isomers.

View Article: PubMed Central - PubMed

ABSTRACT

Abscisic acid (ABA) is a major phytohormone that mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Studies on ABA signaling have progressed rapidly since the recent discovery of PYR/PYL receptor proteins as soluble ABA receptors. In plant cells, the receptor receives ABA to inhibit the phosphatase activity of type 2C protein phosphatase (PP2C), which is the major negative regulator in ABA signaling. SNF1-related protein kinase 2 (SnRK2) is then released from negative regulation by PP2C, turning on ABA signals by the phosphorylation of downstream factors. Insights into the regulation of PYR/PYL receptor proteins is therefore required in order to control drought-stress tolerance in plants. This article reviews the regulatory mechanism of the ABA receptor by ABA and its selective agonist. Structural analyses of PYR/PYL receptors have clearly elucidated the mechanism of ABA perception of the receptor or the mechanism of interaction with PP2C that leads to inhibition of its phosphatase activity. Moreover, the structures of PYR/PYL receptors complexed with pyrabactin, a selective ABA agonist, have provided the structural basis of ABA agonism and antagonism.

No MeSH data available.