Structural Basis of ABA Perception by PYR/PYL/RCAR Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 114

7.1 The Detection of ABA Receptors
As we all know, a receptor, which is usually a protein molecule found on the sur-
face of a cell or in the cytoplasm/nucleus, receives chemical signals from outside
of the cell. The molecule that binds to a receptor is called a ligand, which can
be a peptide or other small molecules like hormone. The receptors have several
features such as: high affinity with ligand; saturable and reversible binding; high
specificity. Abscisic acid (ABA) is a plant hormone that regulates a lot of pro-
cesses including seed germination, development, and abiotic stress tolerance,
especially drought resistance (Kline et al. 2010 ). By binding to the receptors of the
cell, ABA molecule delivers its signal to the downstream messengers, and finally
to the responsive genes, thus regulating the expression of many genes. In the ABA
signaling pathway, the perception by the ABA receptors is mostly important.
Usually, through traditional genetic approach, many proteins involved in hor-
mone signaling are identified through screening of Arabidopis mutants with
reduced or enhanced sensitivity to a hormone. Many hormone receptors were
cloned through this method (Santner and Estelle 2009 ). But for ABA, it did not
work, because ABA receptors were endowed with functional redundancy or plei-
otropic effects including embryo or gamete lethality (Santiago et al. 2012 ). By
using biochemical techniques, ABA-binding proteins CHLH/ABAR/GUN5 were
first found by Zhang group (Shen et al. 2006 ), then G protein-coupled receptors
GCR2 (Liu et al. 2007 ), GTG1/GTG2 (Pandey et al. 2009 ), were also found to
bind ABA by pharmacological evidence. A chemical genetic approach using a syn-
thetic selective ABA agonist, pyrabactin (4-bromo- N -pyridin-2-yl methyl naphtha-
lene-1-sulfonamide), was pursued to identify a family of soluble ABA receptors,
which is named PYRABACTIN RESISTANCE 1 (PYR1)-like (PYL) (Park et al.
2009 ). Mutants were not strongly abscisic acid resistant until the plants lost at
least three members of PYLs family. In an independent study, Ma et al. 2009 also
identified the PYLs family and referred it as the regulatory component of ABA
receptors (RCAR) by a yeast-2-hybrid screen. RCAR interacted with PP2Cs in an
abscisic acid-dependent manner and inhibited its phosphatase activity.
PYR/PYL/RCAR receptors belong to the START/Bet v I superfamily, which
forms a helix-grip fold structural motif containing a seven-stranded ʲ -sheet and
three-to-four ʱ -helices. The helix-grip fold creates a large conserved hydropho-
bic ligand-binding pocket that can bind diverse lipids, hormones, and antibiotics
(Klingler et al. 2010 ). There are 14 PYR/PYL/RCAR family members, named
PYR1, PYL1-PYL13 in Arabidopis , showing functional redundancy.
7.2 Structural Insights into PYR/PYL/RCAR Receptors
The crystal structures of ABA receptors, PYR1, PYL1, PYL2, PYL3, PYL5,
PYL9, PYL10, and PYL13 (Miyazono et al. 2009 ; Santiago et al. 2009 ; Yin et al.
2009 ; Hao et al. 2011 ; Zhang et al. 2012 , 2013 ; Li et al. 2013 ), had been reported