Agriculture Reference
In-Depth Information
we found that the bulkier hydrophobic residue (Leu79) at the entrance of the
ligand-binding pocket in PYL10 provided a platform to dock the pocket-fac-
ing, hydrophobic Leu83 of gate loop, thus facilitating a closed conformation of
the gate loop. On the contrary, the corresponding residue in other PYLs except
PYL13 is valine (see Fig. 7.3 ), which is smaller than leucine. Moreover, a lysine
in PYL10 (Lys80), which prevents PYL10 dimerization in solution (the charged
residue is unfavorable in the hydrophobic interface), is replaced by isoleucine or
valine residues of dimeric PYLs (see Fig. 7.8 b). Therefore, PYL10 exists as a
monomer.
Taken together, in order to achieve ABA-independent inhibition of PP2Cs, PYLs
should be a monomer first. Then, the residues guarding the ligand-binding pocket
of PYLs must be bulky and hydrophobic so that they can make sufficient hydropho-
bic contacts to stabilize a closed gate loop in the absence of ABA. If the biochemi-
cal features of PYLs pockets cannot match these requirements, an amphipathic
ligand, such as ABA or pyrabactin, is required to close the gate loop to the binding
pocket. Recent studies found that PYL13 selectively inhibited PP2CA independ-
ent of ABA, and PYL13 antagonized PYL10 in the ABA-independent inhibition of
PP2Cs because PYL13 and PYL10 were the only two PYLs containing a Leu resi-
due instead of Val residue to facilitate the closure of the gate loop (Li et al. 2013 ).
Although some monomeric PYLs can bind to PP2Cs in the absence of ABA,
their inhibitory is greatly reduced compared with the inhibitory in the presence of
ABA. This can be structurally explained by the fact that ABA enhances the inter-
action between PYLs and PP2Cs by a lot of hydrophobic interactions and water-
mediated hydrogen bonds.
The structural research have revealed how the binding of ABA to PYLs leads
to the inhibition of PP2Cs, but how does this event then convey to other outputs?
An important clue came from the finding of Snf1 (Sucrose-non-fermentation
1)-related kinases subfamily 2 (SnRK2s) as a positive regulator of ABA signaling
(Park et al. 2009 ). The SnRK2s subfamily contains 10 members in Arabidopsis .
In normal growth conditions where cellular ABA levels are low, PP2Cs bind and
dephosphorylate SnRK2s to keep them in inactive state. When stresses come, ele-
vated cellular ABA binds to PYLs, which in turn bind and deactivate the phos-
phatase activity of PP2Cs, and then release the SnRK2s (Boudsocq 2004 ; Belin
et al. 2006 ; Vlad et al. 2009 ). The activated SnRK2s could be self-activated via
auto-phosphorylation, then phosphorylate different downstream targets, such as
the b-ZIP transcription factors and ion channels SLAC1 (slow anion channel 1),
KAT (inward rectifying K + channel) (Geiger et al. 2009 ; Lee et al. 2009 ; Sato
et al. 2009 ), and so on.
SnRK2s contain a Snf1-related kinase domain and a highly acidic C-terminal
segment termed ABA box, which is important for their interactions with PP2Cs
(Boudsocq et al. 2006 ). The solved SnRK2s-PP2Cs complex structure reveals
marked similarity in PP2Cs recognition by SnRK2s and ABA receptors (Soon
et al. 2012 ). In addition to the highly acidic C-terminal region, SnRK2.6 contrib-
utes three separate regions within the kinase domain for binding to HAB1: first,
the activation loop inserts deeply into the catalytic cleft of HAB1 and mimics the
Search WWH ::




Custom Search