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ABA sensitivity (Choi et al. 2005 ). It also interacts with ABF1, ABF2/AREB1,
and ABF3. Two other CDPKs, CPK4 and CPK11, phosphorylates ABF1 and
ABF4/AREB2 and act as positive regulators of ABA response (Zhu et al. 2007 ).
11.5 Modulation of ABFs/AREBs/ABI5 Functions by
Protein Degradation
Protein degradation by proteasome is an important mechanism of regulating the
abundance of regulatory proteins (Vierstra 2009 ), and it plays an important role in
the regulation of ABI5 activity. ABA promotes the accumulation of ABI5 protein
by stabilizing it (Lopez-Molina et al. 2001 ). This stabilization process involves
protein phosphorylation, which is accomplished by above-mentioned kinases.
Thus, protein phosphorylation not only activates the transcriptional activity of
ABI5 but also increases its stability.
Several components of the ubiquitin-26S proteasome system have been
reported to control ABI5 degradation. AFP, which is one of the ABI5-interacting
proteins, facilitates ubiquitin-mediated degradation of ABI5 (Lopez-Molina et al.
2003 ). KEEP ON GOING (KEG), a RING-type E3 ligase, ubiquitinates ABI5 and
promotes its proteasomal degradation to negatively regulate ABA signaling (Liu
and Stone 2010 , 2013 ; Stone et al. 2006 ). Recently, ABF1 and ABF3 also have
been demonstrated to be the substrates of KEG (Chen et al. 2013b ). CUL4-based
E3 ligases, DWA1 and DWA2, act as negative regulators of ABA response by tar-
geting ABI5 for proteasomal degradation (Lee et al. 2010a ). Although not related
to protein degradation directly, ABI5 is a target of the SUMO E3 ligase SIZ1,
which inactivates and stabilizes ABI5 (Miura et al. 2009 ). It has been proposed
that reversible sumoylation of ABI5 by SIZ1 protects inactive form of ABI5 from
degradation. In this scheme of ABI5 activity regulation, ABA activation of ABI5 is
preceded by desumoylation of ABI5.
11.6 ABI3/ABI4
ABI3 and ABI4 have been isolated by map-based cloning (Finkelstein 1994 ;
Giraudat et al. 1992 ). ABI3, which is an ortholog of the maize VP1 (McCarty et al.
1991 ), belongs to the B3-domain family of TFs (Swaminathan et al. 2008 ). ABI3
expression is seed specific, and it plays a key role in the establishment of seed dor-
mancy and seed maturation, together with two other B3 proteins FUS3 and LEC2
(Holdsworth et al. 2008 ; Suzuki and McCarty 2008 ). It also plays an essential role
in embryo degreening process (Delmas et al. 2013 ). ABI3 binds to the Sph/RY ele-
ment (TCCATGCAT) present in seed-specific gene promoters (Monke et al. 2004 ;
Suzuki et al. 1997 ), and the genome-wide analysis of ABI3 regulon indicates that
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