Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 177

Hoth 2011 ; Liu et al. 2011 ; Seo et al. 2012 ). AtCHIP, a U-box-containing E3
ligase, plays important roles in temperature stress tolerance and ABA response.
Overexpression of AtCHIP enhanced plant more sensitive to ABA and low- and
high-temperature treatments compared with wild type. AtCHIP functions as an
E3 ligase of the A subunit of protein phosphatase 2A (PP2A), which functions in
plant response to ABA and stress. In vitro assay showed it seemed that a single
ubiquitin molecule was added to A subunits of PP2A by E3 ligase AtCHIP; author
proposed the possibility that the ubiquitylation event might not lead to the degra-
dation of the A subunits of PP2A; instead it might lead to a change in their interac-
tion with the B or C subunit or to the enzymatic activity of PP2A (Yan et al. 2003 ;
Luo et al. 2006 ). In vivo protein-state analysis should be a key point needed to
support their claim.
9.3.2.3 CRLs Involved in ABA Signaling
The multi-subunit ligases CRLs (CULLIN-RING E3 ubiquitin ligases) are belong
to the biggest E3 family in Arabidopsis. Upon to now several members of this
family were founded to be in ABA pathway. Class I homeobox-Leu Zipper (HD-
ZIP) transcription factor, ATHB6, a target of the ABI1, plays a negative role in
ABA signal pathway in stomatal closure and germination assays (Himmelbach
et al. 2002 ). MATH/BTB proteins (BPMs), acting as adaptors for Cullin3-based
ubiquitin E3 ligase to bind substrate, directly interact with and target ATHB6 for
proteasomal degradation. The knockdown of BPMs and the overexpression of
ATHB6 result in ABA insensitivity. Reducing the function of CUL3 BPM leads to
higher accumulation of ATHB6 proteins (Lechner et al. 2011 ).
Mutants in two DWD genes, DWA1 and DWA2 ( DWD hypersensitive to
ABA1 and 2 ) encoding substrate receptors for CULLIN4-RING E3 ligase,
are hypersensitive to ABA and accumulate higher ABI5 proteins. DWA1 and
DWA2 can interact with ABI5 in vivo. So, DWA1 and DWA2 may work as
the substrate receptors for Cul4 E3 ligase to promote the degradation of ABI5.
Furthermore, cul4 mutant is hypersensitive to ABA and has higher ABI5 pro-
tein compared with wild type after ABA treatment (Lee et al. 2010 ). ABD1
(ABA-hypersensitive DCAF1) is another substrate receptor for CULLIN4-based
E3 ubiquitin ligases, which negatively regulates ABA signaling by binding to
and affecting the ABI5 stability in the nucleus. abd1 mutant is hypersensitive
to ABA during seed germination and seedling growth and leads to higher accu-
mulation of ABI5 (Seo et al. 2014 ). In in vivo Co-IP experiments, ABD1 and
DWA1/2 can bind different isoforms of ABI5. ABD1 interacts with the two iso-
forms of ABI5, while DWA1 and DWA2 interact with the slow migrating form
of ABI5 (Lee et al. 2010 ; Seo et al. 2014 ). Recently, Irigoyen et al. found that
ABA receptors, PYL8, as well as PYL4 and PYL9, can interact with DET1-,
DDB1-ASSOCIATED1 (DDA1), which is the part of COP10-DET1-DDA1
(CDD) complex and can provide substrate for CRL4 ligase. Furthermore, over-
expression of DDA1 promotes the degradation of PYL8 protein and reduces