Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 180

9.5 Conclusions and Perspective
In fact, the synthesis and destruction of proteins are equally important for plant
to grow and survive from adverse conditions. Therefore, by targeting an ABA
receptor or ABA-responsive proteins at the right moments, the ubiquitination and
sumoylation modification modulate the ABA biosynthesis and the sensitivity of
plant responsive to ABA (Fig. 9.2 ). Based on the observations illustrated above,
the following mechanisms are involved, (1) one E3 ligase can target several sub-
strates, (2) one target can be modified by several E3 ligases, (3) the positive regu-
lator and the negative regulator in the same pathway can be regulated by the same
E3 ligase.
For instance, the ubiquitin ligase KEG, being the negative regulator of ABA,
leads to the degradation of ABF1, ABF3, ABI5, and CIPK6, and four proteins
play positive roles in ABA inhibition of seed germination and post-germination
growth. ABF1, ABF3, and ABI5 are bZIP-type transcription factors, and CIPK26
is CIPK which can phosphorylate ABI5 in vitro. KEG is stable in the absence of
ABA, and ABA can promote KEG auto-ubiquitination and degradation by the 26S
proteasome. The reduction of KEG protein leads to the accumulation of ABF1,
ABF3, ABI5, and CIPK6, and subsequent growth arrest of plants. Oppositely
one substrate also could be directly regulated by different E3 or E3 complexes.
Well-studied cell cycle negative regulator, ICK/KRP (p27), both in mammals and
plant, is controlled by different types of E3 ligases in different location in cells. In
nucleus, it is degraded by SCF complex, while in cytosol, it is targeted by RING-
type E3 ligase (Kamura et al. 2004 ; Ren et al. 2008 ; Lai et al. 2009 ). It seems
that the key transcriptional factor ABI5 is regulated by similar manners. The deg-
radation of ABI5 is modulated by KEG in cytosol under lower ABA level, and
by DWA1, DWA2, and ABD1 complexes in nucleus, evidenced by the exclusive
localization of those adaptor proteins in nucleus under higher level of ABA and
stress conditions. Another example is the SUMO E3 ligase SIZ1, the negative reg-
ulator of ABA by targeting two proteins, ABI5 and MYB30, in plant response to
ABA. Sumoylation by SIZ1 can stabilize ABI5 and MYB30. Different from abi5
mutant, myb30 mutants are ABA hypersensitive to ABA during ABA-controlled
germination. Furthermore, sumoylated ABI5 proteins are inactive, but sumoylated
MYB30 proteins are inactive. Therefore, SIZ1 sumoylation of both ABI5 and
MYB30 transcription factors regulate plant ABA response in the parallel way.
Why the same SUMO E3 ligase has different effects on the activity of different
substrates, one possibility is that sumoylation might affect other types of protein
modification, for example, phosphorylation of ABI5 is essential for the activity of
ABI5. This conception need be addressed experimentally.
Besides the SUMO, different types of E2 and E3 ligases were discovered in
ABA signaling, the component of the UPS destruction 26S proteasome com-
plex, RPN10, was also found to be important in ABA signaling. rpn10 - 1 was
found more sensitive to ABA with the effect on the selective stabilization of the
short-lived ABA-signaling protein ABI5 (Smalle et al. 2003 ). Together with all