Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 172

sumoylation play antagonist roles in protein stability, thus act oppositely in the
target protein involved in biological processes. Abscisic acid (ABA) is a key phy-
tohormone to regulate plant growth and development, especially in the aspect
of plant and environmental interaction. Recently, a number of discoveries have
shown that both ubiquitination and sumoylation participate in many aspects of
ABA biology, including regulation of ABA biosynthesis, activation or repres-
sion of ABA response, by regulating the stabilization or the degradation of ABA-
signaling components.
9.2 Protein Ubiquitination and Sumoylation
Ubiquitin, a highly conserved 76-amino acid protein, was covalently ligated to
other proteins to post-translationally regulate the target proteins in eukaryotic
systems. Ubiquitin modified the substrate protein by either one molecule (mon-
oubiquitination) or polymers (polyubiquitination) covalently attached to the tar-
get. Typically, ubiquitin proteasome system (UPS) firstly requires a three-step
sequential enzymatic cascade of E1 (ubiquitin-activating enzyme), E2 (ubiquitin-
conjugating enzyme), and E3 (ubiquitin ligase). Among them, E3s are the most
abundant and very important to define the substrate specificity. For example,
there are 2 E1s-, 37 E2s-, and 8 E2-like proteins, but over 1,500 different E3s in
Arabidopsis genome. According to the mechanisms of action and subunit compo-
sition, E3s were classified to four types in plants, HECT, really interesting new
gene (RING), U-box and Cullin-RING ligases (CRLs). CRLs, the multi-subunit
ligases, are further divided into four subtypes based on a different target recog-
nition site, SCF E3s (S-phase kinase-associated protein1-cullin1-F-Box ligases),
BTB E3s (bric-a-brac-tramtrak-broad complex), DDB E3s (the DNA damage-
binding proteins), and the anaphase-promoting complex (APC) (Vierstra 2009 ).
Most ubiquitinated proteins were broken down by the 26S proteasome system,
which degrade the target and release the linked ubiquitin for reuse.
Besides ubiquitin, there are several functional ubiquitin-like proteins (UBLs)
in eukaryotes, including SUMO, RUB (related to ubiquitin, or Nedd8 in yeast
and animals), ATG8 and ATG12 (autophagy 8 and 12), FUB1 (fau ubiquitin-like
protein 1), URM1 (ubiquitin-related modifier 1), UFM1 (ubiquitin-fold modi-
fier1), and HUB1 (homology to ubiquitin1) (Park et al. 2011 ). Among them,
SUMO is a kind of essential post-translation modification, firstly identified in
yeast (Meluh and Koshland 1995 ), and then exclusively reported in animals and
plants. Although SUMO shares only about 30 % identity with ubiquitin, three-
dimensional structures are highly conserved, constituted by one ʱ helix and four ʲ
strands (Fig. 9.1 , Downes and Vierstra 2005 ; Hochstrasser 2009 ). Only one SUMO
coding gene exists in yeast and four in human, while eight forms of SUMOs
(encoded by SUMO1-8 ) and one SUMO-like pseudogene ( SUMO9 ) are present
in Arabidopsis (Miura et al. 2007 ), implying specific function of various forms
of SUMOs in plants. SUMO preproteins (95-111 amino acids) encoded by these