Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 175

9.3.2.1 RING-type E3 Ligases Involved in ABA Signaling
RING-type E3 ligases were classified due to their conserved cysteine and histidine
domain. Various RING fingers exhibit binding activity toward E2 ubiquitin-con-
jugating enzymes (Ubc's) and also the single RING-type E3 ligase has capacity
to bind and ubiquitinate its substrate(s). This family has around 500 members in
Arabidopsis, the second large family of E3 in Arabidopsis. A couple of RING
finger E3 ligases were discovered in ABA signaling. Abscisic Acid-Insensitive 3
(ABI3), a B3-domain transcription factor, plays a central role in ABA-signaling
pathway. Both biochemical and genetic evidences demonstrated that the abun-
dance of ABI3 is post-translationally regulated by AIP2 (ABI3-interacting pro-
tein2), a RING-type E3 ligase. AIP2 can directly polyubiquitinate ABI3 in vitro.
AIP2 -overexpression plants show lower levels of ABI3 protein and more resistant
to ABA compared with wild type plants, which is similar to the phenotype of abi3 .
Whereas aip2 mutant contains higher ABI3 protein levels and is hypersensitive to
ABA, phenocopying ABI3 -overexpressing plants. Therefore, AIP2 is a negative
regulator through regulating the stability of ABI3 (Zhang et al. 2005 ).
Plants have evolved protective mechanism to survive from adverse environ-
mental conditions during their transition from heterotrophic to autotrophic growth.
This post-germination developmental checkpoint is controlled by ABA, which
induces the accumulation of Abscisic Acid-Insensitive 5 (ABI5) in germination
stage. ABI5, a basic Leu Zipper transcription factor acting downstream of ABI3, is
essential for ABA-dependent post-germinative growth arrest (Lopez-Molina et al.
2001 , 2002 ). ABA-induced ABI5 protein accumulation can be regulated through
both activation of transcription and enhanced protein stability. Enhanced ABI5
protein in seedlings treated with 26S proteasome inhibitors indicates that the deg-
radation of ABI5 is dependent on the UPS (Lopez-Molina et al. 2001 ; Brocard
et al. 2002 ). A RING-type E3 KEEP ON GOING (KEG) is a negative regulator
of ABA signaling, required for keeping the low levels of ABI5 in the absence of
ABA. KEG can interact with and ubiquitinate ABI5 in vitro. Cytoplasmic deg-
radation of ABI5 is mediated by KEG. keg mutant plants accumulated very high
levels of ABI5 and resulted in hypersensitivity to ABA, exhibiting plant growth
arrest immediately after germination in presence of ABA, similar to the phenotype
of ABI5 -overexpressing plants. Loss of ABI5 can partially rescue the keg pheno-
type (Stone et al. 2006 ; Liu and Stone 2013 ). Furthermore, KEG protein levels
are modulated by ABA through self-ubiquitination and subsequent degradation
via 26S proteasome (Liu and Stone 2010 ). Another protein, ABI FIVE BINDING
PROTEIN (AFP) can attenuate ABA signals by promoting ABI5 degradation,
although the mechanism of this process is not clear (Lopez-Molina et al. 2003 ).
In addition to ABI5, two other ABI5-related bZIP transcription factors, named
ABF1 and ABF3, also regulated seed germination and post-germination growth
(Jakoby et al. 2002 ; Finkelstein et al. 2005 ). Similar model to ABI5 degradation
modulated by KEG, the abundance of ABF1 and ABF3 proteins is also affected
by ABA and KEG. KEG can interact with and ubiquitinate ABF1 and ABF3 in
vitro, and the loss of KEG slows the degradation of ABF1 and ABF3 in vivo and in