Biology Reference
In-Depth Information
5.15
Function of ROS in Ubiquitin-Proteasome System
Proteasomes are large protein complexes located in the nucleus and the cytoplasm
(Peters et al.
1994
). The main function of the proteasome is to degrade unneeded or
damaged proteins by proteolysis. Proteasomes regulate the concentration of particu-
lar proteins and degrade misfolded proteins. Proteins are tagged for degradation by
a small protein called ubiquitin (Pickart
2001
). Ubiquitin- and proteasome-mediated
degradation of proteins plays an important role in plant defense signaling system
(Dreher and Callis
2007
; Goritsching et al.
2007
; Yao and Ndoja
2012
). H
2
O
2
triggers the up-regulation of ubiquitin precursor proteins, ubiquitin-conjugating
enzymes (E2), and ubiquitin-protein ligases (E3) in tobacco (Vandenabeele et al.
2003
). An Arabidopsis MAPKKK, MEKK1 has been shown to act downstream of
H
2
O
2.
MEKK1 kinase activity and protein stability was regulated by H
2
O
2
in a
proteasome-dependent manner (Nakagami et al.
2006
).
5.16
ROS May Regulate Expression of Transcription Factors
Expression of several transcription factors has been shown to be regulated by H
2
O
2
(Desikan et al.
2001
; Vandenabeele et al.
2003
). These transcription factors may be
direct targets for redox modifi cation by H
2
O
2
(Hancock et al.
2006
). The transcrip-
tion factors induced by H
2
O
2
include ethylene-responsive element binding protein
(EREBP), MYB-related transcription factor, and zinc fi nger transcription factor in
Arabidopsis
(Desikan et al.
2001
) and WRKY 11 in tobacco (Vranová et al.
2002
).
Vandenabeele et al. (
2003
) identifi ed four main classes of transcription factors
which are regulating downstream gene expression in ROS signaling system in
tobacco: MYB family, WRKY, AP2, and SCARECROW. The identifi ed MYB tran-
scription factor was homologous with the N-terminal MYB domain of a JA-dependent
transcription factor. The induced WRKY transcription factors contained a redox-
sensitive zinc-fi nger DNA-binding domain. Four EREBP/AP2 and ethylene-
responsive transcription factor (ERF1) were also up-regulated together with a
CEO1-like protein. CEO1 is a potential cofactor of EREBP transcription factors
(Vandenabeele et al.
2003
). Activation of these transcription factors may result in
activation of transcription of several defense genes.
5.17
Redox Signaling System
The ROS signal functions are manifested as a consequence of their ability to act as
mobile carriers of an unpaired electron (Forman et al.
2004
). In redox signaling, the
reaction of the ROS with the target protein is reminiscent of on-off signaling associ-
ated with phosphorylation/dephosphorylation events. Redox signaling occurs
when at least one step in a signaling event involves one of its components being
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