Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 413

caused overproduction of ethylene in Arabidopsis . Similarly, mutation of the RUB-
conjugating enzyme RCE1 also increased ethylene production (Larsen and Cancel
2004 ). These results suggest that ubiquitin proteins-cullin RING ligases negatively
regulate biosynthesis of ethylene.
Ethylene signal transduction terminates in a transcription cascade involving
the EIN3/EIL ( E THYLENE IN SENSITIVE 3 / EI N3 - L IKE) and ERF ( E thylene
R esponsive F actor) families of plant-specifi c transcription factors (Potuschak et al.
2003 ; van Loon et al. 2006 ). EIN3 is expressed constitutively, but is unable to accu-
mulate because it is subjected to permanent proteolysis mediated by two
Arabidopsis SCF complex F box proteins called EBF1 and EBF2 (for EIN3-
Binding F box protein 1 and 2). The F-box proteins specifi cally recruit the target
EIN proteins to an SCF ubiquitin ligase for degradation by the proteasome
(Potuschak et al. 2003 ). EIN3 increases in response to increased levels of ethylene
(Guo and Ecker 2003 ). EIN3 is rapidly degraded by ubiquitylation by the SCF EBF1/
EBF2 through a proteasome-mediated pathway, but is stabilized upon ethylene treat-
ment (Guo and Ecker 2003 ; Kepinski and Leyser 2003 ). EIN3 becomes stabilized
after perception of ethylene and acts on its target promoters (Potuschak et al.
2003 ). These observations suggest that ethylene signaling action depends on EIN3
protein stabilization and proteolytic regulation of this protein may affect transcrip-
tion of the defense-related genes (Potuschak et al. 2003 ).
ERFs are plant-specifi c transcription factors detected in tobacco, tomato, and
Arabidopsis. They have been shown to bind nucleotide sequences containing the
GCC box, the core sequence of an ethylene-responsive element of defense genes
and regulate the expression of GCC box-mediated transcriptions (Fujimoto et al.
2000 ). ERF proteins are grouped into three classes based on amino acid sequence
identities within the ERF domain. Class I and class III ERFs act as activators,
whereas class II ERFs act as repressors (Ohta et al. 2000 ; Koyama et al. 2003 ).
Class II ERF repressors down-regulate the transactivation activity of class I and
class II ERFs (Fujimoto et al. 2000 ). Ubiquitin-proteasome system has been shown
to be involved in the repression of class II ERFs (Koyama et al. 2003 ).
In tobacco, the ERF3 gene coding for a class II repressor as well as genes for
activators such as ERF2 and ERF4 were transcriptionally upregulated in response to
ethylene (Ohme-Takagi and Shinshi 1995 ; Kitajima et al. 2000 ). These ERF genes
for both activators and a repressor were both rapidly induced by a fungal elicitor
treatment (Yamamoto et al. 1999 ). A ubiquitin-conjugating enzyme (NtUBC2) was
found to be involved in the repression activity of ERF3 (Koyama et al. 2003 ). The
ubiquitin-conjugation activity of NtUBC2 may be involved in the regulation of
repression activity of ERF3. The NtUBC2 interacted with ERF3 but not with ERF2
or ERF4. This suggests that the mechanism of regulation of the repression activity
of ERF3 is distinct from that of the activation activity of ERF2 and ERF4. Since
ERF repressor can suppress transactivation activity of ERF activators (Koyama
et al. 2003 ), down-regulation of the repression activity of ERF3 by NtUBC2 may be
operating for the induction of the GCC box-mediated transcription of defense genes
(Koyama et al. 2003 ). Thus the interaction between ERF3 and NtUBC2 may be a
critical step in activating transcription of various defense genes.