Plant Molecular Biology 49: 401–409, 2002.
Perrot-Rechenmann and Hagen (Eds.), Auxin Molecular Biology.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
The role of regulated protein degradation in auxin response
Sunethra Dharmasiri and Mark Estelle
Institute for Cellular and Molecular Biology, Section of Molecular Cell and Developmental Biology, University of
Texas at Austin, Austin, TX 7871, USA
Received 1 June 2001; accepted is revised form 9 July 2001
Key words: auxin, cullin, Nedd8, RUB, SCF, ubiquitin
Auxin-regulated gene expression is mediated by two families of transcription factors. The ARF proteins bind
to a conserved DNA sequence called the AuxRE and activate transcription. The Aux/IAA proteins repress ARF
function, presumably by forming dimers with ARF proteins. Recent genetic studies in Arabidopsis indicate that
auxin regulates this system by promoting the ubiquitin-mediated degradation of the Aux/IAA proteins, thus per-
mitting ARF function. Mutations in components of SCF
, a ubiquitin protein ligase (E3) result in stabilization
of Aux/IAA proteins and decreased auxin response. Further, recent biochemical experiments indicate that the
Aux/IAA proteins bind SCF
in an auxin-dependent manner.
Auxin plays a crucial role in diverse aspects of plant
growth and development including tropic responses,
apical dominance in the shoot, lateral root formation
and differentiation of the vascular system. To under-
stand the molecular basis of auxin action, two major
approaches have been used. The ﬁrst approach is to
identify mutants that lack normal auxin responses,
and to determine the genetic basis for these defects
(Hobbie et al., 1994). The second is to take a direct
molecular approach to identify genes and proteins that
are regulated by the auxin signal (Abel and Theologis,
1994; Guilfoyle et al., 1998). Both of these approaches
have provided signiﬁcant insights into auxin signaling.
One of the most exciting developments of the past
few years, at least from our perspective, is the con-
vergence of the two approaches. In this review we will
focus on recent results that indicate auxin response de-
pends on the regulated degradation of a large family of
transcriptional regulators, the Aux/IAA proteins.
Ubiquitin proteasome pathway
Ubiquitin is a small conserved protein that is cova-
lently attached to diverse proteins, usually targeting
them for degradation by 26S proteasome (Hershko
and Ciechanover, 1998). The involvement of regu-
lated protein degradation by the ubiquitin-proteasome
pathway has been demonstrated in a wide variety of
cellular processes (Hershko and Ciechanover, 1998).
The ubiquitin conjugation pathway begins by the
ATP-dependent activation of ubiquitin by a ubiquitin-
activating (E1) enzyme. All E1 enzymes have a con-
served cysteine residue that forms a thiol ester linkage
with the C-terminal glycine of ubiquitin. Usually, E1
enzymes are encoded by a single gene or a small
family of related genes (Hershko and Ciechanover,
1998). The activated ubiquitin molecule is then trans-
ferred onto the second component of the pathway, a
ubiquitin-conjugating (E2) enzyme, again forming a
thiol ester linkage between the terminal glycine of
ubiquitin and a conserved cysteine on the E2 enzyme.
The E2 enzymes are encoded by a large gene family.
In yeast, there are 11 related E2 enzymes while in Ara-
bidopsis there are at least 36 Arabidopsis E2 isoforms
belonging to 12 groups (Hershko and Ciechanover,
1998; R. Vierstra, personal communication). The third
step in the ubiquitination reaction is the transfer of
ubiquitin onto the substrate protein. This step is as-
sisted by an E3 ubiquitin ligase that confers substrate
speciﬁcity to the pathway. The ubiquitinated protein is