RAC GTPases in Tobacco and Arabidopsis Mediate Auxin-Induced Formation of Proteolytically Active Nuclear Protein Bodies That Contain AUX/IAA Proteins

Abstract

Auxin signaling relies on ubiquitin ligase SCFTIR1-mediated 26S proteasome-dependent proteolysis of a large family of short-lived transcription regulators, auxin/indole acetic acid (Aux/IAA), resulting in the derepression of auxin-responsive genes. We have shown previously that a subset of Rac GTPases is activated by auxin, and they in turn stimulate auxin-responsive gene expression. We show here that increasing Rac signaling activity promotes Aux/IAA degradation, whereas downregulating that activity results in the reduction of auxin-accelerated Aux/IAA proteolysis. Observations reported here reveal a novel function for these Rac GTPases as regulators for ubiquitin/26S proteasome-mediated proteolysis and further consolidate their role in auxin signaling. Moreover, our study reveals a cellular process whereby auxin induces and Rac GTPases mediate the recruitment of nucleoplasmic Aux/IAAs into proteolytically active nuclear protein bodies, into which components of the SCFTIR1, COP9 signalosome, and 26S proteasome are also recruited.

Auxin‐induced, SCFTIR1‐mediated poly‐ubiquitination marks AUX/IAA proteins for degradation

“…For a long time now, plant cell protoplasts have been used to assess hormone responses due to their amenability for transformation and their responsiveness to diverse stimuli (Abel and Theologis, 1998;Sheen, 2001). Much of the information gathered on the mechanisms of regulation of Aux/IAA stability has been performed using transiently transformed protoplasts (Ramos et al, 2001;Tiwari et al, 2001Tiwari et al, , 2004, and other important components of the auxin signaling pathway have been functionally characterized in this system including ARFs, SCF TIR1 and RAC GTPases (Guilfoyle et al, 1998;Tao et al, 2005;Wang et al, 2005). In this study, we used Arabidopsis cell suspension protoplasts to demonstrate that auxin-enhanced TIR1-mediated ubiquitination of SHY2/IAA3 and BDL/IAA12 marks these proteins for degradation and leads to auxin-responsive gene expression.…”

Auxin-Binding Protein 1 is a negative regulator of the SCFTIR1/AFB pathway

“…An attractive hypothesis is that the ABP1 pathway would confer auxin conditionality to the interaction between TIR1/AFBs and AUX/IAAs. Such conditionality might result from a change in subcellular or nuclei protein localization of AUX/IAA and/or TIR1/AFB proteins 22,33 , from the presence or absence of another protein or peptide interacting or competing with one of the component of the transcriptional regulatory module [34][35][36][37] or from post-translational modification affecting the relative affinity between TIR1/AFBs 38 and AUX/ IAAs 28,39 . A future challenge will be to elucidate the molecular basis of the action of ABP1 on the SCF TIR1/AFB pathway and to identify ABP1 downstream signalling elements filling the gap between ABP1 and the nuclear-localized SCF TIR1/AFB pathway.…”

“…Determining whether ROP/RAC GTPases, which were shown to act on endocytosis 11,12 , are also part of the ABP1 pathway controlling AUX/IAA stability and gene regulation will be an important question. Interestingly, tobacco and Arabidopsis RAC GTPases were reported to mediate auxin-responsive gene expression 40 and to induce formation of proteolytically active nuclear protein bodies containing AUX/IAA proteins 33 . This would potentially place ROP/RAC GTPases at the node of ABP1 downstream responses.…”

Proteasomes crossing the nuclear border

“…18 Although different lines of evidence support the view that proteasome-mediated degradation of proteins can occur inside the nucleus, it is by no means clear if this is the general rule for nuclear substrates. 19,21,22 One established role of the ubiquitin-proteasome system within the nucleus is the rapid inactivation of regulatory factors, as exemplified by the yeast Matα2 and the human MyoD proteins. 23,24 Degradation of the tumor suppressor p53 was once considered to be exclusively cytoplasmic, but is now thought to occur in both the EXTRA VIEW EXTRA VIEW eukaryotes are characterized by a closed mitosis and maintain an intact nuclear envelope throughout the cell cycle.…”