Model of the TIR1 Pathway for Auxin-Mediated Gene Expression
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The hormone auxin mediates numerous plant responses, some of which have been shown to require transcriptional regulation. This speculative model illustrates a branch of the auxin response pathway mediated by the TIR1 auxin receptor, through which auxin relieves the transcriptional repression of target genes. In the presence of low concentrations of auxin, members of the Aux/IAA family transcriptional repressors bind to ARF proteins and inhibit the transcription of specific auxin response genes. Increased concentrations of auxin cause the Aux/IAA proteins to associate with the TIR1 auxin receptor, leading to their degradation by a proteasome-mediated pathway. This decreases the concentration of Aux/IAA proteins in the nucleus and enables the expression of certain auxin response genes.
Details of Aux/IAA Degradation
The TIR1 auxin receptor is an F-box protein that can associate with an E3 ubiquitin ligase, a large protein complex that binds target proteins and marks them for degradation by catalyzing the covalent attachment of ubiquitin proteins. The particular E3 ubiquitin ligase believed to act in this model is SCFTIR1, a complex that includes a central cullin (CUL1), RBX1, ASK1, and TIR1. The depiction of the SCF is based on structural models of other cullin-based SCFs. Interaction between auxin and TIR1 enhances the binding of Aux/IAA proteins to TIR1; however, the order in which TIR1 and Aux/IAA assemble into an active SCFTIR1 is not yet known.
By analogy with other systems, it is assumed that when Aux/IAA proteins are recruited to the SCF, they become polyubiquitinated. The depiction of this process is based on what is known in other systems. Polyubiquitination is a process that is regulated by the COP9 signalosome, CAND1 protein, and heterodimeric AXR1/ECR1 and RCE, which carry out cycles of RUB de-conjugation/re-conjugation. The momentary dissociation of the SCF prior to subsequent cycles of ubiquitination alludes to the activity of these proteins, which are not explicitly shown. Once polyubiquitinated, a target protein can be recognized by the proteasome. Enzymes in the lid of the proteasome remove the ubiquitin and unfold the protein, allowing it to enter the core. Degradation of the Aux/IAA transcriptional repressor leads to expression of specific auxin response genes.
Learning Resource Type: Animation
Context: Undergraduate upper division, graduate, professional (degree program)
Intended Users: Teacher, learner
Intended Educational Use: Teach, learn, plan
Discipline: Biochemistry, Botany & Plant Science, Developmental Biology, Physiology
Keywords: Auxin, Signal transduction, SCFTIR1, Ubiquitin, Arabidopsis
Format: Shockwave Flash Object (swf file)
Size: 156 KB
Requirements: This animation will play with Macromedia Flash 5 (http://www.macromedia.com/downloads/) or higher.
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Rights: This material may be downloaded for noncommercial, course-teaching purposes only, provided credit to STKE is included by listing the citation for the Teaching Resource.
Citation: M. Laskowski, Model of the TIR1 pathway for auxin-mediated gene expression. Sci. STKE2006, tr1 (2006).