From 2012.igem.org

The Auxin-inductible degron system

Overview

Auxin-inductible degron (AID) system is a new tool to regulate protein expression specifically through induction of the proteolysis system by auxin.
Auxin represents a family of plant hormones that control gene expression during many aspects of growth and development.
Figure 1 show how this system works:

Figure 1: Schematic illustration of the AID system Source:http://www.cosmobio.co.jp/export_e/products/proteins_and_peptides/brs_20110824.asp?entry_id=7884
Auxin family hormones, such as indole-3-acetic acid (IAA, a natural auxin) and 1-naphthaleneacetic acid (NAA, a synthetic auxin), bind to the F-box transport inhibitor response 1 (TIR1) protein and promote the interaction of the E3 ubiquitin ligase SCF-TIR1 (a form of SCF complex containing TIR1) and the auxin or IAA (AUX/IAA) transcription repressors. SCF-TIR1 recruits an E2 ubiquitin conjugating enzyme that then polyubiquitylates AUX/IAAs resulting in rapid degradation of the latter by the proteasome.
The AID plasmid is composed like it shows in this figure:

Figure 2: Schematic composition of AID plasmid Source:http://www.cosmobio.co.jp/export_e/products/proteins_and_peptides/brs_20110824.asp?entry_id=7884
The great advantage is that you can bind a reporter like GFP to aid. That allows you to follow in live the degradation of the target protein:

Figure 3: Immunofluoresence analysis of protein depletion by the AID system Source:“An auxin-based degron system for the rapid depletion of proteins in nonplant cells”, Nature Methods. 2009 Dec;6(12):917-22. Epub 2009 Nov 15
This system can be used to induce rapid depletion of a protein of interest (within 30 min) in the presence of auxin, and the reaction was reversible and tunable. This system worked in budding yeast as well as in cell lines derived from chicken, mouse, hamster, monkey and human, implying that it could be applicable to most eukaryotes, except plant species.

NB: Thanks to the work of Nishimura and al. (“An auxin-based degron system for the rapid depletion of proteins in nonplant cells”, Nature Methods. 2009 Dec;6(12):917-22. Epub 2009 Nov 15) we introduce the AID system in iGem competition. This paper is a great source of knowledge for our project, that’s why a lot of information is extracted from it in this page.


Advantages

AID system shows lot of advantages compared to other degradation based method to control protein expression:

Figure 4: comparison of degradation based methods to control protein expression Source:“An auxin-based degron system for the rapid depletion of proteins in nonplant cells”, Nature Methods. 2009 Dec;6(12):917-22. Epub 2009 Nov 15
Moreover, there is a great advantage of using auxin to control protein expression. Its molecule in only active in plants and appears to be relatively silent in nonplant eukaryotes. Because it’s a small molecule this system allowed us to degrade protein in cytoplasm but also in nucleus. Moreover, this molecule is cheap and 20-500 µM of auxin should be enough to induce maximum depletion for most animal cells. Another advantage is that the culture can be maintained at a constant temperature.
The only limitation of AID system is that it can’t be used to control endogenous proteins without manipulation of the gene of interest.


Applications

With the AID system, we can imagine a lot of application in Synthetic Biology.
This system is very flexible, fast and reversible. Because you just have to introduce the plamid with you target protein, this system is very powerful for protein control. With our new chassis, the xenopus, AID system could be a new source of iGEM projects.
If you want to stop protein effect during a certain time, you just have to put your tadpoles in a auxin bath, it’s not harmful for them.

A great application in future could be the chastity belt.
Indeed, the question of genetically modified organism is very important in Synthetic biology. We’re still searching a response to avoid the dissemination of modified species in nature. With AID system we can imagine a way to avoid this problem.
In tadpoles there are genes responsible of sex determination (DM-W and DMRT1). With degron system is simple to avoid expression of these genes. Like this we can make only male or only female in our lab, avoiding the possibility of reproduction of our species.
Possibilities are vast with AID system but we haven’t the time to explore it. That’s why our work is very useful for iGEM community, we wish new team will work on the AID system and explore the possibilities it procure.
It could be you the next student working on it!
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