# Strategies for Post-Translational Control of Protein Expression and Their Applications
## 4 Conditional Control of Protein Stability Using Genetic Manipulation
### Small-Molecule-Switchable Degrons
They generated dTAG molecules that consist of AP1867, a bumped ligand for FKBP12 F36V, and a ligand for the CRBN E3 Ub ligase (Figure 3d). The dTAG molecules bind to both FKBP12 F36V conjugated to the POI and the CRBN E3 Ub ligase, causing ubiquitination and degradation of the fusion proteins with FKBP12 F36V. Figure 3: Small-molecule-switchable degrons. Concepts of (**a**) DD, (**b**) LID, (**c**) HyT, (**d**) dTAG/ HaloPROTAC, (**e**) AID, and (**f**) IMiD-induced degron. Molecular glues that recruit E3 Ub ligases to the substrate can also be a powerful tool for inducing proximity between a POI and an E3 ligase. One of the advantages of molecular glues, compared with the chimeric bifunctional molecule, is the structural simplicity of the degradation inducer, which provides improved solubility compared with bifunctional molecules, and may be beneficial for therapeutic applications. A representative example of a molecular-glue-based approach is the auxin-inducible degron (AID) system, in which the auxin-dependent degradation pathway is transposed to other eukaryotic cells [54]. The plant hormone auxin (e.g., indole-3-acetic acid (IAA)) binds to the F-box transport inhibitor response 1 (TIR1) protein and induce the interaction between SCF-TIR1 and the auxin or IAA (AUX/IAA) transcription repressors. SCF-TIR1 recruits an E2 Ub-conjugating enzyme, resulting in ubiquitination and degradation. The F-box protein in SCF determines substrate specificity in all eukaryotes, whereas TIR1 and AUX/IAAs exist only in plant species. In cells co-expressing TIR1 and an AID conjugated to POI, auxins recruit SCF-TIR1 to the AID fusion protein, leading to ubiquitination and degradation (Figure 3e). The AID technology can be applied to the study of various organisms, including mice [55,56], yeast [54,57], _Drosophila melanogaster_[58], and _Caenorhabditis elegans_[59]. However, the AID system suffers from leaky degradation in the absence of auxin and requires a high dose of auxin for effective degradation. Recently, the [PERSON] group reported an improved system using auxin-receptor F-box protein \(At\)AFB2 and the short degron miniAA7, to achieve rapid and efficient degradation with minimal basal depletion in the absence of the IAA compound [60]. Furthermore, the [PERSON] and [PERSON] groups improved the AID system using engineered bumped-auxin and holed-TIR1 pairs via a bump-and-hole strategy [56,61]. The improved AID system induces effective degradation at a lower dose of auxin and shows no detectable leaky degradation. There is a distinct type of molecular glue-based technology which relies on thalidomide molecules (called IMIDs). IMIDs recruit the celebron Ub ligase complex to its substrates, such as IKAROS family zinc finger 3 (IKZF3) and Sal-like protein 4 (SALL4), thus resulting in the proteasomal degradation (Figure 3f). The substrate specificity of celebron is dependent on the structure of IMIDs, which achieve the selective degradation of the target protein tagged with IKZF3 or SALL4. The advantage of IMIDs-based-technologies is the use of FDA-approved drugs, such as pomalidomide, which may be a potential benefit for the future therapeutic application of these techniques. Another feature of IMIDs-induced degrons is that their size is relatively small compared with other degrons. [PERSON] et al. screened the minimal IMiD-responsive IKZF3 degron and showed that this peptidic degron can be used to target fusion proteins for degradation in the presence of IMIDs [62]. [PERSON] et al. also developed another IMiD-induced protein degradation system using a SALL4-derived degron [63]. Unlike the IKZF3 degron, the SALL4-based degron peptide can work in either the N- or C-terminus of the target protein.