# Strategies for Post-Translational Control of Protein Expression and Their Applications
## 6 Conclusions  
This review introduced technologies for the post-translational control of protein expression and their applications. Bifunctional molecules that recruit proteolytic pathways to target proteins have been emerging and being applied to drug discovery and development. This is an attractive approach in drug discovery and development because it does not require the functional inhibition of proteins; it only requires binding capacity and its catalytic mechanism of action makes it effective in low-dose treatments. At present, several challenges remain regarding the development of PROTAC molecules, namely, the rational design of bifunctional molecules and the expansion of the use of various types of E3 Biases other than the currently used Biases (VHL, CRBN, cLAPs, and MDM2) [105]. Moreover, the precise mechanism of action of the degradation machinery in several techniques, including AUTAC, remains unclear [106; 107]. A recent report demonstrated that PROTAC-based polyubiquitin could be facilitated by the thyroid hormone receptor-interacting protein 12 (TRIP12), rather than the hijacking of the enzyme by an E3 ligase binder, thus highlighting a role for TRIP12 as an accelerator of PROTAC-directed degradation [108]. Further research on proteolytic pathways may allow the selection of a proteolytic system according to the characteristics of the target protein. Concomitantly, several research groups generated small-molecule- and light-switchable degrons that can be tagged to target proteins by genetic engineering. The conditional regulation of protein expression using degrons would be more suitable for basic research applications than is the method using bifunctional molecules, because it does not require the design of chemical compounds for each target protein. Another attractive research tool is the regulation of endogenous protein expression via the knock-in of degrons into endogenous proteins using CRISPR/Cas9. This strategy is valuable for loss-of-function studies and the generation of disease models in human pluripotent stem cells. However, there is concern that the degron tag may interfere with the function of the target protein. Therefore, methods such as removable degrons and nanobody-based degrons will continue to newly emerge to control the expression level of target proteins while minimizing the impact on their functions. It may be difficult to precisely control protein expression levels simply by controlling protein degradation using degrons. A combination of methods to regulate protein synthesis by controlling transcription may solve this problem. Although gene therapy and cell therapy have safety issues because of genetic modification, it is possible to prevent their side effects using degrons to control the expression of therapeutic proteins [109]. In the future, bifunctional molecules and degrons will be newly developed or improved. Biomedical research and therapeutic applications will follow.  
Conceptualization, Y.U. and Y.M.; investigation, Y.U. and Y.M.; writing--original draft preparation, Y.U.; writing--review and editing, Y.U. and Y.M.; supervision, Y.M.; funding acquisition, Y.M. Both authors have read and agreed to the published version of the manuscript. This research was funded by MEXT KAKENHI Grant Number JP 21H00271.  
Not applicable.  
We thank [PERSON] for helpful discussion on the topic.  
The authors declare no conflict of interest.