Noncanonical Amino Acids in Synthetic Biosafety and Post-translational Modification Studies

Functional analysis of protein post-translational modifications using genetic codon expansion

Post-translational modifications (PTMs) of proteins not only exponentially increase the diversity of proteoforms, but also contribute to dynamically modulating the localization, stability, activity, and interaction of proteins. Understanding the biological consequences and functions of specific PTMs has been challenging for many reasons, including the dynamic nature of many PTMs and the technical limitations to access homogenously modified proteins. The genetic code expansion technology has emerged to provide unique approaches for studying PTMs. Through site-specific incorporation of unnatural amino acids (UAAs) bearing PTMs or their mimics into proteins, genetic code expansion allows the generation of homogenous proteins with site-specific modifications and atomic resolution both in vitro and in vivo. With this technology, various PTMs and mimics have been precisely introduced into proteins. In this review, we summarize the UAAs and approaches that have been recently developed to site-specifically install PTMs and their mimics into proteins for functional studies of PTMs.

Applications of genetic code expansion in studying protein post-translational modification

Various post-translational modifications can naturally occur on proteins, regulating the activity, subcellular localization, interaction, or stability of the proteins. However, it can be challenging to decipher the biological implication or physiological roles of site-specific modifications due to their dynamic and sub-stoichiometric nature. Genetic code expansion method, relying on an orthogonal aminoacyl-tRNA synthetase/tRNA pair, enables site-specific incorporation of non-canonical amino acids. Here we focus on the application of genetic code expansion to study site-specific protein post-translational modification in vitro and in vivo. After a brief introduction, we discuss possibilities of incorporating non-canonical amino acids containing post-translational modifications or their mimics into target proteins. This approach is applicable for Ser/Thr/Tyr phosphorylation, Tyr sulfation and nitration, Lys acetylation and acylation, Lys/His mono-methylation, as well as Arg citrullination. The next section describes the use of a precursor non-canonical amino acid followed by chemical and/or enzymatic reactions to afford the desired modification, such as Cys/Lys acylation, ubiquitin and ubiquitin-like modifications, as well as Lys/Gln methylation. We also discuss means for functional regulation of enzymes involving in post-translational modifications through genetically incorporated non-canonical amino acids. Lastly, the limitations and perspectives of genetic code expansion in studying protein post-translational modification are described.