In a study published in Nucleic Acids Res, a research team led by Prof. ZHOU Xiao-Long from the Center for Excellence in Molecular Cell Science, Shanghai Institute of Biochemistry and Cell Biology of the Chinese Academy of Sciences elucidated the editing mechanism of eukaryotic trans-editing factor AlaX and uncovered the function of AlaX in regulating the fidelity and rate of protein synthesis.
Aminoacyl-tRNA synthetases (aaRSs) ensure both speed and accuracy of mRNA translation by catalyzing tRNA aminoacylation. It is crucial for transmitting the genetic information accurately. Due to the similar side chains of amino acids, it is incidental for aaRSs to ligate the wrong amino acid to tRNA. For example, AlaRS or ThrRS can mischarge tRNAAla or tRNAThr with Ser. AaRSs have evolved the editing activity, which is called cis-editing, to hydrolyze the mischarged tRNAs. Accordingly, AlaRS or ThrRS can remove Ser-tRNAAla or Ser-tRNAThr, respectively, via cis-editing mechanism.
There are some freestanding proteins, which are homologs to the editing domains of aaRSs. Some of factors have been proved to be active in hydrolysis of the mischarged tRNAs generated by the relevant aaRSs. This type of hydrolysis is called trans-editing.
AlaX (encoded by the Aarsd1 gene) is a trans-editing factor in eukaryotes, homologous to the editing domains of AlaRS and ThrRS. Previous studies have revealed that bacterial or archaeal AlaX mainly performs as the trans-editing factor of AlaRS to edit Ser-tRNAAla. However, our understanding of the activity, potential substrate(s), tRNA recognition mechanism and biological function of eukaryotic AlaX remains limited.
In this study, researchers purified and reconstituted the enzyme activity of human and yeast AlaXs. They found that eukaryotic AlaXs were active trans-editing factors with stringent Ser specificity. Eukaryotic AlaX could edit mischarged tRNAs, including Ser-tRNAAla and Ser-tRNAThr. They elucidated the editing mechanism of human AlaX. Loss of yeast or human AlaX induced Ala>Ser and Thr>Ser misincorporation in translation and conferred vulnerability or advantage under different stresses, which could be rescued by the expression of editing-capable AlaX, AlaRS or ThrRS. Unexpectedly, they also found that AlaX could robustly hydrolyze the cognate Ser-tRNASer and the intermediate product Ser-tRNASec in Sec-tRNASec biogenesis, indicating AlaX possibly regulates mRNA translation efficiency under specific conditions.
Overall, the study elucidates the editing mechanism of eukaryotic AlaXs. The study also suggests that eukaryotic AlaX functions as an unprecedented multi-layer trans-editing factor for AlaRS, ThrRS (via Ser-tRNAAla and Ser-tRNAThr hydrolysis), and seryl-tRNA synthetase (SerRS) (via Ser-tRNASer hydrolysis) to maintain the speed and fidelity of mRNA translation.
Contact: ZHOU Xiao-Long
Center for Excellence in Molecular Cell Science, CAS
E-mail: xlzhou@sibcb.ac.cn
Reference: https://doi.org/10.1093/nar/gkae486
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