Human cytoplasmic ProX edits mischarged tRNA^Pro with amino acid but not tRNA specificity

aaRSs (aminoacyl-tRNA synthetases) are responsible for ensuring the fidelity of the genetic code translation by accurately linking a particular amino acid to its cognate tRNA isoacceptor. To ensure accuracy of protein biosynthesis, some aaRSs have evolved an editing process to remove mischarged tRNA. The hydrolysis of the mischarged tRNA usually occurs in an editing domain, which is inserted into or appended to the main body of the aaRS. In addition, autonomous, editing domain-homologous proteins can also trans-edit mischarged tRNA in concert or in compensating for the editing function of its corresponding aaRS. The freestanding ProX is a homologue of the editing domain of bacterial ProRS (prolyl-tRNA synthetase). In the present study, we cloned for the first time a gene encoding HsProX (human cytoplasmic ProX) and purified the expressed recombinant protein. The catalytic specificity of HsProX for non-cognate amino acids and identity elements on tRNA^Pro for editing were also investigated. We found that HsProX could deacylate mischarged Ala-tRNA^Pro, but not Cys-HstRNA_UGG^Pro, and specifically targeted the alanine moiety of Ala-tRNA^Pro. The importance of the CCA⁷⁶ end of the tRNA for deacylation activity and key amino acid residues in HsProX for its editing function were also identified.