Unexpected enzymatic function of an ancient nucleic acid-binding fold

Aminoacyl-tRNA synthetases (ARSs) are indispensable for all living organisms and their associated aminoacyl-tRNA editing domains ensure the fidelity of translation. In eukaryotes, ARSs form a multi-aminoacyl-tRNA synthetase complex (MSC), which is assembled together with several nonsynthetase scaffolding proteins. The MSC found in Trypanosoma brucei (Tb) includes two proteins with oligosaccharide/oligonucleotide-binding (OB) folds-MSC-associated protein 1 (MCP1) and MCP2-and one known trans-editing factor, MCP3, an Ala-tRNA deacylase. The activity of MCP1 was unexplored until now. Our study shows that recombinantly-expressed and purified MCP1 also deacylates Ala-tRNAs despite lacking known tRNA-editing domain homology. Domain deletion studies reveal that the OB-fold houses the catalytic pocket and mutation of any one of three conserved OB-fold residues (K326, R331, S335) abolishes activity. Assays with Saccharomyces cerevisiae Arc1p reveal that MCP1's deacylation activity is conserved across organisms. This discovery explains the 3' CCA-end binding activity of this protein family and uncovers an ancient nucleic acid binding domain's unexpected enzymatic function.