# Unexpected enzymatic function of an ancient nucleic acid-binding fold

## TL;DR

A protein with an ancient nucleic acid-binding fold was found to have an unexpected enzymatic function in deacylating Ala–tRNAs.

## Contribution

Discovery of a novel enzymatic function in a protein with an ancient OB-fold, previously uncharacterized.

## Key findings

- MCP1 deacylates Ala–tRNAs despite lacking known tRNA-editing domain homology.
- The OB-fold in MCP1 contains the catalytic pocket, with three conserved residues critical for activity.
- MCP1's deacylation activity is conserved across organisms, as shown with Saccharomyces cerevisiae Arc1p.

## Abstract

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.

Graphical Abstract