Distinct substrate specificities of the human tRNA methyltransferases TRMT10A and TRMT10B

The tRNA m¹R₉ methyltransferase (Trm10) family is conserved throughout Eukarya and Archaea. Despite the presence of a single Trm10 gene in Archaea and most single-celled eukaryotes, metazoans encode up to three homologs of Trm10. Several disease states correlate with a deficiency in the human homolog TRMT10A, despite the presence of another cytoplasmic enzyme, TRMT10B. Here we investigate these phenomena and demonstrate that human TRMT10A (hTRMT10A) and human TRMT10B (hTRMT10B) are not biochemically redundant. In vitro activity assays with purified hTRMT10A and hTRMT10B reveal a robust activity for hTRMT10B as a tRNA^Asp-specific m¹A₉ methyltransferase and suggest that it is the relevant enzyme responsible for this newly discovered m¹A₉ modification in humans. Moreover, a comparison of the two cytosolic enzymes with multiple tRNA substrates exposes the enzymes' distinct substrate specificities, and suggests that hTRMT10B exhibits a restricted selectivity hitherto unseen in the Trm10 enzyme family. Single-turnover kinetics and tRNA binding assays highlight further differences between the two enzymes and eliminate overall tRNA affinity as a primary determinant of substrate specificity for either enzyme. These results increase our understanding of the important biology of human tRNA modification systems, which can aid in understanding the molecular basis for diseases in which their aberrant function is increasingly implicated.