The tRNA pseudouridine synthase TruB1 regulates the maturation and function of let-7 miRNA

Let-7 is an evolutionary conserved microRNA that mediates post-transcriptional gene silencing to regulate a wide range of biological processes, including development, differentiation, and tumor suppression. Let-7 biogenesis is tightly regulated by several RNA-binding proteins, including Lin28A/B, which represses let-7 maturation. To identify new regulators of let-7, we devised a cell-based functional screen of RNA-binding proteins using a let-7 sensor luciferase reporter and identified the tRNA pseudouridine synthase, TruB1. TruB1 enhanced maturation specifically of let-7 family members. Rather than inducing pseudouridylation of the miRNAs, high-throughput sequencing crosslinking immunoprecipitation (HITS-CLIP) and biochemical analyses revealed direct binding between endogenous TruB1 and the stem-loop structure of pri-let-7, which also binds Lin28A/B. TruB1 selectively enhanced the interaction between pri-let-7 and the microprocessor DGCR8, which mediates miRNA maturation. Finally, TruB1 suppressed cell proliferation, which was mediated in part by let-7. Altogether, we reveal an unexpected function for TruB1 in promoting let-7 maturation.

本研究では、tRNA修飾酵素の一つであるTruB1が、これまで全く予測されなかった Let-7の成熟化の促進を担っていることを突き止めました。TruB1によるLet-7の制御機構は非常に選択性が高いため、がんや発生の病態解明や治療開発につながることが期待されます。また、この制御機構は、TruB1 のシュードウリジン合成酵素活性には依存していませんでした。これまでRNA 修飾酵素の研究は、酵素活性を中心に着目されてきており、今回の結果は今後のRNA研究に影響を与えることが期待されます。