The molecular basis of tRNA selectivity by human pseudouridine synthase 3 (PUS3)
Pseudouridine (?), the isomer of uridine, is ubiquitous in most RNA families, including tRNA, rRNA and mRNA. Pseudouridine synthase 3 (PUS3) catalyzes pseudouridylation of position 38/39 in tRNAs, but whether it can modify other RNA types, including mRNA, remains elusive. Here, we determine the single particle cryo-EM structure of apo and tRNA-bound human PUS3, showing how it forms a symmetric homodimer that recognizes the characteristic L-shape of tRNA across two distinct interaction interfaces. Structure-guided and patient-derived mutations validate our structural findings in complementary biochemical assays. Furthermore, we deleted PUS1 and PUS3 in HEK293 cells and used Pseudo-seq to detect ? sites in the transcriptome. Although PUS1-dependent sites were detectable in tRNA and mRNA, we did not find any evidence for PUS3 modifying other RNA classes than tRNA. In summary, our work provides the molecular basis for PUS3-mediated tRNA modification in humans and aids in understanding how impairments in its activity lead to intellectual disabilities through defects in tRNA modifications. Overall design: Pseudo-seq analysis to detect pseudouridine sites in the human transcriptome using HEK293T cells. Detection of PUS1 and PUS3 dependent pseudouridine sites in the transcriptome using PUS1-/- and PUS3-/- mutant cell lines.