PUSL1

Model summary

A soluble mitochondrial RNA-modifying enzyme that binds structured tRNA and rRNA and catalyzes pseudouridylation, thereby stabilizing RNA architecture central to mitochondrial gene expression. By installing pseudouridine within mitochondrial tRNAs—and potentially within ribosomal RNA—it enhances tRNA folding and decoding fidelity and supports efficient mitochondrial translation. Its architecture forms an RNA-binding catalytic cradle that generates and resolves a covalent intermediate to complete the isomerization reaction within the mitochondrial matrix.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide is dominated by the TruA-class pseudouridine synthase fold: IPR020103 (Pseudouridine synthase, catalytic domain superfamily, residues 12–288) spans the catalytic core, and the N-terminal half is further specified by IPR020094 (Pseudouridine synthase TruA/RsuA/RluB/E/F, N-terminal homologous superfamily, residues 12–125) and IPR020097 (Pseudouridine synthase I, TruA, alpha/beta domain, residues 19–122). The C-terminal half is defined by IPR020095 (Pseudouridine synthase I, TruA, C-terminal homologous superfamily, residues 126–282) and a second TruA alpha/beta domain annotation (IPR020097, residues 168–284). The entire protein is classified within IPR001406 (Pseudouridine synthase I, TruA family, residues 10–297). This ordered N-terminal alpha/beta lobe followed by a C-terminal alpha/beta lobe is the hallmark of TruA-family isomerases that catalyze uridine-to-pseudouridine conversion in structured RNAs. The architecture creates a rigid catalytic cradle that binds RNA and positions a conserved catalytic nucleophile to form and resolve a covalent intermediate, thereby enforcing pseudouridine synthase chemistry.

2. 02

   From this fold, the molecular function follows directly.

   The TruA catalytic superfamily (IPR020103) and the complete TruA family signature (IPR001406) cause pseudouridine synthase activity, aligning with GO:0009982 pseudouridine synthase activity. The extensive RNA-recognition surfaces contributed by the N- and C-terminal alpha/beta domains (IPR020097 and IPR020095) necessitate stable RNA contacts, supporting GO:0003723 RNA binding. Together, these features define an RNA-modifying enzyme that binds and remodels structured tRNA and rRNA substrates.

3. 03

   The biological process emerges from the TruA lineage’s specialization: TruA enzymes typically modify tRNA, often at position 38 or adjacent sites, stabilizing tertiary folds.

   This places the enzyme squarely in GO:0031119 tRNA pseudouridine synthesis. The same fold can act on rRNA, but the TruA family’s canonical role and the presence of tRNA-modification determinants argue that the dominant pathway is tRNA pseudouridylation, with potential auxiliary activity on mitochondrial rRNA.

4. 04

   Cellular location is inferred from the soluble, non-membranous architecture and the presence of mitochondrial targeting context.

   The absence of transmembrane segments and the soluble TruA fold indicate a matrix-localized enzyme. Given that mitochondrial tRNAs and rRNAs require modification within the organelle and that mitochondrial RNA processing occurs in the matrix, the most parsimonious localization is the mitochondrion, consistent with GO:0005739 mitochondrion.

5. 05

   Mechanistically, the enzyme likely binds precursor and mature mitochondrial tRNAs, recognizes their conserved L- and anticodon-loop neighborhoods, and catalyzes uridine isomerization via a covalent intermediate formed by the TruA catalytic residue.

   This modification increases local base-stacking and hydrogen-bonding capacity, stabilizing tRNA structure and tuning decoding properties. In mitochondria, this activity supports efficient translation by ensuring properly folded and functional tRNAs.

6. 06

   From this model, plausible interaction partners are RNA biogenesis and translation factors that handle mitochondrial RNAs and tRNAs.

   The enzyme is poised to transiently associate with mitochondrial tRNA maturation and utilization systems: the mitochondrial pseudouridine synthase pathway (e.g., other RNA-modifying enzymes), the mitochondrial ribosome, and aminoacylation machinery. It may also engage with RNA chaperones and assembly factors that shepherd tRNA and rRNA within the mitochondrial matrix, coordinating modification with ribosome assembly and translation initiation.