RPUSD4

Model summary

An RNA-modifying enzyme that installs pseudouridine within structured RNA, using a conserved catalytic core to isomerize uridine in both cytoplasmic and mitochondrial RNA substrates. By binding and remodeling tRNA and ribosomal RNA, it stabilizes RNA architecture and enhances organellar translation, thereby promoting mitochondrial gene expression and broader RNA-modification pathways. It operates primarily in mitochondria and ribonucleoprotein granules, with a nucleoplasmic pool, and likely functions through transient assemblies that couple pseudouridylation to RNA processing and assembly.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The core of the protein is defined by IPR020103 (Pseudouridine synthase, catalytic domain superfamily, residues 91–364), which frames a classical isomerase fold that catalyzes uridine-to-pseudouridine conversion in RNA. Nested within this scaffold is IPR006145 (Pseudouridine synthase, RsuA/RluA-like domain, residues 105–329), specifying the RluA/RsuA enzyme lineage that binds structured RNA and performs base isomerization via a conserved catalytic nucleophile. The broader family assignment IPR050188 (RluA Pseudouridine Synthase family, residues 54–362) extends N-terminally and consolidates the enzyme’s identity as a site-specific RNA pseudouridine synthase. Critically, IPR006224 (RluA-like conserved site, residues 149–163) marks the catalytic signature that drives the uridine-to-pseudouridine isomerization chemistry. The ordered layout—family-defining N-terminal region feeding into a well-formed RluA catalytic core and its conserved active-site patch—causally establishes a pseudouridine synthase that binds RNA and executes site-specific isomerization.

2. 02

   This RluA-centered architecture dictates molecular function.

   The catalytic core and conserved site produce pseudouridine synthase activity (GO:0009982), while the RsuA/RluA-like domain confers high-affinity RNA recognition consistent with rRNA and tRNA binding. The explicit RNA-binding capacity aligns with tRNA binding (GO:0000049) and large ribosomal subunit rRNA binding (GO:0070180). The extensive surface area and modular RNA-recognition elements also rationalize general protein binding (GO:0005515), reflecting assembly with ribonucleoprotein partners.

3. 03

   From these molecular activities, the biological processes follow.

   The enzyme’s chemistry—uridine isomerization—directly supports mRNA modification (GO:0016556) and mitochondrial mRNA modification (GO:1900864). Its RluA lineage and RNA-binding breadth explain participation in tRNA pseudouridine synthesis (GO:0031119) and the broader pathway of mitochondrial tRNA processing (GO:0090646). By stabilizing RNA structures and enhancing decoding-site integrity in organellar ribosomes, the enzyme’s modification program promotes positive regulation of mitochondrial translation (GO:0070131). The same RNA-centric mechanism can influence nuclear-encoded transcripts and cytoplasmic RNA granules, but the mitochondrial bias of its modification network explains the strongest coupling to mitochondrial gene expression.

4. 04

   Cellular localization emerges from the functional circuitry.

   The enzyme’s RNA-modification outputs feed mitochondrial translation and mRNA maturation, placing it in the mitochondrion (GO:0005739). Its presence in ribonucleoprotein granules (GO:0035770) is consistent with RNA surveillance and processing hubs that handle untranslated mitochondrial RNAs. A nucleoplasmic pool (GO:0005654) is plausible because RNA-modifying enzymes often traffic through nuclear RNA-processing territories, even when their principal activity is mitochondrial.

5. 05

   Mechanistically, the RluA catalytic platform binds structured RNA substrates and flips target uridines into pseudouridines, stabilizing local helices and ribosomal functional centers.

   In mitochondria, this activity fortifies organellar rRNA/tRNA architecture, thereby elevating translation efficiency and fidelity. I hypothesize that the enzyme transiently associates with mitochondrial ribosomal subunits and assembly factors to access nascent rRNA and with mitochondrial tRNA maturation factors to coordinate pseudouridylation with other modifications. In cytoplasmic and nuclear RNA granules, it likely engages general RNA chaperones and ribonucleoprotein assembly factors, using its protein-binding surfaces to dock onto processing modules that channel modified RNAs toward translation pathways.