New Structure Reveals How RNA Modification Machines Coordinate Activity

H/ACA small nucleolar ribonucleoproteins (snoRNPs) are essential cellular machines that modify RNA molecules through pseudouridylation, a process critical for ribosome function, RNA stability, and telomere maintenance. Despite their importance, the detailed structure and coordination mechanisms of these complexes remained unclear until now.

Researchers used cryo-electron microscopy to determine high-resolution structures of endogenous insect H/ACA snoRNPs, revealing that these complexes exist as dimers containing two protomers assembled on two-hairpin H/ACA snoRNA guides. The structures showed specific protein-protein and protein-RNA interactions that coordinate pseudouridylation activity between the two protomers, explaining why eukaryotic H/ACA snoRNAs predominantly contain two hairpin structures rather than one.

The study identified key interprotomer contact hotspots and used biochemical assays to characterize mutations at these interfaces. Several mutations associated with dyskeratosis congenita (DC), a genetic disorder affecting ribosome biogenesis, were found to directly impair pseudouridine formation. These findings provide mechanistic insight into how DC mutations disrupt RNA modification and cellular function.

Additionally, the researchers discovered coordinated structural changes between the proteins Nop10, Nhp2, and the N-terminal extensions of Cbf5 in one protomer that resemble active and inactive conformations. These conformational changes may represent a regulatory mechanism controlling H/ACA snoRNP activity, suggesting that the two protomers can adopt different functional states.

These structural insights advance our understanding of fundamental RNA modification processes and provide a framework for developing therapeutic approaches for diseases involving H/ACA snoRNP dysfunction, including dyskeratosis congenita and potentially cancer, where these pathways are often disrupted.