Interprotomer Communication and Functional Asymmetry in H/ACA snoRNPs

Area of Science:

• Molecular Biology
• Structural Biology
• Genetics

Background:

• H/ACA small nucleolar ribonucleoproteins (H/ACA snoRNPs) are essential for RNA modification, folding, and stability, playing key roles in ribosome biogenesis and telomere maintenance.
• Mutations in H/ACA snoRNP proteins are linked to the genetic disorder Dyskeratosis congenita, highlighting their clinical significance.
• Eukaryotic H/ACA snoRNAs typically feature two hairpin structures, but the structural basis for this preference and its relation to function remained unclear.

Purpose of the Study:

• To elucidate the structure and function of endogenous, catalytically active insect H/ACA snoRNPs.
• To characterize the protein-protein and protein-RNA interactions within the H/ACA snoRNP complex.
• To investigate the impact of Dyskeratosis congenita-associated mutations on H/ACA snoRNP activity and pseudouridine formation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures of H/ACA snoRNPs.
• Biochemical assays to characterize protein-protein and protein-RNA interactions.
• Analysis of mutations associated with Dyskeratosis congenita to understand their functional consequences.

Main Results:

• Multiple cryo-EM structures revealed an asymmetric, dimeric H/ACA snoRNP complex with two protomers bound to a two-hairpin snoRNA.
• Key inter-protomer contact sites were identified, explaining the coordination of pseudouridylation activity and the prevalence of two-hairpin snoRNA structures.
• Mutations in H/ACA proteins linked to Dyskeratosis congenita were found to impair pseudouridine formation, providing mechanistic insight into the disease.
• Coordinated structural changes in specific H/ACA subunits suggest a regulatory mechanism for snoRNP activity.

Conclusions:

• The study provides unprecedented structural detail of active H/ACA snoRNPs, revealing an asymmetric dimeric architecture.
• The findings elucidate the mechanism of coordinated pseudouridylation and shed light on the pathogenesis of Dyskeratosis congenita.
• This work deepens the understanding of H/ACA snoRNP function in critical cellular processes, including RNA modification, ribosome biogenesis, and telomere maintenance.