Usb1 controls U6 snRNP assembly through evolutionarily divergent cyclic phosphodiesterase activities

Area of Science:

• Molecular Biology
• RNA Processing
• Protein Structure

Background:

• U6 small nuclear ribonucleoprotein (snRNP) biogenesis is vital for spliceosome assembly.
• The precise mechanisms governing U6 snRNP maturation remain incompletely understood.

Purpose of the Study:

• To elucidate the structural and enzymatic properties of the U6 RNA processing enzyme Usb1 in yeast and humans.
• To characterize the post-transcriptional assembly pathway of yeast U6 snRNP.
• To propose a comprehensive model for U6 snRNP biogenesis.

Main Methods:

• X-ray crystallography was used to determine the structures of yeast and human Usb1.
• In vitro reconstitution assays were employed to study yeast U6 snRNP assembly.
• Enzymatic activity assays were performed to characterize Usb1's function.

Main Results:

• Yeast Usb1 exhibits unique cyclic phosphodiesterase activity preventing U6 RNA overprocessing, unlike its human counterpart.
• Usb1 processing significantly impacts U6 RNA's affinity for RNA-binding proteins.
• The in vitro reconstitution revealed a complex, multi-protein assembly pathway for yeast U6 snRNP, involving intricate protein-protein interactions.

Conclusions:

• A detailed model for U6 snRNP assembly is proposed, highlighting the cooperative roles of diverse proteins in chaperoning U6 RNA.
• The findings provide critical insights into the evolutionarily conserved yet divergent mechanisms of spliceosome component biogenesis.