Single molecule analysis reveals reversible and irreversible steps during spliceosome activation

Area of Science:

• Molecular Biology
• RNA Processing
• Biochemistry

Background:

• The spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs) and proteins, removes introns from pre-mRNAs.
• Spliceosome activation involves subunit rearrangement to form a catalytic active site, a process not fully understood.
• U4 release and Prp19 complex (NTC) recruitment are critical for stabilizing the active site during activation.

Purpose of the Study:

• To directly observe key events during Saccharomyces cerevisiae spliceosome activation.
• To elucidate the coordination mechanism of spliceosome subunit rearrangement.
• To understand the role of ATP in spliceosome activation pathways.

Main Methods:

• Utilized multi-wavelength colocalization single-molecule spectroscopy.
• Tracked dynamic molecular events in real-time during spliceosome assembly and activation.
• Analyzed the binding and dissociation of U4/U6.U5 tri-snRNP and NTC recruitment.

Main Results:

• Observed two distinct pathways after tri-snRNP binding: assembly reversal or activation via U4 loss.
• Demonstrated that NTC recruitment predominantly follows U4 release.
• Showed ATP stimulates both U4 release and tri-snRNP discard, influencing activation dynamics.

Conclusions:

• Revealed a detailed mechanism for spliceosome activation.
• Indicated that spliceosome efficiency is maintained by cycles of disassembly and reassociation.
• Provided direct evidence for the dynamic nature of spliceosome remodeling.