Small CAG Repeat RNA Forms a Duplex Structure with Sticky Ends That Promote RNA Condensation

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Biomolecular condensation forms essential membraneless organelles, but aberrant condensation is linked to human diseases.
• Ribonucleic acid (RNA) is crucial for condensate formation via phase transitions involving proteins and other RNAs.
• Physicochemical principles of RNA phase transitions, particularly for short RNAs, are not fully understood.

Purpose of the Study:

• Investigate the phase transition behavior of small CAG repeat (sCAG) RNAs, pathogenic factors in Huntington's disease.
• Elucidate the structural and mechanistic basis of sCAG RNA-driven biomolecular condensation.
• Explore the cellular localization and implications of sCAG RNA condensates.

Main Methods:

• Solution nuclear magnetic resonance (NMR) spectroscopy.
• Coarse-grained molecular dynamic (MD) simulations.
• In vitro and in-cell studies of RNA condensation.

Main Results:

• sCAG RNAs (6-7 repeats) undergo phase transition both in vitro and within cells.
• sCAG RNAs form duplex structures with GC-rich 3'-sticky ends that mediate intermolecular crosslinking.
• These structural features promote the formation of RNA condensates, which localize to nuclear speckles in cells.

Conclusions:

• Specific RNA structural motifs, like GC-rich sticky ends, can drive phase transitions and condensate formation, independent of sequence length.
• This finding advances understanding of RNA's role in biomolecular condensation and disease pathogenesis.
• Opens possibilities for designing novel RNA-based biomaterials and therapeutic strategies targeting aberrant RNA condensation.