Stability of a human SWI-SNF remodeled nucleosomal array

Area of Science:

• Chromatin remodeling
• Molecular mechanisms of DNA-protein interactions

Background:

• SWI-SNF is a chromatin remodeling complex crucial for gene regulation.
• ATP-dependent nucleosome alterations are central to SWI-SNF function.
• Understanding the stability and dynamics of remodeled states is key to elucidating its biological roles.

Purpose of the Study:

• To investigate the energetic and kinetic properties of SWI-SNF-mediated nucleosome remodeling.
• To determine the role of ATP hydrolysis and hSWI-SNF binding in stabilizing remodeled states.
• To assess the ability of SWI-SNF to induce topological changes under energetically unfavorable conditions.

Main Methods:

• Utilizing closed circular nucleosomal arrays to study topological changes.
• Monitoring array topology changes in the presence and absence of ATP.
• Assessing the effect of competitor DNA and human SWI-SNF (hSWI-SNF) on reversion kinetics.
• Evaluating SWI-SNF activity without topoisomerase I.

Main Results:

• SWI-SNF remodeling creates a higher-energy state that reverts slowly upon ATP removal (half-life of hours).
• The remodeled state is kinetically trapped, indicating a high energy barrier.
• Competitor DNA accelerates reversion, suggesting hSWI-SNF binding stabilizes the remodeled state, even without ATP hydrolysis.
• SWI-SNF can induce topological changes even in the absence of topoisomerase I, overcoming unfavorable energetics.

Conclusions:

• The remodeled nucleosome state is less stable than the standard state but is kinetically trapped by a high activation energy barrier.
• hSWI-SNF binding plays a significant role in stabilizing the remodeled conformation, independent of ATP hydrolysis.
• SWI-SNF possesses the intrinsic ability to drive topological changes against unfavorable energy landscapes.