A Brownian ratchet model for DNA loop extrusion by the cohesin complex

Area of Science:

• Molecular Biology
• Genetics
• Biophysics

Background:

• The cohesin complex is crucial for sister chromatid cohesion and chromatin domain formation.
• Cohesin's dual roles in DNA topology suggest complex structural mechanisms.
• Understanding cohesin's function is key to comprehending genome stability and regulation.

Purpose of the Study:

• To propose a structure-based model explaining cohesin's DNA binding and loop extrusion activities.
• To elucidate the molecular mechanisms underlying cohesin's topological DNA manipulation.
• To provide a unified model for cohesin's diverse functions in DNA processing.

Main Methods:

• Development of a structure-based model for cohesin function.
• Utilizing molecular-mechanical simulations to study cohesin-DNA interactions.
• Analyzing cohesin's conformational changes during ATP binding and hydrolysis.

Main Results:

• A model where ATP binding induces a DNA gripping state in cohesin.
• ATP hydrolysis drives unidirectional hinge module movement for topological DNA entry.
• Failure of head gate passage leads to loop extrusion via a Brownian ratchet mechanism.
• Simulations accurately recapitulate experimental DNA loop extrusion characteristics.
• The model accounts for various loop extrusion modes, including asymmetric, symmetric, and z-loop formation.

Conclusions:

• Cohesin's distinct activities are explained by a unified structure-based mechanism.
• Biased Brownian motion during loop extrusion has significant implications for chromosomal cohesin function.
• The proposed model provides a framework for future experimental and computational investigations of cohesin.