Nucleosome flipping drives kinetic proofreading and processivity by SWR1

Area of Science:

• Chromatin biology
• Molecular mechanisms of DNA repair and replication
• Epigenetics

Background:

• The SWR1 complex is crucial for replacing canonical histone H2A-H2B dimers with Htz1-H2B dimers in nucleosomes.
• Understanding the mechanism of SWR1-mediated exchange is key to comprehending nucleosome dynamics and epigenetic regulation.

Purpose of the Study:

• To elucidate the step-by-step mechanism of double histone dimer exchange by the SWR1 complex.
• To investigate whether nucleosome release is required for processive double exchange.
• To determine the role of nucleosome flipping and the Swc2 subunit in the SWR1 complex's function.

Main Methods:

• Single-molecule analysis to observe real-time histone dimer exchange.
• Cryo-electron microscopy to visualize structural intermediates and conformational states.
• Biochemical assays to measure binding dwell times and reaction intermediates.

Main Results:

• Demonstrated a two-step, processive double exchange mechanism for H2A-H2B dimers by SWR1, occurring without nucleosome release.
• Observed nucleosomes flipping between distinct states, presenting different faces and histone dimers to SWR1.
• Identified a hexasome intermediate bound in a fixed orientation and highlighted the Swc2 subunit's role in facilitating conformational changes.

Conclusions:

• The SWR1 complex employs a processive mechanism for double dimer exchange, involving nucleosome flipping and specific subunit interactions.
• This mechanism allows SWR1 to efficiently 'proofread' histone dimer identities within nucleosomes.
• The findings provide a detailed molecular understanding of SWR1's role in chromatin remodeling and epigenetic regulation.