Resolution of transcription-induced hexasome-nucleosome complexes by Chd1 and FACT

Area of Science:

• Molecular Biology
• Chromatin Biology
• Biochemistry

Background:

• ATP-dependent chromatin remodelers and histone chaperones maintain nucleosome organization during gene transcription.
• RNA polymerase II (RNAPII) activity can disrupt nucleosome structure, requiring remodeling factors.
• Hexasomes, nucleosome intermediates lacking a histone H2A/H2B dimer, form at transcribed gene regions.

Purpose of the Study:

• To elucidate the mechanism by which the chromatin remodeler Chd1 interacts with hexasome-nucleosome complexes.
• To determine the role of the FACT complex in modulating Chd1 activity on these complexes.
• To provide structural insights into the regulation of chromatin remodeling during transcription.

Main Methods:

• Cryoelectron microscopy (cryo-EM) to determine structures of Chd1 bound to hexasome-nucleosome complexes.
• Biochemical assays to assess Chd1 remodeling activity in the presence and absence of FACT.
• Structural analysis of Chd1-nucleosome interactions.

Main Results:

• Two cryo-EM structures reveal Chd1 bound to hexasome-nucleosome complexes, both before and after H2A/H2B dimer restoration by FACT.
• Chd1 uniquely interacts with the complex, utilizing its ATPase domain to shift the hexasome away from the nucleosome.
• In the absence of the H2A/H2B dimer, Chd1's DNA-binding domain (DBD) is positioned against its ATPase domain, indicating an inhibited state.
• FACT-mediated restoration of the H2A/H2B dimer triggers a conformational change that displaces the DBD and activates Chd1 remodeling.

Conclusions:

• Chd1 remodeling activity is regulated by the presence of the inner H2A/H2B histone dimer.
• FACT acts as a histone chaperone to restore the dimer, thereby stimulating Chd1.
• These findings reveal a mechanism for how Chd1 and FACT cooperate to maintain chromatin organization and facilitate transcription by RNAPII.