A Dimeric Structural Scaffold for PRC2-PCL Targeting to CpG Island Chromatin

Area of Science:

• Epigenetics and chromatin biology
• Structural biology
• Molecular mechanisms of gene regulation

Background:

• Polycomb Repressive Complex 2 (PRC2) is essential for epigenetic gene silencing.
• Accessory subunits mediate PRC2 recruitment to CpG island (CGI) chromatin.
• PRC2 self-association has been previously implicated but structurally uncharacterized.

Purpose of the Study:

• To elucidate the structural basis of PRC2 complex assembly and self-association.
• To understand the role of accessory subunits in PRC2 dimerization.
• To investigate the functional consequences of PRC2 dimerization on chromatin binding and histone modification.

Main Methods:

• X-ray crystallography to determine the structure of SUZ12-RBBP4 complex with accessory subunits.
• Biochemical assays to assess PRC2 dimerization and DNA binding.
• In vitro assays to measure histone H3K27 trimethylation (H3K27me3).
• Analysis in mouse embryonic stem cells to evaluate in vivo function.

Main Results:

• The crystal structure reveals a dimeric PRC2 architecture formed by domain swapping of RBBP4 and SUZ12 C2 domain.
• Accessory subunits MTF2 and PHF19 stabilize the PRC2 dimer at the dimer interface.
• AEBP2 binding disrupts the PRC2 dimer.
• PRC2 dimerization enhances binding to CGI DNA in vitro.
• Loss of PRC2 dimerization impairs H3K27me3 at developmental gene loci in mouse embryonic stem cells.

Conclusions:

• PRC2 intrinsically forms dimers via a novel domain-swapping mechanism involving SUZ12 and RBBP4.
• Accessory proteins differentially regulate PRC2 dimerization, impacting its function.
• PRC2 dimerization is critical for efficient H3K27me3 deposition and developmental gene regulation.