Read-write mechanisms of H2A ubiquitination by Polycomb repressive complex 1

Area of Science:

• Epigenetics
• Molecular Biology
• Structural Biology

Background:

• Epigenetic inheritance relies on maintaining silent chromatin domains through histone modifications.
• Histone H2A lysine 119 monoubiquitination (H2AK119Ub) is crucial for silencing but is diluted during DNA replication.
• The Polycomb repressive complex 1 (PRC1) is responsible for re-establishing H2AK119Ub, but its mechanism is unclear.

Purpose of the Study:

• To elucidate the mechanism by which non-canonical PRC1 containing RYBP (ncPRC1^RYBP) restores H2AK119Ub.
• To characterize the structural and functional aspects of the ncPRC1^RYBP read-write mechanism.
• To understand the role of ncPRC1^RYBP in maintaining cellular memory and epigenetic regulation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of ncPRC1^RYBP.
• Functional assays to assess the E3 ligase activity of ncPRC1^RYBP.
• Biochemical approaches to study nucleosome binding and ubiquitination.

Main Results:

• ncPRC1^RYBP binds asymmetrically to H2AK119Ub nucleosomes, with RYBP's zinc-finger domain recognizing existing H2AK119Ub.
• This asymmetrical binding positions RING domains for intra- and inter-nucleosome H2AK119Ub restoration.
• A positive-feedback loop mechanism for H2AK119Ub re-establishment was identified.

Conclusions:

• ncPRC1^RYBP employs a unique read-write mechanism to efficiently restore H2AK119Ub.
• This mechanism is vital for sustaining silent chromatin domains and cellular memory.
• The findings provide critical insights into epigenetic regulation and the function of Polycomb complexes.