A Two-Step Mechanism for Creating Stable, Condensed Chromatin with the Polycomb Complex PRC1

Area of Science:

• Molecular Biology
• Chromatin Biology
• Epigenetics

Background:

• The Polycomb Repressive Complex 1 (PRC1) is crucial for gene regulation via chromatin modification.
• Two key subunits, PSC and Ph, play distinct roles in PRC1's function.
• PSC's C-terminal region (PSC-CTR) is vital for in vitro chromatin compaction, while Ph's sterile alpha motif (SAM) drives in vivo condensate formation.

Purpose of the Study:

• To elucidate the combined functions of PSC and Ph in PRC1-mediated chromatin organization.
• To investigate how PSC's chromatin bridging and Ph's phase separation activities cooperate.
• To understand the formation of stable chromatin architectures by PRC1.

Main Methods:

• In vitro analysis of PRC1 with large chromatin templates.
• Structural analysis of PRC1-chromatin interactions.
• Investigating the effects of Ph variants (mini-Ph) and PRC1 lacking Ph (PRC1ΔPh) on chromatin condensates.

Main Results:

• PRC1 bridges chromatin into extensive fibrillar networks, primarily driven by PSC-CTR.
• Ph's SAM domain facilitates chromatin phase separation, forming condensates.
• PRC1ΔPh treatment of mini-Ph chromatin condensates results in stable, non-fusing clusters with high compaction.

Conclusions:

• PRC1 utilizes a sequential mechanism involving Ph-driven phase separation and PSC-CTR-mediated bridging to establish unique, stable chromatin architectures.
• This coordinated action creates highly compacted chromatin regions with extraordinary stability.
• These stable chromatin structures may play a significant role in maintaining long-term transcriptional states.