Insulator function and topological domain border strength scale with architectural protein occupancy

Area of Science:

• Genomics
• Molecular Biology
• Epigenetics

Background:

• Eukaryotic genomes are organized into topologically associating domains (TADs).
• TAD borders are enriched for architectural proteins involved in insulator function.
• Most architectural protein sites are within TADs, suggesting border sites are a distinct functional subclass.

Purpose of the Study:

• To investigate the genome-wide distribution and function of architectural proteins.
• To understand how TADs are established and the role of architectural protein binding sites.
• To differentiate between border-associated and non-border architectural protein binding sites.

Main Methods:

• Genome-wide mapping of Drosophila architectural protein target sites.
• Analysis of TFIIIC and condensin complex binding profiles.
• Reporter-based enhancer-blocking assays and endogenous domain border strength measurements.
• Comparative analyses in mouse and human stem cells.

Main Results:

• Architectural proteins form dense clusters at TAD borders.
• Co-binding of architectural proteins at high occupancy sites correlates with strong insulator activity and TAD partitioning.
• Clustering of architectural proteins is conserved across species and may explain tissue-invariant TADs.
• Low occupancy sites are associated with gene-specific regulation within TADs.

Conclusions:

• Architectural protein occupancy exists on a spectrum, correlating with chromosome topology and regulatory potential.
• High occupancy sites mediate robust TAD partitioning and insulator function.
• Low occupancy sites are involved in gene regulation within TADs.