SIR proteins create compact heterochromatin fibers

Area of Science:

• Molecular Biology
• Epigenetics
• Chromatin Biology

Background:

• Heterochromatin, a silenced chromatin state, is crucial for genomic stability and development.
• Its complex structure has historically made characterization challenging.
• In budding yeast, SIR proteins (Sir3, Sir2-4 complex) are key to heterochromatin assembly, with Sir2-4 mediating recruitment and histone H4 deacetylation.

Purpose of the Study:

• To characterize the stoichiometry and conformation of reconstituted SIR chromatin fibers.
• To elucidate the structural requirements for heterochromatin compaction in budding yeast.

Main Methods:

• Reconstitution of chromatin fibers using the complete complement of SIR proteins.
• Analysis via sedimentation velocity, molecular modeling, and atomic force microscopy.

Main Results:

• SIR chromatin fibers reconstituted with all SIR proteins are highly compact, unlike those with Sir3 alone.
• Fiber condensation is dependent on the integrity of histone H4 lysine 16 (H4K16) and an interaction between Sir3 and Sir4.
• A model is proposed where Sir3 dimers bridge nucleosomes, stabilized by Sir2-4 heterotetramers.

Conclusions:

• The complete SIR complex, not Sir3 alone, drives significant heterochromatin compaction.
• Histone H4K16 acetylation status and Sir3-Sir4 interaction are critical for stable heterochromatin structure.
• A detailed molecular model explains the mechanism of SIR-mediated chromatin fiber compaction.