Staphylococcus aureus protease. A probe of exposed, nonbasic histone sequences in nucleosomes

Area of Science:

• Molecular Biology
• Chromatin Structure
• Protein Digestion

Background:

• Nucleosome cores and chromatin are fundamental units of DNA packaging in eukaryotes.
• Histones are the primary proteins involved in organizing DNA into nucleosomes.
• Understanding histone accessibility is crucial for deciphering gene regulation.

Purpose of the Study:

• To investigate the digestion patterns of chicken erythrocyte histones by Staphylococcus aureus protease.
• To identify exposed nonbasic amino acid residues in core histones within nucleosome structures.
• To map the accessibility of specific histone proteins (H1, H5, H3, H2b, H2a, H4) in nucleosomes.

Main Methods:

• Limited digestion of chicken erythrocyte nucleosome cores and chromatin using Staphylococcus aureus protease.
• Protease specificity for Glu-X bonds was utilized to probe exposed acidic residues.
• Electrophoresis of histone fragments to analyze digestion products and identify cleavage sites.

Main Results:

• Staphylococcus aureus protease readily degraded H1, H5, and H3; moderately degraded H2b; and slightly degraded H2a and H4.
• Electrophoresis revealed accessible nonbasic cleavage sites primarily in H3 (glutamic acids at positions 51, 60, 73, 94, 97) and one in H2b.
• Histones H2a and H4 showed minimal accessibility, while H5 was cleaved near its N-terminus.

Conclusions:

• Nonbasic core histone sequences are largely buried within the nucleosome interior.
• Histone H3 exhibits increased accessibility, likely due to its interaction with the ends of core DNA.
• This differential accessibility provides insights into the structural organization and DNA-binding properties of core histones.