We have developed an approach termed PUB-NChIP (proximity utilizing biotinylation with native ChIP) to purify and study the protein composition of chromatin in proximity to a nuclear protein of interest. It is based on coexpression of (1) a protein of interest, fused with the bacterial biotin ligase BirA, together with (2) a histone fused to a biotin acceptor peptide (BAP), which is specifically biotinylated by BirA-fusion in the proximity of the protein of interest. Using the RAD18 protein as a model, we demonstrate that the RAD18-proximal chromatin is enriched in some H4 acetylated species. Moreover, the RAD18-proximal chromatin containing a replacement histone H2AZ has a different pattern of H4 acetylation. Finally, biotin pulse-chase experiments show that the H4 acetylation pattern starts to resemble the acetylation pattern of total H4 after the proximity of chromatin to RAD18 has been lost.
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