Stability of nucleosomes containing homogenously ubiquitylated H2A and H2B prepared using semisynthesis.

Post-translational modifications (PTMs) of histones are an essential feature in the dynamic regulation of chromatin. One of these modifications, ubiquitylation, has been speculated to directly influence the stability of the nucleosome, which represents the basic building block of chromatin. Here we report a strategy for the semisynthesis of site-specifically ubiquitylated histone H2A (uH2A). This branched protein was generated through a three-piece expressed protein ligation approach including a traceless ligation at valine. uH2A could be efficiently incorporated into nucleosomes, thereby opening the way to detailed biochemical and biophysical studies on the function of this PTM. Accordingly, we used uH2A, as well as a previously generated ubiquitylated H2B, in chaperone-coupled nucleosome stability assays to demonstrate that the direct effect of ubiquitylated histones on nucleosomal stability is in fact modest.