Presence of non-histone proteins in nucleosomes

It has been established that nucleosomes are made of histones and DNA fragments. The purpose of this work to establish whether some non-histone proteins are also present in these chromatin subunits. We have found that nucleosome preparations contain phosphorylated non-histone proteins and protein kinases by sucrose gradient analysis. In order to establish whether these proteins are actually bound to nucleosomes or if they represent unbound or aggregated proteins, the following experiments were performed. (a) Free non-histone proteins and proteins released from chromatin by DNase overdigestion were analyzed by sucrose gradient centrifugation. No phosphoproteins but some phosvitin kinase activity was found in the part of the gradient which contained the nucleosomes. It could be assumed that part of the phosphoproteins are bound to nucleosomes. (b) A digestion of nucleosomes with DNase I suppressed the phosvitin kinase activity in the 11-S region of the gradient. (c) High ionic strength, which extracted non-histone proteins, suppressed the phosvitin kinase activity in the nucleosome region. Part of phosvitin kinase and of nuclear phosphoproteins are therefore bound to nucleosomes and are released by nuclease digestion and by high ionic strength.