A protein, which facilitates assembly of a nucleosome-like structure in vitro, was previously partially purified from mouse FM3A cells [Ishimi, Y. et al. (1983) J. Biochem. (Tokyo) 94, 735–744]. The protein has been purified to approximately 80% from FM3A cells by using histone-Sepharose column chromatography. It sedimented at 4.6 S and had a molecular mass of 53kDa. A preincubation of core histones with the 53-kDa peptide before DNA addition was necessary for the nucleosome assembly. The 53-kDa peptide bound to core histones and formed a 12-S complex. This complex contained stoichiometrical amounts of the 53-kDa peptide and core histones, and the core histones in this complex were composed of equal amounts of H2A, H2B. H3 and H4 histones. The nucleosomes were assembled by adding pBR322 DNA to the 12-S complex. When mononucleosome DNA and core histones were mixed in the presence of the 53-kDa peptide, formation of a 10.5-S complex was observed. The complex contained DNA and core histones in equal amounts, while no 53-kDa peptide was detected in the complex.
From above results it is suggested that the 53-kDa peptide facilitates nucleosome assembly by mediating formation of histone octamer and transferring it to DNA. Rat antibody against the 53-kDa peptide did not bind to nucleoplasmin from Xenopus eggs. The relationship between the 53-kDa peptide and nucleoplasmin is discussed.