Further Studies on Basic Nucleoproteins from the Cellular Slime Mold Dictyostelium discoideum

The nuclei from Dictyostelium discoideum were isolated using a number of detergents. Microscopically clean and intact nuclei were obtained using Kyro EOB, Cemulsol NPT 12 and Triton X-100. While density gradient purification of Kyro-EOB-isolated nuclei led to some loss of basic nucleoproteins, Triton and Cemulsol-isolated nuclei gave relatively consistent patterns of basic nucleoproteins. The variabilities are discussed. No reproducible differences were found between basic nucleoproteins of vegetative and differentiating cells. The basic nucleoproteins were separated on sodium dodecylsulfate as well as acid/urea gels and compared with calf thymus histones. The perchloric-acid-soluble protein (fraction 1) which comigrates with histone H1 on acid/urea gels migrates faster than H1 on dodecylsulfate gels (Mr 19000–20000). The ethanol/HCl fraction (‘arginine-rich’) contained most of bands 3 and 4 and all of band 5; the latter comigrating with H4 on acid/urea gels. The amino acid compositions of whole basic nucleoprotein, fraction 1 protein and the ethanol/HCl-soluble fraction were determined. The whole basic nucleoprotein contained a high proportion of arginine and lysine, although like many lower eukaryotes the basic/acidic amino acid ratio was relatively low (1.18). The ethanol/HCl-soluble fraction contained about 13.7 mol lysine and 9 mol arginine per 100 mol total residues compared 9.6 and 12 mol/100 mol respectively in a similar fraction from calf thymus histones. The fraction 1 protein is a lysine-rich protein, but contains more arginine and less lysine than calf thymus H1. Furthermore, it contains a much higher content of acidic (or amide) residues than H1. Hence, it resembles the two lysine-rich basic nucleoproteins of Physarum polycephalum. Tryptic peptide maps of fraction 1 protein and H1 were compared. Some 14 spots were superimposable and one group of five small, highly charged peptides showed a striking correspondence. The possibility that they occur in a conserved region of the protein involved in DNA binding is considered. The similarity between fraction 1 protein and the calf thymus high-mobility group proteins is discussed.