Co-existence of two different types of soluble histone complexes in nuclei of Xenopus laevis oocytes

The nuclear pool of soluble histones in Xenopus laevis oocytes is organized into two major types of acidic histone complexes separable by sucrose density gradient centrifugation. One type of complex sediments at 5 S (Mr approximately 120,000), is isoelectric at pH 4.6, and contains histones H3 and/or H4 tightly bound to one polypeptide of a pair of very acidic polypeptides, designated N1 and N2 (Kleinschmidt, J. A., and Franke, W. W. (1982) Cell 29, 799-809). This complex can be selectively immunoprecipitated by guinea pig antibodies against purified protein N1/N2. In contrast, a larger complex of 7 S contains four histones and nucleoplasmin (the purified protein exists as a pentamer of a polypeptide of Mr approximately 30,000), is isoelectric over the pH range of 5-7, and can be immunoprecipitated by nucleoplasmin antibodies. Its relative molecular weight of 130,000-170,000, as determined by gel filtration, sucrose density gradient centrifugation, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the cross-linked complexes, excludes the association of a histone octamer with nucleoplasmin. In addition to histones H2A and H2B, two histones (designated H3 and H4) which are similar in their electrophoretic mobilities to histones H3 and H4 but have lower isoelectric pH values are enriched in immuno-precipitates obtained with nucleoplasmin antibodies. Cross-linking of complexes present in intact nuclei, using 1% formaldehyde at near-physiological ionic strength and pH, indicates the coexistence of these two soluble histone complexes in the living cell. In chromatin assembly experiments using SV 40 DNA, both histone fractions are able to transfer histones to DNA, resulting in an increase of DNA superhelicity and the formation of beaded nucleoprotein complexes of nucleosome-like morphology. The common principle governing both types of complexes, i.e. the association of one or two histone molecules with a karyophilic large acidic histone-binding protein is emphasized. We discuss the possible role of these complexes in storing histones utilized in chromatin assembly during early amphibian embryogenesis as well as the possible existence of similar complexes, albeit at lower concentrations, in somatic cells.