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

Association of nucleosome core particle DNA with different histone oligomers. Transfer of histones between DNA-(H2A,H2B) and DNA-(H3,H4) complexes

In non-denaturing low ionic strength gels, the titration of core DNA with H2A,H2B produces five well-defined bands. Quantitative densitometry and cross-linking experiments indicate that these bands are due to the successive binding of H2A,H2B dimers to core DNA. Only two bands are obtained with DNA-(H3,H4) samples. The slower of these bands is broad and presumably corresponds to two complexes containing one and two H3,H4 tetramers, respectively. In gels of higher ionic strength, DNA-(H2A,H2B) samples produce an ill-defined band, suggesting that the lifetime of the complexes containing H2A,H2B is relatively short. However, the low intensity of the free DNA band observed in these gels indicates that most of the DNA is associated with H2A,H2B. In agreement with this, our results obtained using different techniques (sedimentation, cross-linking, trypsin and nuclease digestions, and thermal denaturation) demonstrate that the association of H2A,H2B with core DNA occurs in free solution in both the absence and presence of NaCl (0.1 to 0.2 M). The low mobilities of DNA-(H2A,H2B) complexes, together with sedimentation and DNase I digestion results, indicate that the DNA in these complexes is not folded into the compact structure found in the core particle. Furthermore, non-denaturing gels have been used to study the dynamic properties of DNA-(H2A,H2B) and DNA-(H3,H4) complexes in 0.2 M-NaCl. Our results show that: (1) H2A,H2B and H3,H4 can associate, respectively, with DNA-(H3,H4) and DNA-(H2A,H2B) to produce complexes containing the four core histones; (2) DNA-(H2A,H2B) and DNA-(H3,H4) are able to transfer histones to free core DNA; (3) an exchange of histone pairs takes place between DNA-(H2A,H2B) and DNA-(H3,H4) and produces complexes with the same histone composition as that of the normal nucleosome core particle; and (4) although both histone pairs can exchange, histones H2A,H2B show a higher tendency than H3,H4 to migrate from one incomplete core particle to another. The complexes produced in these reactions have the same compact structure as reconstituted core particles containing the four core histones. Our kinetic results are consistent with a reaction mechanism in which the transfer of histones involves direct contacts between the reacting complexes. The possible participation of these spontaneous reactions on the mechanism of nucleosome assembly is discussed.

Kinetics of accumulation and depletion of soluble newly synthesized histone in the reciprocal regulation of histone and DNA synthesis

Procedures are presented which permit the identification and analysis of cellular histone that is not bound to chromatin. This histone, called soluble histone, could be distinguished from that bound to chromatin by the state of H4 modification and the lack of H2A ubiquitination. Changes in the levels of newly synthesized soluble histone were analyzed with respect to the balance between histone and DNA synthesis in hamster ovary cells. Pulse-chase protocols suggested that the chase of newly synthesized histone from the soluble fraction into chromatin may have two kinetic components with half-depletion times of about 1 and 40 min. When protein synthesis was inhibited, the pulse-chase kinetics of newly synthesized histone from the solubl fraction into chromatin were not significantly altered from those of the control. However, in contrast to the control, when protein synthesis was inhibited, DNA synthesis was also inhibited with kinetics similar to those of the chase of newly synthesized histone from the soluble fraction. There was a rapid decrease in the rate of DNA synthesis with a half-deceleration time of 1 min down to about 30% of the control rate, followed by a slower decrease with an approximate half-deceleration time of 40 min. When DNA synthesis was inhibited, newly synthesized histone accumulated in the soluble fraction, but H2A and H2B continued to complex with chromatin at a significant rate. Soluble histone in G1 cells showed the same differential partitioning of H4/H3 and H2A/H2B between the soluble and chromatin-bound fractions as was found in cycling cells with inhibited DNA synthesis. These results support a unified model of reciprocal regulatory mechanisms between histone and DNA synthesis in the assembly of chromatin.

DNA cloning and amino acid sequence determination of a major constituent protein of mammalian nucleoli. Correspondence of the nucleoplasmin-related protein NO38 to mammalian protein B23

Using a cDNA probe encoding the nucleolar protein NO38 of Xenopus laevis, we have isolated clones that code for the corresponding mammalian protein from cDNA libraries of mouse embryonal carcinoma and fetal liver cells. The murine cDNA-derived amino acid sequence defines a polypeptide of 292 amino acids (including the initial methionine) of a total molecular weight of 32,560 and identifies a single approximately 1.5 kb mRNA on Northern blot hybridization. This polypeptide, which is highly homologous to the Xenopus protein NO38, displays an organization in three major domains: (1) an aminoterminal portion of 119 amino acids, which shows a striking homology to nucleoplasmin of Xenopus; (2) a central portion of 68 amino acids that contains two extended acidic domains, a shorter of 13 residues and a longer of 29 residues, separated by an interval enriched in positively charged amino acids; (3) a carboxyterminal portion of 105 amino acids, which is almost identical to the reported partial amino acid sequence of human and rat nucleolar protein termed B23. The sequence comparisons show that the murine protein is the mammalian counterpart to the nucleolar protein NO38 of Xenopus and is compatible with the idea that both proteins NO38 represent the amphibian and murine equivalents to the human and rat nucleolar phosphoprotein B23. Special sequence features and predicted conformations of this protein are discussed in relation to the specific localization and the possible functions of this major nucleolar protein.

Nuclear protein migration involves two steps: rapid binding at the nuclear envelope followed by slower translocation through nuclear pores

When injected into the cytoplasm of Vero cells, nucleoplasmin rapidly concentrates in a narrow layer around the nuclear envelope and then accumulates within the nucleus. Transport into the nucleus can be reversibly arrested at the perinuclear stage by metabolic inhibitors or by chilling. Nucleoplasmin-coated colloidal gold particles concentrate around the nuclear envelope of Vero cells or Xenopus oocytes, and by electron microscopy of oocytes appear to be associated with fibrils attached to nuclear pore complexes. Perinuclear accumulation is not observed for the nonmigrating nucleoplasmin core fragment or nonnuclear proteins. We propose two steps in nuclear migration of proteins: rapid binding around the nuclear envelope, possibly to pore-associated fibrils, followed by slower, energy-dependent translocation through nuclear pores.

The metabolism of ribosomal proteins microinjected into the oocytes of Xenopus laevis

When the total proteins from Xenopus laevis 60 S ribosomal subunits (TP60) were 3H-labeled in vitro and injected back into X. laevis oocytes, most 3H-TP60 are integrated into the cytoplasmic 60 S subunits via the nucleus during 16 h of incubation. In the oocytes whose rRNA synthesis is inhibited, 3H-TP60 are rapidly degraded with a half-life of 2-3 h. This degradation ceased as soon as rRNA synthesis was resumed, suggesting that ribosomal proteins unassociated with nascent rRNA are unstable in the oocytes. The degradation of 3H-TP60 in the absence of RNA synthesis was inhibited by iodoacetamide, a cysteine protease inhibitor, resulting in the accumulation of 3H-TP60 in the nucleus reaching about a threefold concentration in the cytoplasm. Considering the results with enucleated oocytes, we suggest that the X. laevis nucleus has a limited capacity to accumulate ribosomal proteins in an active manner but that those ribosomal proteins accumulated in excess over rRNA synthesis are degraded by a cysteine protease in the nucleus. By contrast, ribosomal proteins from Escherichia coli only equilibrate between the nucleus and the cytoplasm and are degraded by serine protease(s) in the cytoplasm without being integrated in the form of ribosomes in the nucleus.

Two complexes that contain histones are required for nucleosome assembly in vitro: role of nucleoplasmin and N1 in Xenopus egg extracts

The composition and function of histone storage complexes of Xenopus eggs have been investigated using monoclonal antibodies. We show that core histones are contained in two distinct complexes: H2A and H2B are associated with nucleoplasmin, and H3 and H4 are associated with nuclear protein N1. Immunodepletion analyses demonstrate that both complexes are required for nucleosome core assembly by extracts in vitro, the product being a simple sum of the histones from each complex. In addition, the majority of the stored H2A is shown to be an unusual form that migrates close to the position of H3 by SDS-polyacrylamide gel electrophoresis and resembles a variant synthesized in a cell-cycle-independent manner in mammalian cells.

Biochemical characterization of the soluble form of the junctional plaque protein, plakoglobin, from different cell types

A polypeptide of identical molecular mass (Mr 83,000) and charge to desmosomal plakoglobin from bovine snout epidermis was identified in soluble and pelletable fractions from diverse tissues and cells of different mammalian species, including cells and tissues devoid of desmosomes (e.g. endothelial, retinal, lenticular cells, fibroblasts). The protein, however, was not detected in erythrocytes and platelets and in myeloma cells, nor in smooth muscle tissue. In all cells examined, the plakoglobin soluble upon cell lysis in buffers of near-physiological pH and ionic strength (21-31% of the total plakoglobin in the different cell types) was found to exist in a distinct molecular form. On sucrose gradient centrifugation it appeared at about 7 S and gel filtration chromatography revealed a Stokes radius of about 5.0 nm, from which an Mr of about 170,000 was estimated. By using isoelectric focusing under non-denaturing conditions, soluble approximately equal to 7-S plakoglobin had an isoelectric point at about pH 5.3. The plaque-bound and the soluble form of plakoglobin were indistinguishable by electrical charge and molecular mass, regardless of the source, indicating molecular identity. Cross-linking of soluble proteins with cupric 1,10-phenanthroline resulted in the formaton of a cross-linked product of plakoglobin with similar physical properties as the native approximately equal to 7-S particle, which is compatible with the interpretation that the soluble plakoglobin particle is a dimer. While a major proportion of the plakoglobin in the desmosomal plaque was resistant to various extraction procedures, plakoglobin present in the plaques of non-desmosome-containing cells and tissues was readily extractable under low and high salt conditions. This indicates that differences exist in the binding of plakoglobin to desmosomal plaques and the plaques of non-demosomal junctions.

Cloning of nucleoplasmin from Xenopus laevis oocytes and analysis of its developmental expression

Nucleoplasmin is the most abundant protein in the nucleus of Xenopus laevis oocytes. We cloned a cDNA coding for nucleoplasmin from an expression library of immature Xenopus laevis oocytes. The deduced amino acid sequence shows that the carboxyl terminus is very hydrophilic and contains an unusual stretch of 12 glutamic acid residues, which is consistent with one of the proposed functions of nucleoplasmin--that of promoting chromatin assembly. The last 50 amino acids are lysine- and alanine-rich and contain short stretches of homology to histone H1. These regions could be involved in interactions with nucleosomes. The levels of nucleoplasmin mRNA and protein during oogenesis and embryogenesis were investigated using Northern blots, Western blots, and in situ hybridization to oocyte sections. The mRNA is detected during oogenesis but not during embryogenesis, suggesting that nucleoplasmin may be an exclusively maternally expressed gene. However, the protein is present throughout embryogenesis and undergoes pronounced changes in its level of phosphorylation during maturation of the oocyte and just after midblastula transition. These results support the notion that nucleoplasmin is not only important in oocytes but also plays a major role during the rapid cleavages of early embryogenesis.

Binding mode of nucleosome-assembly protein (AP-I) and histones

Studies were made on the binding mode of the nucleosome-assembly protein AP-I with histones H2A + H2B and/or H3 + H4. Histones H2A + H2B bound with AP-I to form a 7-S complex which contained twice as much AP-I (by weight) as histones. Histone H3 + H4 formed an 8-S complex with AP-I. The 7-S and 8-S complexes did not form a new complex when mixed, but significant amounts of two histone pairs were assembled into a 12-S complex on mixing the (H2A + H2B)--AP-I complex (7-S) with free H3 + H4. In contrast, when the (H3 + H4)--AP-I complex (8-S) was incubated with free H2A + H2B, almost no assembly occurred, but the 7-S complex of H2A + H2B was newly formed. Binding studies by enzyme-linked immunosorbent assay showed that AP-I bound with H2A + H2B faster than with H3 + H4. From these results, it is suggested that AP-I has a higher binding affinity for histone H2A + H2B than for H3 + H4, and that the 7-S complex is an intermediate in formation of the 12-S octamer complex (H2A + H2B + H3 + H4)2.

The clam embryo protein cyclin A induces entry into M phase and the resumption of meiosis in Xenopus oocytes

Fertilized clam embryos synthesize several new cell-cycle-related proteins. The cloned cDNA and derived amino acid sequences of one of these, cyclin A, are presented here. Immunoblots with an anti-cyclin A antibody reveal that cyclin A is undetectable in oocytes, appears within 15 min of fertilization, and is destroyed near the end of each meiosis and mitosis. We directly tested the ability of cyclin A to induce M phase by injecting SP6 cyclin A mRNA into Xenopus oocytes, which are arrested at the G2/M border of first meiosis. The injected mRNA was translated, with the result that the Xenopus oocytes entered meiosis. These findings indicate that the rise in cyclin A plays a direct and natural role in driving cells into M phase.

Massive phosphorylation distinguishes Xenopus laevis nucleoplasmin isolated from oocytes or unfertilized eggs

Nucleoplasmin isolated from unfertilized Xenopus laevis eggs possesses an in vitro chromatin assembly activity which is superior to nucleoplasmin isolated from oocytes. It is demonstrated here that the two forms of the protein differ in the amount of attached phosphate, with the egg protein possessing nearly 20 phosphate groups per protein monomer and the oocyte protein possessing less than 10 phosphate groups per monomer. A kinase preparation from unfertilized eggs is shown to be capable of modifying oocyte nucleoplasmin so that it displays the electrophoretic heterogeneity of egg nucleoplasmin. Furthermore, when the egg protein is treated with phosphatase and repurified, the chromatin assembly activity deteriorates to the level of the oocyte protein.

Xenopus nucleoplasmin: egg vs. oocyte

Nucleoplasmin has been purified from either oocytes or unfertilized eggs of the frog, Xenopus laevis. We find that the pentameric form of egg nucleoplasmin exhibits an apparent molecular mass approximately 15 000 daltons larger than its oocyte counterpart upon sodium dodecyl sulfate (SDS)-acrylamide gel electrophoresis. Egg nucleoplasmin monomers are more heterogeneous, substantially more acidic, and overall larger in apparent molecular weight than oocyte nucleoplasmin monomers when analyzed by isoelectric focusing or SDS gel electrophoresis. Protease digestions indicate that the structural differences between egg and oocyte nucleoplasmin are primarily confined to the N-terminal halves of the proteins. The structural diversity observed is accompanied by a difference in the ability of nucleoplasmin from the two sources to act as a nucleosome assembly agent in vitro. Egg nucleoplasmin efficiently promotes the formation of nucleosomes onto circular pBR322 DNA in vitro at physiological ionic strength and at physiological histone:DNA ratios, while oocyte nucleoplasmin is markedly deficient in serving as an in vitro chromatin assembly agent under all conditions which we have tested. Treatment of egg nucleoplasmin in vitro with alkaline phosphatase demonstrates that the structural diversity between egg and oocyte nucleoplasmin results primarily from extensive additional phosphorylation of the egg protein. The relevance of nucleoplasmin phosphorylation in leading to differences in the chromatin assembly activity of this protein both in vitro and in vivo is considered.

Histones and their modifications

Histones constitute the protein core around which DNA is coiled to form the basic structural unit of the chromosome known as the nucleosome. Because of the large amount of new histone needed during chromosome replication, the synthesis of histone and DNA is regulated in a complex manner. During RNA transcription and DNA replication, the basic nucleosomal structure as well as interactions between nucleosomes must be greatly altered to allow access to the appropriate enzymes and factors. The presence of extensive and varied post-translational modifications to the otherwise highly conserved histone primary sequences provides obvious opportunities for such structural alterations, but despite concentrated and sustained effort, causal connections between histone modifications and nucleosomal functions are not yet elucidated.