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

Double maternal-effect: duplicated nucleoplasmin 2 genes, npm2a and npm2b, with essential but distinct functions are shared by fish and tetrapods

BACKGROUND

Nucleoplasmin 2 (npm2) is an essential maternal-effect gene that mediates early embryonic events through its function as a histone chaperone that remodels chromatin. Recently, two npm2 (npm2a and npm2b) genes have been annotated in zebrafish. Thus, we examined the evolution of npm2a and npm2b in a variety of vertebrates, their potential phylogenetic relationships, and their biological functions using knockout models via the CRISPR/cas9 system.

RESULTS

We demonstrated that the two npm2 duplicates exist in a wide range of vertebrates, including sharks, ray-finned fish, amphibians, and sauropsids, while npm2a was lost in coelacanth and mammals, as well as some specific teleost lineages. Using phylogeny and synteny analyses, we traced their origins to the early stages of vertebrate evolution. Our findings suggested that npm2a and npm2b resulted from an ancient local gene duplication, and their functions diverged although key protein domains were conserved. We then investigated their functions by examining their tissue distribution in a wide variety of species and found that they shared ovarian-specific expression, a key feature of maternal-effect genes. We also demonstrated that both npm2a and npm2b are maternally-inherited transcripts in vertebrates, and that they play essential, but distinct, roles in early embryogenesis using zebrafish knockout models. Both npm2a and npm2b function early during oogenesis and may play a role in cortical granule function that impact egg activation and fertilization, while npm2b is also involved in early embryogenesis.

CONCLUSION

These novel findings will broaden our knowledge on the evolutionary history of maternal-effect genes and underlying mechanisms that contribute to vertebrate reproductive success. In addition, our results demonstrate the existence of a newly described maternal-effect gene, npm2a, that contributes to egg competence, an area that still requires further comprehension.

A NuRD Complex from Xenopus laevis Eggs Is Essential for DNA Replication during Early Embryogenesis

DNA replication in the embryo of Xenopus laevis changes dramatically at the mid-blastula transition (MBT), with Y RNA-independent random initiation switching to Y RNA-dependent initiation at specific origins. Here, we identify xNuRD, an MTA2-containing assemblage of the nucleosome remodeling and histone deacetylation complex NuRD, as an essential factor in pre-MBT Xenopus embryos that overcomes a functional requirement for Y RNAs during DNA replication. Human NuRD complexes have a different subunit composition than xNuRD and do not support Y RNA-independent initiation of DNA replication. Blocking or immunodepletion of xNuRD inhibits DNA replication initiation in isolated nuclei in vitro and causes inhibition of DNA synthesis, developmental delay, and embryonic lethality in early embryos. xNuRD activity declines after the MBT, coinciding with dissociation of the complex and emergence of Y RNA-dependent initiation. Our data thus reveal an essential role for a NuRD complex as a DNA replication factor during early Xenopus development.

Shaping Chromatin in the Nucleus: The Bricks and the Architects

Chromatin organization in the nucleus provides a vast repertoire of information in addition to that encoded genetically. Understanding how this organization impacts genome stability and influences cell fate and tumorigenesis is an area of rapid progress. Considering the nucleosome, the fundamental unit of chromatin structure, the study of histone variants (the bricks) and their selective loading by histone chaperones (the architects) is particularly informative. Here, we report recent advances in understanding how relationships between histone variants and their chaperones contribute to tumorigenesis using cell lines and Xenopus development as model systems. In addition to their role in histone deposition, we also document interactions between histone chaperones and other chromatin factors that govern higher-order structure and control DNA metabolism. We highlight how a fine-tuned assembly line of bricks (H3.3 and CENP-A) and architects (HIRA, HJURP, and DAXX) is key in adaptation to developmental and pathological changes. An example of this conceptual advance is the exquisite sensitivity displayed by p53-null tumor cells to modulation of HJURP, the histone chaperone for CENP-A (CenH3 variant). We discuss how these findings open avenues for novel therapeutic paradigms in cancer care.

A Cell Free Assay to Study Chromatin Decondensation at the End of Mitosis

During the vertebrate cell cycle chromatin undergoes extensive structural and functional changes. Upon mitotic entry, it massively condenses into rod shaped chromosomes which are moved individually by the mitotic spindle apparatus. Mitotic chromatin condensation yields chromosomes compacted fifty-fold denser as in interphase. During exit from mitosis, chromosomes have to re-establish their functional interphase state, which is enclosed by a nuclear envelope and is competent for replication and transcription. The decondensation process is morphologically well described, but in molecular terms poorly understood: We lack knowledge about the underlying molecular events and to a large extent the factors involved as well as their regulation. We describe here a cell-free system that faithfully recapitulates chromatin decondensation in vitro, based on mitotic chromatin clusters purified from synchronized HeLa cells and X. laevis egg extract. Our cell-free system provides an important tool for further molecular characterization of chromatin decondensation and its co-ordination with processes simultaneously occurring during mitotic exit such as nuclear envelope and pore complex re-assembly.

Nuclear Reformation at the End of Mitosis

Cells have developed highly sophisticated ways to accurately pass on their genetic information to the daughter cells. In animal cells, which undergo open mitosis, the nuclear envelope breaks down at the beginning of mitosis and the chromatin massively condenses to be captured and segregated by the mitotic spindle. These events have to be reverted in order to allow the reformation of a nucleus competent for DNA transcription and replication, as well as all other nuclear processes occurring in interphase. Here, we summarize our current knowledge of how, in animal cells, the highly compacted mitotic chromosomes are decondensed at the end of mitosis and how a nuclear envelope, including functional nuclear pore complexes, reassembles around these decondensing chromosomes.

Maternally inherited npm2 mRNA is crucial for egg developmental competence in zebrafish

The molecular mechanisms underlying and determining egg developmental competence remain poorly understood in vertebrates. Nucleoplasmin (Npm2) is one of the few known maternal effect genes in mammals, but this maternal effect has never been demonstrated in nonmammalian species. A link between developmental competence and the abundance of npm2 maternal mRNA in the egg was previously established using a teleost fish model for egg quality. The importance of maternal npm2 mRNA for egg developmental competence remains unknown in any vertebrate species. In the present study, we aimed to characterize the contribution of npm2 maternal mRNA to early developmental success in zebrafish using a knockdown strategy. We report here the oocyte-specific expression of npm2 and maternal inheritance of npm2 mRNA in zebrafish eggs. The knockdown of the protein translated from this maternal mRNA results in developmental arrest before the onset of epiboly and subsequent embryonic death, a phenotype also observed in embryos lacking zygotic transcription. Npm2 knockdown also results in impaired transcription of the first-wave zygotic genes. Our results show that npm2 is also a maternal effect gene in a nonmammalian vertebrate species and that maternally inherited npm2 mRNA is crucial for egg developmental competence. We also show that de novo protein synthesis from npm2 maternal mRNA is critical for developmental success beyond the blastula stage and required for zygotic genome activation. Finally, our results suggest that npm2 maternal mRNA is an important molecular factor of egg quality in fish and possibly in all vertebrates.

The intrinsically disordered distal face of nucleoplasmin recognizes distinct oligomerization states of histones

The role of Nucleoplasmin (NP) as a H2A-H2B histone chaperone has been extensively characterized. To understand its putative interaction with other histone ligands, we have characterized its ability to bind H3-H4 and histone octamers. We find that the chaperone forms distinct complexes with histones, which differ in the number of molecules that build the assembly and in their spatial distribution. When complexed with H3-H4 tetramers or histone octamers, two NP pentamers form an ellipsoidal particle with the histones located at the center of the assembly, in stark contrast with the NP/H2A-H2B complex that contains up to five histone dimers bound to one chaperone pentamer. This particular assembly relies on the ability of H3-H4 to form tetramers either in solution or as part of the octamer, and it is not observed when a variant of H3 (H3C110E), unable to form stable tetramers, is used instead of the wild-type protein. Our data also suggest that the distal face of the chaperone is involved in the interaction with distinct types of histones, as supported by electron microscopy analysis of the different NP/histone complexes. The use of the same structural region to accommodate all type of histones could favor histone exchange and nucleosome dynamics.

Maternal control of mouse preimplantation development

Mammalian preimplantation development is a process of dedifferentiation from the terminally differentiated eggs to the totipotent blastomeres at the cleavage stage, and then to the pluripotent cells at the blastocyst stage. Maternal factors that accumulate during oogenesis dominate early preimplantation development until the embryonic factors gain control after the activation of the embryonic genome. Recently, a handful of maternal factors that are encoded by the maternal-effect genes have been characterized in genetically modified mouse models. These factors are shown to participate in many aspects of preimplantation development, such as the degradation of maternal macromolecues, epigenetic modification, protein translation, cellular signaling transduction, and cell compaction. Even so, little is known about the interactions between different maternal factors. In this chapter, we have summarized the functions of known maternal factors and hopefully this will lead to a better understanding of the regulation of preimplantation embryogenesis by the maternal regulatory network.

Protein arginine methyltransferase Prmt5-Mep50 methylates histones H2A and H4 and the histone chaperone nucleoplasmin in Xenopus laevis eggs

Histone proteins carry information contained in post-translational modifications. Eukaryotic cells utilize this histone code to regulate the usage of the underlying DNA. In the maturing oocytes and eggs of the frog Xenopus laevis, histones are synthesized in bulk in preparation for deposition during the rapid early developmental cell cycles. During this key developmental time frame, embryonic pluripotent chromatin is established. In the egg, non-chromatin-bound histones are complexed with storage chaperone proteins, including nucleoplasmin. Here we describe the identification and characterization of a complex of the protein arginine methyltransferase 5 (Prmt5) and the methylosome protein 50 (Mep50) isolated from Xenopus eggs that specifically methylates predeposition histones H2A/H2A.X-F and H4 and the histone chaperone nucleoplasmin on a conserved motif (GRGXK). We demonstrate that nucleoplasmin (Npm), an exceedingly abundant maternally deposited protein, is a potent substrate for Prmt5-Mep50 and is monomethylated and symmetrically dimethylated at Arg-187. Furthermore, Npm modulates Prmt5-Mep50 activity directed toward histones, consistent with a regulatory role for Npm in vivo. We show that H2A and nucleoplasmin methylation appears late in oogenesis and is most abundant in the laid egg. We hypothesize that these very abundant arginine methylations are constrained to pre-mid blastula transition events in the embryo and therefore may be involved in the global transcriptional repression found in this developmental time frame.

New insights into the nucleophosmin/nucleoplasmin family of nuclear chaperones

Basic proteins and nucleic acids are assembled into complexes in a reaction that must be facilitated by nuclear chaperones in order to prevent protein aggregation and formation of non-specific nucleoprotein complexes. The nucleophosmin/nucleoplasmin (NPM) family of chaperones [NPM1 (nucleophosmin), NPM2 (nucleoplasmin) and NPM3] have diverse functions in the cell and are ubiquitously represented throughout the animal kingdom. The importance of this family in cellular processes such as chromatin remodeling, genome stability, ribosome biogenesis, DNA duplication and transcriptional regulation has led to the rapid growth of information available on their structure and function. The present review covers different aspects related to the structure, evolution and function of the NPM family. Emphasis is placed on the long-term evolutionary mechanisms leading to the functional diversification of the family members, their role as chaperones (particularly as it pertains to their ability to aid in the reprogramming of chromatin), and the importance of NPM2 as an essential component of the amphibian chromatin remodeling machinery during fertilization and early embryonic development.

The characterization of amphibian nucleoplasmins yields new insight into their role in sperm chromatin remodeling

Background

Nucleoplasmin is a nuclear chaperone protein that has been shown to participate in the remodeling of sperm chromatin immediately after fertilization by displacing highly specialized sperm nuclear basic proteins (SNBPs), such as protamine (P type) and protamine-like (PL type) proteins, from the sperm chromatin and by the transfer of histone H2A-H2B. The presence of SNBPs of the histone type (H type) in some organisms (very similar to the histones found in somatic tissues) raises uncertainty about the need for a nucleoplasmin-mediated removal process in such cases and poses a very interesting question regarding the appearance and further differentiation of the sperm chromatin remodeling function of nucleoplasmin and the implicit relationship with SNBP diversity The amphibians represent an unique opportunity to address this issue as they contain genera with SNBPs representative of each of the three main types: Rana (H type); Xenopus (PL type) and Bufo (P type).

Results

In this work, the presence of nucleoplasmin in oocyte extracts from these three organisms has been assessed using Western Blotting. We have used mass spectrometry and cloning techniques to characterize the full-length cDNA sequences of Rana catesbeiana and Bufo marinus nucleoplasmin. Northern dot blot analysis shows that nucleoplasmin is mainly transcribed in the egg of the former species. Phylogenetic analysis of nucleoplasmin family members from various metazoans suggests that amphibian nucleoplasmins group closely with mammalian NPM2 proteins.

Conclusion

We have shown that these organisms, in striking contrast to their SNBPs, all contain nucleoplasmins with very similar primary structures. This result has important implications as it suggests that nucleoplasmin's role in chromatin assembly during early zygote development could have been complemented by the acquisition of a new function of non-specifically removing SNBPs in sperm chromatin remodeling. This acquired function would have been strongly determined by the constraints imposed by the appearance and differentiation of SNBPs in the sperm.

Chromatin decondensation and nuclear reprogramming by nucleoplasmin

Somatic cell nuclear cloning has repeatedly demonstrated striking reversibility of epigenetic regulation of cell differentiation. Upon injection into eggs, the donor nuclei exhibit global chromatin decondensation, which might contribute to reprogramming the nuclei by derepressing dormant genes. Decondensation of sperm chromatin in eggs is explained by the replacement of sperm-specific histone variants with egg-type histones by the egg protein nucleoplasmin (Npm). However, little is known about the mechanisms of chromatin decondensation in somatic nuclei that do not contain condensation-specific histone variants. Here we found that Npm could widely decondense chromatin in undifferentiated mouse cells without overt histone exchanges but with specific epigenetic modifications that are relevant to open chromatin structure. These modifications included nucleus-wide multiple histone H3 phosphorylation, acetylation of Lys 14 in histone H3, and release of heterochromatin proteins HP1beta and TIF1beta from the nuclei. The protein kinase inhibitor staurosporine inhibited chromatin decondensation and these epigenetic modifications with the exception of H3 acetylation, potentially linking these chromatin events. At the functional level, Npm pretreatment of mouse nuclei facilitated activation of four oocyte-specific genes from the nuclei injected into Xenopus laevis oocytes. Future molecular elucidation of chromatin decondensation by Npm will significantly contribute to our understanding of the plasticity of cell differentiation.

Large scale real-time PCR analysis of mRNA abundance in rainbow trout eggs in relationship with egg quality and post-ovulatory ageing

The mRNA levels of 39 target genes were monitored in unfertilized eggs of 14 rainbow trout sampled the day of ovulation and again 5, 14, and 21 days later. For all 56 collected egg batches, an egg sample was fertilized to estimate egg quality by monitoring embryonic development. Remaining eggs were used for RNA extraction and subsequent real-time PCR analysis. A significant drop of egg quality was observed when eggs were held in the body cavity for 14 or 21 days post-ovulation (dpo). During the same period, eight transcripts (nucleoplasmin or Npm2, ferritin H, tubulin beta, JNK1, cyclin A1, cyclin A2, cathepsin Z, and IGF2) exhibited a differential abundance at one or several collection time(s). Interestingly, we observed higher levels of cyclins A1 and A2 mRNAs in eggs taken 5 days post-ovulation than in eggs taken, from the same females, at the time of ovulation. In addition, seven transcripts exhibited a differential abundance between low quality and high quality eggs. Low quality eggs were characterized by lower levels of Npm2, tubulin beta, and IGF1 transcripts. In contrast, keratins 8 and 18, cathepsin Z, and prostaglandin synthase 2 were more abundant in low quality eggs than in high quality eggs. In this study, we have demonstrated differences in mRNA levels in the rainbow trout egg that are reflective of developmental competence differences induced by post-ovulatory ageing. The putative role of these transcripts in post-ovulatory ageing-induced egg quality defects is discussed with special attention for corresponding cellular functions.

The slender lobes gene, identified by retarded mushroom body development, is required for proper nucleolar organization in Drosophila

The nucleolus dynamically alters its shape through the assembly and disassembly of a variety of nucleolar components in proliferating cells. While the nucleolus is known to function in vital cellular events, little is known about how its components are correctly assembled. Through the analysis of a Drosophila mutant that exhibits a reduced number of mushroom body (MB) neurons in the brain, we reveal that the slender lobes (sle) gene encodes a novel nuclear protein that affects nucleolar organization during development. In sle mutant neuroblasts, the nucleolus was packed more tightly, forming a dense sphere, and the nucleolar proteins fibrillarin and Nop60B were abnormally distributed in the interphase nucleolus. Moreover, another nucleolar marker, Aj1 antigen, was localized to the center of the nucleolus in a manner complementary to the Nop60B distribution, and also formed a large aggregate in the cytoplasm. While developmental defects were limited to a few tissues in sle mutants, including MBs and nurse cells, the altered organization of the nucleolar components were evident in most developing tissues. Therefore, we conclude that Sle is a general factor of nuclear architecture in Drosophila that is required for the correct organization of the nucleolus during development.

Analysis of the stability and function of nucleoplasmin through cysteine mutants

Xenopus laevis nucleoplasmin is a pentameric nuclear chaperone. The relation between the structure and the multifunctional aspects of the molecule has not yet been clearly established. In the course of analysing a C-terminally His-tagged recombinant version of the region equivalent to the trypsin resistant core (r-NP142) of the molecule, we found that this domain exhibited a substantially decreased oligomerization potential. To better understand the role of the three cysteines of nucleoplasmin on its pentameric functional structure, we have selectively mutated these residues to serine and generated three mutants (C15S, C35S, and C45S) both for the complete recombinant nucleoplasmin (r-NP) and the truncated r-NP142 non-tagged forms. We demonstrate that there are no disulphide bridges stabilizing either the monomer or the pentamer. Neither C15S nor C35S has any structural effects, while the mutation C45S abolishes the ability of r-NP142 to pentamerize. This structural impairment suggests that hydrophobic interactions of Cys 45 are critical for the stability of the protein. Our studies allow to analyse for the first time the structural and functional properties of nucleoplasmin in its monomeric form.

Mutation of the small acidic tract A1 drastically reduces nucleoplasmin activity

Xenopus laevis nucleoplasmin is a molecular chaperone that mediates sperm decondensation and nucleosome assembly. Nucleoplasmin has three acidic tracts (A1, A2 and A3) and until recent years the long polyglutamic tract A2 was thought to be the binding site for basic proteins. However, the latest publications in this field show that neither A2 nor A3 is indispensable for histone and sperm-specific protein binding. In this work, we show that the mutation of only four acidic amino acid residues of the small A1 tract drastically reduces nucleoplasmin decondensing activity, pointing out this region as the potential binding site for sperm proteins.

H1oo: a pre-embryonic H1 linker histone in search of a function

The mouse oocyte-specific linker histone H1oo (1) constitutes a novel mammalian homologue of the oocyte-specific linker histone B4 of the frog and of the cs-H1 linker histone of the sea urchin; (2) is expressed as early as the germinal vesicle (PI) stage oocyte, persisting into the MII stage oocyte, the oocytic polar bodies, and the 2-cell embryo, extinction becoming apparent at the 4-8 cell embryonic stage; and (3) may play a key role in the control of gene expression during oogenesis and early embryogenesis, presumably through the perturbation of chromatin structure.

Roles of NPM2 in chromatin and nucleolar organization in oocytes and embryos

Upon fertilization, remodeling of condensed maternal and paternal gamete DNA occurs to form the diploid genome. In Xenopus laevis, nucleoplasmin 2 (NPM2) decondenses sperm DNA in vitro. To study chromatin remodeling in vivo, we isolated mammalian NPM2 orthologs. Mouse NPM2 accumulates in oocyte nuclei and persists in preimplantation embryos. Npm2 knockout females have fertility defects owing to failed preimplantation embryo development. Although sperm DNA decondensation proceeds without NPM2, abnormalities are evident in oocyte and early embryonic nuclei. These defects include an absence of coalesced nucleolar structures and loss of heterochromatin and deacetylated histone H3 that normally circumscribe nucleoli in oocytes and early embryos, respectively. Thus, Npm2 is a maternal effect gene critical for nuclear and nucleolar organization and embryonic development.

Nucleoplasmin interaction with protamines. Involvement of the polyglutamic tract

Different recombinant forms of nucleoplasmin including truncations at the carboxyl-terminal end of the molecule (r-NP121, r-NP142) have been expressed and purified. All of them were found to oligomerize, forming pentameric complexes which, according to their hydrodynamic properties, can be modeled by oblate ellipsoids of constant width. In this model, the highly charged carboxyl ends appear to be arranged around a pentameric core along the plane defined by the major axes of the ellipsoid. Importantly, all the recombinant forms appear to be able to decondense protamine-containing sperm nuclei. However, although the stoichiometry with which protamines bind to these forms appears to be constant (2.5 mol of protamine/mol of nucleoplasmin pentamer), the efficiency with which they remove protamines from the sperm DNA decreases in the following order: o-NP > r-NP142 > or = r-NP >> r-NP121. Therefore, the main polyglutamic tract of nucleoplasmin (which is absent in r-NP121), while enhancing the efficiency of protamine removal, is not indispensable for sperm chromatin decondensation in the biological model we have used.

Cloning, expression and nuclear localization of human NPM3, a member of the nucleophosmin/nucleoplasmin family of nuclear chaperones

BACKGROUND

Studies suggest that the related proteins nucleoplasmin and nucleophosmin (also called B23, NO38 or numatrin) are nuclear chaperones that mediate the assembly of nucleosomes and ribosomes, respectively, and that these activities are accomplished through the binding of basic proteins via their acidic domains. Recently discovered and less well characterized members of this family of acidic phosphoproteins include mouse nucleophosmin/nucleoplasmin 3 (Npm3) and Xenopus NO29. Here we report the cloning and initial characterization of the human ortholog of Npm3.

RESULTS

Human genomic and cDNA clones of NPM3 were isolated and sequenced. NPM3 lies 5.5 kb upstream of FGF8 and thus maps to chromosome 10q24-26. In addition to amino acid similarities, NPM3 shares many physical characteristics with the nucleophosmin/nucleoplasmin family, including an acidic domain, multiple potential phosphorylation sites and a putative nuclear localization signal. Comparative analyses of 14 members of this family from various metazoans suggest that Xenopus NO29 is a candidate ortholog of human and mouse NPM3, and they further group both proteins closer with the nucleoplasmins than with the nucleophosmins. Northern blot analysis revealed that NPM3 was strongly expressed in all 16 human tissues examined, with especially robust expression in pancreas and testis; lung displayed the lowest level of expression. An analysis of subcellular fractions of NIH3T3 cells expressing epitope-tagged NPM3 revealed that NPM3 protein was localized solely in the nucleus.

CONCLUSIONS

Human NPM3 is an abundant and widely expressed protein with primarily nuclear localization. These biological activities, together with its physical relationship to the chaparones nucleoplasmin and nucleophosmin, are consistent with the proposed function of NPM3 as a molecular chaperone functioning in the nucleus.

The crystal structure of nucleoplasmin-core: implications for histone binding and nucleosome assembly

The efficient assembly of histone complexes and nucleosomes requires the participation of molecular chaperones. Currently, there is a paucity of data on their mechanism of action. We now present the structure of an N-terminal domain of nucleoplasmin (Np-core) at 2.3 A resolution. The Np-core monomer is an eight-stranded beta barrel that fits snugly within a stable pentamer. In the crystal, two pentamers associate to form a decamer. We show that both Np and Np-core are competent to assemble large complexes that contain the four core histones. Further experiments and modeling suggest that these complexes each contain five histone octamers which dock to a central Np decamer. This work has important ramifications for models of histone storage, sperm chromatin decondensation, and nucleosome assembly.

A variant of nuclear localization signal of bipartite-type is required for the nuclear translocation of hypoxia inducible factors (1alpha, 2alpha and 3alpha)

Hypoxia inducible factors (HIF1, 2 and 3), consisting of alpha and beta subunits, play an essential role in various responses to hypoxia. Nuclear entry of alpha subunits is a necessary step for the formation of DNA-binding complex with beta subunit, which is constitutively localized in the nucleus. We show here that the nuclear accumulation of HIF2alpha induced by hypoxia is mediated through a novel variant of bipartite-type nuclear localization signal (NLS) in the C-terminus of the protein, which has an unusual length of spacer sequence between two adjacent basic domains. We further show that when the ubiquitin-proteasome system was deficient or inhibited, HIF2alpha accumulated in the nucleus even under normoxia, also mediated through the bipartite NLS. These findings indicate that the protein stability is critical for the nuclear localization of HIF2alpha and hypoxia is not a necessary factor for the process. Importantly, the NLS of HIF2alpha is also conserved in the other HIF family members, HIF1alpha and HIF3alpha. Mutational analyses proved that the NLS mediating the nuclear localization of HIF1alpha is indeed bipartite-, but not monopartite-type as thought before. Our results suggest that the newly identified NLS is crucial for the functional regulation of HIF family.

A mammalian oocyte-specific linker histone gene H1oo: homology with the genes for the oocyte-specific cleavage stage histone (cs-H1) of sea urchin and the B4/H1M histone of the frog

Oocytes and early embryos of multiple (non-mammalian) species lack the somatic form of the linker histone H1. To the best of our knowledge, a mammalian oocyte-specific linker (H1) histone(s) has not, as yet, been reported. We have uncovered the cDNA in question in the course of a differential screening (suppression subtractive hybridization (SSH)) project. Elucidation of the full-length sequence of this novel 1.2 kb cDNA led to the identification of a 912 bp open reading frame. The latter encoded a novel 34 kDa linker histone protein comprised of 304 amino acids, tentatively named H1oo. Amino acid BLAST analysis revealed that H1oo displayed the highest sequence homology to the oocyte-specific B4 histone of the frog, the respective central globular (putative DNA binding) domains displaying 54% identity. Substantial homology to the cs-H1 protein of the sea urchin oocyte was also apparent. While most oocytic mRNAs corresponding to somatic linker histones are not polyadenylated (and remain untranslated), the mRNAs of (non-mammalian) oocyte-specific linker histones and of mammalian H1oo, are polyadenylated, a process driven by the consensus signal sequence, AAUAAA, detected in the 3′-untranslated region of the H1oo cDNA. Our data suggest that the mouse oocyte-specific linker histone H1oo (1) constitutes a novel mammalian homolog of the oocyte-specific linker histone B4 of the frog and of the cs-H1 linker histone of the sea urchin; (2) is expressed as early as the GV (PI) stage oocyte, persisting into the MII stage oocyte, the oocytic polar bodies, and the two-cell embryo, extinction becoming apparent at the four- to eight-cell embryonic stage; and (3) may play a key role in the control of gene expression during oogenesis and early embryogenesis, presumably through the perturbation of chromatin structure.

Regulation of ectodermal and excretory function by the C. elegans POU homeobox gene ceh-6

Caenorhabditis elegans has three POU homeobox genes, unc-86, ceh-6 and ceh-18. ceh-6 is the ortholog of vertebrate Brn1, Brn2, SCIP/Oct6 and Brn4 and fly Cf1a/drifter/ventral veinless. Comparison of C. elegans and C. briggsae CEH-6 shows that it is highly conserved. C. elegans has only three POU homeobox genes, while Drosophila has five that fall into four families. Immunofluorescent detection of the CEH-6 protein reveals that it is expressed in particular head and ventral cord neurons, as well as in rectal epithelial cells, and in the excretory cell, which is required for osmoregulation. A deletion of the ceh-6 locus causes 80% embryonic lethality. During morphogenesis, embryos extrude cells in the rectal region of the tail or rupture, indicative of a defect in the rectal epithelial cells that express ceh-6. Those embryos that hatch are sick and develop vacuoles, a phenotype similar to that caused by laser ablation of the excretory cell. A GFP reporter construct expressed in the excretory cell reveals inappropriate canal structures in the ceh-6 null mutant. Members of the POU-III family are expressed in tissues involved in osmoregulation and secretion in a number of species. We propose that one evolutionary conserved function of the POU-III transcription factor class could be the regulation of genes that mediate secretion/osmoregulation.

Somatic linker histone H1 is present throughout mouse embryogenesis and is not replaced by variant H1 degrees

A striking feature of early embryogenesis in a number of organisms is the use of embryonic linker histones or high mobility group proteins in place of somatic histone H1. The transition in chromatin composition towards somatic H1 appears to be correlated with a major increase in transcription at the activation of the zygotic genome. Previous studies have supported the idea that the mouse embryo essentially follows this pattern, with the significant difference that the substitute linker histone might be the differentiation variant H1 degrees, rather than an embryonic variant. We show that histone H1 degrees is not a major linker histone during early mouse development. Instead, somatic H1 was present throughout this period. Though present in mature oocytes, somatic H1 was not found on maternal metaphase II chromatin. Upon formation of pronuclear envelopes, somatic H1 was rapidly incorporated onto maternal and paternal chromatin, and the amount of somatic H1 steadily increased on embryonic chromatin through to the 8-cell stage. Microinjection of somatic H1 into oocytes, and nuclear transfer experiments, demonstrated that factors in the oocyte cytoplasm and the nuclear envelope, played central roles in regulating the loading of H1 onto chromatin. Exchange of H1 from transferred nuclei onto maternal chromatin required breakdown of the nuclear envelope and the extent of exchange was inversely correlated with the developmental advancement of the donor nucleus.

Origin and evolution of the regulatory gene male-specific lethal-3

Dosage compensation in Drosophila is mediated by genes known as "male-specific lethals" (msls). Several msls, including male-specific lethal-3 (msl-3), encode proteins of unknown function. We cloned the Drosophila virilis msl-3 gene. Using the information provided by the sequences of the Drosophila melanogaster and D. virilis genes, we found that sequences of other species can be aligned along their entire lengths with msl-3. Among them, there are genes in yeasts (the Schizosaccharomyces pombe Alp13 gene, as well as a putative Alp13 homolog, found in Saccharomyces cerevisae) and in mammals (MRG15 and MSL3L1 and their relatives) plus uncharacterized sequences of the nematode Caenorhabditis elegans and the plants Arabidopsis thaliana, Lycopersicon esculentum, and Zea mays. A second Drosophila gene of this family has also been found. It is thus likely that msl-3-like genes are present in all eukaryotes. Phylogenetic analyses suggest that msl-3 is orthologous to the mammalian MSL3L1 genes, while the second Drosophila melanogaster gene (which we have called Dm MRG15) is orthologous to mammalian MRG15. These analyses also suggest that the msl-3/MRG15 duplication occurred after the fungus/animal split, while an independent duplication occurred in plants. The proteins encoded by these genes have similar structures, including a putative chromodomain close to their N-terminal end and a putative leucine zipper at their C-terminus. The possible functional roles of these proteins are discussed.

Control of gene expression in Xenopus early development

We examine the control of gene expression before and through the MBT in Xenopus laevis. The generalized repression of transcription that occurs before the midblastula transition (MBT) is regulated by a dynamic competition between chromatin and transcription complex assembly. Conditions favoring the access of basal factors (TBP) or transactivators can overcome this transcriptional repression. Changes in DNase I hypersensitivity patterns of the chromatin during early development show that it is more accessible to DNase I before the MBT (and by extension to other DNA interacting proteins) than after the MBT. We also show that at the level of genomic domains, organization of the chromatin matrix attachment sites is random before MBT. We propose that these three components, chromatin domain structure, DNA accessibility, and the transcription complex-chromatin dynamic competition, combine to regulate transcription in the embryo before and through the MBT.

Functional discontinuities in prothymosin alpha caused by caspase cleavage in apoptotic cells

Our study examines the effect of apoptosis on prothymosin alpha, an abundant, nuclear protein intimately involved with proliferation of all mammalian cells. When HeLa cells were treated with actinomycin D, with etoposide, or with staurosporine following synchronization with hydroxyurea, they underwent apoptosis based on several specific criteria, including fragmentation of DNA and activation of specific caspases. Similarly treated NIH3T3 cells arrested and displayed no indicators of apoptosis. In HeLa, but not in NIH3T3 cells, prothymosin alpha levels declined precipitously and a truncated version of the protein was formed. The following observations implicate caspase activity: (1) The truncated polypeptide arose only in the treated HeLa cell cultures. (2) The appearance of the truncated polypeptide coincided with the activation of caspase 3 and the cleavage of poly(ADP-ribose) polymerase, a known caspase substrate. (3) Carbobenzoxy-DEVD-fluoromethylketone, a cell-permeable caspase 3 inhibitor, blocked cleavage and degradation of prothymosin alpha. (4) The same inhibitor, when added to mixed extracts of apoptotic and normal cells, prevented cleavage of intact prothymosin alpha. (5) Recombinant caspase 3 and, to a much lesser extent, caspase 7 truncated purified prothymosin alpha. (6) In HeLa cells, cleavage occurred at three overlapping caspase 3-like sites with the consensus sequence D-X-X-D and released 10 to 14 residues from the carboxyl terminus, including the core nuclear localization signal. Two immediate consequences of the cleavage were observed: truncated prothymosin alpha was no longer confined to the nucleus and it was deficient in phosphate. These data suggest that the disabling of prothymosin alpha is a significant event in apoptosis. J. Cell. Physiol. 182:256-268, 2000. Published 2000 Wiley-Liss, Inc.

Functional modules in ribosomal protein L5 for ribonucleoprotein complex formation and nucleocytoplasmic transport

Ribosomal protein L5 forms a small, extraribosomal complex with 5 S ribosomal RNA, referred to as the 5 S ribonucleoprotein complex, which shuttles between nucleus and cytoplasm in Xenopus oocytes. Mapping elements in L5 that mediate nuclear protein import defines three separate such activities (L5-nuclear localization sequence (NLS)-1, -2, and -3), which are functional in both oocytes and somatic cells. RNA binding activity involves N-terminal as well as C-terminal elements of L5. In contrast to the full-length protein, none of the individual NLSs carrying L5 fragments are able to allow for the predominating accumulation in the nucleoli that is observed with the full-length protein. The separate L5-NLSs differ in respect to two activities. Firstly, only L5-NLS-1 and -3, not L5-NLS-2, are capable of promoting the nuclear transfer of a heterologous, covalently attached ribonucleoprotein complex. Secondly, only L5-NLS-1 is able to bind strongly to a variety of different import receptors; those that recognize L5-NLS-2 and -3 have yet to be identified.

Mechanism of salmon sperm decondensation by nucleoplasmin

Removal of protamine from DNA-protamine (salmine, protamine from salmon sperm) complexes by nucleoplasmin was examined and compared with that of poly-L-glutamic acid (PLGA) using turbidity and ethidium bromide (EB) treatment methods. When nucleoplasmin or PLGA was added to a DNA-protamine complex solution, turbidity was decreased and the amount of EB intercalated into DNA was increased. These results suggest that nucleoplasmin and PLGA can remove protamine from DNA-protamine complexes. The effect of nucleoplasmin was more potent than that of PLGA. Direct interaction of nucleoplasmin with protamine was confirmed by mixing experiments using circular dichroism (CD) and fluorescence spectroscopies. Results suggest that nucleoplasmin is bound to protamine in a 1:1 ratio and that Trp126 is located near a hydrophilic region containing a polyglutamic acid tract of nucleoplasmin which was obviously influenced by its binding with protamine. It would appear that the polyglutamic acid tract in nucleoplasmin plays a critical role for binding with protamine.

Does prothymosin alpha affect the phosphorylation of elongation factor 2?

Prothymosin alpha is a small, acidic, essential nuclear protein that plays a poorly defined role in the proliferation and survival of mammalian cells. Recently, Vega et al. proposed that exogenous prothymosin alpha can specifically increase the phosphorylation of eukaryotic elongation factor 2 (eEF-2) in extracts of NIH3T3 cells (Vega, F. V., Vidal, A., Hellman, U., Wernstedt, C., and Domínguez, F. (1998) J. Biol. Chem. 273, 10147-10152). Using similar lysates prepared by four methods (detergent lysis, Dounce homogenization, digitonin permeabilization, and sonication) and three preparations of prothymosin alpha, one of which was purified by gentle means (the native protein, and a histidine-tagged recombinant prothymosin alpha expressed either in bacteria or in COS cells), we failed to find a response. A reconstituted system composed of eEF-2, recombinant eEF-2 kinase, calmodulin, and calcium was also unaffected by prothymosin alpha. However, unlike our optimized buffer, Vega's system included a phosphatase inhibitor, 50 mM fluoride, which when evaluated in our laboratories severely reduced phosphorylation of all species. Under these conditions, any procedure that decreases the effective fluoride concentration will relieve the inhibition and appear to activate. Our data do not support a direct relationship between the function of prothymosin alpha and the phosphorylation of eEF-2.

Folding and unfolding of a giant duplex-DNA in a mixed solution with polycations, polyanions and crowding neutral polymers

To understand the conformational behavior of a giant duplex-DNA chain in a mixed solution with various biopolymers with different state of ionization, the higher-order structure of the DNA chain was analyzed with a fluorescence microscope in the presence of polycations (poly-arginine), polyanions (poly-glutamic acid), and neutral polymers (poly-ethylene glycol) as a model for cellular environment. Concentrated medium with neutral polymer induced the discrete folding transition of the DNA. At the threshold condition for the transition, addition of small amounts of either the polycation or the polyanion caused marked structural changes in the folded DNAs. Based on thermodynamic considerations on the experimental results, profile of free energy of a single giant DNA chain was depicted with respect to the size, or the expansion factor alpha, in the three-dimensional structure of the DNA. The effect of the neural crowding polymer on the degree of folding of a single giant DNA chain is discussed in a semi-quantitative manner.

Physicochemical and functional comparison of Xenopus laevis nucleoplasmin obtained from oocytes and from overexpression in bacteria

We compare the physicochemical and functional characteristics of nucleoplasmin obtained from Xenopus laevis oocytes and by bacterial overexpression of a plasmid containing the nucleoplasmin gene. The comparison shows that, while the secondary structure of the protein is not affected by the method used to obtain this protein, the bacterial expressed form exhibits a marked tendency to form large aggregates and an impaired ability to displace protamines from sperm nuclei. These results add a word of caution to the indiscriminate use, in functional or structural (crystallographic) studies, of bacterially overproduced proteins that have been end-terminally tagged with polyhistidine.

The histone binding protein nucleoplasmin does not facilitate binding of transcription factor IIIA to nucleosomal Xenopus laevis 5S rRNA genes

In an attempt to understand the mechanism by which transcription factors compete with histone octamers for cognate binding sites in chromatin, the effect of the histone binding protein nucleoplasmin on the binding of TFIIIA to nucleosomal 5S rRNA genes was tested. In this study, it was shown that, despite the previously reported nucleosome remodeling ability of nucleoplasmin, the binding of TFIIIA to nucleosomal DNA cannot be facilitated by this protein. Furthermore, it was demonstrated that nucleoplasmin cannot overcome nucleosome mediated repression of transcription of reconstituted 5S rRNA genes. In contrast to earlier work, this study used a homologous system composed of the 5S rRNA gene, nucleoplasmin, and TFIIIA from Xenopus laevis.

Decondensation of the mouse sperm nucleus within the interphase nucleus

Sperm nuclei incorporated into the cytoplasm (ooplasm) of fertilised mouse eggs at the pronuclear stage remain condensed, whereas those injected into male or female pronuclei decondense. Similarly, sperm nuclei injected into germinal vesicles of immature oocytes or the nuclei of 2-cell embryos decondense, while those entering the cytoplasm of these oocytes/embryos do not. These facts seem to suggest that factors necessary for the decondensation of sperm nucleus are present in interphase nuclei and are released into the ooplasm during nuclear envelope breakdown. Nucleoplasmin, which is synthesised in the cytoplasm and accumulated within the nucleus, is likely a major candidate for these factors.

Prothymosin alpha in vivo contains phosphorylated glutamic acid residues

Human and monkey prothymosin alpha contain activated carbonyl groups on glutamic acid residues. Three lines of evidence indicate the existence of unusual phosphates. 1) Prothymosin alpha continued to be metabolically labeled with [32P]orthophosphoric acid despite a mutation at Ser1, the sole site of phosphate in purified bovine prothymosin alpha (Sburlati, A. R., De La Rosa, A., Batey, D. W., Kurys, G. L., Manrow, R. E., Pannell, L. K., Martin, B. M., Sheeley, D. M., and Berger, S. L. (1993) Biochemistry 32, 4587-4596). 2) Immediately upon cell lysis, the pH stability curves of metabolically labeled native [32P]prothymosin alpha or a [32P]histidine-tagged variant resembled the pH stability curve of acetyl phosphate. 3) After a brief incubation at pH 7, these curves changed from a pattern diagnostic for an acyl phosphate to that characteristic of a serine or threonine phosphate, an observation consistent with transfer of phosphate in vitro. Our data indicate that most of prothymosin alpha's phosphates are subject instantaneously to hydrolysis, based on the observation that greater than 90% of the phosphate initially found at pH 7 disappeared at the extremes of pH. Rapid loss of phosphate was not affected by the presence of phosphatase inhibitors including 50 mM sodium fluoride, 1 mM okadaic acid, and 0.5 mM calyculin A. The amount of phosphate missing could not be ascertained, but the trifling amount recovered on Ser or Thr depended heavily on conditions favoring the transient survival of labile phosphate. Further analysis using COS cells lysed in the presence of sodium borohydride showed that: 1) phosphate recovered on prothymosin alpha decreased 8-fold when lysates were treated with borohydride; 2) the reagent caused 4-8 glutamic acid residues/molecule to vanish; 3) using [3H]NaBH4, label was introduced into proline, a product derived from reductive cleavage of phosphoglutamate; and 4) [3H]proline was localized almost exclusively to a peptide with pronounced homology to the histone binding site of nucleoplasmin, a chromatin remodeling protein found in Xenopus laevis. Our data demonstrate that prothymosin alpha is energy-rich by virtue of stoichiometric amounts of glutamyl phosphate.

Cloning and developmental expression of Xenopus cDNAs encoding the Enhancer of split groucho and related proteins

The two full-length cDNAs encoding ESG1 (Enhancer of split groucho) and related AES (Amino Enhancer of split) proteins of 767 and 197 amino acids, respectively, were cloned and sequenced from the African frog Xenopus laevis. The amino acid sequence of Xenopus ESG1 protein had 61% identity to the full-length Drosophila groucho. Xenopus AES protein exhibited 91%, 58% and 48% identity to the mouse AES, amino-terminal regions of Xenopus ESG1 and Drosophila groucho, respectively. Northern blot analysis showed that widespread RNA expression of ESG1 of 2.8 kb, ESG2 of 3.6 kb and AES of 2.2 kb transcripts were seen in adult tissues, whereas ESG1 and AES transcripts of 2.8 kb and 2.2 kb, respectively, were ubiquitously expressed in the developing embryos. The overall structural relationships of ESG and AES proteins among human, mouse, rat, Xenopus and Drosophila were analysed.

A domain distinct from nucleoplasmin's nuclear localization sequence influences its transport

We constructed mutants of the prototypical, nuclear-accumulating protein nucleoplasmin and used them in both in vivo and in vitro nuclear transport assays to search for transport-influencing domains distinct from this protein's recognized nuclear localization sequence. We identified the polyglutamic acid tract on the amino flank of the nuclear localization sequence as being involved in two stages of nuclear transport. This poly-glu tract is required for the facilitated translocation of nucleoplasmin through the nuclear pore complex, and it also enhances the subsequent binding of nucleoplasmin within the nucleus.

Conformation of nucleoplasmin and its interaction with DNA-protamine complex as a simple model of fish sperm nuclei

Nucleoplasmin was isolated from Xenopus laevis eggs and purified by an improved method using an open column. Its conformation was investigated spectrophotometrically by UV, CD and fluorescence. It was shown that alpha-helix content of nucleoplasmin was 30-40%, and one of the two tryptophan residues in nucleoplasmin located in the hydrophobic surroundings and the other in the relatively hydrophilic surroundings. The isolated nucleoplasmin was found to decondense sperm nuclei of salmon also, suggesting a possibility of the existence of nucleoplasmin-like protein in fish as well. Collapse of the protamine (salmine)-DNA complex as a simple model for fish sperm nuclei by nucleoplasmin was directly observed by measuring OD320 of aqueous protamine-DNA mixtures. This is a molecular level observation for the removal of protamine from DNA-protamine complex.

ATP-facilitated chromatin assembly with a nucleoplasmin-like protein from Drosophila melanogaster

To gain a better understanding of the factors that can mediate chromatin assembly, we have purified and cloned a core histone-binding protein from Drosophila melanogaster embryos. This protein resembles Xenopus laevis nucleoplasmin, and it has therefore been termed dNLP, for Drosophila nucleoplasmin-like protein. dNLP is a nuclear protein that is present throughout development. Both purified native and recombinant dNLP bind to core histones and can function in the assembly of approximately regularly spaced nucleosomal arrays in a reaction that additionally requires DNA, purified core histones, ATP, and a partially purified fraction (containing at least one other assembly activity). We also analyzed the properties of an N-terminally truncated version of dNLP, termed dNLP-S, and found that the deletion of the N-terminal 31 residues of dNLP results in a loss of the specificity of the interaction of dNLP with core histones. We then compared the abilities of dNLP and Drosophila nucleosome assembly protein-1 (dNAP-1) to promote the decondensation of Xenopus sperm chromatin, a process that can be mediated by nucleoplasmin. We observed that dNAP-1, but not dNLP, was able to promote the decondensation of sperm chromatin. These and other data collectively suggest that dNLP may participate in parallel with other histone-binding proteins such as dNAP-1 in the assembly of chromatin.

Nucleosome assembly: the CAF and the HAT

Recent data argue strongly that a protein complex termed chromatin assembly factor-1 (CAF-I) plays a major role in de novo nucleosome assembly during DNA replication. Human CAF-I deposits newly synthesized, acetylated histones onto replicated DNA in vitro and localizes to sites of DNA replication in S-phase cells. Specific lysines of the histones used for nucleosome assembly are acetylated; in the past year the first gene encoding a histone acetyltransferase was cloned. However, mechanistic links between histone acetylation and nucleosome assembly have not been established in vivo or in vitro.

Hyperphosphorylation of nucleoplasmin facilitates Xenopus sperm decondensation at fertilization

Previous studies showed that the nuclear phosphoprotein nucleoplasmin performs the first stage of chromatin decondensation of Xenopus sperm at fertilization. It binds and removes sperm basic proteins replacing them with histones. We now show that this activity depends upon the massive hyperphosphorylation of nucleoplasmin that occurs when oocytes mature into eggs. Egg extracts or purified hyperphosphorylated egg nucleoplasmin decondense sperm chromatin and remove sperm basic proteins much faster than oocyte extracts or hypophosphorylated oocyte nucleoplasmin. Furthermore, dephosphorylation of egg nucleoplasmin slows sperm decondensation and prevents basic protein removal from sperm chromatin. We conclude that hyperphosphorylation of nucleoplasmin is used to modulate the rapid changes in chromatin structure that accompany early development in Xenopus.

Do products of the myc proto-oncogene play a role in transcriptional regulation of the prothymosin alpha gene?

The Myc protein has been reported to activate transcription of the rat prothymosin alpha gene by binding to an enhancer element or E box (CACGTG) located in the first intron (S. Gaubatz et al., Mol. Cell. Biol. 14:3853-3862, 1994). The human prothymosin alpha gene contains two such motifs: in the promoter region at kb -1.2 and in intron 1, approximately 2 kb downstream of the transcriptional start site in a region which otherwise bears little homology to the rat gene. Using chloramphenicol acetyltransferase (CAT) reporter constructs driven either by the 5-kb human prothymosin alpha promoter or by a series of truncated promoters, we showed that removal of the E-box sequence had no effect on transient expression of CAT activity in mouse L cells. When intron 1 of the prothymosin alpha gene was inserted into the most extensive promoter construct downstream of the CAT coding region, a diminution in transcription, which remained virtually unchanged upon disruption of the E boxes, was observed. CAT constructs driven by the native prothymosin alpha promoter or the native promoter and intron were indifferent to Myc; equivalent CAT activity was observed in the presence of ectopic normal or mutant Myc genes. Similarly, expression of a transiently transfected wild-type prothymosin alpha gene as the reporter was not affected by a repertoire of myc-derived genes, including myc itself and dominant or recessive negative myc mutants. In COS-1 cells, equivalent amounts of the protein were produced from transfected prothymosin alpha genes regardless of whether genomic E boxes were disrupted, intron 1 was removed, or a repertoire of myc-derived genes was included in the transfection cocktail. More importantly, cotransfection of a dominant negative Max gene failed to reduce transcription of the endogenous prothymosin alpha gene in COS cells or the wild-type transfected gene in COS or L cells. Taken together, the data do not support the idea that Myc activates transcription of the intact human prothymosin alpha gene or reporter constructs that mimic its structure. Rather, they suggest that the human prothymosin alpha promoter and downstream elements are buffered so as to respond poorly, if at all, to transient fluctuations in transcription factors which regulate other genes.

Nucleoplasmin associates with and is phosphorylated by casein kinase II

Nucleoplasmin is a phosphorylated nuclear-accumulating protein. We report herein that the kinetics of its cytoplasm-->nucleus transport are affected by its degree of phosphorylation. Therefore, we sought to identify any protein kinase which specifically associates with nucleoplasmin. We discovered that nucleoplasmin co-isolates by two independent methods (immunoabsorption and chromatography) in a complex including a kinase which phosphorylates nucleoplasmin. The co-purifying kinase is casein kinase II-like because: (i) it phosphorylates casein; (ii) its phospho-transferase activity can be competed out by GTP; (iii) it is stimulated by polylysine; and (iv) it is inhibited by heparin. Moreover, a polyclonal antibody to the alpha (38 kDa) and alpha' (36 kDa) catalytic subunits of casein kinase II specifically recognizes 38 and 36 kDa polypeptides in the nucleoplasmin-complex, and a specific inhibitor of casein kinase II inhibits nucleoplasmin's nuclear transport. Additionally, we found that phosphorylation of nucleoplasmin by its associated casein kinase II is strongly inhibited by histones and that, in addition to nucleoplasmin, another protein (p100) in the nucleoplasmin-complex is phosphorylated by casein kinase II.