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

Expression and purification of the full murine NPM2 and study of its interaction with protamines and histones

Mouse nucleoplasmin M.NPM2 was recombinantly expressed and the protein consisting of the complete sequence was purified and characterized. Similar to its Xenopus laevis X.NPM2 counterpart, the protein forms stable pentameric complexes and exhibits an almost undistinguishable hydrodynamic ionic strength-dependent unfolding behavior. The interaction of N.PM2 with histones and mouse P1/P2 protamines revealed that these chromosomal proteins bind preferentially to the distal part of the nucleoplasmin pentamer. Moreover, the present work highlights the critical role played by histones H2B and H4 in the association of the histone H2A-H2B dimers and histone octamer with nucleoplasmin.

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Characterization of the entire mouse M.NPM2 protein.

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Determination of sites of interaction of M.NPM2 with histones and mouse protamines.

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Use of crosslinking mass spectrometry to determine protein-protein interactions.

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Analysis of the C-terminal NPM2 unfolding.

sNASP, a histone H1-specific eukaryotic chaperone dimer that facilitates chromatin assembly

NASP has been described as a histone H1 chaperone in mammals. However, the molecular mechanisms involved have not yet been characterized. Here, we show that this protein is not only present in mammals but is widely distributed throughout eukaryotes both in its somatic and testicular forms. The secondary structure of the human somatic version consists mainly of clusters of alpha-helices and exists as a homodimer in solution. The protein binds nonspecifically to core histone H2A-H2B dimers and H3-H4 tetramers but only forms specific complexes with histone H1. The formation of the NASP-H1 complexes is mediated by the N- and C-terminal domains of histone H1 and does not involve the winged helix domain that is characteristic of linker histones. In vitro chromatin reconstitution experiments show that this protein facilitates the incorporation of linker histones onto nucleosome arrays and hence is a bona fide linker histone chaperone.

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.

Nucleoplasmin: a nuclear chaperone

In this article, we briefly review the structural and functional information currently available on nucleoplasmin. Special emphasis is placed on the discussion of the molecular mechanism involved in the sperm chromatin remodelling activity of this protein. A model is proposed based on current crystallographic data, recent biophysical and functional studies, as well as in the previously available information.

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.

Activation mechanism of the nuclear chaperone nucleoplasmin: role of the core domain

Nucleoplasmin (NP) mediates nucleosome assembly by removing basic proteins from sperm chromatin and exchanging them with histones. This function is modulated by phosphorylation of NP at multiple sites. NP is pentameric, each monomer consisting of two domains: a core, which forms a stable ring-like pentamer, and a tail, that holds a polyglutamic tract and the nuclear localization signal. In the present study, we have explored the role of the core domain in the functionality of NP. Despite lacking the poly-Glu region, a putative binding site for basic proteins, the isolated core domain of the hyperphosphorylated protein isolated from eggs of Xenopus laevis is able to bind sperm basic proteins and decondense chromatin, in contrast to the inactive, non-phosphorylated recombinant core. This activity can be reproduced artificially in the recombinant core domain through mutation of putative phosphorylation sites to aspartate, thus mimicking the charge effect of phosphorylation. The mutated residues locate in flexible or loop regions exposed on the "distal face" of the core pentamer, where a short acidic region is also found, indicating that phosphorylation might activate the core domain of NP by generating a strong localized negative potential. Our results show that the phosphorylated core domain of NP is active in chromatin decondensation, thus it could contribute together with the poly-Glu containing tail in displaying a binding surface for sperm basic proteins on the NP pentamer.

Interaction of nucleoplasmin with core histones

Nucleoplasmin is one of the most abundant proteins in Xenopus laevis oocytes, and it has been involved in the chromatin remodeling that takes place immediately after fertilization. This molecule has been shown to be responsible for the removal of the sperm-specific proteins and deposition of somatic histones onto the male pronuclear chromatin. To better understand the latter process, we have used sedimentation velocity, sedimentation equilibrium, and sucrose gradient fractionation analysis to show that the pentameric form of nucleoplasmin binds to a histone octamer equivalent consisting of equal amounts of the four core histones, H2A, H2B, H3, and H4, without any noticeable preference for any of these proteins. Removal of the histone N-terminal "tail" domains or the major C-terminal polyglutamic tracts of nucleoplasmin did not alter these binding properties. These results indicate that interactions other than those electrostatic in nature (likely hydrophobic) also play a critical role in the formation of the complex between the negatively charged nucleoplasmin and positively charged histones. Although the association of histones with nucleoplasmin may involve some ionic interactions, the interaction process is not electrostatically driven.

NBP-45, a novel nucleosomal binding protein with a tissue-specific and developmentally regulated expression

Here we characterize a novel murine nuclear protein, which we named NBP-45, that is related to the ubiquitous nuclear proteins HMG-14/-17, binds specifically to nucleosome core particles, and can function as a transcriptional activator. NBP-45 mRNA is expressed at low levels and in variable amounts in all mouse tissues tested but is especially abundant in RNA extracted from 7-day-old mouse embryos, suggesting that it functions in early embryonic development. NBP-45 is composed of 406 amino acids and is encoded by a single size transcript. The region spanning the N-terminal 85 amino acids contains three segments that are highly homologous to functionally important domains in the HMG-14/-17 protein family: the nuclear localization signal, the nucleosome binding domain, and the chromatin unfolding domain. The protein region spanning the C-terminal 321 amino acids has a 42% content of negatively charged residues. The first 23 amino acids contain a region necessary for nuclear entry of the protein, the region spanning residues 12-40 is the main nucleosomal binding domain of the protein, and the negatively charged, C-terminal domain is necessary for transcription activation. The functional domains of NBP-45 are indicative of a nuclear protein that binds to nucleosomes, thereby creating a chromatin region of high local negative charge. Our studies establish the nucleosomal binding domain as a protein motif that is present in other than just the ubiquitous HMG-14/-17 proteins. We suggest that the nucleosomal binding domain motif is a protein module that facilitates binding to nucleosomes in chromatin.