Nucleoplasmin associates with and is phosphorylated by casein kinase II

Fly Fishing for Histones: Catch and Release by Histone Chaperone Intrinsically Disordered Regions and Acidic Stretches

Chromatin is the complex of eukaryotic DNA and proteins required for the efficient compaction of the nearly 2-meter-long human genome into a roughly 10-micron-diameter cell nucleus. The fundamental repeating unit of chromatin is the nucleosome: 147bp of DNA wrapped about an octamer of histone proteins. Nucleosomes are stable enough to organize the genome yet must be dynamically displaced and reassembled to allow access to the underlying DNA for transcription, replication, and DNA damage repair. Histone chaperones are a non-catalytic group of proteins that are central to the processes of nucleosome assembly and disassembly and thus the fluidity of the ever-changing chromatin landscape. Histone chaperones are responsible for binding the highly basic histone proteins, shielding them from non-specific interactions, facilitating their deposition onto DNA, and aiding in their eviction from DNA. Although most histone chaperones perform these common functions, recent structural studies of many different histone chaperones reveal that there are few commonalities in their folds. Importantly, sequence-based predictions show that histone chaperones are highly enriched in intrinsically disordered regions (IDRs) and acidic stretches. In this review, we focus on the molecular mechanisms underpinning histone binding, selectivity, and regulation of these highly dynamic protein regions. We highlight new evidence suggesting that IDRs are often critical for histone chaperone function and play key roles in chromatin assembly and disassembly pathways.

Developmentally Regulated Post-translational Modification of Nucleoplasmin Controls Histone Sequestration and Deposition

Nucleoplasmin (Npm) is an abundant histone chaperone in vertebrate oocytes and embryos. During embryogenesis, regulation of Npm histone binding is critical for its function in storing and releasing maternal histones to establish and maintain the zygotic epigenome. Here, we demonstrate that Xenopus laevis Npm post-translational modifications (PTMs) specific to the oocyte and egg promote either histone deposition or sequestration, respectively. Mass spectrometry and Npm phosphomimetic mutations used in chromatin assembly assays identified hyperphosphorylation on the N-terminal tail as a critical regulator for sequestration. C-terminal tail phosphorylation and PRMT5-catalyzed arginine methylation enhance nucleosome assembly by promoting histone interaction with the second acidic tract of Npm. Electron microscopy reconstructions of Npm and TTLL4 activity toward the C-terminal tail demonstrate that oocyte- and egg-specific PTMs cause Npm conformational changes. Our results reveal that PTMs regulate Npm chaperoning activity by modulating Npm conformation and Npm-histone interaction, leading to histone sequestration in the egg.

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.

Vertebrate nucleoplasmin and NASP: egg histone storage proteins with multiple chaperone activities

Recent reviews have focused on the structure and function of histone chaperones involved in different aspects of somatic cell chromatin metabolism. One of the most dramatic chromatin remodeling processes takes place immediately after fertilization and is mediated by egg histone storage chaperones. These include members of the nucleoplasmin (NPM2/NPM3), which are preferentially associated with histones H2A-H2B in the egg and the nuclear autoantigenic sperm protein (NASP) families. Interestingly, in addition to binding and providing storage to H3/H4 in the egg and in somatic cells, NASP has been shown to be a unique genuine chaperone for histone H1. This review revolves around the structural and functional roles of these two families of chaperones whose activity is modulated by their own post-translational modifications (PTMs), particularly phosphorylation. Beyond their important role in the remodeling of paternal chromatin in the early stages of embryogenesis, NPM and NASP members can interact with a plethora of proteins in addition to histones in somatic cells and play a critical role in processes of functional cell alteration, such as in cancer. Despite their common presence in the egg, these two histone chaperones appear to be evolutionarily unrelated. In contrast to members of the NPM family, which share a common monophyletic evolutionary origin, the different types of NASP appear to have evolved recurrently within different taxa.

Histone chaperones link histone nuclear import and chromatin assembly

Histone chaperones are proteins that shield histones from nonspecific interactions until they are assembled into chromatin. After their synthesis in the cytoplasm, histones are bound by different histone chaperones, subjected to a series of posttranslational modifications and imported into the nucleus. These evolutionarily conserved modifications, including acetylation and methylation, can occur in the cytoplasm, but their role in regulating import is not well understood. As part of histone import complexes, histone chaperones may serve to protect the histones during transport, or they may be using histones to promote their own nuclear localization. In addition, there is evidence that histone chaperones can play an active role in the import of histones. Histone chaperones have also been shown to regulate the localization of important chromatin modifying enzymes. This review is focused on the role histone chaperones play in the early biogenesis of histones, the distinct cytoplasmic subcomplexes in which histone chaperones have been found in both yeast and mammalian cells and the importins/karyopherins and nuclear localization signals that mediate the nuclear import of histones. We also address the role that histone chaperone localization plays in human disease. This article is part of a Special Issue entitled: Histone chaperones and chromatin assembly.

Recognition of nucleoplasmin by its nuclear transport receptor importin α/β: insights into a complete import complex

Nuclear import of the pentameric histone chaperone nucleoplasmin (NP) is mediated by importin α, which recognizes its nuclear localization sequence (NLS), and importin β, which interacts with α and is in charge of the translocation of the NP/α/β complex through the nuclear pore. Herein, we characterize the assembly of a functional transport complex formed by full-length NP with importin α/β. Isothermal titration calorimetry (ITC) was used to analyze the thermodynamics of the interactions of importin α with β, α with NP, and the α/β heterodimer with NP. Our data show that binding of both importin α and α/β to NP is governed by a favorable enthalpic contribution and that NP can accommodate up to five importin molecules per NP pentamer. Phosphomimicking mutations of NP, which render the protein active in histone chaperoning, do not modulate the interaction with importin. Using small-angle X-ray scattering, we model the α/β heterodimer, NP/α, and NP/α/β solution structures, which reveal a glimpse of a complete nuclear import complex with an oligomeric cargo protein. The set of alternative models, equally well fitting the scattering data, yields asymmetric elongated particles that might represent consecutive geometries the complex can adopt when stepping through the nuclear pore.

Protein kinase CK2 phosphorylation of EB2 regulates its function in the production of Epstein-Barr virus infectious viral particles

The Epstein-Barr Virus (EBV) early protein EB2 (also called BMLF1, Mta, or SM) promotes the nuclear export of a subset of early and late viral mRNAs and is essential for the production of infectious virions. We show here that in vitro, protein kinase CK2alpha and -beta subunits bind both individually and, more efficiently, as a complex to the EB2 N terminus and that the CK2beta regulatory subunit also interacts with the EB2 C terminus. Immunoprecipitated EB2 has CK2 activity that phosphorylates several sites within the 80 N-terminal amino acids of EB2, including Ser-55, -56, and -57, which are localized next to the nuclear export signal. EB2S3E, the phosphorylation-mimicking mutant of EB2 at these three serines, but not the phosphorylation ablation mutant EB2S3A, efficiently rescued the production of infectious EBV particles by HEK293(BMLF1-KO) cells harboring an EB2-defective EBV genome. The defect of EB2S3A in transcomplementing 293(BMLF1-KO) cells was not due to impaired nucleocytoplasmic shuttling of the mutated protein but was associated with a decrease in the cytoplasmic accumulation of several late viral mRNAs. Thus, EB2-mediated production of infectious EBV virions is regulated by CK2 phosphorylation at one or more of the serine residues Ser-55, -56, and -57.

Phosphorylation of both nucleoplasmin domains is required for activation of its chromatin decondensation activity

Nucleoplasmin (NP) is a histone chaperone involved in nucleosome assembly, chromatin decondensation at fertilization, and apoptosis. To carry out these activities NP has to interact with different types of histones, an interaction that is regulated by phosphorylation. Here we have identified a number of phosphorylated residues by mass spectrometry and generated mutants in which these amino acids are replaced by Asp to mimic the effect of phosphorylation. Our results show that, among the eight phosphoryl groups experimentally detected, four are located at the flexible N terminus, and the rest are found at the tail domain, flanking the nuclear localization signal. Phosphorylation-mimicking mutations render a recombinant protein as active in chromatin decondensation as hyperphosphorylated NP isolated from Xenopus laevis eggs. Comparison of mutants in which the core and tail domains of the protein were independently or simultaneously "activated" indicates that activation or phosphorylation of both protein domains is required for NP to efficiently extract linker-type histones from chromatin.

A putative transcriptional elongation factor hIws1 is essential for mammalian cell proliferation

Iws1 has been implicated in transcriptional elongation by interaction with RNA polymerase II (RNAP II) and elongation factor Spt6 in budding yeast Saccharomyces cerevisiae, and association with transcription factor TFIIS in mammalian cells, but its role in controlling cell growth and proliferation remains unknown. Here we report that the human homolog of Iws1, hIws1, physically interacts with protein arginine methyltransferases PRMT5 which methylates elongation factor Spt5 and regulates its interaction with RNA polymerase II. Gene-specific silencing of hIws1 by RNA interference reveals that hIws1 is essential for cell viability. GFP fusion protein expression approaches demonstrate that the hIws1 protein is located in the nucleus, subsequently, two regions harbored within the hIws1 protein are demonstrated to contain nuclear localization signals (NLSs). In addition, mouse homolog of hiws1 is found to express ubiquitously in various tissues.

Regulation of thyroid hormone receptor alpha2 RNA binding and subcellular localization by phosphorylation

Thyroid hormone receptor alpha2 (TRalpha2) is an alternative splice product of the TRalpha primary transcript whose unique carboxyl terminus does not bind T3 or activate transcription. The physiological function of TRalpha2 is unknown. We have found that TRalpha2 is a single stranded RNA binding protein and that the RNA binding domain localizes to a 41 amino acid region immediately distal to the second zinc finger. TRalpha2 contains a single protein kinase CK2 phosphorylation site in its amino terminus and potentially nine CK2 sites in its unique carboxyl terminus. In vitro CK2 treatment of TRalpha2 eliminated its RNA binding. Mutational analysis indicated that phosphorylations at the N- and C-terminal sites both contribute to this inhibitory effect. Cellular localization studies demonstrated that phosphorylated TRalpha2 is primarily cytoplasmic, whereas unphosphorylated TRalpha2 is primarily nuclear. Since RNA binding is a property of unphosphorylated TRalpha2, the TRalpha2-RNA interaction likely represents a nuclear function of TRalpha2.

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.

Nuclear import and activity of histone deacetylase in Xenopus oocytes is regulated by phosphorylation

Most of the histone deacetylase (HDAC) activity detected in oocytes and early embryos of Xenopus can be accounted for by the presence of a protein complex that contains the maternal HDACm protein. This complex appears to fulfil the conditions required of a 'deposition' histone deacetylase, its primary function being to deacetylate the core histones incorporated into newly-synthesized chromatin during the rapid cell cycles leading up to blastula. A major event in the assembly and accumulation of the HDAC complex is the translocation of the HDACm protein into the germinal vesicle during oogenesis. Here we examine the features of HDACm that are responsible for its nuclear uptake and enzyme activity, identifying the charged C-terminal domain as a target for modification by phosphorylation. Whereas, one phosphorylation site lying within the putative nuclear localization signal, T445, is required for efficient nuclear import of a GST-carboxy-tail fusion, two others, S421 and S423, appear to effect release from the import receptors. Although overexpression of recombinant HDACm in oocytes leads to premature condensation of endogenous chromatin, this effect is abrogated in vivo by mutation of S421A and S423A. Thus, both translocation and activity of HDACm appear to be regulated by specific phosphorylation events. These results have implications for techniques involving the transfer of somatic nuclei into enucleated oocytes.

Sequestration of thermogenic transcription factors in the cytoplasm during development of brown adipose tissue

Transcription factors that regulate gene expression during adipogenesis also control the expression of genes of thermogenesis in brown adipose tissue, in particular, the mitochondrial uncoupling protein gene (Ucp1). There is evidence that a plasticity exists among adipocytes in which activation of the Ucp1 gene together with mitochondrial biogenesis can increase the brown adipocyte character of white fat. To understand this process, we have characterized the changes in transcription that occur in interscapular brown adipocytes during development. We have found dramatic reductions in both DNA-binding activity to probes and immunoreactive protein for peroxisome proliferator-activated receptor, retinoid X receptor, CCAAT/enhancer binding protein, and cAMP-response element-binding protein regulatory motifs in nuclear extracts when mice reach adulthood. Exposure of adult mice to the cold, which reactivates Ucp1 expression, leads to a re-accumulation of factors in the nucleus. We propose that transcription factors are sequestered in the cytoplasm as mice age under conditions of reduced thermogenesis. Changes in isoform sub-types for peroxisome proliferator-activated receptor-gamma and cAMP-response element-binding proteins indicate an additional level of control on gene expression during thermogenesis. The increased movement of the RIIbeta protein kinase A regulatory subunit into the nucleus with age suggests a mechanism for regulating the phosphorylation of transcription factors in the nucleus in response to the thermogenic requirements of the animal. Nuclear factor-kappaB has been used as a model to demonstrate that the nuclear localization of transcription factors in brown fat are reduced during post-natal development. Furthermore, it was found by immunofluorescence that adrenergic stimulation of primary adipocyte cultures causes an increase of both the protein kinase A catalytic alpha-subunit and nuclear factor-kappaB into the nucleus.

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.

Protein kinase CK2 phosphorylates the Fas-associated factor FAF1 in vivo and influences its transport into the nucleus

We previously identified the Fas-associated factor FAF1 as an in vitro substrate of protein kinase CK2 and determined Ser289 and Ser291 as phosphorylation sites. Here we demonstrate that these two serine residues are the only sites phosphorylated by CK2 in vitro, and that at least one site is phosphorylated in vivo. Furthermore, we analyzed putative physiological functions of FAF1 phosphorylation. The ability of FAF1 to potentiate Fas-induced apoptosis is not influenced by the FAF1 phosphorylation status; however, the nuclear import of a phosphorylation-deficient FAF1 mutant was delayed in comparison to wild-type FAF1.

The GTP-binding domain of class II transactivator regulates its nuclear export

The transcriptional coactivator class II transactivator (CIITA), although predominantly localized in the nucleus, is also present in the cytoplasm. The subcellular distribution of CIITA is actively regulated by the opposing actions of nuclear export and import. In this study, we show that nuclear export is negatively regulated by the GTP-binding domain (GBD; aa 421-561) of CIITA: mutation or deletion of the GBD markedly increased export of CIITA from the nucleus. Remarkably, a CIITA GBD mutant binds CRM1/exportin significantly better than does wild-type CIITA, leading to the conclusion that GTP is a negative regulator of CIITA nuclear export. We also report that, in addition to the previously characterized N- and C-terminal nuclear localization signal elements, there is an additional N-terminal nuclear localization activity, present between aa 209 and 222, which overlaps the proline/serine/threonine-rich domain of CIITA. Thus, fine-tuning of the nucleocytoplasmic distribution of coactivator proteins involved in transcription is an active and dynamic process that defines a novel mechanism for controlling gene regulation.

Prointerleukin-16 contains a functional CcN motif that regulates nuclear localization

The immunomodulatory cytokine interleukin-16 (IL-16) represents the secreted C-terminus of a larger precursor, pro-IL-16. Following cleavage by caspase 3, the residual N-terminal domain translocates into the nucleus, inducing G(0)/G(1) cell cycle arrest. We have previously identified a classical bipartite nuclear localization sequence (NLS) in the N-terminal domain of pro-IL-16. We now show that N-terminal to the NLS domain of pro-IL-16 are protein kinase CK2 substrate and cdc2 kinase substrate sites which, along with the NLS, constitute a dual phosphorylation-regulated CcN motif which regulates nuclear localization of pro-IL-16. In addition, we demonstrate that mutation of either site is associated with impairment of the N-terminal domain's ability to induce G(0)/G(1) cell cycle arrest. This is the first description of a functional CcN motif in a cytokine precursor.

Nuclear import and DNA-binding activity of RFX1. Evidence for an autoinhibitory mechanism

RFX1 binds and regulates the enhancers of a number of viruses and cellular genes. RFX1 belongs to the evolutionarily conserved RFX protein family that shares a DNA-binding domain and a conserved C-terminal region. In RFX1 this conserved region mediates dimerization, and is followed by a unique C-terminal tail, containing a highly acidic stretch. In HL-60 cells nuclear translocation of RFX1 is regulated by protein kinase C with unknown mechanisms. By confocal fluorescence microscopy, we have identified a nonclassical nuclear localization signal (NLS) at the extreme C-terminus. The adjacent 'acidic region', which showed no independent NLS activity, potentiated the function of the NLS. Subcellular fractionation showed that the tight association of RFX1 with the nucleus is mediated by its DNA-binding domain and enhanced by the dimerization domain. In contrast, the acidic region inhibited nuclear association, by down-regulating the DNA-binding activity of RFX1. These data suggest an autoinhibitory interaction, which may regulate the function of RFX1 at the level of DNA binding. The C-terminal tail thus constitutes a composite localization domain, which on the one hand mediates nuclear import of RFX1, and on the other hand inhibits its association with the nucleus and binding to DNA. The participation of the acidic region in both activities suggests a mechanism by which the nuclear import and DNA-binding activity of RFX1 may be coordinately regulated by phosphorylation by kinases such as PKC.

Conserved regions in the Epstein-Barr virus leader protein define distinct domains required for nuclear localization and transcriptional cooperation with EBNA2

Epstein-Barr virus (EBV) EBNA-LP is a latent protein whose function is not fully understood. Recent studies have shown that EBNA-LP may be an important EBNA2 cofactor by enhancing EBNA2 stimulation of the latency C and LMP-1 promoters. To further our understanding of EBNA-LP function, we have introduced a series of mutations into evolutionarily conserved regions and tested the mutant proteins for the ability to enhance EBNA2 stimulation of the latency C and LMP-1 promoters. Three conserved regions (CR1 to CR3) are located in the repeat domains that are essential for the EBNA2 cooperativity function. In addition, three serine residues are also well conserved in the repeat domains. Clustered alanine mutations were introduced into CR1 to CR3, and the conserved serines were also changed to alanine residues in an EBNA-LP with two repeats, which is the minimal protein able to cooperate with EBNA2. Mutations introduced into CR1a had no effect on EBNA-LP function, while mutations introduced into CR1b resulted in EBNA-LP with slightly decreased activity. Mutations in CR1c and CR2 resulted in proteins that no longer localized exclusively to the nucleus and also had no EBNA2 cooperation activity. Mutations introduced into conserved serines S5/71 resulted in proteins with slightly higher activity, while mutations introduced into conserved serines S35/101 or in CR3 (which contains S60/126) resulted in EBNA-LP proteins with substantially reduced activity. The potential karyophilic signals within EBNA-LP CR1c and CR2 were also examined by introducing oligonucleotides encoding these positively charged amino acid groupings into a cytoplasmic test protein, herpes simplex virus DeltaIE175, and by examining the intracellular localization of the resulting proteins. This assay identified a strong nuclear localization signal between EBNA-LP amino acids 43 and 50 (109 to 117 in the second W repeat) comprising CR2, while EBNA-LP amino acids 29 to 36 (91 to 98 in the second W repeat) were unable to function independently as a nuclear localization signal. However, a combination of amino acids 29 to 50 resulted in more efficient nuclear localization than with amino acids 43 to 50 alone. These results indicate that EBNA-LP has a bipartite nuclear localization signal and that efficient nuclear localization is essential for EBNA2 cooperativity function. Interestingly, EBNA-LP with only a single repeat localized exclusively to the cytoplasm, providing an explanation for why this isoform has no activity. In addition, two conserved serine residues which are distinct from nuclear import functions are important for EBNA2 cooperativity function.

Phosphorylation protects sperm-specific histones H1 and H2B from proteolysis after fertilization

At intermediate stages of male pronucleus formation, sperm-derived chromatin is composed of hybrid nucleoprotein particles formed by sperm H1 (SpH1), dimers of sperm H2A-H2B (SpH2A-SpH2B), and a subset of maternal cleavage stage (CS) histone variants. At this stage in vivo, the CS histone variants are poly(ADP-ribosylated), while SpH2B and SpH1 are phosphorylated. We have postulated previously that the final steps of sperm chromatin remodeling involve a cysteine-protease (SpH-protease) that degrades sperm histones in a specific manner, leaving the maternal CS histone variants unaffected. More recently we have reported that the protection of CS histones from degradation is determined by the poly(ADP-ribose) moiety of these proteins. Because of the selectivity displayed by the SpH-protease, the coexistence of a subset of SpH together with CS histone variants at intermediate stages of male pronucleus remodeling remains intriguing. Consequently, we have investigated the phosphorylation state of SpH1 and SpH2B in relation to the possible protection of these proteins from proteolytic degradation. Histones H1 and H2B were purified from sperm, phosphorylated in vitro using the recombinant alpha-subunit of casein kinase 2, and then used as substrates in the standard assay of the SpH-protease. The phosphorylated forms of SpH1 and SpH2B were found to remain unaltered, while the nonphosphorylated forms were degraded. On the basis of this result, we postulate a novel role for the phosphorylation of SpH1 and SpH2B that occurs in vivo after fertilization, namely to protect these histones against degradation at intermediate stages of male chromatin remodeling.

Effects of polyamines, polyamine synthesis inhibitors, and polyamine analogs on casein kinase II using Myc oncoprotein as substrate

Polyamines, casein kinase II (CKII), and the myc oncogene are directly involved in the regulation of molecular events in cell proliferation, differentiation, and apoptosis. Each is increased in rapidly growing cancer cells. In our current study, we showed that the Km values for purified CKII were similar for casein and Myc oncoprotein under a variety of assay conditions, and that specific natural and synthetic polyamines stimulated CKII phosphorylation of Myc oncoprotein 2- to 20-fold via increases in Vmax. When polyamine synthesis inhibitors and analogs were studied with this purified enzyme system, two polyamine analogs (N1,N12-bis-(ethyl)-spermine [BESpm] and 1,19-bis-(ethylamino)-5,10,15, triazononadecane [BE4X4]), which did not affect basal enzyme activity, did prevent (or inhibit) polyamine-stimulated CKII activity by approximately 70 and 85 percent, respectively. Because the Myc oncoprotein transactivates several genes for key proteins involved in the regulation of cellular proliferation, including the omithine decarboxylase gene (rate-limiting enzyme of polyamine synthesis), we suggest that there may be linkages between polyamines, CKII, and Myc in the control of cellular proliferation. We also suggest that the anticancer drugs BESpm and BE4X4 may inhibit cancer cell proliferation partially through interference with the above-suggested CKII linkages.

Maternal histone deacetylase is accumulated in the nuclei of Xenopus oocytes as protein complexes with potential enzyme activity

Reversible acetylation of core histones plays an important regulatory role in transcription and replication of chromatin. The acetylation status of chromatin is determined by the equilibrium between activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs). The Xenopus protein HDACm shows sequence homology to other putative histone deacetylases, but its mRNA is expressed only during early development. Both HDACm protein and acetylated non-chromosomal histones are accumulated in developing oocytes, indicating that the key components for histone deposition into new chromatin during blastula formation are in place by the end of oogenesis. Here we show that the 57 kDa HDACm protein undergoes steady accumulation in the nucleus, where it is organized in a multiprotein complex of approx. 300 kDa. A second, major component of the nuclear complex is the retinoblastoma-associated protein p48 (RbAp48/46), which may be used as an adaptor to contact acetylated histones in newly assembled chromatin. The nuclear complex has HDAC activity that is sensitive to trichostatin A, zinc ions and phosphatase treatment. The 57 kDa protein serves as a marker for total HDAC activity throughout oogenesis and early embryogenesis. The active HDACm complex and its acetylated histone substrates appear to be kept apart until after chromatin assembly has taken place. However, recombinant HDACm, injected into the cytoplasm of oocytes, not only is translocated to the nucleus, but also is free to interact with the endogenous chromatin.

Novel low molecular weight microtubule-associated protein-2 isoforms contain a functional nuclear localization sequence

Known high and low molecular weight (LMW) MAP2 protein isoforms result from alternative splicing of the MAP2 gene. Contrary to previous reports that MAP2 is neural-specific, we recently identified MAP2 mRNA and protein in somatic and germ cells of rat testis, and showed the predominant testicular isoform is LMW. Although cytoplasmic in neural tissue, MAP2 appeared predominantly nuclear in germ cells using immunohistochemistry. We sought to determine whether this unexpected localization was due to the inclusion of exon 10 within novel LMW MAP2 isoforms. Normally excluded from the LMW MAP2c, exon 10 harbors a putative CcN motif, comprising a nuclear localization sequence (NLS) flanked by regulatory phosphorylation sites for protein kinase CK2 and cdc2 kinase. Characterization of MAP2 mRNA in adult and immature brain and testis, by reverse transcriptase-polymerase chain reaction/Southern analysis and Northern blot, identified novel LMW forms containing exons 10 and 11, previously detected only in high molecular weight MAP2a and 2b. The MAP2 NLS targeted a large heterologous protein to the nucleus, as demonstrated using bacterially expressed MAP2-CcN-beta-galactosidase fusion protein and an in vitro nuclear import assay. Antibodies raised against the fusion protein produced a testicular immunohistochemical staining pattern correlating with MAP2 protein distribution in the nucleus of most germ cells, and precipitated both approximately 70-kDa and >220-kDa proteins recognized by the commercial MAP2-specific HM2 monoclonal antibody, supporting our hypothesis of a novel LMW MAP2 isoform. These results demonstrate the presence of a functional NLS in MAP2 and indicate that novel LMW MAP2 isoforms may be targeted to the nucleus in both neural and non-neuronal tissues.

The 37-amino-acid interdomain of dengue virus NS5 protein contains a functional NLS and inhibitory CK2 site

The dengue virus NS5 RNA-dependent RNA polymerase has been detected in the nucleus of virus-infected mammalian cells. We demonstrate here for the first time using in vitro and in vivo assay systems that the 37-amino-acid linker interdomain of NS5 (residues 369 to 405) contains a nuclear localization sequence (NLS) which is capable of targeting b-galactosidase to the nucleus. Further, we show that the linker is recognized by subunits of the NLS-binding importin complex with an affinity similar to that of the bipartite NLS of the retinoblastoma protein and, in analogous fashion to proteins such as the SV40 large tumor antigen, contains a functional protein kinase CK2 phosphorylation site (threonine 395). Interestingly, this site appears to inhibit NS5 nuclear targeting, probably through a cytoplasmic retention mechanism. The linker may have an important role in targeting NS5 to the nucleus in a regulated manner during the dengue virus infectious cycle.

Protein kinase CK2 and its role in cellular proliferation, development and pathology

Protein kinase CK2 is a pleiotropic, ubiquitous and constitutively active protein kinase that can use both ATP and GTP as phosphoryl donors with specificity for serine/threonine residues in the vicinity of acidic amino acids. Recent results show that the enzyme is involved in transcription, signaling, proliferation and in various steps of development. The tetrameric holoenzyme (alpha2beta2) consists of two catalytic alpha-subunits and two regulatory beta-subunits. The structure of the catalytic subunit with the fixed positioning of the activation segment in the active conformation through its own aminoterminal region suggests a regulation at the transcriptional level making a regulation by second messengers unlikely. The high conservation of the catalytic subunit from yeast to man and its role in the tetrameric complex supports this notion. The regulatory beta-subunit has been far less conserved throughout evolution. Furthermore the existence of different CK2beta-related proteins together with the observation of deregulated CK2beta levels in tumor cells and the reported association of CK2beta protein with key proteins in signal transduction, e.g. A-Raf, Mos, pg90rsk etc. are suggestive for an additional physiological role of CK2beta protein beside being the regulatory compound in the tetrameric holoenzyme.

Negative charge at the protein kinase CK2 site enhances recognition of the SV40 large T-antigen NLS by importin: effect of conformation

SV40 large tumor-antigen (T-ag) nuclear import is enhanced by the protein kinase CK2 (CK2) site (Ser111Ser112) flanking the nuclear localization sequence (NLS). Here we use site-directed mutagenesis to examine the influence of negative charge and conformation at the site on T-ag nuclear import and recognition by the NLS-binding importin subunits. Negative charge through aspartic acid in place of Ser111 simulated CK2 phosphorylation in enhancing nuclear accumulation to levels well above those of proteins lacking a functional CK2 site. This was shown to be through enhancement of T-ag NLS recognition by importin using an ELISA-based assay. Asp112-substituted mutants containing proline at positions 109, 110 (wild-type position) or 111 were compared to assess the role of conformation at the CK2 site. Maximal nuclear import of the protein with Pro109 was lower than that of the Pro110 derivative, with the Pro111 variant even lower, these differences also being attributable to effects on importin binding. All results indicate a correlation of the initial nuclear import rate with the importin binding affinity, demonstrating that NLS recognition by importin is a key rate-determining step in nuclear import.

An engineered site for protein kinase C flanking the SV40 large T-antigen NLS confers phorbol ester-inducible nuclear import

Nuclear import of simian virus SV40 large tumour antigen (T-ag) is enhanced by the protein kinase CK2 (CK2) site flanking the nuclear localisation sequence (NLS). We report here that replacement of this site with a consensus site for protein kinase C (PK-C) can alter the regulation of T-ag nuclear import and render it inducible by phorbol ester. Measurement of nuclear import kinetics using fluorescently labelled proteins and confocal laser scanning microscopy show that the introduced PK-C site is functional in enhancing T-ag nuclear import compared to a protein lacking the CK2 site. Treatment with the PK-C activator phorbol 12-myristate 13-acetate (PMA) further increases the level of maximal nuclear accumulation and the initial nuclear import rate. This engineered PMA-responsive NLS may have application in targeting of molecules of interest to the nucleus in response to agents stimulating PK-C.

The cAMP-dependent protein kinase site (Ser312) enhances dorsal nuclear import through facilitating nuclear localization sequence/importin interaction

Control over the nuclear import of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation and transformation. The Drosophila TF Dorsal shares with other rel TF family members the fact that it contains a phosphorylation site for the cAMP-dependent protein kinase (PKA) 22 amino acids N-terminal to the nuclear localization signal (NLS) at amino acids 335-340. This study examines for the first time the nuclear import kinetics of Dorsal fusion proteins in rat hepatoma cells in vivo and in vitro. Nuclear uptake was found to be not only NLS-dependent, but also strongly dependent on the PKA site, whereby substitution of Ser312 by either Ala or Glu using site-directed mutagenesis severely reduced nuclear accumulation. Exogenous cAMP or PKA catalytic subunit significantly enhanced the nuclear import of wild-type proteins both in vivo and in vitro. Using a direct binding assay, the molecular basis of PKA site enhancement of Dorsal fusion protein nuclear import was determined to be PKA site-mediated modulation of NLS recognition by the importin 58/97 complex. The physiological relevance of these results is supported by the observation that Drosophila embryos expressing PKA site Dorsal mutant variants were impaired in development. We conclude that the Dorsal NLS and PKA site constitute a phosphorylation-regulated NLS essential to Dorsal function and able to function in heterologous mammalian cell systems, where phosphorylation modulates the affinity of NLS recognition by importin.

Signals mediating nuclear targeting and their regulation: application in drug delivery

The recent progress with respect to understanding the signals mediating the transport of proteins in both directions through the NPC, and cellular proteins interacting with these signals to effect the transport process has made possible a number of advances in terms of the use of this information in a clinical setting. In particular, our knowledge of the mechanism of regulation of the process, and of how we may exploit the cellular transport machinery itself in a therapeutic situation, especially where there may be transport pathways specific to particular viruses, has advanced considerably. In this context, this review expounds current understanding of the signals conferring targeting to the nucleus, and their practical and potential use in delivering molecules of interest to the nucleus in a clinical context. It also deals with targeting signals conferring nuclear protein export/ shuttling between nuclear and cytoplasmic compartments as well as with those conferring nuclear or cytoplasmic retention, and with the specific mechanisms regulating the activity of these signals, and in particular those regulating signal-dependent nuclear protein import. Detailed understanding of the processes of signal-mediated nuclear protein import/export and its regulation enables the considered application and optimization of approaches to target molecules of interest, such as plasmid DNA or toxic molecules, efficiently to the nucleus according to need in a clinical or research context, and enhance the expression or efficiency of their action, respectively. The use of nuclear targeting signals in this context is reviewed, and future possibilities in terms of the application of our growing understanding of nuclear transport and its regulation are discussed.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

The R1 subunit of herpes simplex virus ribonucleotide reductase is a good substrate for host cell protein kinases but is not itself a protein kinase

The N terminus of the R1 subunit of herpes simplex virus type 2 ribonucleotide reductase is believed to be a protein kinase domain mainly because the R1 protein was phosphorylated in a protein kinase assay on blot. Using Escherichia coli and adenovirus expression vectors to produce R1, we found that, whereas the reductase activity of both recombinant proteins was similar, efficient phosphorylation of R1 and casein in the presence of Mg2+ was obtained only with the R1 purified from eukaryotic cells. Phosphorylation of this R1, in solution or on blot, results mainly from the activity of casein kinase II (CKII), a co-purifying protein kinase. Labeling on blot occurs from CKII leakage off the membrane and its subsequent high affinity binding to in vivo CKII-phosphorylated R1. CKII target sites were mapped to an acidic serine-rich segment of the R1 N terminus. Improvement in purification of the R1 expressed in eukaryotic cells nearly completely abolished its phosphorylation potential. An extremely low level of phosphorylation observed in the presence of Mn2+ with the R1 produced in E. coli was probably due to an unidentified prokaryotic protein kinase. These results provide evidence that the herpes simplex virus type 2 R1 does not possess an intrinsic protein kinase activity.

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.

Npm3: a novel, widely expressed gene encoding a protein related to the molecular chaperones nucleoplasmin and nucleophosmin

We report the cloning and initial characterization of the cDNAs, gene, and pseudogene of Npm3, a novel murine gene that encodes a protein related to the nuclear chaperone phosphoproteins, nucleoplasmin and nucleophosmin. Npm3 is located approximately 5 kb upstream of Fgf8 on mouse Chromosome 19 and consists of six exons spanning 2 kb. The first five exons code for an acidic protein of 19.0 kDa that contains a potential nuclear localization signal and potential phosphorylation sites for several kinases. Npm3 was expressed in all mouse tissues examined. On the basis of the similarity of Npm3 to nucleoplasmin and nucleophosmin in amino acid sequence, protein features, and exon structure, we propose that Npm3 is a new member of, and may share basic functions with, the nucleoplasmin/ nucleophosmin family of molecular chaperone proteins.

Nucleolar protein B23 stimulates nuclear import of the HIV-1 Rev protein and NLS-conjugated albumin

Nucleolar phosphoprotein B23 is a putative ribosome assembly factor with a relatively high affinity for peptides containing sequences of nuclear localization signals (NLSs) of the SV40 T-antigen type [Szebeni, A., Herrera, J. E., & Olson, O. J. (1995) Biochemistry 34, 8037-8042]. The effects of protein B23 on nuclear import were determined by an in vitro assay [Dean, D. A., & Kasamatsu, H. (1994) J. Biol. Chem. 269, 4910-4916] using NLS peptide-conjugated bovine serum albumin (NLS-BSA) or the HIV-1 Rev protein as substrates for import into isolated rat liver nuclei. The import was ATP-dependent and inhibited by wheat germ agglutinin or by an antibody against p97, a component of the nuclear import system. The rate of import of either substrate was increased if protein B23 was added to the incubation medium. Similar enhancements of import were seen with both isoforms (B23.1 and B23.2). The stimulatory effect on Rev protein import was saturable with maximum stimulation (2-3-fold) at a molar ratio of protein B23:Rev of approximately 1:1. Phosphorylation of protein B23.1 by casein kinase II produced an additional doubling of the import rate. This effect was not seen if protein B23.1 was phosphorylated with a cdc2 type protein kinase. Mutant forms of protein B23.1 in which the nuclear localization signal was either deleted or altered did not stimulate import of the substrates. These results suggest that protein B23 plays a role as an accessory factor in the nuclear import of the NLS-containing proteins and that phosphorylation at sites in the highly acidic segments of the protein enhances the stimulatory effect.

Casein kinase II binds to and phosphorylates cytoplasmic dynein

We have isolated a 27-kDa protein that binds to cytoplasmic dynein. Microsequencing of a 17-amino acid peptide of this polypeptide yielded a sequence which completely matched the predicted sequence of the beta subunit of casein kinase II, a highly conserved serine/threonine kinase. Affinity chromatography using a dynein column indicates that both the alpha and beta subunits of casein kinase II are retained by the column from rat brain cytosol. Although dynactin is also bound to the column, casein kinase II is not a dynactin subunit. Casein kinase II does not co-immunoprecipitate with dynactin, and it binds to a dynein intermediate chain column which has been preblocked with excess p150(Glued), a treatment that inhibits the binding of dynactin from cytosol. Bacterially expressed and purified rat dynein intermediate chain can be phosphorylated by casein kinase II in vitro. Further, native cytoplasmic dynein purified from rat brain can also be phosphorylated by casein kinase II in vitro. We propose that CKII may be involved in the regulation of dynein function possibly by altering its cargo specificity or its ability to interact with dynactin.

A consensus cAMP-dependent protein kinase (PK-A) site in place of the CcN motif casein kinase II site simian virus 40 large T-antigen confers PK-A-mediated regulation of nuclear import

The regulation of nuclear protein transport by phosphorylation plays a central role in gene expression in eukaryotic cells. We previously showed that nuclear import of SV40 large tumor antigen (T-ag) fusion proteins is regulated by the CcN motif, comprising phosphorylation sites for casein kinase II and the cyclin-dependent kinase cdc2, together with the nuclear localization signal. Regulation of nuclear uptake by CcN motif kinase sites also holds true for the yeast transcription factor SWI5 and the Xenopus nuclear phosphoprotein nucleoplasmin. To test directly whether a kinase site other than those of the CcN motif could regulate nuclear import of T-ag, the CcN motif casein kinase II site, which markedly increases the rate of T-ag nuclear import, was replaced by a consensus site for the cAMP-dependent protein kinase (PK-A) using site-directed mutagenesis. The resultant fusion protein could be specifically phosphorylated by PK-A in vitro and in cell extracts. Nuclear import of the fluorescently labeled protein was analyzed in the HTC rat hepatoma cell line both in vivo (microinjected cells) and in vitro (mechanically perforated cells) in the presence and the absence of cAMP and/or PK-A catalytic subunit using confocal laser scanning microscopy. In vitro PK-A-prephosphorylated protein was also tested. All results indicated that the rate of nuclear import was increased by phosphorylation at the PK-A site (2-5-fold), demonstrating that kinases other than those of the CcN motif can regulate nuclear import in response to stimulatory signals. The phosphorylation-regulated nuclear localization signal derived here represents an important first step toward developing a signal conferring inducible nuclear targeting of molecules of interest.

Cyclin-dependent kinase site-regulated signal-dependent nuclear localization of the SW15 yeast transcription factor in mammalian cells

Control over the nuclear transport of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation, transformation and signal transduction. The Saccharomyces cerevisiae TF SWI5 is excluded from the nucleus in a cell cycle-dependent fashion, mediated by phosphorylation by the cyclin-dependent kinase (cdk) CDC28. Nuclear entry occurs in G1. beta-galactosidase fusion proteins carrying SWI5 amino acids 633-682, including the nuclear localization sequence (NLS: Lys-Lys-Tyr-Glu-Asn-Val-Val-Ile-Lys-Arg-Ser-Pro-Arg-Lys-Arg-Gly-Arg-Pro- Arg-Lys655) were analyzed for subcellular localization in appropriate temperature-sensitive yeast strains blocked in G1 or G2/M using indirect immunofluorescence, and for nuclear import kinetics in living rat hepatoma or Vero African green monkey kidney cells microinjected with fluorescently labeled bacterially expressed protein and quantitative confocal laser microscopy. Cell cycle-dependent nuclear localization in yeast was both NLS and cdk site-dependent, whereby mutation of the cdk site serines (Ser646 and Ser664) to alanine resulted in constitutive nuclear localization. In mammalian cells, the SWI5 fusion proteins were similarly transported to the nucleus in an NLS-dependent fashion, while the mutation to Ala of the cdk site serines increased the maximal level of nuclear accumulation from about 1- to over 8-fold. We suggest that phosphorylation at the cdk sites inhibits nuclear transport of SWI5, consistent with our previous observations for the inhibition of SV40 large tumor antigen nuclear transport by phosphorylation by the cdk cdc2. The results indicate for the first time that a yeast NLS and, fascinatingly, its regulatory mechanisms are functional in higher eukaryotes, implying the universal nature of regulatory signals for protein transport to the nucleus.