Identification of a novel nucleolar localization signal and a degradation signal in Survivin-deltaEx3: a potential link between nucleolus and protein degradation

Mechanisms of gene regulation by histone degradation in adaptation of yeast: an overview of recent advances

Histones are important component of eukaryotic cells chromatin and consist of arginine and lysine residues. Histones play an important role in the protection of DNA. Their contents significantly affect high-level chromatin structure formation, gene expression, DNA replication, and other important life activities. Protein degradation is an important regulatory mechanism of histone content. Recent studies have revealed that modification of amino acid sequence is directly related to histone breakdown. In addition, histone degradation is closely related to covalent modifications, such as ubiquitination and acetylation, which are considered to be driving factors in gene regulation. Gene regulation is an important mechanism in adaptation to the environment and survival of species. With the introduction of highly efficient technology, various mutations in histones have been identified in yeast. In the field of epigenetics and the transmission of chromatin states, two widely used model organisms are the budding yeast Saccharomyces cerevisiae and Schizosaccharomyces pombe. Higher eukaryotes can use their silent loci to maintain their epigenetic states and providing the base to investigate mechanisms underlying development. Therfore, both species have contributed a plethora of information on these mechanisms in both yeast and higher eukaryotes. This study focuses on the role of histone modifications in controlling telomeric silencing in Saccharomyces cerevisiae and centromeric silencing in S. pombe as examples of genetic loci that demonstrate epigenetic inheritance. In view of recent advances, this review focuses on the post-translational modification of histone amino acid residues and reviews the relationship between histone degradation and amino acid residue modification.

The nucleolus functions as the compartment for histone H2B protein degradation

Histones are main components of chromatin, and the protein levels of histones significantly affect chromatin assembly. However, how histone protein levels are regulated, especially whether and how histones are degraded, is largely unclear. Here, we found that histone H2B is mainly degraded through the proteasome-mediated pathway, and the lysine-120 site of H2B is essential for its K48-linked polyubiquitination and degradation. Moreover, the degradation-impaired H2BK120R mutant shows an increased nucleolus localization, and inhibition of the proteasome results in an elevated nucleolus distribution of wild-type H2B, which is similar to that of H2BK120R mutants. More importantly, the nucleolus fractions can ubiquitinate and degrade the purified H2B in vitro, suggesting that the nucleolus, in addition to its canonical roles regulating ribosome genesis and protein translation, likely associates with H2B degradation. Therefore, these findings revealed a novel mechanism for the regulation of H2B degradation in which a nucleolus-associated proteasome pathway is involved.

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Histone H2B can be polyubiquitinated at the lysine 120 residue

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The degradation of histone H2B is achieved via the ubiquitination-proteasome pathway

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The nucleolus regulates the protein degradation of histone H2B

Ubiquitin Modification of the Epstein-Barr Virus Immediate Early Transactivator Zta

The Epstein-Barr virus (EBV) immediate early transactivator Zta plays a key role in regulating the transition from latency to the lytic replication stages of EBV infection. Regulation of Zta is known to be controlled through a number of transcriptional and posttranscriptional events. Here, we show that Zta is targeted for ubiquitin modification and that this can occur in EBV-negative and in EBV-infected cells. Genetic studies show critical roles for both an amino-terminal region of Zta and the basic DNA binding domain of Zta in regulating Zta ubiquitination. Pulse-chase experiments demonstrate that the bulk population of Zta is relatively stable but that at least a subset of ubiquitinated Zta molecules are targeted for degradation in the cell. Mutation of four out of a total of nine lysine residues in Zta largely abrogates its ubiquitination, indicating that these are primary ubiquitination target sites. A Zta mutant carrying mutations at these four lysine residues (lysine 12, lysine 188, lysine 207, and lysine 219) cannot induce latently infected cells to produce and/or release infectious virions. Nevertheless, this mutant can induce early gene expression, suggesting a possible defect at the level of viral replication or later in the lytic cascade. As far as we know, this is the first study that has investigated the targeting of Zta by ubiquitination or its role in Zta function.IMPORTANCE Epstein-Barr virus (EBV) is a ubiquitous human pathogen and associated with various human diseases. EBV undergoes latency and lytic replication stages in its life cycle. The transition into the lytic replication stage, at which virus is produced, is mainly regulated by the viral gene product, Zta. Therefore, the regulation of Zta function becomes a central issue regarding viral biology and pathogenesis. Known modifications of Zta include phosphorylation and sumoylation. Here, we report the role of ubiquitination in regulating Zta function. We found that Zta is subjected to ubiquitination in both EBV-infected and EBV-negative cells. The ubiquitin modification targets 4 lysine residues on Zta, leading to both mono- and polyubiquitination of Zta. Ubiquitination of Zta affects the protein's stability and likely contributes to the progression of viral lytic replication. The function and fate of Zta may be determined by the specific lysine residue being modified.

Roles of ASYMMETRIC LEAVES2 (AS2) and Nucleolar Proteins in the Adaxial-Abaxial Polarity Specification at the Perinucleolar Region in Arabidopsis

Leaves of Arabidopsis develop from a shoot apical meristem grow along three (proximal-distal, adaxial-abaxial, and medial-lateral) axes and form a flat symmetric architecture. ASYMMETRIC LEAVES2 (AS2), a key regulator for leaf adaxial-abaxial partitioning, encodes a plant-specific nuclear protein and directly represses the abaxial-determining gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3). How AS2 could act as a critical regulator, however, has yet to be demonstrated, although it might play an epigenetic role. Here, we summarize the current understandings of the genetic, molecular, and cellular functions of AS2. A characteristic genetic feature of AS2 is the presence of a number of (about 60) modifier genes, mutations of which enhance the leaf abnormalities of as2. Although genes for proteins that are involved in diverse cellular processes are known as modifiers, it has recently become clear that many modifier proteins, such as NUCLEOLIN1 (NUC1) and RNA HELICASE10 (RH10), are localized in the nucleolus. Some modifiers including ribosomal proteins are also members of the small subunit processome (SSUP). In addition, AS2 forms perinucleolar bodies partially colocalizing with chromocenters that include the condensed inactive 45S ribosomal RNA genes. AS2 participates in maintaining CpG methylation in specific exons of ETT/ARF3. NUC1 and RH10 genes are also involved in maintaining the CpG methylation levels and repressing ETT/ARF3 transcript levels. AS2 and nucleolus-localizing modifiers might cooperatively repress ETT/ARF3 to develop symmetric flat leaves. These results raise the possibility of a nucleolus-related epigenetic repression system operating for developmental genes unique to plants and predict that AS2 could be a molecule with novel functions that cannot be explained by the conventional concept of transcription factors.

Unraveling novel survivin mRNA transcripts in cancer cells using an in-house developed targeted high-throughput sequencing approach

The human baculoviral IAP repeat containing 5 (BIRC5), also known as survivin, is a conserved member of the inhibitor of apoptosis protein (IAPs) family, which is normally expressed during embryonic and fetal development. Although the expression levels of survivin are low in terminally differentiated cells and/or tissues, they can be found notably increased in certain pathological conditions including malignant tumors. Conventional cloning and sequencing techniques have already confirmed that alternative splicing events of the survivin pre-mRNA result in five distinct alternative transcript variants. In the present study, however, we implemented an innovative, in-house developed, targeted DNA-seq assay to identify novel survivin alternative transcript variants with increased depth and coverage that high-throughput sequencing approaches offer. Bioinformatics analysis of the derived NGS datasets unveiled several novel splice junctions between annotated exons of survivin gene as well as the existence of a novel exon of 117 nt, spanning between the annotated exons 3 and 3B. Validation of the NGS findings with PCR-based assays, using variant-specific primers, led to the identification of fourteen novel survivin alternative splice variants (BIRC5 v.4 - v.17), which demonstrate wide expression profiles in a broad established panel of human cell lines. Although the presented novel findings provide a crystal-clear overview of the survivin mRNAs that are actually generated from the pre-mRNA, future studies should focus on the impending necessity of characterizing the biological function of all novel alternative transcript variants as well as the putative protein isoforms. Such studies will further contribute to our understanding of how the balance between survivin isoforms regulate malignant cell proliferation and apoptosis, providing novel diagnostic, prognostic and predictive biomarkers as well as therapeutic targets.

The formation of perinucleolar bodies is important for normal leaf development and requires the zinc-finger DNA-binding motif in Arabidopsis ASYMMETRIC LEAVES2

In Arabidopsis, the ASYMMETRIC LEAVES2 (AS2) protein plays a key role in the formation of flat symmetric leaves via direct repression of the abaxial gene ETT/ARF3. AS2 encodes a plant-specific nuclear protein that contains the AS2/LOB domain, which includes a zinc-finger (ZF) motif that is conserved in the AS2/LOB family. We have shown that AS2 binds to the coding DNA of ETT/ARF3, which requires the ZF motif. AS2 is co-localized with AS1 in perinucleolar bodies (AS2 bodies). To identify the amino acid signals in AS2 required for formation of AS2 bodies and function(s) in leaf formation, we constructed recombinant DNAs that encoded mutant AS2 proteins fused to yellow fluorescent protein. We examined the subcellular localization of these proteins in cells of cotyledons and leaf primordia of transgenic plants and cultured cells. The amino acid signals essential for formation of AS2 bodies were located within and adjacent to the ZF motif. Mutant AS2 that failed to form AS2 bodies also failed to rescue the as2-1 mutation. Our results suggest the importance of the formation of AS2 bodies and the nature of interactions of AS2 with its target DNA and nucleolar factors including NUCLEOLIN1. The partial overlap of AS2 bodies with perinucleolar chromocenters with condensed ribosomal RNA genes implies a correlation between AS2 bodies and the chromatin state. Patterns of AS2 bodies in cells during interphase and mitosis in leaf primordia were distinct from those in cultured cells, suggesting that the formation and distribution of AS2 bodies are developmentally modulated in plants.

Nucleolar Translocation of Histone Deacetylase 2 Is Involved in Regulation of Transcriptional Silencing in the Cat Germinal Vesicle

Histone deacetylase 2 (HDAC2) is a key transcriptional coregulator that is suspected to play a role during oogenesis. It is known that RNA transcription in the cat germinal vesicle (GV) stops during folliculogenesis at the late antral follicle stage and is unrelated to histone deacetylation or chromatin condensation. The objective of the present study was to determine if and how HDAC2 participates in transcription regulation in the cat GV. Spatiotemporal HDAC2 protein expression was examined by immunostaining oocytes from primary to large antral follicles. HDAC2 was detected in the majority of GVs within oocytes from early, small, and large antral follicles. At early and small antral stages, HDAC2 was found primarily in the GV's nucleoplasm. There then was a significant shift in HDAC2 localization into the nucleolus, mostly in oocytes from large antral follicles. Assessments revealed that transcription was active in oocytes that contained nucleoplasm-localized HDAC2, whereas nucleolar-bound HDAC2 was associated with loss of both global transcription and ribosomal RNA presence at all antral stages. When oocytes were exposed to the HDAC inhibitor valproic acid, results indicated that HDAC regulated transcriptional activity in the nucleoplasm, but not in the nucleolus. Collective results suggest that nucleolar translocation of HDAC2 is associated with transcriptional silencing in the GV, thereby likely contributing to an oocyte's acquisition of competence.

Dictyostelium discoideum has a highly Q/N-rich proteome and shows an unusual resilience to protein aggregation

Many protein-misfolding diseases are caused by proteins carrying prion-like domains. These proteins show sequence similarity to yeast prion proteins, which can interconvert between an intrinsically disordered and an aggregated prion state. The natural presence of prions in yeast has provided important insight into disease mechanisms and cellular proteostasis. However, little is known about prions in other organisms, and it is not yet clear whether the findings in yeast can be generalized. Using bioinformatics tools, we show that Dictyostelium discoideum has the highest content of prion-like proteins of all organisms investigated to date, suggesting that its proteome has a high overall aggregation propensity. To study mechanisms regulating these proteins, we analyze the behavior of several well-characterized prion-like proteins, such as an expanded version of human huntingtin exon 1 (Q103) and the prion domain of the yeast prion protein Sup35 (NM), in D. discoideum. We find that these proteins remain soluble and are innocuous to D. discoideum, in contrast to other organisms, where they form cytotoxic cytosolic aggregates. However, when exposed to conditions that compromise molecular chaperones, these proteins aggregate and become cytotoxic. We show that the disaggregase Hsp101, a molecular chaperone of the Hsp100 family, dissolves heat-induced aggregates and promotes thermotolerance. Furthermore, prion-like proteins accumulate in the nucleus, where they are targeted by the ubiquitin-proteasome system. Our data suggest that D. discoideum has undergone specific adaptations that increase the proteostatic capacity of this organism and allow for an efficient regulation of its prion-like proteome.

Identification of FOXM1 as a therapeutic target in B-cell lineage acute lymphoblastic leukaemia

Despite recent advances in the cure rate of acute lymphoblastic leukaemia (ALL), the prognosis for patients with relapsed ALL remains poor. Here we identify FOXM1 as a candidate responsible for an aggressive clinical course. We show that FOXM1 levels peak at the pre-B-cell receptor checkpoint but are dispensable for normal B-cell development. Compared with normal B-cell populations, FOXM1 levels are 2- to 60-fold higher in ALL cells and are predictive of poor outcome in ALL patients. FOXM1 is negatively regulated by FOXO3A, supports cell survival, drug resistance, colony formation and proliferation in vitro, and promotes leukemogenesis in vivo. Two complementary approaches of pharmacological FOXM1 inhibition-(i) FOXM1 transcriptional inactivation using the thiazole antibiotic thiostrepton and (ii) an FOXM1 inhibiting ARF-derived peptide-recapitulate the findings of genetic FOXM1 deletion. Taken together, our data identify FOXM1 as a novel therapeutic target, and demonstrate feasibility of FOXM1 inhibition in ALL.

Molecular characterizations of subcellular localization signals in the nucleocapsid protein of porcine epidemic diarrhea virus

The nucleolus is a dynamic subnuclear structure, which is crucial to the normal operation of the eukaryotic cell. The porcine epidemic diarrhea virus (PEDV), coronavirus nucleocapsid (N) protein, plays important roles in the process of virus replication and cellular infection. Virus infection and transfection showed that N protein was predominately localized in the cytoplasm, but also found in the nucleolus in Vero E6 cells. Furthermore, by utilizing fusion proteins with green fluorescent protein (GFP), deletion mutations or site-directed mutagenesis of PEDV N protein, coupled with live cell imaging and confocal microscopy, it was revealed that, a region spanning amino acids (aa), 71–90 in region 1 of the N protein was sufficient for nucleolar localization and R87 and R89 were critical for its function. We also identified two nuclear export signals (NES, aa221–236, and 325–364), however, only the nuclear export signal (aa325–364) was found to be functional in the context of the full-length N protein. Finally, the activity of this nuclear export signal (NES) was inhibited by the antibiotic Lepomycin B, suggesting that N is exported by a chromosome region maintenance 1-related export pathway.

Differential localization and high expression of SURVIVIN splice variants in human embryonic stem cells but not in differentiated cells implicate a role for SURVIVIN in pluripotency

The BIRC5 gene encodes the oncofetal protein SURVIVIN, as well as four additional splice variants (ΔEx3, 2B, 3B and 2α). SURVIVIN, an inhibitor of apoptosis, is also a chromosomal passenger protein (CPP). Previous results have demonstrated that SURVIVIN is expressed at high levels in embryonic stem cells and inhibition of SURVIVIN function results in apoptosis, however these studies have not investigated the other four splice variants. In this study, we demonstrate that all variants are expressed at significantly higher levels in human embryonic stem (hES) cells than in differentiated cells. We examined the subcellular localization of the three most highly expressed variants. SURVIVIN displayed canonical CPP localization in mitotic cells and cytoplasmic localization in interphase cells. In contrast, SURVIVIN-ΔEx3 and SURVIVIN-2B did not localize as a CPP; SURVIVIN-ΔEx3 was found constitutively in the nucleus while SURVIVIN-2B was distributed along the chromosomes during mitosis and also to the mitotic spindle poles. We used inducible shRNA against SURVIVIN to inhibit expression in a titratable fashion. Using this system, we reduced the mRNA levels of these three variants to approx. 40%, resulting in a concomitant reduction of OCT4 and NANOG mRNA, suggesting a role for the SURVIVIN variants in pluripotency.

Clinico-pathologic relevance of Survivin splice variant expression in cancer

Survivin is a member of the inhibitor of apoptosis (IAP) family and has multifunctional properties that include aspects of proliferation, invasion and cell survival control. Survivin is a promising candidate for targeted cancer therapy as its expression is associated with poor clinical outcome, more aggressive clinico-pathologic features, and resistance to radiation and chemotherapy. In the present review the different properties of the Survivin splice variants are discussed and their activities correlated with different aspects of cancer cell biology, to include subcellular location. Special emphasis is placed on our current understanding of these Survivin splice variants influence on each other and on the phenotypic responses to therapy that they may control.

Plant WEE1 kinase is cell cycle regulated and removed at mitosis via the 26S proteasome machinery

In yeasts and animals, premature entry into mitosis is prevented by the inhibitory phosphorylation of cyclin-dependent kinase (CDK) by WEE1 kinase, and, at mitosis, WEE1 protein is removed through the action of the 26S proteasome. Although in higher plants WEE1 function has been confirmed in the DNA replication checkpoint, Arabidopsis wee1 insertion mutants grow normally, and a role for the protein in the G2/M transition during an unperturbed plant cell cycle is yet to be confirmed. Here data are presented showing that the inhibitory effect of WEE1 on CDK activity in tobacco BY-2 cell cultures is cell cycle regulated independently of the DNA replication checkpoint: it is high during S-phase but drops as cells traverse G2 and enter mitosis. To investigate this mechanism further, a yeast two-hybrid screen was undertaken to identify proteins interacting with Arabidopsis WEE1. Three F-box proteins and a subunit of the proteasome complex were identified, and bimolecular fluorescence complementation confirmed an interaction between AtWEE1 and the F-box protein SKP1 interacting partner 1 (SKIP1). Furthermore, the AtWEE1-green fluorescent protein (GFP) signal in Arabidopsis primary roots treated with the proteasome inhibitor MG132 was significantly increased compared with mock-treated controls. Expression of AtWEE1-YFP(C) (C-terminal portion of yellow fluorescent protein) or AtWEE1 per se in tobacco BY-2 cells resulted in a premature increase in the mitotic index compared with controls, whereas co-expression of AtSKIP1-YFP(N) negated this effect. These data support a role for WEE1 in a normal plant cell cycle and its removal at mitosis via the 26S proteasome.

NOA36 protein contains a highly conserved nucleolar localization signal capable of directing functional proteins to the nucleolus, in mammalian cells

NOA36/ZNF330 is an evolutionarily well-preserved protein present in the nucleolus and mitochondria of mammalian cells. We have previously reported that the pro-apoptotic activity of this protein is mediated by a characteristic cysteine-rich domain. We now demonstrate that the nucleolar localization of NOA36 is due to a highly-conserved nucleolar localization signal (NoLS) present in residues 1-33. This NoLS is a sequence containing three clusters of two or three basic amino acids. We fused the amino terminal of NOA36 to eGFP in order to characterize this putative NoLS. We show that a cluster of three lysine residues at positions 3 to 5 within this sequence is critical for the nucleolar localization. We also demonstrate that the sequence as found in human is capable of directing eGFP to the nucleolus in several mammal, fish and insect cells. Moreover, this NoLS is capable of specifically directing the cytosolic yeast enzyme polyphosphatase to the target of the nucleolus of HeLa cells, wherein its enzymatic activity was detected. This NoLS could therefore serve as a very useful tool as a nucleolar marker and for directing particular proteins to the nucleolus in distant animal species.

Chk2 phosphorylation of survivin-DeltaEx3 contributes to a DNA damage-sensing checkpoint in cancer

Survivin is an oncogene that functions in cancer cell cytoprotection and mitosis. Here we report that differential expression in cancer cells of a C-terminal splice variant of survivin, termed survivin-ΔEx3, is tightly associated with aggressive disease and markers of unfavorable prognosis. In contrast to other survivin variants, survivin-ΔEx3 localized exclusively to nuclei in tumor cells and was phosphorylated at multiple residues by the checkpoint kinase Chk2 during DNA damage. Mutagenesis of the Chk2 phosphorylation sites enhanced the stability of survivin-ΔEx3 in tumor cells, inhibited the expression of phosphorylated H2AX (γH2AX) in response to double-strand DNA breaks, and impaired growth after DNA damage. DNA damage induced Chk2 phosphorylation, stabilization of p53, induction of the cyclin-dependent kinase inhibitor p21, and homologous recombination-induced repair were not affected. In vivo, active Chk2 was detected at the earliest stages of the colorectal adenoma-to-carcinoma transition, persisted in advanced tumors, and correlated with increased survivin expression. Together, our findings suggest that Chk2-mediated phosphorylation of survivin-ΔEx3 contributes to a DNA damage-sensing checkpoint that may affect cancer cell sensitivity to genotoxic therapies.

L-Ilf3 and L-NF90 traffic to the nucleolus granular component: alternatively-spliced exon 3 encodes a nucleolar localization motif

Ilf3 and NF90, two proteins containing double-stranded RNA-binding domains, are generated by alternative splicing and involved in several functions. Their heterogeneity results from posttranscriptional and posttranslational modifications. Alternative splicing of exon 3, coding for a 13 aa N-terminal motif, generates for each protein a long and short isoforms. Subcellular fractionation and localization of recombinant proteins showed that this motif acts as a nucleolar localization signal. Deletion and substitution mutants identified four arginines, essential for nucleolar targeting, and three histidines to stabilize the proteins within the nucleolus. The short isoforms are never found in the nucleoli, whereas the long isoforms are present in the nucleoplasm and the nucleoli. For Ilf3, only the posttranslationally-unmodified long isoform is nucleolar, suggesting that this nucleolar targeting is abrogated by posttranslational modifications. Confocal microscopy and FRAP experiments have shown that the long Ilf3 isoform localizes to the granular component of the nucleolus, and that L-Ilf3 and L-NF90 exchange rapidly between nucleoli. The presence of this 13 aminoacid motif, combined with posttranslational modifications, is responsible for the differences in Ilf3 and NF90 isoforms subcellular localizations. The protein polymorphism of Ilf3/NF90 and the various subcellular localizations of their isoforms may partially explain the various functions previously reported for these proteins.

p14(ARF) inhibits the functions of adenovirus E1A oncoprotein

The tumour suppressor ARF (alternative reading frame) is one of the most important oncogenic stress sensors. ARF provides an 'oncogenic checkpoint' function through both p53-dependent and p53-independent mechanisms. In the present study, we demonstrate a novel p53-independent interaction between p14(ARF) and the adenovirus oncoprotein E1A. p14(ARF) inhibits E1A transcriptional function and promotes ubiquitination-dependent degradation of E1A. p14(ARF) overexpression relocalizes E1A into the nucleolus and inhibits E1A-induced cellular DNA replication independent of p53. Knockdown of endogenous p14(ARF) increases E1A transactivation. In addition, E1A can competitively inhibit ARF-Mdm2 (murine double minute 2) complex formation. These results identify a novel binding partner of p14(ARF) and reveal a mutually inhibitory interaction between p14(ARF) and E1A. We speculate that the ARF-E1A interaction may represent an additional host defence mechanism to limit viral replication. Alternatively, the interaction may allow adenovirus to sense the functional state of p53 in host cells, and fine-tune its own replication activity to prevent the triggering of a detrimental host response.

Nucleolar localization and identification of nuclear/nucleolar localization signals of the calmodulin-binding protein nucleomorphin during growth and mitosis in Dictyostelium

The calmodulin-binding protein nucleomorphin isoform NumA1 is a nuclear number regulator in Dictyostelium that localizes to intra-nuclear patches adjacent to the nuclear envelope and to a lesser extent the nucleoplasm. Earlier studies have shown similar patches to be nucleoli but only three nucleolar proteins have been identified in Dictyostelium. Here, actinomycin-D treatment caused the loss of NumA1 localization, while calcium and calmodulin antagonists had no effect. In keeping with a nucleolar function, NumA1 moved out of the presumptive nucleoli during mitosis redistributing to areas within the nucleus, the spindle fibers, and centrosomal region before re-accumulating in the presumptive nucleoli at telophase. Together, these data verify NumA1 as a true nucleolar protein. Prior to this study, the dynamics of specific nucleolar proteins had not been determined during mitosis in Dictyostelium. FITC-conjugated peptides equivalent to presumptive nuclear localization signals within NumA1 localized to nucleoli indicating that they also act as nucleolar localization signals. To our knowledge, these represent the first precisely defined nucleolar localization signals as well as the first nuclear/nucleolar localization signals identified in Dictyostelium. Together, these results reveal that NumA1 is a true nucleolar protein and the only nucleolar calmodulin-binding protein identified in Dictyostelium. The possible use of nuclear/nucleolar localization signal-mediated drug targeting to nucleoli is discussed.

Characterisation of histone variant distribution in human embryonic stem cells by transfection of in vitro transcribed mRNA

Recent studies, primarily in mouse embryonic stem cells, have highlighted the unique chromatin state of pluripotent stem cells, including the incorporation of histone variants into specific genomic locations, and its role in facilitating faithful expression of genes during development. However, there is little information available on the expression and subcellular localisation of histone variants in human embryonic stem cells (hESCs). In this study, we confirmed the expression of a panel of histone variant genes in several hESC lines and demonstrated the utility of transfection of in vitro transcribed, epitope-tagged mRNAs to characterise the subcellular localisation of these proteins. The subcellular localisations of variant histone H3 (CENP-A, H3.3), H2A (MACROH2A, H2AX, H2AZ, H2ABBD) and H1 (H1A, HB, H1C, H1D) were examined, revealing distinct nuclear localisation profiles for each protein. These data highlight the differences between murine (m) ESCs and hESCs, including the presence of a MACROH2A-enriched inactive X chromosome in undifferentiated XX hESC lines. We also provide the first evidence for MACROH2A accumulation on the Y-chromosome in XY hESCs.

Ribosome biogenesis: from structure to dynamics

In this chapter we describe the status of the research concerning the nucleolus, the major nuclear body. The nucleolus has been recognized as a dynamic organelle with many more functions than one could imagine. In fact, in addition to its fundamental role in the biogenesis of preribosomes, the nucleolus takes part in many other cellular processes and functions, such as the cell-cycle control and the p53 pathway: the direct or indirect involvement of the nucleolus in these various processes makes it sensitive to their alteration. Moreover, it is worth noting that the different nucleolar factors participating to independent mechanisms show different dynamics of association/disassociation with the nucleolar body.

Role of nuclear bodies in apoptosis signalling

Promyelocytic leukemia nuclear bodies (PML NBs) are dynamic macromolecular multiprotein complexes that recruit and release a plethora of proteins. A considerable number of PML NB components play vital roles in apoptosis, senescence regulation and tumour suppression. The molecular basis by which PML NBs control these cellular responses is still just beginning to be understood. In addition to PML itself, numerous further tumour suppressors including transcriptional regulator p53, acetyl transferase CBP (CREB binding protein) and protein kinase HIPK2 (homeodomain interacting protein kinase 2) are recruited to PML NBs in response to genotoxic stress or oncogenic transformation and drive the senescence and apoptosis response by regulating p53 activity. Moreover, in response to death-receptor activation, PML NBs may act as nuclear depots that release apoptotic factors, such as the FLASH (FLICE-associated huge) protein, to amplify the death signal. PML NBs are also associated with other nuclear domains including Cajal bodies and nucleoli and share apoptotic regulators with these domains, implying crosstalk between NBs in apoptosis regulation. In conclusion, PML NBs appear to regulate cell death decisions through different, pathway-specific molecular mechanisms.

Identification and characterization of a novel gene differentially expressed in zebrafish cross-subfamily cloned embryos

Background

Cross-species nuclear transfer has been shown to be a potent approach to retain the genetic viability of a certain species near extinction. However, most embryos produced by cross-species nuclear transfer were compromised because that they were unable to develop to later stages. Gene expression analysis of cross-species cloned embryos will yield new insights into the regulatory mechanisms involved in cross-species nuclear transfer and embryonic development.

Results

A novel gene, K31, was identified as an up-regulated gene in fish cross-subfamily cloned embryos using SSH approach and RACE method. K31 complete cDNA sequence is 1106 base pairs (bp) in length, with a 342 bp open reading frame (ORF) encoding a putative protein of 113 amino acids (aa). Comparative analysis revealed no homologous known gene in zebrafish and other species database. K31 protein contains a putative transmembrane helix and five putative phosphorylation sites but without a signal peptide. Expression pattern analysis by real time RT-PCR and whole-mount in situ hybridization (WISH) shows that it has the characteristics of constitutively expressed gene. Sub-cellular localization assay shows that K31 protein can not penetrate the nuclei. Interestingly, over-expression of K31 gene can cause lethality in the epithelioma papulosum cyprinid (EPC) cells in cell culture, which gave hint to the inefficient reprogramming events occurred in cloned embryos.

Conclusion

Taken together, our findings indicated that K31 gene is a novel gene differentially expressed in fish cross-subfamily cloned embryos and over-expression of K31 gene can cause lethality of cultured fish cells. To our knowledge, this is the first report on the determination of novel genes involved in nucleo-cytoplasmic interaction of fish cross-subfamily cloned embryos.

The clinical significance of splice variants and subcellular localisation of survivin in non-small cell lung cancers

Survivin is a member of the inhibitor of apoptosis protein family. Survivin has splice variants with different biological functions associated with tumorigenesis. We investigated 134 non-small cell lung cancers (NSCLCs) to study the clinical significance of wild-type survivin, survivin-2B, and survivin-deltaEx3. Real-time PCR analyses were performed for their gene expressions. The subcellular localisation of survivin proteins was evaluated by immunohistochemistry. The Ki-67 proliferation index and the apoptotic index were also evaluated. The survivin-deltaEx3 gene expression was significantly higher in stage II–III than in stage I (P=0.0174), and significantly correlated with the nuclear pan-survivin expression (P<0.0001). The Ki-67 index was significantly higher in wild-type survivin-positive tumours (P<0.0001), survivin-deltaEx3-positive tumours (P<0.0001), and tumours with positive expression of the nuclear pan-survivin (P=0.0047). In contrast, the apoptotic index was significantly lower only in wild-type survivin-positive tumours (P<0.0001). Thus, the wild-type survivin gene expression was associated with apoptotic inhibition and tumour proliferation. Furthermore, the survivin-deltaEx3 gene expression was strongly associated with tumour proliferation, especially in advanced stage NSCLCs. In contrast, the survivin-2B gene expression did not correlate with tumour proliferation or tumour apoptosis.

Nucleolus: the fascinating nuclear body

Nucleoli are the prominent contrasted structures of the cell nucleus. In the nucleolus, ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins. RNA polymerase I synthesizes the ribosomal RNAs and this activity is cell cycle regulated. The nucleolus reveals the functional organization of the nucleus in which the compartmentation of the different steps of ribosome biogenesis is observed whereas the nucleolar machineries are in permanent exchange with the nucleoplasm and other nuclear bodies. After mitosis, nucleolar assembly is a time and space regulated process controlled by the cell cycle. In addition, by generating a large volume in the nucleus with apparently no RNA polymerase II activity, the nucleolus creates a domain of retention/sequestration of molecules normally active outside the nucleolus. Viruses interact with the nucleolus and recruit nucleolar proteins to facilitate virus replication. The nucleolus is also a sensor of stress due to the redistribution of the ribosomal proteins in the nucleoplasm by nucleolus disruption. The nucleolus plays several crucial functions in the nucleus: in addition to its function as ribosome factory of the cells it is a multifunctional nuclear domain, and nucleolar activity is linked with several pathologies. Perspectives on the evolution of this research area are proposed.

Nucleolar localization of parafibromin is mediated by three nucleolar localization signals

Parafibromin is a putative tumor suppressor encoded by HRPT2 and implicated in parathyroid tumorigenesis. We previously reported a functional bipartite nuclear localization signal (NLS) at residues 125-139. We now demonstrate that parafibromin exhibits nucleolar localization, mediated by three nucleolar localization signals (NoLS) at resides 76-92, 192-194 and 393-409. These NoLS represent clusters of basic amino acids arginine and lysine, similar to those found in other nucleolar proteins, as well as being characteristic of NLSs. While parafibromin's bipartite NLS is the primary determinant of nuclear localization, it does not mediate nucleolar localization. In contrast, the three identified NoLSs play only a minor role in nuclear localization, but are critical for the nucleolar localization of parafibromin.

Subcellular distribution of human RDM1 protein isoforms and their nucleolar accumulation in response to heat shock and proteotoxic stress

The RDM1 gene encodes a RNA recognition motif (RRM)-containing protein involved in the cellular response to the anti-cancer drug cisplatin in vertebrates. We previously reported a cDNA encoding the full-length human RDM1 protein. Here, we describe the identification of 11 human cDNAs encoding RDM1 protein isoforms. This repertoire is generated by alternative pre-mRNA splicing and differential usage of two translational start sites, resulting in proteins with long or short N-terminus and a great diversity in the exonic composition of their C-terminus. By using tagged proteins and fluorescent microscopy, we examined the subcellular distribution of full-length RDM1 (renamed RDM1alpha), and other RDM1 isoforms. We show that RDM1alpha undergoes subcellular redistribution and nucleolar accumulation in response to proteotoxic stress and mild heat shock. In unstressed cells, the long N-terminal isoforms displayed distinct subcellular distribution patterns, ranging from a predominantly cytoplasmic to almost exclusive nuclear localization, suggesting functional differences among the RDM1 proteins. However, all isoforms underwent stress-induced nucleolar accumulation. We identified nuclear and nucleolar localization determinants as well as domains conferring cytoplasmic retention to the RDM1 proteins. Finally, RDM1 null chicken DT40 cells displayed an increased sensitivity to heat shock, compared to wild-type (wt) cells, suggesting a function for RDM1 in the heat-shock response.

Viral protein apoptin as a molecular tool and therapeutic bullet: implications for cancer control

The chicken anemia virus-derived protein apoptin induces apoptosis in human tumor cells via a p53-independent pathway, while leaving normal cells intact. Moreover, apoptin treatment in preclinical animal studies leads to reduced tumor growth or remission without a detectable effect on healthy tissues. Apoptin is activated by a still unknown tumor-specific kinase activity. The mode of action of apoptin is under intense investigation, as certain features make it a promising tool for discovering early events in tumorigenesis, identifying druggable targets for antitumor treatment and possibly serving as an antitumor therapy in itself.

Identification of a common subnuclear localization signal

Proteins share peptidic sequences, such as a nuclear localization signal (NLS), which guide them to particular membrane-bound compartments. Similarities have also been observed within different classes of signals that target proteins to membrane-less subnuclear compartments. Common localization signals affect spatial and temporal subcellular organization and are thought to allow the coordinated response of different molecular networks to a given signaling cue. Here we identify a higher-order and predictive code, {[RR(I/L)X(3)r]((n, n > or = 1))+[L(phi/N)(V/L)]((n,n>1))}, that establishes high-affinity interactions between a group of proteins and the nucleolus in response to a specific signal. This position-independent code is referred to as a nucleolar detention signal regulated by H(+) (NoDS(H+)) and the class of proteins includes the cIAP2 apoptotic regulator, VHL ubiquitylation factor, HSC70 heat shock protein and RNF8 transcription regulator. By identifying a common subnuclear targeting consensus sequence, our work reveals rules governing the dynamics of subnuclear organization and ascribes new modes of regulation to several proteins with diverse steady-state distributions and dynamic properties.

Nuclear survivin is associated with disease recurrence and poor survival in patients with cutaneous malignant melanoma

AIMS

Survivin is expressed in neoplastic cells and appears to be associated with resistance to therapy and shorter survival in various types of tumours. The aim of the present study was to determine whether nuclear or cytoplasmic expression of survivin is related to disease recurrence and overall survival of patients with Stage I and II melanoma according to the American Joint Committee on Cancer (AJCC) staging system.

METHODS AND RESULTS

Immunohistochemistry was performed on formalin-fixed paraffin-embedded sections of primary cutaneous melanoma from 50 patients. Survival rates were estimated using the Kaplan-Meier method and compared using the log rank test. Association of clinical variables (gender, age, tumour location, thickness, Clark level and AJCC stage) with survivin expression was analysed by Fisher's exact test. Patients with nuclear immunoreactivity for survivin had an increased risk of disease recurrence during the first three postoperative years (P < 0.05) and of death (P < 0.05). Cytoplasmic immunoreactivity was not correlated with either survival or clinical variables.

CONCLUSIONS

Nuclear presence of survivin may be an independent biomarker for disease recurrence and overall survival in patients with Stage I and II melanoma.

Rat homolog of PinX1 is a nucleolar protein involved in the regulation of telomere length

Human PinX1 involves in regulation of telomere length. Here, we describe the function of a rat homolog of PinX1. Rat PinX1 (rPinX1) was cloned from WB-F344, a rat hepatic stem-like epithelial cell. It encodes a protein of 331 amino acids with 70% homology to human PinX1 and 91% homology to mouse. Northern analysis revealed that rPinX1 is expressed in both somatic and germ tissues, most abundantly in heart, liver and testis. Co-localization with a nucleolar protein, fibrillarin, showed that rPinX1 resides in the nucleolus. Analysis of truncated mutants revealed that an internal K,E/D region seems to be important for nucleolar localization. A stable cell line expressing rPinX1 was established in NIH3T3, a mouse-transformed embryonic fibroblast cell line, and stable cells were subcultured for more than 150 population doublings. The growth of stable rPinX1 cells slowed down at late passages, and a fraction of these cells exhibited increased size and stained positively for senescence-associated beta-galactosidase. Overexpression of rPinX1 in NIH3T3 cells resulted in gradual telomere shortening over successive passages. However, the telomeric 3' overhang was not altered by PinX1 expression. This study demonstrates that a rat homolog of human PinX1 is a nucleolar protein, and that overexpression of rPinX1 induces cellular senescence and telomere shortening, but has no effect on 3' overhang length. The function of PinX1 in regulating telomere length is conserved in rodents, and this study may provide insight into the mechanism by which a nucleolar protein can regulate telomere length.

Trafficking motifs in the SARS-coronavirus nucleocapsid protein

The severe acute respiratory syndrome-coronavirus nucleocapsid (N) protein is involved in virus replication and modulation of cell processes. In this latter respect control may in part be achieved through the sub-cellular localisation of the protein. N protein predominately localises in the cytoplasm (the site of virus replication and assembly) but also in the nucleus/nucleolus. Using a combination of live-cell and confocal microscopy coupled to mutagenesis we identified a cryptic nucleolar localisation signal in the central part of the N protein. In addition, based on structural comparison to the avian coronavirus N protein, a nuclear export signal was identified in the C-terminal region of the protein.

RNA viruses: hijacking the dynamic nucleolus

The nucleolus is a dynamic subnuclear structure that is crucial to the successful functioning of a cell. Its functions include ribosomal RNA synthesis, cell growth and cell-cycle control as well as responding to cellular stress.

Recent studies show that the nucleolus is not a steady-state structure but instead is made up of numerous protein–protein and protein–nucleic-acid interactions that are constantly changing in response to the metabolic conditions of the cell.

Many different viruses target the nucleolus to disrupt host-cell function and to recruit cellular proteins to aid in virus replication.

The study of viral-protein trafficking to the nucleolus and the interaction of viral proteins with nucleolar proteins is providing many insights into the cell biology of the nucleolus.

Because the nucleolus is fundamental to the life cycle of many viruses, disrupting the interaction between the nucleolus and the virus could lead to the design of novel therapeutic strategies.

RNA viruses, particularly positive-strand RNA viruses, interact with the nucleolus to usurp host-cell functions and recruit nucleolar proteins to facilitate virus replication. Here, Julian Hiscox reviews the latest data on RNA-virus interactions with this dynamic subnuclear structure.

The nucleolus is a dynamic subnuclear structure with roles in ribosome subunit biogenesis, mediation of cell-stress responses and regulation of cell growth. The proteome and structure of the nucleolus are constantly changing in response to metabolic conditions. RNA viruses interact with the nucleolus to usurp host-cell functions and recruit nucleolar proteins to facilitate virus replication. Investigating the interactions between RNA viruses and the nucleolus will facilitate the design of novel anti-viral therapies, such as recombinant vaccines and therapeutic molecular interventions, and also contribute to a more detailed understanding of the cell biology of the nucleolus.

A B23-interacting sequence as a tool to visualize protein interactions in a cellular context

We report the characterization of a nucleolar localization sequence (NoLS) that targets the green fluorescent protein (GFP) into the granular component (GC) of nucleoli. This NoLS interacts in vitro specifically and directly with the major nucleolar protein B23 and more precisely with the region of B23 including the two acidic stretches. The affinity of NoLS for B23 is stronger than that of the HIV-1 Rev protein in vitro. Moreover, B23-NoLS interaction also occurs in vivo. Indeed, (1) NoLS confers on the GFP the behavior of B23 throughout the cell cycle, (2) the GFP-NoLS fusion and B23 remain colocalized after drug treatments, (3) a selective delocalization of B23 from nucleoli to nucleoplasm induces a concomitent delocalization of the GFP-NoLS fusion, and (4) the fusion of NoLS to fibrillarin makes it possible to colocalize fibrillarin and B23. Interestingly, by fusing NoLS to fibrillarin, both fibrillarin and the fibrillarin partner Nop56 are mislocalized in the GC of nucleoli. Similarly, by fusing the NoLS to MafG, part of the nuclear transcription factor NF-E2 composed of both MafG and p45 NF-E2, NF-E2 is redirected from the nucleoplasm to the nucleoli. Thus, we propose that the NoLS may be used as a tool to visualize and prove protein interactions in a cellular context.

Survivin subcellular localization in high-grade astrocytomas: simultaneous expression in both nucleus and cytoplasm is negative prognostic marker

OBJECTIVE

Subcellularly localized (nuclear and/or cytoplasmic) survivin has various functions, and correlates with prognosis of malignant tumors. However, there have been no reports about the significance of subcellularly localized survivin in high-grade astrocytomas. The aim of the present study was to examine the relationship between prognosis and subcellular localization of survivin in high-grade astrocytoma.

METHODS

We immunohistochemically examined the pattern of subcellular localization of survivin expression (nuclear, cytoplasmic, or both) in 51 patients with high-grade astrocytoma (19 anaplastic astrocytomas; 32 glioblastomas). We statistically examined the relationship between survivin localization and prognosis, using multivariate analysis including other clinicopathological factors (age, sex, WHO grade, extent of resection, MIB-1 labeling index, and expression of p53 and epidermal growth factor receptor).

RESULTS

All specimens stained positive for survivin: localized in nucleus only (nuclear-positive group), 10 cases (20%); localized in cytoplasm only (cytoplasmic-positive group), 23 cases (45%); simultaneous expression in nucleus and cytoplasm (nuclear-cytoplasmic group), 19 cases (35%). There was no significant difference in prognosis between the nuclear-positive group and cytoplasmic-positive group (P=0.796). However, the nuclear-cytoplasmic group had significantly shorter overall survival than the nuclear-positive group and the cytoplasmic-positive group (P<0.0001).

CONCLUSIONS

We found that simultaneous expression of survivin in both the nucleus and cytoplasm is an important prognostic factor for high-grade astrocytoma. The present findings indicate that subcellular localization of survivin expression is a reliable prognostic factor for patients with this tumor.

Dissecting the role of endothelial SURVIVIN DeltaEx3 in angiogenesis

The identification of alternative splice variants of Survivin that possess distinct functions from those originally identified for the main Survivin isoform has greatly increased the complexity of our understanding of the role of Survivin in different cells. Previous functional studies of the Survivin splice variants have been performed almost exclusively in cancer cells. However, Survivin has increasingly been implicated in other normal physiologic and pathophysiologic processes, including angiogenesis. In this study, we dissect the involvement of Survivin DeltaEx3 in angiogenesis. We show by confocal microscopy that a pool of endothelial Survivin DeltaEx3 is localized to membrane ruffles. We also demonstrate that Survivin DeltaEx3 is the Survivin splice variant responsible for modulating angiogenesis in vitro, in tube formation assays, and in vivo, in an in vivo angiogenesis assay. Our data indicate that Survivin DeltaEx3 may regulate angiogenesis via several mechanisms including cell invasion, migration, and Rac1 activation. Our findings identify a novel pathway regulating angiogenesis through Survivin DeltaEx3 and a novel mechanism for Rac1 activation during angiogenesis. In conclusion, our results provide new insights into the regulation of endothelial cell homeostasis and angiogenesis by the Survivin proteins.

Survivin study: an update of "what is the next wave"?

Studies on survivin over the past 2-3 years have shown that survivin possesses multiple subcellular localizations and is a multifunctional molecule involved in many aspects of cellular processes and/or behaviors. The subcellular localization and function of the survivin splice variants, however, have not yet been well elucidated. We have, therefore, provided additional observations on several survivin splice variants for further exploration. This review article will update the role of survivin, and its splice variants in the mitosis/cell cycle, apoptosis, tumorigenesis, chemoprevention, drug/radiation resistance, and cancer therapeutics.

Potential roles for ubiquitin and the proteasome during ribosome biogenesis

We have investigated the possible involvement of the ubiquitin-proteasome system (UPS) in ribosome biogenesis. We find by immunofluorescence that ubiquitin is present within nucleoli and also demonstrate by immunoprecipitation that complexes associated with pre-rRNA processing factors are ubiquitinated. Using short proteasome inhibition treatments, we show by fluorescence microscopy that nucleolar morphology is disrupted for some but not all factors involved in ribosome biogenesis. Interference with proteasome degradation also induces the accumulation of 90S preribosomes, alters the dynamic properties of a number of processing factors, slows the release of mature rRNA from the nucleolus, and leads to the depletion of 18S and 28S rRNAs. Together, these results suggest that the UPS is probably involved at many steps during ribosome biogenesis, including the maturation of the 90S preribosome.

Developmental expression of pluripotency determining factors in caprine embryos: Novel pattern of NANOG protein localization in the nucleolus

Transcription factors, POU5F1/OCT4 and NANOG, whose expression is restricted to the inner cell mass (ICM) in mouse and human blastocysts, are used to characterize undifferentiated embryonic stem cells (ESC) in vitro. However, POU5F1 may not be a useful marker in domestic animals due to its expression in both ICM and trophectoderm (TE), while NANOG mRNA and protein expression have only been described fully in mice. In an effort to identify ESC markers for domestic animals, expression patterns of NANOG, POU5F1, and the cell surface markers (SSEA1, SSEA4, TRA-1-60, TRA-1-81) were examined in preimplantation goat embryos, a species that has proven to be a superior choice for the production of transgenic proteins in milk (biopharming). Our results indicate that while goat embryos express POU5F1, SSEA1, and SSEA4 proteins, their expression is not strictly restricted to the ICM. In a unique staining pattern, NANOG protein was localized to the nucleoplasm and nucleoli in ICM cells, but was localized strictly to nucleoli in TE. This pattern may reflect down-regulation of protein by sequestration/degradation utilizing a nucleolar mechanism known to operate in stem cells. Furthermore, NANOG mRNA in TE was also significantly down-regulated as compared with that in ICM. Taken together, this novel expression pattern of NANOG in goat preimplantation embryos suggests that NANOG could serve as marker of pluripotency in goats and may be useful in derivation and characterization of caprine ESC. This study is the first to characterize both NANOG mRNA and protein expression in any species other than the mouse.

Delineation and modelling of a nucleolar retention signal in the coronavirus nucleocapsid protein

Unlike nuclear localization signals, there is no obvious consensus sequence for the targeting of proteins to the nucleolus. The nucleolus is a dynamic subnuclear structure which is crucial to the normal operation of the eukaryotic cell. Studying nucleolar trafficking signals is problematic as many nucleolar retention signals (NoRSs) are part of classical nuclear localization signals (NLSs). In addition, there is no known consensus signal with which to inform a study. The avian infectious bronchitis virus (IBV), coronavirus nucleocapsid (N) protein, localizes to the cytoplasm and the nucleolus. Mutagenesis was used to delineate a novel eight amino acid motif that was necessary and sufficient for nucleolar retention of N protein and colocalize with nucleolin and fibrillarin. Additionally, a classical nuclear export signal (NES) functioned to direct N protein to the cytoplasm. Comparison of the coronavirus NoRSs with known cellular and other viral NoRSs revealed that these motifs have conserved arginine residues. Molecular modelling, using the solution structure of severe acute respiratory (SARS) coronavirus N‐protein, revealed that this motif is available for interaction with cellular factors which may mediate nucleolar localization. We hypothesise that the N‐protein uses these signals to traffic to and from the nucleolus and the cytoplasm.

A Wiring of the Human Nucleolus

Recent proteomic efforts have created an extensive inventory of the human nucleolar proteome. However, approximately 30% of the identified proteins lack functional annotation. We present an approach of assigning function to uncharacterized nucleolar proteins by data integration coupled to a machine-learning method. By assembling protein complexes, we present a first draft of the human ribosome biogenesis pathway encompassing 74 proteins and hereby assign function to 49 previously uncharacterized proteins. Moreover, the functional diversity of the nucleolus is underlined by the identification of a number of protein complexes with functions beyond ribosome biogenesis. Finally, we were able to obtain experimental evidence of nucleolar localization of 11 proteins, which were predicted by our platform to be associates of nucleolar complexes. We believe other biological organelles or systems could be "wired" in a similar fashion, integrating different types of data with high-throughput proteomics, followed by a detailed biological analysis and experimental validation.

p53 is involved in clearance of ionizing radiation-induced RAD51 foci in a human colon cancer cell line

We have investigated p53-related differences in cellular response to DNA damaging agents, focusing on p53s effects on RAD51 protein level and sub-cellular localization post exposure to ionizing radiation. In a human colon cancer cell line, HCT116 and its isogenic p53-/- subcell line we show here p53-independent RAD51 foci formation but interestingly the resolution of RAD51 foci showed clear p53 dependence. In p53 wt cells, but not in p53-/- cells, RAD51 protein level decreased 48 h post irradiation and fluorescence immunostaining showed resolution of RAD51 foci and relocalization of RAD51 to nucleoli at time points corresponding to the decrease in RAD51 protein level. Both cell lines rejoined DNA double strand breaks efficiently with similar kinetics and p53 status did not influence sensitivity to DNA damaging agents. We suggest that p53 has a role in RAD51 clearance post DSB repair and that nucleoli might be sites of RAD51 protein degradation.

Molecular analysis of survivin isoforms: evidence that alternatively spliced variants do not play a role in mitosis

Survivin is a protein with proposed roles in cell division and apoptosis. Transcripts encoding splice variants of human survivin have been described and their expression correlated with cancer progression. As survivin forms homodimers in vitro, it has been suggested that these isoforms could interfere with wild type function by forming heterodimers. Here we show that survivin-2beta and survivin-deltaEx3 can interact with wild type survivin but have reduced affinity for the partner protein of survivin, borealin, and thus do not localize with the chromosomal passenger complex in vivo. Furthermore, we demonstrate that overexpression of survivin-2beta-green fluorescent protein (GFP) or survivin-deltaEx3-GFP does not impede cell cycle progression. We also report that wild type survivin, but not survivin-2beta-GFP or survivin-deltaEx3-GFP, can rescue cell proliferation inhibited by small interfering RNA-mediated survivin depletion. These data suggest that, despite their ability to interact with wild type survivin, neither of these isoforms acts as its competitor during mitosis nor has an essential function.