Telomerase RNA accumulates in Cajal bodies in human cancer cells

The proteins of intra-nuclear bodies: a data-driven analysis of sequence, interaction and expression

Background

Cajal bodies, nucleoli, PML nuclear bodies, and nuclear speckles are morpohologically distinct intra-nuclear structures that dynamically respond to cellular cues. Such nuclear bodies are hypothesized to play important regulatory roles, e.g. by sequestering and releasing transcription factors in a timely manner. While the nucleolus and nuclear speckles have received more attention experimentally, the PML nuclear body and the Cajal body are still incompletely characterized in terms of their roles and protein complement.

Results

By collating recent experimentally verified data, we find that almost 1000 proteins in the mouse nuclear proteome are known to associate with one or more of the nuclear bodies. Their gene ontology terms highlight their regulatory roles: splicing is confirmed to be a core activity of speckles and PML nuclear bodies house a range of proteins involved in DNA repair. We train support-vector machines to show that nuclear proteins contain discriminative sequence features that can be used to identify their intra-nuclear body associations. Prediction accuracy is highest for nucleoli and nuclear speckles. The trained models are also used to estimate the full protein complement of each nuclear body. Protein interactions are found primarily to link proteins in the nuclear speckles with proteins from other compartments. Cell cycle expression data provide support for increased activity in nucleoli, nuclear speckles and PML nuclear bodies especially during S and G₂ phases.

Conclusions

The large-scale analysis of the mouse nuclear proteome sheds light on the functional organization of physically embodied intra-nuclear compartments. We observe partial support for the hypothesis that the physical organization of the nucleus mirrors functional modularity. However, we are unable to unambiguously identify proteins' intra-nuclear destination, suggesting that critical drivers behind of intra-nuclear translocation are yet to be identified.

Box H/ACA small ribonucleoproteins

Box H/ACA RNAs represent an abundant, evolutionarily conserved class of small noncoding RNAs. All H/ACA RNAs associate with a common set of proteins, and they function as ribonucleoprotein (RNP) enzymes mainly in the site-specific pseudouridylation of ribosomal RNAs (rRNAs) and small nuclear RNAs (snRNAs). Some H/ACA RNPs function in the nucleolytic processing of precursor rRNA (pre-rRNA) and synthesis of telomeric DNA. Thus, H/ACA RNPs are essential for three fundamental cellular processes: protein synthesis, mRNA splicing, and maintenance of genome integrity. Recently, great progress has been made toward understanding of the biogenesis, intracellular trafficking, structure, and function of H/ACA RNPs.

Specificity and stoichiometry of subunit interactions in the human telomerase holoenzyme assembled in vivo

The H/ACA motif of human telomerase RNA (hTR) directs specific pathways of endogenous telomerase holoenzyme assembly, function, and regulation. Similarities between hTR and other H/ACA RNAs have been established, but differences have not been explored even though unique features of hTR H/ACA RNP assembly give rise to telomerase deficiency in human disease. Here, we define hTR H/ACA RNA and RNP architecture using RNA accumulation, RNP affinity purification, and primer extension activity assays. First, we evaluate alternative folding models for the hTR H/ACA motif 5' hairpin. Second, we demonstrate an unanticipated and surprisingly general asymmetry of 5' and 3' hairpin requirements for H/ACA RNA accumulation. Third, we establish that hTR assembles not one but two sets of all four of the H/ACA RNP core proteins, dyskerin, NOP10, NHP2, and GAR1. Fourth, we address a difference in predicted specificities of hTR association with the holoenzyme subunit WDR79/TCAB1. Together, these results complete the analysis of hTR elements required for active RNP biogenesis and define the interaction specificities and stoichiometries of all functionally essential human telomerase holoenzyme subunits. This study uncovers unexpected similarities but also differences between telomerase and other H/ACA RNPs that allow a unique specificity of telomerase biogenesis and regulation.

Nuclear bodies: random aggregates of sticky proteins or crucibles of macromolecular assembly?

The principles of self-assembly and self-organization are major tenets of molecular and cellular biology. Governed by these principles, the eukaryotic nucleus is composed of numerous subdomains and compartments, collectively described as nuclear bodies. Emerging evidence reveals that associations within and between various nuclear bodies and genomic loci are dynamic and can change in response to cellular signals. This review will discuss recent progress in our understanding of how nuclear body components come together, what happens when they form, and what benefit these subcellular structures may provide to the tissues or organisms in which they are found.

Telomeres and telomerase in cancer

Myriad genetic and epigenetic alterations are required to drive normal cells toward malignant transformation. These somatic events commandeer many signaling pathways that cooperate to endow aspiring cancer cells with a full range of biological capabilities needed to grow, disseminate and ultimately kill its host. Cancer genomes are highly rearranged and are characterized by complex translocations and regional copy number alterations that target loci harboring cancer-relevant genes. Efforts to uncover the underlying mechanisms driving genome instability in cancer have revealed a prominent role for telomeres. Telomeres are nucleoprotein structures that protect the ends of eukaryotic chromosomes and are particularly vulnerable due to progressive shortening during each round of DNA replication and, thus, a lifetime of tissue renewal places the organism at risk for increasing chromosomal instability. Indeed, telomere erosion has been documented in aging tissues and hyperproliferative disease states-conditions strongly associated with increased cancer risk. Telomere dysfunction can produce the opposing pathophysiological states of degenerative aging or cancer with the specific outcome dictated by the integrity of DNA damage checkpoint responses. In most advanced cancers, telomerase is reactivated and serves to maintain telomere length and emerging data have also documented the capacity of telomerase to directly regulate cancer-promoting pathways. This review covers the role of telomeres and telomerase in the biology of normal tissue stem/progenitor cells and in the development of cancer.

Novel role of nucleostemin in the maintenance of nucleolar architecture and integrity of small nucleolar ribonucleoproteins and the telomerase complex

Nucleostemin (NS) is a nucleolar protein involved in the regulation of cell proliferation. Both overexpression and knockdown of NS increase the activity of the tumor suppressor protein p53, resulting in cell cycle arrest. In addition, NS regulates processing of pre-rRNA and consequently the level of total protein synthesis. Here, we describe a previously uncharacterized function of NS in the maintenance of the tripartite nucleolar structure as well as the integrity of small nucleolar ribonucleoproteins (snoRNPs). NS is also necessary to maintain the telomerase complex which shares common protein subunits with the H/ACA box snoRNPs. First, immunofluorescence microscopy and electron microscopy demonstrated that knockdown of NS disorganized the nucleolar architecture, in particular, the dense fibrillar component where snoRNPs are localized. Second, gel filtration chromatography and immunoprecipitation indicated that NS depletion leads to dissociation of the components of snoRNPs and the telomerase complex. Third, NS depletion reduced both telomerase activity and the cellular level of pseudouridine, an H/ACA snoRNP-mediated modification of rRNA and other RNAs that are important for their folding and stability. These morphological, biochemical and functional studies demonstrate that NS plays an important role to maintain nucleolar structure and function on a more fundamental level than previously thought.

Genetic and environmental factors influencing human diseases with telomere dysfunction

Both genetic and environmental factors have been implicated in the mechanism underlying the pathogenesis of serious and fatal forms of human blood disorder (acquired aplastic anemia, AA) and lung disease (idiopathic pulmonary fibrosis, IPF). We and other researchers have recently shown that naturally occurring mutations in genes encoding the telomere maintenance complex (telomerase) may predispose patients to the development of AA or IPF. Epidemiological data have shown that environmental factors can also cause and/or exacerbate the pathogenesis of these diseases. The exact mechanisms that these germ-line mutations in telomere maintenance genes coupled with environmental insults lead to ineffective hematopoiesis in AA and lung scarring in IPF are not well understood, however. In this article, we provide a summary of evidence for environmental and genetic factors influencing the diseases. These studies provide important insights into the interplay between environmental and genetic factors leading to human diseases with telomere dysfunction.

The regulation of telomerase in oncogenesis

The influence that the expression of the human (glial-derived neurotrophic factor (GDNF)) neurotrophic factor has on the morphology and proliferative activity of embryonic stem cells (SC) of a mouse with R1 lineage, as well as their ability to form embroid bodies (EB), has been studied. Before that, using a PCR (polymerase chain reaction) coupled with reverse transcription, it was shown that, in this very lineage of the embryonic SC, the expression of the receptors' genes is being fulfilled for the neurotropfic RET and GFRα1 glia factor. The mouse's embryonic SC lineage has been obtained, transfected by the human GDNF gene, and has been fused with the "green" fluorescent protein (GFP) gene. The presence of the expression of the human GDNF gene in the cells was shown by northern hybridization and the synthesis of its albuminous product by immunocitochemical coloration with the use of specific antibodies. The reliable slowing-down of the embriod-body formation by the embryonic SC transfected by the GDNF gene has been shown. No significant influence of the expression of the GDNF gene on the morphology and the proliferative activity of the transfected embryonic SCs has been found when compared with the control ones.

Human PinX1 mediates TRF1 accumulation in nucleolus and enhances TRF1 binding to telomeres

Human PinX1 (hPinX1) is known to interact with telomere repeat binding factor 1 (TRF1) and telomerase. Here, we report that hPinX1 regulates the nucleolar accumulation and telomeric association of TRF1. In HeLa, HA-hPinX1 was co-localized with fibrillarin, a nucleolar protein, in 51% of the transfected cells and was present in the nucleoplasm of the remaining 48%. Mutant analysis showed that the C-terminal region was important for nucleolar localization, while the N-terminus exhibited an inhibitory effect on nucleolar localization. Unlike HA- and Myc-hPinX1, GFP-hPinX1 resided predominantly in the nucleolus. Nuclear hPinX1 bound to telomeres and other repeat sequences as well but, despite its interaction with TRF1, nucleolar hPinX1 did not bind to telomeres. Nucleolar hPinX1 forced endogenous TRF1 accumulation in the nucleolus. Furthermore, TRF1 binding to telomeres was upregulated in cells over-expressing hPinX1. In an ALT cell line, WI-38 VA-13, TRF1 did not co-localize with hPinX1 in the nucleoli. In summary, hPinX1 likely interacts with TRF1 in both the nucleolus and the nucleoplasm, and excess hPinX1 results in increased telomere binding of TRF1. The PinX1 function of mediating TRF1 nucleolar accumulation is absent from ALT cells, suggesting that it might be telomerase-dependent.

Telomerase regulation in hematological cancers: a matter of stemness?

Human telomerase is a nuclear ribonucleoprotein enzyme complex that catalyzes the synthesis and extension of telomeric DNA. This enzyme is highly expressed and active in most malignant tumors while it is usually not or transiently detectable in normal somatic cells, suggesting that it plays an important role in cellular immortalization and tumorigenesis. As most leukemic cells are generally telomerase-positive and have often shortened telomeres, our understanding of how telomerase is deregulated in these diseases could help to define novel therapies targeting the telomere/telomerase complex. Nonetheless, considering that normal hematopoietic stem cells and some of their progeny do express a functional telomerase, it is tempting to consider such an activity in leukemias as a sustained stemness feature and important to understand how telomere length and telomerase activity are regulated in the various forms of leukemias.

A human telomerase holoenzyme protein required for Cajal body localization and telomere synthesis

Telomerase is a ribonucleoprotein (RNP) complex that synthesizes telomere repeats in tissue progenitor cells and cancer cells. Active human telomerase consists of at least three principal subunits, including the telomerase reverse transcriptase, the telomerase RNA (TERC), and dyskerin. Here, we identify a holoenzyme subunit, TCAB1 (telomerase Cajal body protein 1), that is notably enriched in Cajal bodies, nuclear sites of RNP processing that are important for telomerase function. TCAB1 associates with active telomerase enzyme, established telomerase components, and small Cajal body RNAs that are involved in modifying splicing RNAs. Depletion of TCAB1 by using RNA interference prevents TERC from associating with Cajal bodies, disrupts telomerase-telomere association, and abrogates telomere synthesis by telomerase. Thus, TCAB1 controls telomerase trafficking and is required for telomere synthesis in human cancer cells.

Structure and functional studies of the CS domain of the essential H/ACA ribonucleoparticle assembly protein SHQ1

H/ACA ribonucleoprotein particles are essential for ribosomal RNA and telomerase RNA processing and metabolism. Shq1p has been identified as an essential eukaryotic H/ACA small nucleolar (sno) ribonucleoparticle (snoRNP) biogenesis and assembly factor. Shq1p is postulated to be involved in the early biogenesis steps of H/ACA snoRNP complexes, and Shq1p depletion leads to a specific decrease in H/ACA small nucleolar RNA levels and to defects in ribosomal RNA processing. Shq1p contains two predicted domains as follows: an N-terminal CS (named after CHORD-containing proteins and SGT1) or HSP20-like domain, and a C-terminal region of high sequence homology called the Shq1 domain. Here we report the crystal structure and functional studies of the Saccharomyces cerevisiae Shq1p CS domain. The structure consists of a compact anti-parallel beta-sandwich fold that is composed of two beta-sheets containing four and three beta-strands, respectively, and a short alpha-helix. Deletion studies showed that the CS domain is required for the essential functions of Shq1p. Point mutations in residues Phe-6, Gln-10, and Lys-80 destabilize Shq1p in vivo and induce a temperature-sensitive phenotype with depletion of H/ACA small nucleolar RNAs and defects in rRNA processing. Although CS domains are frequently found in co-chaperones of the Hsp90 molecular chaperone, no interaction was detected between the Shq1p CS domain and yeast Hsp90 in vitro. These results show that the CS domain is essential for Shq1p function in H/ACA snoRNP biogenesis in vivo, possibly in an Hsp90-independent manner.

The Cajal body

The Cajal body, originally identified over 100 years ago as a nucleolar accessory body in neurons, has come to be identified with nucleoplasmic structures, often quite tiny, that contain coiled threads of the marker protein, coilin. The interaction of coilin with other proteins appears to increase the efficiency of several nuclear processes by concentrating their components in the Cajal body. The best-known of these processes is the modification and assembly of U snRNPs, some of which eventually form the RNA splicing machinery, or spliceosome. Over the last 10 years, research into the function of Cajal bodies has been greatly stimulated by the discovery that SMN, the protein deficient in the inherited neuromuscular disease, spinal muscular atrophy, is a Cajal body component and has an essential role in the assembly of spliceosomal U snRNPs in the cytoplasm and their delivery to the Cajal body in the nucleus.

Nucleolar localization of TERT is unrelated to telomerase function in human cells

Telomerase maintains telomere length and has been implicated in both aging and carcinogenesis of human cells. This enzyme is a specialized ribonucleoprotein (RNP) complex, minimally consisting of two essential components: the protein catalytic subunit TERT (telomerase reverse transcriptase) and the integral RNA moiety TR (telomerase RNA, TERC). Both TERT and TR have been found to localize to nucleoli within the nucleus, leading to the suggestion of nucleoli as the site for telomerase RNP biogenesis in human cells. However, whether this statement is true or not has not yet been convincingly demonstrated. Here, we identify that residues 965-981 of the human TERT polypeptide constitute an active nucleolar-targeting signal (NTS) essential for mediating human TERT nucleolar localization. Mutational inactivation of this NTS completely disrupted TERT nucleolar translocation in both normal and malignant human cells. Most interestingly, such a TERT mutant still retained the capacity to activate telomerase activity, maintain telomere length and extend the life-span of cellular proliferation, as does wild-type TERT, in BJ cells (normal fibroblasts). Therefore, our data suggest that TERT nucleolar localization is unrelated to telomerase function in human cells.

Insight into the Protein Components of the Box H/ACA RNP

Among eukaryotic organisms a vast majority of Box H/ACA ribonucleoproteins (RNPs) are responsible for the post-transcriptional introduction of pseudouridine (Psi) into ribosomal RNAs (rRNA) and spliceosomal small nuclear RNAs (snRNA), thus influencing protein translation and pre-mRNA splicing, respectively. Additionally, a few distinct Box H/ACA RNPs are involved in the processing of rRNA, and the stabilization of vertebrate telomerase RNA. Thus, whether directly or indirectly, Box H/ACA RNPs impact major steps of gene expression, as well as play a role in maintaining genome integrity. Box H/ACA RNPs each consist of a unique Box H/ACA RNA and a set of four common core proteins. While the RNA component is responsible for dictating site-specificity, the four core proteins impact numerous aspects of RNP function including both stability and catalytic potential. Interestingly, mutations have been identified in the core proteins of the Box H/ACA RNP, resulting in a rare inherited bone marrow failure syndrome referred to as dyskeratosis congenita. This review discusses our current understanding of the roles of the protein components of the Box H/ACA RNP, and provides a framework to understand how mutations in the Box H/ACA RNP contribute to disease pathology.

Telomerase reverse transcriptase is required for the localization of telomerase RNA to cajal bodies and telomeres in human cancer cells

Telomere maintenance by telomerase is critical for the unlimited division potential of most human cancer cells. The two essential components of human telomerase, telomerase RNA (hTR) and telomerase reverse transcriptase (hTERT), are recruited from distinct subnuclear sites to telomeres during S phase. Throughout the remainder of the cell cycle hTR is found primarily in Cajal bodies. The localization of hTR to Cajal bodies and telomeres is specific to cancer cells where telomerase is active and is not observed in primary cells. Here we show that the trafficking of hTR to both telomeres and Cajal bodies depends on hTERT. RNA interference-mediated depletion of hTERT in cancer cells leads to loss of hTR from both Cajal bodies and telomeres without affecting hTR levels. In addition, expression of hTERT in telomerase-negative cells (including primary and ALT cancer cell lines) induces hTR to localize to both sites. Factors that did not stimulate hTR localization in our experiments include increased hTR RNA levels and Cajal body numbers, and expression of SV40 large T antigen and oncogenic Ras. Our findings suggest that the trafficking of telomerase to Cajal bodies and telomeres in cancer cells correlates with and depends on the assembly of the enzyme.

Proteome alteration induced by hTERT transfection of human fibroblast cells

Background

Telomerase confers cellular immortality by elongating telomeres, thereby circumventing the Hayflick limit. Extended-life-span cells have been generated by transfection with the human telomerase reverse transcriptase (hTERT) gene. hTERT transfected cell lines may be of outstanding interest to monitor the effect of drugs targeting the telomerase activity. The incidence of hTERT gene transfection at the proteome level is a prerequisite to that purpose. The effect of the transfection has been studied on the proteome of human fibroblast (WI38). Cytosolic and nuclear fractions of WI38 cells, empty vector transfected WI38 (WI38-HPV) and hTERT WI38 cells were submitted to a 2D-DIGE (Two-Dimensional Differential In-Gel Electrophoresis) analysis. Only spots that had a similar abundance in WI38 and WI38-HPV, but were differentially expressed in WI38 hTERT were selected for MS identification. This method directly points to the proteins linked with the hTERT expression. Number of false positive differentially expressed proteins has been excluded by using control WI38-HPV cells. The proteome alteration induced by hTERT WI38 transfection should be taken into account in subsequent use of the cell line for anti-telomerase drugs evaluation.

Results

2D-DIGE experiment shows that 57 spots out of 2246 are significantly differentially expressed in the cytosolic fraction due to hTERT transfection, and 38 were confidently identified. In the nuclear fraction, 44 spots out of 2172 were selected in the differential proteome analysis, and 14 were identified. The results show that, in addition to elongating telomeres, hTERT gene transfection has other physiological roles, among which an enhanced ER capacity and a potent cell protection against apoptosis.

Conclusion

We show that the methodology reduces the complexity of the proteome analysis and highlights proteins implicated in other processes than telomere elongation. hTERT induced proteome changes suggest that telomerase expression enhances natural cell repair mechanisms and stress resistance probably required for long term resistance of immortalized cells. Thus, hTERT transfected cells can not be only consider as an immortal equivalent to parental cells but also as cells which are over-resistant to stresses. These findings are the prerequisite for any larger proteomics aiming to evaluate anti-telomerase drugs proteome alteration and thus therapeutics induced cell reactions.

The role of the plant nucleolus in pre-mRNA processing

The nucleolus is a multifunctional compartment of the eukaryotic nucleus. Besides its well-recognised role in transcription and processing of ribosomal RNA and the assembly of ribosomal subunits, the nucleolus has functions in the processing and assembly of a variety of RNPs and is involved in cell cycle control and senescence and as a sensor of stress. Historically, nucleoli have been tenuously linked to the biogenesis and, in particular, export of mRNAs in yeast and mammalian cells. Recently, data from plants have extended the functions in which the plant nucleolus is involved to include transcriptional gene silencing as well as mRNA surveillance and nonsense-mediated decay, and mRNA export. The nucleolus in plants may therefore have important roles in the biogenesis and quality control of mRNAs.

Physiological assembly and activity of human telomerase complexes

Telomerase is a specialized reverse transcriptase conserved throughout almost all eukaryotic life. It plays a fundamental role in genome maintenance, adding back the telomeric DNA repeats lost from chromosome ends due to incomplete replication or damage. The protein and RNA subunits of telomerase fold and function in a co-dependent manner to establish a high fidelity of telomeric repeat synthesis. Over the past two decades, studies of telomerase have uncovered previously unanticipated levels of complexity in its assembly, activity and regulation. This review describes the current understanding of telomerase in humans, with particular focus on telomerase biogenesis and regulation in its cellular context.

Human telomerase RNA accumulation in Cajal bodies facilitates telomerase recruitment to telomeres and telomere elongation

Telomerase is required for telomere maintenance and is responsible for the immortal phenotype of cancer cells. How telomerase is assembled and reaches telomeres in the context of nuclear architecture is not understood. Recently, the telomerase RNA subunit (hTR) was shown to accumulate in Cajal bodies (CBs), subnuclear structures implicated in ribonucleoprotein maturation. However, the functional relevance of this localization for telomerase was unknown. hTR localization to CBs requires a short sequence motif called the CAB box. Here, we reconstitute telomerase in human cells and determine the effects of CAB box mutations on telomere biology. We demonstrate that mutant hTR, which fails to accumulate in CBs, is fully capable of forming catalytically active telomerase in vivo but is strongly impaired in telomere extension. The functional deficiency is accompanied by a decreased association of telomerase with telomeres. Collectively, these data identify subnuclear localization as an important regulatory mechanism for telomere length homeostasis in human cells.

Structural and functional characterization of human telomerase RNA processing and cajal body localization signals

The RNA component of human telomerase (hTR) includes H/ACA and CR7 domains required for 3' end processing, localization, and accumulation. The terminal loop of the CR7 domain contains the CAB box (ugAG) required for targeting of scaRNAs to Cajal bodies (CB) and an uncharacterized sequence required for accumulation and processing. To dissect out the contributions of the CR7 stem loop to hTR processing and localization, we solved the solution structures of the 3' terminal stem loops of hTR CR7 and U64 H/ACA snoRNA, and the 5' terminal stem loop of U85 C/D-H/ACA scaRNA. These structures, together with analysis of localization, processing, and accumulation of hTRs containing nucleotide substitutions in the CR7 domain, identified the sequence and structural requirements of the hTR processing and CB localization signals and showed that these signals are functionally independent. Further, 3' end processing was found to be a prerequisite for translocation of hTR to CBs.

Noncoding RNAs of the H/ACA family

The H/ACA RNAs are an abundant family of trans-acting, noncoding RNAs found in eukaryotes and archaea. More than 100 H/ACA RNAs are known to exist in humans. The function of the majority of the identified H/ACA RNAs is to guide sites-pecific pseudouridylation of ribosomal RNA. In eukaryotes, H/ACA RNAs also mediate the processing of pre-rRNA, provide the template for telomere synthesis, and guide pseudouridylation of other classes of target RNAs (e.g., small nuclear RNAs [snRNAs]). Thus, currently, the H/ACA RNAs are known to be integrally involved in the production of both ribosomes and spliceosomes, and in the maintenance of chromosome integrity. In addition, dozens of H/ACA RNAs have been identified for which no function has yet been determined. The H/ACA RNAs select and present substrate molecules via base pairing. All H/ACA RNAs contain conserved sequence elements (box H and box ACA) and assemble with a core set of four proteins to form functional ribonucleoprotein complexes (RNPs). Mutations in key RNA and protein components of H/ACA RNPs result in dyskeratosis congenita, a serious multisystem genetic disease. Impressive progress has been made very recently in understanding the biogenesis, trafficking, and function of H/ACA RNPs.

Expression and suppression of human telomerase RNA

Telomeres are maintained by the ribonucleoprotein (RNP) enzyme telomerase, which replenishes telomeres through its unique mechanism of internal RNA-templated addition of telomeric DNA. Telomerase is active in most human cancers, typically because its core protein subunit, TERT, is up-regulated. Although the major known function of telomerase in cancer is to replenish telomeric DNA and maintain cell immortality, the regulation of the RNA component of telomerase is not well understood. In the course of investigations that have implicated telomerase RNA in key aspects of cancer progression, including metastasis, we explored some of the cis-acting elements affecting telomerase RNA expression and knockdown. The expression efficiency and subsequent RNA processing to produce the mature hTER differed considerably among various promoters. Together with other results, these findings establish that the crucial elements of the hTER gene affecting RNA-processing efficiency to produce the mature hTER RNA are the promoter and internal telomerase RNA-coding sequences.

An unconventional human Ccr4-Caf1 deadenylase complex in nuclear cajal bodies

mRNA deadenylation is a key process in the regulation of translation and mRNA turnover. In Saccharomyces cerevisiae, deadenylation is primarily carried out by the Ccr4p and Caf1p/Pop2p subunits of the Ccr4-Not complex, which is conserved in eukaryotes including humans. Here we have identified an unconventional human Ccr4-Caf1 complex containing hCcr4d and hCaf1z, distant human homologs of yeast Ccr4p and Caf1p/Pop2p, respectively. The hCcr4d-hCaf1z complex differs from conventional Ccr4-Not deadenylase complexes, because (i) hCaf1z and hCcr4d concentrate in nuclear Cajal bodies and shuttle between the nucleus and cytoplasm and (ii) the hCaf1z subunit, in addition to rapid deadenylation, subjects substrate RNAs to slow exonucleolytic degradation from the 3' end in vitro. Exogenously expressed hCaf1z shows both of those activities on reporter mRNAs in human HeLa cells and stimulates general mRNA decay when restricted to the cytoplasm by deletion of its nuclear localization signal. These observations suggest that the hCcr4d-hCaf1z complex may function either in the nucleus or in the cytoplasm after its nuclear export, to degrade polyadenylated RNAs, such as mRNAs, pre-mRNAs, or those RNAs that are polyadenylated prior to their degradation in the nucleus.

Dyskerin expression influences the level of ribosomal RNA pseudo-uridylation and telomerase RNA component in human breast cancer

Dyskerin is a nucleolar protein, altered in dyskeratosis congenita, which carries out two separate functions, both fundamental for proliferating cells. One function is the pseudo-uridylation of ribosomal RNA (rRNA) molecules, necessary for their processing, and the other is the stabilization of the telomerase RNA component, necessary for telomerase activity. A significant feature of dyskeratosis congenita is an increased susceptibility to cancer; so far, however, no data have been reported on dyskerin changes in human tumours. Therefore, in this study, the distribution of dyskerin in a large series of human tumours from the lung, breast, and colon, as well as from B-cell lymphomas, was analysed by immunohistochemistry. Dyskerin proved never to be lost or delocalized outside the nucleolus. A quantitative analysis of dyskerin mRNA expression was then performed in 70 breast carcinomas together with the evaluation of telomerase RNA component levels and rRNA pseudo-uridylation. Dyskerin mRNA levels were highly variable and directly associated with both telomerase RNA component levels and rRNA pseudo-uridylation. Dyskerin gene silencing in the MCF-7 human breast carcinoma cell line reduced telomerase activity and rRNA pseudo-uridylation. Significantly, patients with low dyskerin expression were characterized by a better clinical outcome than those with a high dyskerin level. These data indicate that dyskerin is not lost in human cancers and that the levels of its expression and function are associated with tumour progression.

The biogenesis and regulation of telomerase holoenzymes

Chromosome stability requires a dynamic balance of DNA loss and gain in each terminal tract of telomeric repeats. Repeat addition by a specialized reverse transcriptase, telomerase, has an important role in maintaining this equilibrium. Insights that have been gained into the cellular pathways for biogenesis and regulation of telomerase ribonucleoproteins raise new questions, particularly concerning the dynamic nature of this unique polymerase.

Immunodetection of human telomerase reverse-transcriptase (hTERT) re-appraised: nucleolin and telomerase cross paths

The involvement of telomerase in cellular immortalization and senescence has often been assessed by means of telomerase expression at the RNA level and quantification of telomerase activity by the telomeric repeat amplification protocol assay. However, these methods either neglected the existence of various telomerase splice variants, or ignored the nonconventional functions of telomerase independent of its ability to elongate and maintain telomere length. Immunodetection of telomerase is now being recognized as a necessary approach to precisely elucidate its roles in oncogenesis and senescence. A few antibodies directed against the catalytic subunit of the human telomerase (hTERT) are currently used but their specificity is not always demonstrated. A survey of the literature showed inconsistencies and led us to comparatively re-evaluate the most frequently used antibodies. Surprisingly, mass spectrometry, two-dimensional gel analysis and immunofluorescent experiments revealed that the most frequently used hTERT immunoprobe, a mouse monoclonal antibody that was claimed to be directed against an hTERT protein epitope, in fact recognizes nucleolin rather than telomerase. Our findings have interesting implications regarding the biology of nucleolin and telomerase in the context of pathophysiological investigations recently carried out.

hTERT immunopositivity patterns in the normal brain and in astrocytic tumors

Accumulating data about the impact of hTERT in astrocytic tumor carcinogenesis and recent evidence about its association with disease outcome prompt the evaluation of this molecule with methods applicable in routine pathology practice. In this study, we investigated hTERT protein expression with immunohistochemistry (IHC) and the NCL-hTERT antibody in 49 astrocytic tumors. Results were validated with the assessment of hTERT mRNA (relative quantification, identification of splice variants, in situ hybridization). Specific nuclear hTERT immunostaining patterns (IPs) were characterized as patterns As (single large dot) and Am (multiple dots) without nucleoplasm staining and pattern B (nucleoplasm staining with or without dots), corresponding to low and high relative hTERT expression values (P<0.0001). Low- and high-grade astrocytic tumors were found positive for hTERT in 74 and 85% of cases, respectively. Heterogeneity in the distribution of hTERT-positive cells was observed in all tumors. The prevailing nuclear IPs differed significantly between pilocytic astrocytomas (pattern As) and the rest of histologic types up to glioblastoma (patterns Am and B) (P<0.0001). The described nuclear IPs were also observed in non-neoplastic cells. Positive endothelial cells were found in astrocytic tumors of all grades, even when tumor cells showed no hTERT immunoreactivity. A subset of mature normal neurons was positive for hTERT (pattern As), suggesting a role for this molecule in neuronal maintenance in the adult brain. The nuclear hTERT IPs described here may reflect the functional status of non-neoplastic brain and neoplastic astrocytic cells and support the model of a continuum in the development of glioblastomas from diffuse fibrillary astrocytomas.

Pumping RNA: nuclear bodybuilding along the RNP pipeline

Cajal bodies (CBs) are nuclear subdomains involved in the biogenesis of several classes of small ribonucleoproteins (RNPs). A number of recent advances highlight progress in the understanding of the organization and dynamics of CB components. For example, a class of small Cajal body-specific (sca) RNPs has been discovered. Localization of scaRNPs to CBs was shown to depend on a conserved RNA motif. Intriguingly, this motif is also present in mammalian telomerase RNA and the evidence suggests that assembly of the active form of telomerase RNP occurs in and around CBs during S phase. Important steps in the assembly and modification of spliceosomal RNPs have also been shown to take place in CBs. Additional experiments have revealed the existence of kinetically distinct subclasses of CB components. Finally, the recent identification of novel markers for CBs in both Drosophila and Arabidopsis not only lays to rest questions about the evolutionary conservation of these nuclear suborganelles, but also should enable forward genetic screens for the identification of new components and pathways involved in their assembly, maintenance and function.

A distant coilin homologue is required for the formation of cajal bodies in Arabidopsis

Cajal bodies (CBs) are subnuclear bodies that are widespread in eukaryotes, being found in mammals, many other vertebrates and in all plant species so far examined. They are mobile structures, moving, fusing, and budding within the nucleus. Here we describe a screen for Arabidopsis mutants with altered CBs and describe mutants that have smaller Cajal bodies (ncb-2, ncb-3), lack them altogether (ncb-1), have increased numbers of CBs (pcb) or have flattened CBs (ccb). We have identified the gene affected in the ncb mutants as a distant homolog of the vertebrate gene that encodes coilin (At1g13030) and have termed the resulting protein Atcoilin. A T-DNA insertional mutant in this gene (ncb-4) also lacks Cajal bodies. Overexpression of Atcoilin cDNA in ncb-1 restores Cajal bodies, which recruit U2B'' as in the wild type, but which are, however, much larger than in the wild type. Thus we have shown that At1g13030 is required for Cajal body formation in Arabidopsis, and we hypothesize that the level of its expression is correlated with Cajal body size. The Atcoilin gene is unaffected in pcb and ccb, suggesting that other genes can also affect CBs.

Nuclear localization and dynamic properties of the Marek's disease virus oncogene products Meq and Meq/vIL8

Marek's disease virus (MDV) is an avian herpesvirus that causes T-cell lymphomas and immune suppression in susceptible chickens. At least one gene product, MDV Eco Q-encoded protein (Meq), is essential for the oncogenicity of MDV. Alternative splicing permits the meq gene to give rise to two major transcripts encoding proteins designated Meq and Meq/vIL8. Meq is a basic leucine zipper protein capable of modulating transcription. The Meq/vIL8 protein retains a modified leucine zipper, along with the mature receptor-binding portion of vIL8, but lacks the domain of Meq responsible for transcriptional modulation. In this report, we describe studies using fusions between either Meq or Meq/vIL8 and fluorescent proteins to characterize the distribution and properties of these products in chicken embryo fibroblasts (CEFs). Meq and Meq/vIL8 both localized to the nucleoplasm, nucleoli, and Cajal bodies of transfected cells. Similar distributions were found for fluorescent fusion proteins and native Meq or Meq/vIL8. Fluorescence recovery after photobleaching and photoactivatable green fluorescent protein revealed that Meq exhibited mobility properties similar to those of other transcription factors, while Meq/vIL8 was far less mobile. In addition, fluorescence resonance energy transfer studies indicated the formation of Meq/vIL8 homodimers in CEFs. Time lapse studies revealed the coordinated elimination of a portion of Meq and Meq/vIL8 from the nucleus. Our data provide new insight regarding the dynamic cellular properties of two forms of a herpesvirus-encoded oncoprotein and suggest that these forms may have fundamentally different functions in MDV-infected cells.

Human telomerase and Cajal body ribonucleoproteins share a unique specificity of Sm protein association

Cajal bodies are nuclear structures that host RNA modification and assembly reactions. Some RNAs transit Cajal bodies, while others must concentrate in Cajal bodies to function. Here we report that at least a subfraction of human telomerase RNA and individual resident Cajal body RNAs is associated with Sm proteins. Surprisingly, of seven Sm proteins assembled into a heteroheptameric ring, only a subset copurifies telomerase and Cajal body ribonucleoproteins. We show that a Cajal body RNA localization motif determines this specificity. These discoveries expand the cellular repertoire of Sm protein assemblies and their involvement in ribonucleoprotein localization and function.

Subnuclear organelles: new insights into form and function

The cell nucleus is a complex and highly dynamic environment with many functionally specialized regions of substructure that form and maintain themselves in the absence of membranes. Relatively little is known about the basic physical properties of the nuclear interior or how domains within the nucleus are structurally and functionally organized and interrelated. Here, we summarize recent data that shed light on the structural and functional properties of three prominent subnuclear organelles--nucleoli, Cajal bodies (CBs) and speckles. We discuss how these findings impact our understanding of the guiding principles of nuclear organization and various types of human disease.

Biogenesis and intranuclear trafficking of human box C/D and H/ACA RNPs

Box C/D and H/ACA snoRNAs represent two abundant groups of small noncoding RNAs. The majority of box C/D and H/ACA snoRNAs function as guide RNAs in the site-specific 2'-O-methylation and pseudouridylation of rRNAs, respectively. The box C/D snoRNAs associate with fibrillarin, Nop56, Nop58, and 15.5K/NHPX proteins to form functional snoRNP particles, whereas all box H/ACA snoRNAs form complexes with the dyskerin, Nop10, Nhp2, and Gar1 snoRNP proteins. Recent studies demonstrate that the biogenesis of mammalian snoRNPs is a complex process that requires numerous trans-acting factors. Most vertebrate snoRNAs are posttranscriptionally processed from pre-mRNA introns, and the early steps of snoRNP assembly are physically and functionally coupled with the synthesis or splicing of the host pre-mRNA. The maturing snoRNPs follow a complicated intranuclear trafficking process that is directed by transport factors also involved in nucleocytoplasmic RNA transport. The human telomerase RNA (hTR) carries a box H/ACA RNA domain that shares a common Cajal-body-specific localization element with a subclass of box H/ACA RNAs, which direct pseudouridylation of spliceosomal snRNAs in the Cajal body. However, besides concentrating in Cajal bodies, hTR also accumulates at a small, structurally distinct subset of telomeres during S phase. This suggests that a cell-cycle-dependent, dynamic localization of hTR to telomeres may play an important regulatory role in human telomere synthesis.

Cell cycle-regulated trafficking of human telomerase to telomeres

Telomerase synthesizes telomeres at the ends of human chromosomes during S phase. The results presented here suggest that telomerase activity may be regulated by intranuclear trafficking of the key components of the enzyme in human cells. We examined the subcellular localization of endogenous human telomerase RNA (hTR) and telomerase reverse transcriptase (hTERT) in HeLa cervical carcinoma cells. Throughout most of the cell cycle, we found that the two essential components of telomerase accumulate at intranuclear sites separate from telomeres. However, during S phase, both hTR and hTERT are specifically recruited to subsets of telomeres. The localization of telomerase to telomeres is dynamic, peaking at mid-S phase. We also found complex associations of both hTR and hTERT with nucleoli and Cajal bodies during S phase, implicating both structures in the biogenesis and trafficking of telomerase. Our results mark the first observation of human telomerase at telomeres and provide a mechanism for the cell cycle-dependent regulation of telomere synthesis in human cells.

Cell cycle-dependent recruitment of telomerase RNA and Cajal bodies to human telomeres

Telomerase is a ribonucleoprotein enzyme that counteracts replicative telomere erosion by adding telomeric sequence repeats onto chromosome ends. Despite its well-established role in telomere synthesis, telomerase has not yet been detected at telomeres. The RNA component of human telomerase (hTR) resides in the nucleoplasmic Cajal bodies (CBs) of interphase cancer cells. Here, in situ hybridization demonstrates that in human HeLa and Hep2 S phase cells, besides accumulating in CBs, hTR specifically concentrates at a few telomeres that also accumulate the TRF1 and TRF2 telomere marker proteins. Surprisingly, telomeres accumulating hTR exhibit a great accessibility for in situ oligonucleotide hybridization without chromatin denaturation, suggesting that they represent a structurally distinct, minor subset of HeLa telomeres. Moreover, we demonstrate that more than 25% of telomeres accumulating hTR colocalize with CBs. Time-lapse fluorescence microscopy demonstrates that CBs moving in the nucleoplasm of S phase cells transiently associate for 10-40 min with telomeres. Our data raise the intriguing possibility that CBs may deliver hTR to telomeres and/or may function in other aspects of telomere maintenance.

Cajal bodies: a long history of discovery

This review surveys what is known about the structure and function of the subnuclear domains called Cajal bodies (CBs). The major focus is on CBs in mammalian cells but we provide an overview of homologous CB structures in other organisms. We discuss the protein and RNA components of CBs, including factors recently found to associate in a cell cycle-dependent fashion or under specific metabolic or stress conditions. We also consider the dynamic properties of both CBs and their molecular components, based largely on recent data obtained thanks to the advent of improved in vivo detection and imaging methods. We discuss how these data contribute to an understanding of CB functions and highlight major questions that remain to be answered. Finally, we consider the interesting links that have emerged between CBs and alterations in nuclear structure apparent in a range of human pathologies, including cancer and inherited neurodegenerative diseases. We speculate on the relationship between CB function and molecular disease.

Tim50a, a nuclear isoform of the mitochondrial Tim50, interacts with proteins involved in snRNP biogenesis

Background

The Cajal body (CB) is a nuclear suborganelle involved in the biogenesis of small nuclear ribonucleoproteins (snRNPs), which are vital for pre-mRNA splicing. Newly imported Sm-class snRNPs traffic through CBs, where the snRNA component of the snRNP is modified, and then target to other nuclear domains such as speckles and perichromatin fibrils. It is not known how nascent snRNPs localize to the CB and are released from this structure after modification. The marker protein for CBs, coilin, may play a role in snRNP biogenesis given that it can interact with snRNPs and SMN, the protein mutated in Spinal Muscular Atrophy. Loss of coilin function in mice leads to significant viability and fertility problems and altered CB formation.

Results

In this report, we identify a minor isoform of the mitochondrial Tim50, Tim50a, as a coilin interacting protein. The Tim50a transcript can be detected in some cancer cell lines and normal brain tissue. The Tim50a protein differs only from Tim50 in that it contains an additional 103 aa N-terminal to the translation start of Tim50. Importantly, a putative nuclear localization signal is found within these 103 residues. In contrast to Tim50, which localizes to the cytoplasm and mitochondria, Tim50a is strictly nuclear and is enriched in speckles with snRNPs. In addition to coilin, Tim50a interacts with snRNPs and SMN. Competition binding experiments demonstrate that coilin competes with Sm proteins of snRNPs and SMN for binding sites on Tim50a.

Conclusion

Tim50a may play a role in snRNP biogenesis given its cellular localization and protein interaction characteristics. We hypothesize that Tim50a takes part in the release of snRNPs and SMN from the CB.

The many facets of H/ACA ribonucleoproteins

The H/ACA ribonucleoproteins (RNPs) are known as one of the two major classes of small nucleolar RNPs. They predominantly guide the site-directed pseudouridylation of target RNAs, such as ribosomal and spliceosomal small nuclear RNAs. In addition, they process ribosomal RNA and stabilize vertebrate telomerase RNA. Taken together, the function of H/ACA RNPs is essential for ribosome biogenesis, pre-mRNA splicing, and telomere maintenance. Every cell contains 100-200 different species of H/ACA RNPs, each consisting of the same four core proteins and one function-specifying H/ACA RNA. Most of these RNPs reside in nucleoli and Cajal bodies and mediate the isomerization of specific uridines to pseudouridines. Catalysis of the reaction is mediated by the putative pseudouridylase NAP57 (dyskerin, Cbf5p). Unexpectedly, mutations in this housekeeping enzyme are the major determinants of the inherited bone marrow failure syndrome dyskeratosis congenita. This review details the many diverse functions of H/ACA RNPs, some yet to be uncovered, with an emphasis on the role of the RNP proteins. The multiple functions of H/ACA RNPs appear to be reflected in the complex phenotype of dyskeratosis congenita.

A novel EB-1/AIDA-1 isoform, AIDA-1c, interacts with the Cajal body protein coilin

Background

Cajal bodies (CBs) are nuclear suborganelles that play a role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), which are crucial for pre-mRNA splicing. Upon nuclear reentry, Sm-class snRNPs localize first to the CB, where the snRNA moiety of the snRNP is modified. It is not clear how snRNPs target to the CB and are released from this structure after their modification. Coilin, the CB marker protein, may participate in snRNP biogenesis given that it can interact with snRNPs and SMN. SMN is crucial for snRNP assembly and is the protein mutated in the neurodegenerative disease Spinal Muscular Atrophy. Coilin knockout mice display significant viability problems and altered CB formation. Thus characterization of the CB and its associated proteins will give insight into snRNP biogenesis and clarify the dynamic organization of the nucleus.

Results

In this report, we identify a novel protein isoform of EB-1/AIDA-1, termed AIDA-1c, that interacts with the CB marker protein, coilin. Northern and nested PCR experiments reveal that the AIDA-1c isoform is expressed in brain and several cancer cell lines. Competition binding experiments demonstrate that AIDA-1c competes with SmB' for coilin binding sites, but does not bind SMN. When ectopically expressed, AIDA-1c is predominantly nuclear with no obvious accumulations in CBs. Interestingly, another EB-1/AIDA-1 nuclear isoform, AIDA-1a, does not bind coilin in vivo as efficiently as AIDA-1c. Knockdown of EB-1/AIDA-1 isoforms by siRNA altered Cajal body organization and reduced cell viability.

Conclusion

These data suggest that specific EB-1/AIDA-1 isoforms, such as AIDA-1c, may participate in the regulation of nucleoplasmic coilin protein interactions in neuronal and transformed cells.

Functional characterization of telomerase RNA variants found in patients with hematologic disorders

Human telomerase uses a specific cellular RNA, called hTERC, as the template to synthesize telomere repeats at chromosome ends. Approximately 10% to 15% of patients with aplastic anemia or other bone marrow failure syndromes are carriers of hTERC sequence variants whose functional significance, in most cases, is unknown. We screened 10 reported and 2 newly discovered hTERC variants from such patients and found that 10 of these negatively affected telomerase enzymatic function when they were used to reconstitute telomerase enzymatic function in human cells. Most functional deficits were due to perturbations of hTERC secondary structure and correlated well with the degrees of telomere shortening and reduced telomerase activity observed in peripheral blood lymphocytes of the representative patients. We also found no evidence of dominant-negative activity in any of the mutants. Therefore, loss of telomerase activity and of telomere maintenance resulting from inherited hTERC mutations may limit marrow stem cell renewal and predispose some patients to bone marrow failure.

A simple method allowing DIC imaging in conjunction with confocal microscopy

Current optical methods to collect Nomarski differential interference contrast (DIC) or phase images with a transmitted light detector (TLD) in conjunction with confocal laser scanning microscopy (CLSM) can be technically challenging and inefficient. We describe for the first time a simple method that combines the use of the commercial product QPm (Iatia, Melbourne Australia) with brightfield images collected with the TLD of a CLSM, generating DIC, phase, Zernike phase, dark-field or Hoffman modulation contrast images. The brightfield images may be collected at the same time as the confocal images. This method also allows the calculation of contrast-enhanced images from archival data. The technique described here allows for the creation of contrast-enhanced images such as DIC or phase, without compromising the intensity or quality of confocal images collected simultaneously. Provided the confocal microscope is equipped with a motorized z-drive and a TLD, no hardware or optical modifications are required. The contrast-enhanced images are calculated with software using the quantitative phase-amplitude microscopy technique (Barone-Nugent et al., 2002). This technique, being far simpler during image collection, allows the microscopist to concentrate on their confocal imaging and experimental procedures. Unlike conventional DIC, this technique may be used to calculate DIC images when cells are imaged through plastic, and without the use of expensive strain-free objective lenses.

The unusually large Plasmodium telomerase reverse-transcriptase localizes in a discrete compartment associated with the nucleolus

Telomerase replicates chromosome ends, a function necessary for maintaining genome integrity. We have identified the gene that encodes the catalytic reverse transcriptase (RT) component of this enzyme in the malaria parasite Plasmodium falciparum (PfTERT) as well as the orthologous genes from two rodent and one simian malaria species. PfTERT is predicted to encode a basic protein that contains the major sequence motifs previously identified in known telomerase RTs (TERTs). At ∼2500 amino acids, PfTERT is three times larger than other characterized TERTs. We observed remarkable sequence diversity between TERT proteins of different Plasmodial species, with conserved domains alternating with hypervariable regions. Immunofluorescence analysis revealed that PfTERT is expressed in asexual blood stage parasites that have begun DNA synthesis. Surprisingly, rather than at telomere clusters, PfTERT typically localizes into a discrete nuclear compartment. We further demonstrate that this compartment is associated with the nucleolus, hereby defined for the first time in P.falciparum.

Telomerase and cancer stem cells

Telomerase is critical for the integrity of stem cell compartments. Mutations in telomerase components lead to telomere shortening and hematopoietic stem cell failure in autosomal dominant dyskeratosis congenita and aplastic anemia. Telomerase activity is readily detected in most cancers but not in adult somatic cells. The telomere hypothesis for cancer states that telomerase is reactivated in late stages of carcinogenesis. However, recent evidence has suggested a stem cell origin for certain cancers, implying that the genetic alterations that lead to cancer accumulate in tissue-specific stem cells and not in adult somatic cells. In these cancers, stem cells would already have telomerase and it would not need to be reactivated. Here, we reconsider the telomere hypothesis in view of this evidence and propose that, rather than telomerase reactivation, enzyme activity may increase in later stages of carcinogenesis due to increased expression or efficient assembly of telomerase components. Understanding these mechanisms will refine approaches to telomerase inhibition in cancer.

PHAX and CRM1 are required sequentially to transport U3 snoRNA to nucleoli

To better understand intranuclear-targeting mechanisms, we have studied the transport of U3 snoRNA in human cells. Surprisingly, we found that PHAX, the snRNA export adaptor, is highly enriched in complexes containing m7G-capped U3 precursors. In contrast, the export receptor CRM1 is predominantly bound to TMG-capped U3 species. In agreement, PHAX does not export m7G-capped U3 precursors because their caps become hypermethylated in the nucleus. Inactivation of PHAX and CRM1 shows that U3 first requires PHAX to reach Cajal bodies, and then CRM1 to be routed from there to nucleoli. Furthermore, PHAX also binds the precursors of U8 and U13 box C/D snoRNAs and telomerase RNA. PHAX was previously shown to discriminate between small versus large RNAs during export. Our data indicate that the role of PHAX in determining the identity of small RNAs extends to nonexported species, and this appears critical to promote their transport within the nucleus.

Nucleolin interacts with telomerase

Telomerase is a specialized reverse transcriptase composed of core RNA and protein subunits which plays essential roles in maintaining telomeres in actively dividing cells. Recent work indicates that telomerase shuttles between subcellular compartments during assembly and in response to specific stimuli. In particular, telomerase colocalizes with nucleoli in normal human fibroblasts. Here, we show that nucleolin, a major nucleolar phosphoprotein, interacts with telomerase and alters its subcellular localization. Nucleolin binds the human telomerase reverse transcriptase subunit (hTERT) through interactions with its RNA binding domain 4 and carboxyl-terminal RGG domain, and this binding also involves the telomerase RNA subunit hTERC. The protein-protein interaction between nucleolin and hTERT is critical for the nucleolar localization of hTERT. These findings indicate that interaction of hTERT and nucleolin participates in the dynamic intracellular localization of telomerase complex.