人端粒酶催化亚基(hTERT)基因及其表达调控

端粒酶是一种核糖核蛋白复合物 ,能引起染色体的末端结构端粒的完全复制。端粒作为一种保护性结构 ,是由短的重复ＤＮＡ序列组成。在人体中这种序列为ＴＴＡＧＧＧ ,其平均长度为 5～ 1 5ｋｂ[1] ,细胞每经过一次分裂端粒缩短 50～ 2 0 0ｂｐ ,这种分子侵蚀作用使得细胞的分裂次数有了生理限制 ,从而限制了体细胞的寿命。一种逃避这种限制的机制是端粒酶的激活 ,因为端粒酶能弥补端粒的缩短 ,因此端粒酶被认为与细胞的永生化、肿瘤发生和细胞衰老密切相关。近来 ,组成人端粒酶复合物的 3个主要成分已被鉴定。人端粒酶ＲＮＡ成分 (ｈＴＲ)提…     

The N Terminus of the OB Domain of Telomere Protein TPP1 Is Critical for Telomerase Action

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人端粒酶催化亚基基因启动子调控的肿瘤细胞特异表达载体

为获得端粒酶阳性肿瘤细胞特异表达载体用于癌症的基因治疗 ,克隆并构建了人端粒酶催化亚基 (hTERT)基因启动子调控的萤光素酶报告载体 .用脂质体转染法将其分别转染肿瘤细胞和正常细胞 ,检测其在肿瘤细胞和正常细胞中的转录活性 .hTERT启动子在所检测的 4种端粒酶阳性的肿瘤细胞中具有明显的转录活性 ,平均为阳性对照的 4 4 3% ;而在端粒酶阴性的正常人胚肺成纤维细胞中则无明显的转录活性 .提示hTRET启动子的转录活性在端粒酶阳性的肿瘤细胞中明显上调 ,由hTERT启动子构建的载体可能是一种新颖和有前景的肿瘤细胞特异性表达的基因治疗载体     

Human Immunodeficiency Virus Type 1 Gag Polyprotein Multimerization Requires the Nucleocapsid Domain and RNA and Is Promoted by the Capsid-Dimer Interface and the Basic Region of Matrix Protein

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein directs the formation of virions from productively infected cells. Many gag mutations disrupt virion assembly, but little is known about the biochemical effects of many of these mutations. Protein-protein interactions among Gag monomers are believed to be necessary for virion assembly, and data suggest that RNA may modify protein-protein interactions or even serve as a bridge linking Gag polyprotein monomers. To evaluate the primary sequence requirements for HIV-1 Gag homomeric interactions, a panel of HIV-1 Gag deletion mutants was expressed in bacteria and evaluated for the ability to associate with full-length Gag in vitro. The nucleocapsid protein, the major RNA-binding domain of Gag, exhibited activity comparable to that of the complete polyprotein. In the absence of the nucleocapsid protein, relatively weak activity was observed that was dependent upon both the capsid-dimer interface and basic residues within the matrix domain. The relevance of the in vitro findings was confirmed with an assay in which nonmyristylated mutant Gags were assessed for the ability to be incorporated into virions produced by wild-type Gag expressed in trans. Evidence of the importance of RNA for Gag-Gag interaction was provided by the demonstration that RNase impairs the Gag-Gag interaction and that HIV-1 Gag interacts efficiently with Gags encoded by distantly related retroviruses and with structurally unrelated RNA-binding proteins. These results are consistent with models in which Gag multimerization involves indirect contacts via an RNA bridge as well as direct protein-protein interactions.     

端粒酶催化亚基基因在小鼠 胚胎发育早期的转录活性

用 RT-PCR 引物分别扩增成年昆明 (KM) 小鼠睾丸、脾脏、肾脏、肝脏和胸腺组织的总 RNA 发现,端粒酶催化亚基基因 tert 在这些组织中都有转录,目标产物正确组装到 PMD 18-T 载体后测序,结果与已知 cDNA 序列一致 . PMSG/hCG 超数排卵方法获得 KM 小鼠成熟卵母细胞和 CZB 溶液体外培养的胚胎 (KM ♀× KM ♂ ) ,用酸性 Tyrode's 溶液消化透明带后,采用巢式 RT-PCR ,同时分析 tert 基因和持家基因 hprt 的转录发现,对于单个样品来说 , 全部卵母细胞 (15 h-post hCG , 10/10) 都存在 hprt 转录本,其中,只有 40% (4/10) 还同时存在 tert 转录本 . 原核形成初期 (20 h-post hCG , 6/6) 和原核晚期 (30 h post-hCG , 8/8) 的受精卵,以及发育至 2-C 早期的胚胎 (35 h-post hCG , 7/7) 都不转录 tert 基因,只有 hprt mRNA 存在; 2-C 晚期 (50 h-post hCG) 时,两个基因同时转录 (4/8) 和一个基因单独转录 (4/8) 的胚胎各占 50% ;从 4-C 阶段 (65 h-post hCG , 4/4) 开始,包括 8-C 阶段 (75 h-post hCG , 4/4) ,桑椹胚阶段 (93 h-post hCG , 4/4) ,直至囊胚阶段 (118 h-post hCG , 4/4) ,所有的胚胎都同时转录 tert 和 hprt 基因,而且转录水平明显升高 . 以 20 枚胚胎量为模板进行 RT-PCR 发现,原核早期,原核晚期的胚胎中仍然没有 tert 基因转录,只有 hprt mRNA ,但是,在 2-C 早期胚胎中同时检测到了 hprt 和 tert 两种 mRNA. 结果表明,持家基因 hprt 在成熟卵母细胞受精前后,以及胚胎早期发育过程中均存在转录本 . 40% 卵母细胞中存在的 tert mRNA 在受精后很快降解,检测不到;胚胎基因组在 2-C 早期开始转录 tert mRNA ,转录水平逐渐上升 . 结果暗示,小鼠胚胎的基因组 DNA 在 2-C 早期开始启动,功能基因 tert 也在此时开始转录,可能与胚胎发育初期的染色体保护有关 .     

Functional Consequences of Deletions of the N Terminus of the [epsilon] Subunit of the Chloroplast ATP Synthase

The [epsilon] subunit of the chloroplast ATP synthase functions in part to prevent wasteful ATP hydrolysis by the enzyme. In addition, [epsilon] together with the remainder of the catalytic portion of the synthase (CF1) is required to block the nonproductive leak of protons through the membrane-embedded component of the synthase (CFO). Mutant [epsilon] subunits of the spinach (Spinacia oleracea) chloroplast ATP synthase that lack 5, 11, or 20 amino acids from their N termini ([epsilon]-[delta]5N, [epsilon]-[delta]11N, and [epsilon]-[delta]20N, respectively), were overexpressed as inclusion bodies. Using a procedure that resulted in the folding of full-length, recombinant [epsilon] in a biologically active form, none of these truncated forms resulted in [epsilon] that inhibited the ATPase activity of CF1 deficient in [epsilon], CF1(-[epsilon]). Yet, the [epsilon]-[delta]5N and [epsilon]-[delta]11N peptides significantly inhibited the ATPase activity of CF1(-[epsilon]) bound to CFO in NaBr-treated thylakoids. Although full-length [epsilon] rapidly inhibited the ATPase activity of CF1(-[epsilon]) in solution or bound to CFO, an extended period was required for the truncated forms to inhibit membrane-bound CF1(-[epsilon]). Despite the fact that [epsilon]-[delta]5N significantly inhibited the ATPase activity of CF1(-[epsilon]) bound to CFO, it did not block the proton conductance through CFO in NaBr-treated thylakoids reconstituted with CF1(-[epsilon]). Based on selective proteolysis and the binding of 8-anilino-1-naphthalene sulfonic acid, each of the truncated peptides gained significant secondary structure after folding. These results strongly suggest (a) that the N terminus of [epsilon] is important in its binding to CF1, (b) that CF0 stabilizes [epsilon] binding to the entire ATP synthase, and (c) that the N terminus may play some role in the regulation of proton flux through CFO.     

Two Inactive Fragments of the Integral RNA Cooperate To Assemble Active Telomerase with the Human Protein Catalytic Subunit (hTERT) In Vitro

We have mapped the 5' and 3' boundaries of the region of the human telomerase RNA (hTR) that is required to produce activity with the human protein catalytic subunit (hTERT) by using in vitro assembly systems derived from rabbit reticulocyte lysates and human cell extracts. The region spanning nucleotides +33 to +325 of the 451-base hTR is the minimal sequence required to produce levels of telomerase activity that are comparable with that made with full-length hTR. Our results suggest that the sequence approximately 270 bases downstream of the template is required for efficient assembly of active telomerase in vitro; this sequence encompasses a substantially larger portion of the 3' end of hTR than previously thought necessary. In addition, we identified two fragments of hTR (nucleotides +33 to +147 and +164 to +325) that cannot produce telomerase activity when combined separately with hTERT but can function together to assemble active telomerase. These results suggest that the minimal sequence of hTR can be divided into two sections, both of which are required for de novo assembly of active telomerase in vitro.     

特异性人端粒酶催化亚单位基因小干扰RNA对HeLa细胞生长的抑制作用

通过设计并化学合成人端粒酶催化亚单位(hTERT)特异性siRNA,观察其对hTERT表达水平及肿瘤细胞生长的影响。将hTERT-siRNA以脂质体法转染入HeLa细胞,应用RT-PCR、实时定量TRAP、Western印迹、软琼脂克隆形成实验、荷瘤裸鼠肿瘤内注射等方法检测细胞内hTERTmRNA、蛋白质表达水平及对肿瘤细胞生长的影响。RT-PCR、实时定量TRAP和Western印迹的结果显示hTERT-siRNA明显降低了HeLa细胞内hTERT的mRNA及蛋白质表达水平并伴随有端粒酶活性的下降。克隆形成实验表明hTERT-siRNA组的体外肿瘤形成能力受到抑制。荷瘤裸鼠肿瘤内注射hTERT-siRNA使肿瘤平均体积显著小于对照组。TUNEL凋亡检测表明hTERT-siRNA转染组的凋亡率明显高于对照组。研究表明hTERT特异性siRNA可以明显抑制HeLa细胞内hTERT的表达水平,对其生长有明显抑制作用,是一种有前途的肿瘤治疗新方法。     

Inter-Species Complementation of the Translocon Beta Subunit Requires Only Its Transmembrane Domain

In eukaryotes, proteins enter the secretory pathway through the translocon pore of the endoplasmic reticulum. This protein translocation channel is composed of three major subunits, called Sec61α, β and γ in mammals. Unlike the other subunits, the β subunit is dispensable for translocation and cell viability in all organisms studied. Intriguingly, the knockout of the Sec61β encoding genes results in different phenotypes in different species. Nevertheless, the β subunit shows a high level of sequence homology across species, suggesting the conservation of a biological function that remains ill-defined. To address its cellular roles, we characterized the homolog of Sec61β in the fission yeast Schizosaccharomyces pombe (Sbh1p). Here, we show that the knockout of sbh1 ⁺ results in severe cold sensitivity, increased sensitivity to cell-wall stress, and reduced protein secretion at 23°C. Sec61β homologs from Saccharomyces cerevisiae and human complement the knockout of sbh1 ⁺ in S. pombe. As in S. cerevisiae, the transmembrane domain (TMD) of S. pombe Sec61β is sufficient to complement the phenotypes resulting from the knockout of the entire encoding gene. Remarkably, the TMD of Sec61β from S. cerevisiae and human also complement the gene knockouts in both yeasts. Together, these observations indicate that the TMD of Sec61β exerts a cellular function that is conserved across species.     

Functional Dissection of the Catalytic Carboxyl-Terminal Domain of Origin Recognition Complex Subunit 1 (PfORC1) of the Human Malaria Parasite Plasmodium falciparum

Origin recognition complex subunit 1 (ORC1) is essential for DNA replication in eukaryotes. The deadly human malaria parasite Plasmodium falciparum contains an ORC1/CDC6 homolog with several interesting domains at the catalytic carboxyl-terminal region that include a putative nucleoside triphosphate-binding and hydrolysis domain, a putative PCNA-interacting-protein (PIP) motif, and an extreme C-terminal region that shows poor homology with other ORC1 homologs. Due to the unavailability of a dependable inducible gene expression system, it is difficult to study the structure and function of essential genes in Plasmodium. Using a genetic yeast complementation system and biochemical experiments, here we show that the putative PIP domain in ORC1 that facilitates in vitro physical interaction with PCNA is functional in both yeast (Saccharomyces cerevisiae) and Plasmodium in vivo, confirming its essential biological role in eukaryotes. Furthermore, despite having less sequence homology, the extreme C-terminal region can be swapped between S. cerevisiae and P. falciparum and it binds to DNA directly, suggesting a conserved role of this region in DNA replication. These results not only provide us a useful system to study the function of the essential genes in Plasmodium, they help us to identify the previously undiscovered unique features of replication proteins in general.Origin recognition complex subunit 1 (ORC1), the largest subunit among the ORC components is essential for DNA replication initiation in eukaryotes. ORC1 has a regulatory function in DNA replication since it comes on and off chromatin during cell cycle. Human ORC binds to chromatin during G₁ phase of the cell cycle, followed by degradation of ORC1 by a ubiquitin-mediated pathway. ORC1 reappears during M phase, and it binds to DNA at the onset of G₁ phase (21). In mammalian cells, monoubiquitination and phosphorylation may also lead to the subcellular localization of ORC1 to control DNA replication (24). In the case of Xenopus laevis, ORC1 is bound to chromatin during early interphase but it is destabilized later with the loading of MCM proteins on chromatin (23). While in Drosophila melanogaster, the level of ORC1 is developmentally regulated (2), the murine ORC1 binds to specific locus in the ribosomal RNA in a cell-cycle-dependent manner (29). Interestingly, in the yeast Saccharomyces cerevisiae, although ORC is tightly bound to chromatin throughout the cell cycle, another pre-replication complex (pre-RC) protein, CDC6, comes on and off chromatin, ensuring the control of DNA replication during cell cycle (7, 22). The role of S. cerevisiae ORC1 (ScORC1) in ORC-DNA binding and modulating ScORC function has been described recently using high- resolution electron microscopy of ScORC (5).ORC1 proteins consist of two highly conserved domains: the N-terminal regulatory domain that contains the bromo-adjacent homology domain and the C-terminal catalytic domain that contains the AAA⁺ ATPase domain. The bromo-adjacent homology domain is involved in the regulation of gene expression through protein-protein interaction (4). It also facilitates the binding of ORC1 to the replication origin (21). The AAA⁺ ATPase domain that binds and hydrolyzes ATP is essential for DNA replication in several organisms (11, 27).Plasmodium falciparum, the causative agent of human malaria, contains an ortholog of ORC1/CDC6, although there is no separate CDC6 protein in Plasmodium. The homology of P. falciparum ORC1 (PfORC1) with other ORC1 counterparts is predominantly confined to the C-terminal region containing the putative nucleoside triphosphate (NTP)-binding and hydrolysis domain (residues 784 to 1014) (see Fig. S1 in the supplemental material). The N-terminal region (residues 1 to 783) and the extreme C-terminal region of PfORC1 (residues 1015 to 1189) exhibit poor homology with other ORC1 counterparts (14, 17) (see Fig. S1 in the supplemental material). The latter domain of PfORC1 (residues 1015 to 1189) may have a unique role in DNA binding since the crystal structure of archaeal (Aeropyrum pernix and Sulfolobus solfataricus) ORC1/CDC6-like protein along with origin DNA suggests that the extreme C-terminal region of this protein forms a wing-helix domain that binds to DNA (6, 10). Similarly, another member of the pre-RC, Cdc6, also contains a wing-helix domain at the extreme C terminus (15). It remains to be explored further whether the extreme C-terminal region of ORC1 will be responsible for origin DNA binding in eukaryotes.During the asexual blood-stage P. falciparum developmental cycle, PfORC1 is expressed during the ring stage, colocalizes with the P. falciparum replication foci marker proliferating cell nuclear antigen (PfPCNA1) during the replicating-trophozoite stage, and is degraded completely at the late schizont stage, suggesting its regulatory role in Plasmodium DNA replication (12). Interestingly, the presence of a putative PCNA-interacting protein (PIP) motif in PfORC1 (residues 913 to 920) (see Fig. ​Fig.5A5A and see Fig. S1A in the supplemental material) further supports the colocalization of PfORC1 and PCNA during DNA replication. The putative PIP domain was identified in different ORC1 homologs, including ScORC1, suggesting its conserved yet unidentified role in DNA replication (12). PCNA interacts with various proteins, like DNA polymerase, Fen1, CDT1, MCM10, etc., with diverse roles ranging from DNA replication to ubiquitination of various proteins leading to their regulation (19).Open in a separate windowFIG. 5.Role of the PIP domain in cell viability and interaction with PCNA. (A) Schematic diagrams of wild-type (Wt) ScORC1, PfORC1, chimera I, and their different mutant (Mut) forms (as indicated on the right). The sequence and amino acid coordinates for the PIP box are shown. Asterisks show the residues mutated for the study. (B) Yeast complementation assay to show the PIP domain is essential for yeast as well as chimera proteins. The yeast ORC1 swapper strain was transformed with the constructs described above, and the viability of the yeast cells was tested following a spot test after serial dilution in the absence (−FOA) or presence (+FOA) of FOA selection. The results indicate that the wild-type PIP box is important for survival for ScORC1 as well as chimera I construct. (C) Pull-down experiments using beads containing the wild-type or PIP mutant form of MBP-PfORC1C or the MBP control proteins in the presence of His₆-PfPCNA as described in Materials and Methods. Western blot analysis using anti-PfPCNA antibodies shows the specific binding of wild-type PfORC1C with PfPCNA. The bottom panel shows the Coomassie-stained gel following protein transfer as a loading control. The arrowheads show the purified MBP fusion proteins (top) or MBP alone. (D) Pull-down experiments using yeast proteins. The pull-down experiments were performed using soluble His₆-ScPCNA protein and the wild-type or PIP mutant form of MBP-ScORC1C protein bound on beads as described above for Plasmodium proteins followed by Western blot analysis using anti-His polyclonal antibodies. The results indicate the strong affinity of ScPCNA toward the wild-type ScORC1C compared to the mutant form of the protein. The bottom panel shows the Coomassie-stained gel following protein transfer, and the arrowhead indicates the position of the respective proteins. (E) ATPase activity of different proteins. The ATPase assay was performed as described in Materials and Methods using the wild-type or PIP mutant form of MBP-PfORC1C or MBP-PfORC1C (ATPase mutant) or MBP, and the relative ATPase activity of each protein was plotted accordingly. The results indicate that the activities of the wild-type and PIP mutant forms of PfORC1C do not differ significantly.The presence of a putative NTP-binding domain, a putative PIP motif, and a unique extreme C-terminal region raises the issue of whether these domains have any functional relevance in PfORC1. It is extremely difficult to perform structure-function studies in Plasmodium due to the unavailability of a dependable inducible gene expression system. This is due to the time-consuming and poor transfection efficiency in P. falciparum. Moreover, an inducible gene expression system often requires expression of the transgene (transactivator) and the gene of interest under different promoters and use of different selectable markers when used episomally, causing considerable hindrance in regulation of gene expression. All of these exercises also may result in leaky expression of the gene of interest instead of tight regulation.In order to dissect the functional domains of PfORC1, we adopted a yeast genetic complementation approach along with biochemical experiments. Earlier, genetic complementation experiments in yeast were performed for detailed structure-function analysis of P. falciparum proteins like dihydrofolate reductase and histone-acetyltransferase GCN5 (9, 28). Using a chimera approach for yeast genetic complementation, we found that the putative NTP-binding domain, the PIP motif, and the extreme C-terminal region of PfORC1 are truly functional in the yeast heterologous system, suggesting their important role in DNA replication.These findings offer a useful tool to study the structure and function of essential proteins in P. falciparum that allows us to identify novel functional domains in ORC1 with a conserved role in DNA replication.     

Activity of the Human Telomerase Catalytic Subunit (hTERT) Gene Promoter Could Be Increased by the SV40 Enhancer

Telomerase is a ribonucleoprotein complex of which the function is to add telomeric repeats to chromosomal ends. Telomerase consists of two essential components, the telomerase RNA template (hTR) and the catalytic subunit (hTERT). hTERT is expressed only in cells and tissues positive for telomerase activity, i.e., tumor and fetal cells. The aim of this study is to test the increased telomerase promoter activity for cancer gene therapy in adenovirus vector. We cloned the hTERT promoter in place of the SV40 promoter in the pGL3-contol vector to be increased by the SV40 enhancer sequences, resulting in strong expression of luc+ only in telomerase positive cancer cells. Then we transfected the constructed plasmid into a normal human cell line and several cancer cell lines. Through these experiments, we identified the selective and increased expression of the luciferase gene controlled by the hTERT promoter and the SV40 enhancer in the telomerase positive cancer cell lines. To investigate the possibility of utilizing the hTERT promoter and the SV40 enhancer in targeted cancer gene therapy, we constructed an adenovirus vector expressing HSV-TK controlled by the hTERT promoter and the SV40 enhancer for the induction of specific telomerase positive cancer cell death. NSCLC cells infected by Ad-hT-TK-enh were more significantly suppressed and induced apoptosis than those infected by Ad-hT-TK. Telomerase is activated in 80～90% of cancers, so adenovirus with increasing telomerase promoter activity might be used for targeted cancer gene therapy using suicide genes. These results show that the hTERT promoter and the SV40 enhancer might be used for targeted cancer gene therapy.     

The Impact of Dyskeratosis Congenita Mutations on the Structure and Dynamics of the Human Telomerase RNA Pseudoknot Domain

Abstract

The pseudoknot domain is a functionally crucial part of telomerase RNA and influences the activity and stability of the ribonucleoprotein complex. Autosomal dominant dyskeratosis congenita (DKC) is an inherited disease that is linked to mutations in telomerase RNA and impairs telomerase function. In this paper, we present a computational prediction of the influence of two base DKC mutations on the structure, dynamics, and stability of the pseudoknot domain. We use molecular dynamics simulations, MM-GBSA free energy calculations, static analysis, and melting simulations analysis. Our results show that the DKC mutations stabilize the hairpin form and destabilize the pseudoknot form of telomerase RNA. Moreover, the P3 region of the predicted DKC-mutated pseudoknot structure is unstable and fails to form as a defined helical stem. We directly compare our predictions with experimental observations by calculating the enthalpy of folding and melting profiles for each structure. The enthalpy values are in very good agreement with values determined by thermal denaturation experiments. The melting simulations and simulations at elevated temperatures show the existence of an intermediate structure, which involves the formation of two UU base pairs observed in the hairpin form of the pseudoknot domain.     

Norovirus Translation Requires an Interaction between the C Terminus of the Genome-linked Viral Protein VPg and Eukaryotic Translation Initiation Factor 4G

Viruses have evolved a variety of mechanisms to usurp the host cell translation machinery to enable translation of the viral genome in the presence of high levels of cellular mRNAs. Noroviruses, a major cause of gastroenteritis in man, have evolved a mechanism that relies on the interaction of translation initiation factors with the virus-encoded VPg protein covalently linked to the 5′ end of the viral RNA. To further characterize this novel mechanism of translation initiation, we have used proteomics to identify the components of the norovirus translation initiation factor complex. This approach revealed that VPg binds directly to the eIF4F complex, with a high affinity interaction occurring between VPg and eIF4G. Mutational analyses indicated that the C-terminal region of VPg is important for the VPg-eIF4G interaction; viruses with mutations that alter or disrupt this interaction are debilitated or non-viable. Our results shed new light on the unusual mechanisms of protein-directed translation initiation.     

HeLa细胞端粒酶最适反应条件及活性重建

端粒是真核细胞染色体末端的重复ＤＮＡ序列 ,其生物学功能是防止染色体ＤＮＡ降解、末端融合、非正常重组和染色体的缺失[1] .由于存在“末端复制问题” ,随着老化人体细胞端粒重复序列长度不断缩短 ,但在生殖细胞中由于端粒酶的存在 ,端粒序列并不缩短 .端粒酶是由蛋白质和ＲＮＡ构成的核蛋白 ,是依赖ＲＮＡ的ＤＮＡ聚合酶 ,在ＤＮＡ3’端合成端粒重复序列[2 ] .研究表明 ,在 85 %～ 95 %的人肿瘤细胞中可以检测到端粒酶的活性[3 ,4 ] ,而在正常体细胞中除生殖细胞和造血干细胞等极少数细胞中存在端粒酶活性外 ,均检测不到端粒酶活性 ,这…