An ATPase/helicase complex is an essential cofactor for oncogenic transformation by c-Myc

Crystal structure of the human AAA+ protein RuvBL1

RuvBL1 is an evolutionarily highly conserved eukaryotic protein belonging to the AAA(+)-family of ATPases (ATPase associated with diverse cellular activities). It plays important roles in essential signaling pathways such as the c-Myc and Wnt pathways in chromatin remodeling, transcriptional and developmental regulation, and DNA repair and apoptosis. Herein we present the three-dimensional structure of the selenomethionine variant of human RuvBL1 refined using diffraction data to 2.2A of resolution. The crystal structure of the hexamer is formed of ADP-bound RuvBL1 monomers. The monomers contain three domains, of which the first and the third are involved in ATP binding and hydrolysis. Although it has been shown that ATPase activity of RuvBL1 is needed for several in vivo functions, we could only detect a marginal activity with the purified protein. Structural homology and DNA binding studies demonstrate that the second domain, which is unique among AAA(+) proteins and not present in the bacterial homolog RuvB, is a novel DNA/RNA-binding domain. We were able to demonstrate that RuvBL1 interacted with single-stranded DNA/RNA and double-stranded DNA. The structure of the RuvBL1.ADP complex, combined with our biochemical results, suggest that although RuvBL1 has all the structural characteristics of a molecular motor, even of an ATP-driven helicase, one or more as yet undetermined cofactors are needed for its enzymatic activity.

Balance of Yin and Yang: ubiquitylation-mediated regulation of p53 and c-Myc

Protein ubiquitylation has been demonstrated to play a vital role not only in mediating protein turnover but also in modulating protein activity. The stability and activity of the tumor suppressor p53 and of the oncoprotein c-Myc are no exception. Both are regulated through independent ubiquitylation by several E3 ubiquitin ligases. Interestingly, p53 and c-Myc are functionally connected by some of these E3 enzymes and their regulator ARF, although these proteins play opposite roles in controlling cell growth and proliferation. The balance of this complex ubiquitylation network and its disruption during oncogenesis will be the topics of this review.

Mechanism of transcriptional activation by the Myc oncoproteins

The Myc family proteins are potent oncogenes that can activate and repress a very large number of cellular target genes. The amino terminus of Myc contains a transactivation domain that can recruit a number of nuclear cofactors with diverse activities. Functional studies link transactivation to the ability of Myc to promote normal cell proliferation and for oncogenic transformation. The biochemical mechanism of Myc-mediated transactivation has revealed a wide range of effects on chromatin and basal transcription. This review summarizes recent advances in understanding the function of Myc as a transcriptional activator and the role of this activity in Myc biological activities.

Evolutionary relationships and structural mechanisms of AAA+ proteins

Complex cellular events commonly depend on the activity of molecular "machines" that efficiently couple enzymatic and regulatory functions within a multiprotein assembly. An essential and expanding subset of these assemblies comprises proteins of the ATPases associated with diverse cellular activities (AAA+) family. The defining feature of AAA+ proteins is a structurally conserved ATP-binding module that oligomerizes into active arrays. ATP binding and hydrolysis events at the interface of neighboring subunits drive conformational changes within the AAA+ assembly that direct translocation or remodeling of target substrates. In this review, we describe the critical features of the AAA+ domain, summarize our current knowledge of how this versatile element is incorporated into larger assemblies, and discuss specific adaptations of the AAA+ fold that allow complex molecular manipulations to be carried out for a highly diverse set of macromolecular targets.

The histidine triad protein Hint1 triggers apoptosis independent of its enzymatic activity

Hint1 is a member of the evolutionarily conserved family of histidine triad proteins that acts as a haplo-insufficient tumor suppressor inducing spontaneous tumor formation in Hint+/- and Hint-/- mouse models. However, the molecular mechanisms for the tumor-suppressing activity are poorly defined. In this respect, we have recently shown that Hint1, by interaction with Pontin and Reptin, inhibits T-cell factor/beta-catenin-mediated transcription of Wnt target genes. In this study, we have found that, after transient transfection with Hint1, SW480 and MCF-7 cells undergo apoptosis as analyzed by pro-caspase-3 and poly(ADP-ribose) polymerase cleavage, M30 CytoDEATH staining, cytochrome c release, and DNA fragmentation enzyme-linked immunosorbent assay. Hint1 is involved in the regulation of apoptotic pathways by inducing an up-regulation of p53 expression coinciding with an up-regulation of the proapoptotic factor Bax and a concomitant down-regulation of the apoptosis inhibitor Bcl-2. Bad and Puma levels remained unchanged. Further analyses revealed that Hint1 is associated with the Bax promoter and is a component of the Tip60 histone acetyltransferase complex and, in this context, appears to be involved in the regulation of Bax expression. Knockdown of Hint1 by short hairpin RNA resulted in down-regulation of p53 and Bax but had no effect on Bcl-2 expression. A mutant Hint1 (H112N) protein defective in enzymatic activity as an AMP-NH2 hydrolase was not impaired in induction of apoptosis, suggesting that the Hint1 pro-apoptotic activity is independent of the Hint1 enzymatic activity.

Drosophila Reptin and other TIP60 complex components promote generation of silent chromatin

Histone acetyltransferase (HAT) complexes have been linked to activation of transcription. Reptin is a subunit of different chromatin-remodeling complexes, including the TIP60 HAT complex. In Drosophila, Reptin also copurifies with the Polycomb group (PcG) complex PRC1, which maintains genes in a transcriptionally silent state. We demonstrate genetic interactions between reptin mutant flies and PcG mutants, resulting in misexpression of the homeotic gene Scr. Genetic interactions are not restricted to PRC1 components, but are also observed with another PcG gene. In reptin homozygous mutant cells, a Polycomb response-element-linked reporter gene is derepressed, whereas endogenous homeotic gene expression is not. Furthermore, reptin mutants suppress position-effect variegation (PEV), a phenomenon resulting from spreading of heterochromatin. These features are shared with three other components of TIP60 complexes, namely Enhancer of Polycomb, Domino, and dMRG15. We conclude that Drosophila Reptin participates in epigenetic processes leading to a repressive chromatin state as part of the fly TIP60 HAT complex rather than through the PRC1 complex. This shows that the TIP60 complex can promote the generation of silent chromatin.

GAS41 is required for repression of the p53 tumor suppressor pathway during normal cellular proliferation

GAS41 is a common subunit of the TIP60 and SRCAP complexes and is essential for cell growth and viability. Here, we report that GAS41 is required for repression of the p53 tumor suppressor pathway during normal cellular proliferation. Either GAS41 small interfering RNA-mediated knockdown of GAS41 expression or specific interruptions of the carboxy-terminal coiled-coil motif of the GAS41 protein activate the p53 tumor suppressor pathway, as evidenced by p53 up-regulation, p53 serine-15 phosphorylation, and p21 transcriptional activation. Activation of the p53 pathway does not result from changes in TIP60 complex assembly or TIP60 coactivator functions for p53, since a TIP60 complex containing a coiled-coil mutant of GAS41 retains the same composition and histone acetyltransferase activity as its wild-type counterpart and since mutant GAS41 does not compromise ectopic p53-dependent transcriptional activation in a reporter gene assay. Finally, we demonstrate that GAS41 is prebound to the promoters of two p53 tumor suppressor pathway genes (p21 and p14ARF) in normal unstressed cells but is dissociated from both promoters in response to stress signals that activate p53. Our data suggest that GAS41 plays a role in repressing the p53 tumor suppressor pathway during the normal cell cycle by a TIP60-independent mechanism.

Myc/Max/Mad in invertebrates: the evolution of the Max network

The Myc proto-oncogenes, their binding partner Max and their antagonists from the Mad family of transcriptional repressors have been extensively analysed in vertebrates. However, members of this network are found in all animals examined so far. Several recent studies have addressed the physiological function of these proteins in invertebrate model organisms, in particular Drosophila melanogaster. This review describes the structure of invertebrate Myc/Max/Mad genes and it discusses their regulation and physiological functions, with special emphasis on their essential role in the control of cellular growth and proliferation.

The Mlx Network: Evidence for a Parallel Max-Like Transcriptional Network That Regulates Energy Metabolism

Recent experiments suggest the existence of a transcriptional network that functions in parallel to the canonical Myc/Max/Mad transcriptional network. Unlike the Myc/Max/Mad network, our understanding of this network is still in its infancy. At the center of this network is a Max-like protein called Mlx; hence we have called this network the Mlx network. Like Max, Mix interacts with transcriptional repressors and transcriptional activators, namely the Mad family and the Mondo family, respectively. Similar to Max-containing heterodimers, Mlx-containing heterodimers recognize CACGTG E-box elements, suggesting that the transcriptional targets of these two networks may overlap. Supporting this hypothesis, we have observed genetic interactions between the Drosophila melanogaster orthologs of Myc and Mondo. In higher eukaryotes, two proteins, MondoA and MondoB/CHREBP/WBSCR14, constitute the Mondo family. At present little is known about the transcriptional targets of MondoA; however, pyruvate kinase is a putative target of MondoB/CHREBP/WBSCR14, suggesting a function for the Mondo family in glucose and/or lipid metabolism. Finally, unlike the predominant nuclear localization of Myc family proteins, both Mondo family members localize to the cytoplasm. Therefore, while the Myc and Mondo families may share some biological functions, it is likely each family is under distinct regulatory control.

Quantitative production of macrophages or neutrophils ex vivo using conditional Hoxb8

Differentiation mechanisms and inflammatory functions of neutrophils and macrophages are usually studied by genetic and biochemical approaches that require costly breeding and time-consuming purification to obtain phagocytes for functional analysis. Because Hox oncoproteins enforce self-renewal of factor-dependent myeloid progenitors, we queried whether estrogen-regulated Hoxb8 (ER-Hoxb8) could immortalize macrophage or neutrophil progenitors that would execute normal differentiation and normal innate immune function upon ER-Hoxb8 inactivation. Here we describe methods to derive unlimited quantities of mouse macrophages or neutrophils by immortalizing their respective progenitors with ER-Hoxb8 using different cytokines to target expansion of different committed progenitors. ER-Hoxb8 neutrophils and macrophages are functionally superior to those produced by many other ex vivo differentiation models, have strong inflammatory responses and can be derived easily from embryonic day 13 (e13) fetal liver of mice exhibiting embryonic-lethal phenotypes. Using knockout or small interfering RNA (siRNA) technologies, this ER-Hoxb8 phagocyte maturation system represents a rapid analytical tool for studying macrophage and neutrophil biology.

N-Myc functions in transcription and development

N-Myc is a member of the Myc family of proteins, which are best known for their potent oncogenic activities and association with a large proportion of human cancers. Intense scrutiny of the oncogenic properties of Myc family proteins over the last several decades has revealed a great deal about their transcriptional and oncogenic activities. Myc proteins have broad effects on transcription and can stimulate a variety of cell behaviors that contribute to the malignant phenotype. N-Myc and c-Myc also play essential functions during embryonic development, and loss of these proteins has deleterious effects in most, if not all, tissues and organ systems. What remains to be fully unraveled is the relationship between the diverse activities associated with deregulated and overexpressed Myc and their normal roles during embryonic development and tissue homeostasis. In this review I summarize our understanding of the transcriptional activities of Myc family proteins and the roles of N-myc in morphogenesis, particularly as they relate to cellular proliferation and apoptosis.

Transcriptional targets of hepatocyte growth factor signaling and Ki-ras oncogene activation in colorectal cancer

Both Ki-ras mutation and hepatocyte growth factor (HGF) receptor Met overexpression occur at high frequency in colon cancer. This study investigates the transcriptional changes induced by Ki-ras oncogene and HGF/Met signaling activation in colon cancer cell lines in vitro and in vivo. The model system used in these studies included the DLD-1 colon cancer cell line with a mutated Ki-ras allele, and the DKO-4 cell line generated from DLD-1, with its mutant Ki-ras allele inactivated by targeted disruption. These cell lines were transduced with cDNAs of full-length Met receptor. Microarray transcriptional profiling was conducted on cell lines stimulated with HGF, as well as on tumor xenograft tissues. Overlapping genes between in vitro and in vivo microarray data sets were selected as a subset of HGF/Met and Ki-ras oncogene-regulated targets. Using the Online Predicted Human Interaction Database, novel HGF/Met and Ki-ras regulated proteins with putative functional linkage were identified. Novel proteins identified included histone acetyltransferase 1, phosphoribosyl pyrophosphate synthetase 2, chaperonin containing TCP1, subunit 8, CSE1 chromosome segregation 1-like (yeast)/cellular apoptosis susceptibility (mammals), CCR4-NOT transcription complex, subunit 8, and cyclin H. Transcript levels for these Met-signaling targets were correlated with Met expression levels, and were significantly elevated in both primary and metastatic human colorectal cancer samples compared to normal colorectal mucosa. These genes represent novel Met and/or Ki-ras transcriptionally coregulated genes with a high degree of validation in human colorectal cancers.

The histidine triad protein Hint1 interacts with Pontin and Reptin and inhibits TCF-beta-catenin-mediated transcription

Pontin and Reptin previously were identified as nuclear beta-catenin interaction partners that antagonistically modulate beta-catenin transcriptional activity. In this study, Hint1/PKCI, a member of the evolutionary conserved family of histidine triad proteins, was characterised as a new interaction partner of Pontin and Reptin. Pull-down assays and co-immunoprecipitation experiments show that Hint1/PKCI directly binds to Pontin and Reptin. The Hint1/PKCI-binding site was mapped to amino acids 214-295 and 218-289 in Pontin and Reptin, respectively. Conversely, Pontin and Reptin bind to the N-terminus of Hint1/PKCI. Moreover, by its interaction with Pontin and Reptin, Hint1/PKCI is associated with the LEF-1/TCF-beta-catenin transcription complex. In this context, Hint1/PKCI acts as a negative regulator of TCF-beta-catenin transcriptional activity in Wnt-transfected cells and in SW480 colon carcinoma cells as shown in reporter gene assays. Consistent with these observations, Hint1/PKCI represses expression of the endogenous target genes cyclin D1 and axin2 whereas knockdown of Hint1/PKCI by RNA interference increases their expression. Disruption of the Pontin/Reptin complex appears to mediate this modulatory effect of Hint1/PKCI on TCF-beta-catenin-mediated transcription. These data now provide a molecular mechanism to explain the tumor suppressor function of Hint1/PKCI recently suggested from the analysis of Hint1/PKCI knockout mice.

Expression, purification, crystallization and preliminary X-ray analysis of the human RuvB-like protein RuvBL1

RuvBL1, an evolutionary highly conserved protein related to the AAA+ family of ATPases, has been crystallized using the hanging-drop vapour-diffusion method at 293 K. The crystals are hexagonal and belong to space group P6, with unit-cell parameters a = b = 207.1, c = 60.7 A and three molecules in the asymmetric unit.

Genes associated with the formation of germ cells from embryonic stem cells in cultures containing different glucose concentrations

In a previous study, we established a system for visualizing the development of germ cells from mouse embryonic stem (ES) cells in culture using knock-in ES clones in which visual reporter genes were expressed from the mouse vasa homolog, Mvh. While assessing various culture conditions, we found that germ-cell formation was markedly depressed in low glucose medium. Using a repeated polymerase chain reaction (PCR) subtraction method, we identified genes that were differentially expressed in low versus high glucose media. Three genes that were predominantly expressed in high glucose medium, thioredoxin-interacting protein (Txnip), pituitary tumor-transforming gene 1 (Pttg), and RuvB-like protein 2 (RuvBl2), were further investigated. These genes were also found to be highly expressed in adult and embryonic gonads, and RuvBl2 in particular, which encodes an ATP-dependent DNA helicase, was specifically detected in the spermatocytes and spermatids of the adult testis as well as in primordial germ cells. Furthermore, using a green fluorescent protein (GFP) fusion construct, we found that RuvBl2 was expressed in both the nucleus and cytoplasm of testicular germ cells. These findings suggest a possible relationship between glucose metabolism and germ-cell development.

The ubiquitin ligase HectH9 regulates transcriptional activation by Myc and is essential for tumor cell proliferation

The Myc oncoprotein forms a binary activating complex with its partner protein, Max, and a ternary repressive complex that, in addition to Max, contains the zinc finger protein Miz1. Here we show that the E3 ubiquitin ligase HectH9 ubiquitinates Myc in vivo and in vitro, forming a lysine 63-linked polyubiquitin chain. Miz1 inhibits this ubiquitination. HectH9-mediated ubiquitination of Myc is required for transactivation of multiple target genes, recruitment of the coactivator p300, and induction of cell proliferation by Myc. HectH9 is overexpressed in multiple human tumors and is essential for proliferation of a subset of tumor cells. Our results suggest that site-specific ubiquitination regulates the switch between an activating and a repressive state of the Myc protein, and they suggest a strategy to interfere with Myc function in vivo.

Proteome analysis of a human uveal melanoma primary cell culture by 2-DE and MS

We present here the first proteomics analysis of uveal melanoma (UM) cells. These cells represent a good model for the identification of polypeptide markers, which could be developed as diagnostic tools. UM is the most common primary intraocular tumour in adults. In contrast to other cancers, the survival rate of patients with this malignancy has changed little over the past few decades; a better understanding of the molecular biology of UM oncogenesis and metastasis is needed to build the basis for the identification of novel drug targets. In the study presented here, proteins from a UM primary cell culture were separated by 2-DE using a pI 3-10 gradient; 270 spots were analysed by LC-MS/MS, identifying 683 proteins derived from 393 different genes. Of those, 69 (18%) are related to cancer processes involving cell division, proliferation, invasion, metastasis, oncogenesis, drug resistance and others. To our knowledge, 96% of the proteins identified, including 16 hypothetical proteins, have never been reported in UM before. This study represents the first step towards the establishment of a UM protein database as a valuable resource for the study of this malignancy.

Relocalization of human chromatin remodeling cofactor TIP48 in mitosis

TIP48 is a highly conserved eukaryotic AAA+ protein which is an essential cofactor for several complexes involved in chromatin acetylation and remodeling, transcriptional and developmental regulation and nucleolar organization and trafficking. We show that TIP48 abundance in HeLa cells did not change during the cell cycle, nor did its distribution in various biochemical fractions. However, we observed distinct changes in the subcellular localization of TIP48 during M phase using immunofluorescence microscopy. Our studies demonstrate that in interphase cells TIP48 was found mainly in the nucleus and exhibited a distinct localization in the nuclear periphery. As the cells entered mitosis, TIP48 was excluded from the condensing chromosomes but showed association with the mitotic apparatus. During anaphase, some TIP48 was detected in the centrosome colocalizing with tubulin but the strongest staining appeared in the mitotic equator associated with the midzone central spindle. Accumulation of TIP48 in the midzone and the midbody was observed in late telophase and cytokinesis. This redeployment of TIP48 during anaphase and cytokinesis was independent of microtubule assembly. The relocation of endogenous TIP48 to the midzone/midbody under physiological conditions suggests a novel and distinct function for TIP48 in mitosis and possible involvement in the exit of mitosis.

Cancer therapeutics: targeting the dark side of Myc

The potent Myc oncoprotein plays a pivotal role as a regulator of tumorigenesis in numerous human cancers of diverse origin. Experimental evidence shows that inhibiting Myc significantly halts tumour cell growth and proliferation. This review summarises recent progress in understanding the function of Myc as a transcription factor, with emphasis on key protein interactions and target gene regulation. In addition, major advances in drug development aimed at eliminating Myc are described, including antisense and triple helix forming oligonucleotides, porphyrins and siRNA. Future anti-Myc strategies are also discussed that inhibit Myc at the level of expression and/or function. Targeting the dark side of Myc with novel therapeutic agents promises to have a profound impact in combating cancer.

A human T-cell lymphotropic virus type 1 enhancer of Myc transforming potential stabilizes Myc-TIP60 transcriptional interactions

The human T-cell lymphotropic virus type 1 (HTLV-1) infects and transforms CD4+ lymphocytes and causes adult T-cell leukemia/lymphoma (ATLL), an aggressive lymphoproliferative disease that is often fatal. Here, we demonstrate that the HTLV-1 pX splice-variant p30II markedly enhances the transforming potential of Myc and transcriptionally activates the human cyclin D2 promoter, dependent upon its conserved Myc-responsive E-box enhancer elements, which are associated with increased S-phase entry and multinucleation. Enhancement of c-Myc transforming activity by HTLV-1 p30II is dependent upon the transcriptional coactivators, transforming transcriptional activator protein/p434 and TIP60, and it requires TIP60 histone acetyltransferase (HAT) activity and correlates with the stabilization of HTLV-1 p30II/Myc-TIP60 chromatin-remodeling complexes. The p30II oncoprotein colocalizes and coimmunoprecipitates with Myc-TIP60 complexes in cultured HTLV-1-infected ATLL patient lymphocytes. Amino acid residues 99 to 154 within HTLV-1 p30II interact with the TIP60 HAT, and p30II transcriptionally activates numerous cellular genes in a TIP60-dependent or TIP60-independent manner, as determined by microarray gene expression analyses. Importantly, these results suggest that p30II functions as a novel retroviral modulator of Myc-TIP60-transforming interactions that may contribute to adult T-cell leukemogenesis.

Transcriptional regulation and transformation by Myc proteins

Myc genes are key regulators of cell proliferation, and their deregulation contributes to the genesis of most human tumours. Recently, a wealth of data has shed new light on the biochemical functions of Myc proteins and on the mechanisms through which they function in cellular transformation.

Myc interacts genetically with Tip48/Reptin and Tip49/Pontin to control growth and proliferation during Drosophila development

The transcription factor dMyc is the sole Drosophila ortholog of the vertebrate c-myc protooncogenes and a central regulator of growth and cell-cycle progression during normal development. We have investigated the molecular basis of dMyc function by analyzing its interaction with the putative transcriptional cofactors Tip48/Reptin (Rept) and Tip49/Pontin (Pont). We demonstrate that Rept and Pont have conserved their ability to bind to Myc during evolution. All three proteins are required for tissue growth in vivo, because mitotic clones mutant for either dmyc, pont,or rept suffer from cell competition. Most importantly, pont shows a strong dominant genetic interaction with dmyc that is manifested in the duration of development, rates of survival and size of the adult animal and, in particular, of the eye. The molecular basis for these effects may be found in the repression of certain target genes, such as mfas, by dMyc:Pont complexes. These findings indicate that dMyc:Pont complexes play an essential role in the control of cellular growth and proliferation during normal development.

Transgenic mice expressing a truncated form of CREB-binding protein (CBP) exhibit deficits in hippocampal synaptic plasticity and memory storage

Deletions, translocations, or point mutations in the CREB-binding protein (CBP) gene have been associated with Rubinstein-Taybi Syndrome; a human developmental disorder characterized by retarded growth and reduced mental function. To examine the role of CBP in memory, transgenic mice were generated in which the CaMKII alpha promoter drives expression of an inhibitory truncated CBP protein in forebrain neurons. Examination of hippocampal long-term potentiation (LTP), a form of synaptic plasticity thought to underlie memory storage, revealed significantly reduced late-phase LTP induced by dopamine-regulated potentiation in hippocampal slices from CBP transgenic mice. However, four-train induced late-phase LTP is normal. Behaviorally, CBP transgenic mice exhibited memory deficits in spatial learning in the Morris water maze and deficits in long-term memory for contextual fear conditioning, two hippocampus-dependent tasks. Together, these results demonstrate that CBP is involved in specific forms of hippocampal synaptic plasticity and hippocampus-dependent long-term memory formation.

Transcriptional regulation of a metastasis suppressor gene by Tip60 and β-catenin complexes

Defining the molecular strategies that integrate diverse signalling pathways in the expression of specific gene programmes that are critical in homeostasis and disease remains a central issue in biology. This is particularly pertinent in cancer biology because downregulation of tumour metastasis suppressor genes is a common occurrence, and the underlying molecular mechanisms are not well established. Here we report that the downregulation of a metastasis suppressor gene, KAI1, in prostate cancer cells involves the inhibitory actions of beta-catenin, along with a reptin chromatin remodelling complex. This inhibitory function of beta-catenin-reptin requires both increased beta-catenin expression and recruitment of histone deacetylase activity. The coordinated actions of beta-catenin-reptin components that mediate the repressive state serve to antagonize a Tip60 coactivator complex that is required for activation; the balance of these opposing complexes controls the expression of KAI1 and metastatic potential. The molecular mechanisms underlying the antagonistic regulation of beta-catenin-reptin and the Tip60 coactivator complexes for the metastasis suppressor gene, KAI1, are likely to be prototypic of a selective downregulation strategy for many genes, including a subset of NF-kappaB target genes.

Pontin and Reptin regulate cell proliferation in early Xenopus embryos in collaboration with c-Myc and Miz-1

Pontin (Tip49) and Reptin (Tip48) are highly conserved components of multimeric protein complexes important for chromatin remodelling and transcription. They interact with many different proteins including TATA box binding protein (TBP), beta-catenin and c-Myc and thus, potentially modulate different pathways. As antagonistic regulators of Wnt-signalling, they control wing development in Drosophila and heart growth in zebrafish. Here we show that the Xenopus xPontin and xReptin in conjunction with c-Myc regulate cell proliferation in early development. Overexpression of xPontin or xReptin results in increased mitoses and bending of embryos, which is mimicked by c-Myc overexpression. Furthermore, the knockdown of either xPontin or xReptin resulted in embryonic lethality at late gastrula stage, which is abrogated by the injection of c-Myc-RNA. The N-termini of xPontin and xReptin, which mediate the mitogenic effect were mapped to contain c-Myc interaction domains. c-Myc protein promotes cell cycle progression either by transcriptional activation through the c-Myc/Max complex or by repression of cyclin dependent kinase inhibitors (p21, p15) through c-Myc/Miz-1 interaction. Importantly, xPontin and xReptin exert their mitogenic effect through the c-Myc/Miz-1 pathway as dominant negative Miz-1 and wild-type c-Myc but not a c-Myc mutant deficient in Miz-1 binding could rescue embryonic lethality. Finally, promoter reporter studies revealed that xPontin and xReptin but not the N-terminal deletion mutants enhance p21 repression by c-Myc. We conclude that xPontin and xReptin are essential genes regulating cell proliferation in early Xenopus embryogenesis through interaction with c-Myc. We propose a novel function of xPontin and xReptin as co-repressors in the c-Myc/Miz-1 pathway.

Protein profile of the HeLa cell line

HeLa cells are widely used for all kinds of in vitro studies in biochemistry, biology and medicine. Knowledge on protein expression is limited and no comprehensive study on the proteome of this cell type has been reported so far. We applied proteomics technologies to analyze the proteins of the HeLa cell line. The proteins were analyzed by two-dimensional (2D) gel electrophoresis and identified by matrix-assisted laser desorption ionization mass spectrometry (MS) on the basis of peptide mass fingerprinting, following in-gel digestion with trypsin. Approximately 3000 spots, excised from six two-dimensional gels, were analyzed. The analysis resulted in the identification of about 1200 proteins that were the products of 297 different genes. The HeLa cell database includes proteins with important functions and unknown functions, representing today one of the largest two-dimensional databases for eukaryotic proteomes and forming the basis for future expressional studies at the protein level.

N and C-terminal Sub-regions in the c-Myc Transactivation Region and their Joint Role in Creating Versatility in Folding and Binding

The proto-oncogene c-myc governs the expression of a number of genes targeting cell growth and apoptosis, and its expression levels are distorted in many cancer forms. The current investigation presents an analysis by proteolysis, circular dichroism, fluorescence and Biacore of the folding and ligand-binding properties of the N-terminal transactivation domain (TAD) in the c-Myc protein. A c-Myc sub-region comprising residues 1-167 (Myc1-167) has been investigated that includes the unstructured c-Myc transactivation domain (TAD, residues 1-143) together with a C-terminal segment, which appears to promote increased folding. Myc1-167 is partly helical, binds both to the target proteins Myc modulator-1 (MM-1) and TATA box-binding protein (TBP), and displays the characteristics of a molten globule. Limited proteolysis divides Myc1-167 in two halves, by cleaving in a predicted linker region between two hotspot mutation regions: Myc box I (MBI) and Myc box II (MBII). The N-terminal half (Myc1-88) is unfolded and does not alone bind to target proteins, whereas the C-terminal half (Myc92-167) has a partly helical fold and specifically binds both MM-1 and TBP. Although this might suggest a bipartite organization in the c-Myc TAD, none of the N and C-terminal fragments bind target protein with as high affinity as the entire Myc1-167, or display molten globule properties. Furthermore, merely linking the MBI with the C-terminal region, in Myc38-167, is not sufficient to achieve binding and folding properties as in Myc1-167. Thus, the entire N and C-terminal regions of c-Myc TAD act in concert to achieve high specificity and affinity to two structurally and functionally orthogonal target proteins, TBP and MM-1, possibly through a mechanism involving molten globule formation. This hints towards understanding how binding of a range of targets can be accomplished to a single transactivation domain.

Evolutionary history and higher order classification of AAA+ ATPases

The AAA+ ATPases are enzymes containing a P-loop NTPase domain, and function as molecular chaperones, ATPase subunits of proteases, helicases or nucleic-acid-stimulated ATPases. All available sequences and structures of AAA+ protein domains were compared with the aim of identifying the definitive sequence and structure features of these domains and inferring the principal events in their evolution. An evolutionary classification of the AAA+ class was developed using standard phylogenetic methods, analysis of shared sequence and structural signatures, and similarity-based clustering. This analysis resulted in the identification of 26 major families within the AAA+ ATPase class. We also describe the position of the AAA+ ATPases with respect to the RecA/F1, helicase superfamilies I/II, PilT, and ABC classes of P-loop NTPases. The AAA+ class appears to have undergone an early radiation into the clamp-loader, DnaA/Orc/Cdc6, classic AAA, and "pre-sensor 1 beta-hairpin" (PS1BH) clades. Within the PS1BH clade, chelatases, MoxR, YifB, McrB, Dynein-midasin, NtrC, and MCMs form a monophyletic assembly defined by a distinct insert in helix-2 of the conserved ATPase core, and additional helical segment between the core ATPase domain and the C-terminal alpha-helical bundle. At least 6 distinct AAA+ proteins, which represent the different major clades, are traceable to the last universal common ancestor (LUCA) of extant cellular life. Additionally, superfamily III helicases, which belong to the PS1BH assemblage, were probably present at this stage in virus-like "selfish" replicons. The next major radiation, at the base of the two prokaryotic kingdoms, bacteria and archaea, gave rise to several distinct chaperones, ATPase subunits of proteases, DNA helicases, and transcription factors. The third major radiation, at the outset of eukaryotic evolution, contributed to the origin of several eukaryote-specific adaptations related to nuclear and cytoskeletal functions. The new relationships and previously undetected domains reported here might provide new leads for investigating the biology of AAA+ ATPases.

Rvb1p/Rvb2p recruit Arp5p and assemble a functional Ino80 chromatin remodeling complex

The Rvb1p and Rvb2p (or TIP48 and TIP49) nuclear ATP binding proteins are universally conserved in eukaryotes and essential for viability of yeasts. Rvbp associate with each other as a double hexamer, with YHR034c and with two complexes involved in chromatin remodeling, Ino80.com and Swr1.com. Loss of Rvb1p or Ino80p affects many yeast promoters similarly. Rvbp are not essential for the recruitment of Ino80p to promoters but are essential for the catalytic activity of Ino80.com. Loss of Rvbp leads to loss of the functionally critical Arp5p in Ino80.com. Rvb2p associates with Arp5p in vitro in a reaction dependent on the presence of ATP and Ino80p. Therefore, Rvbp are required for the structural and functional integrity of the Ino80 chromatin remodeling complex.

Genome-wide analysis of gene expression in neuroblastomas detected by mass screening

Neuroblastoma (NB) is the most common extracranial solid tumor of childhood and the third most common pediatric cancer. Although numerous factors including patient age, disease stage and genetic abnormalities have been shown to be predictive of outcome, the mechanisms responsible for the highly variable clinical behavior of this tumor remain largely unknown. In order to gain new insights into the biology of this tumor, we performed microarray analysis and compared the gene expression patterns of NB detected by mass screening, characterized by highly probable spontaneous regression, versus stage 4 NB with poor prognosis. The bioinformatics analysis revealed a set of 19 discriminatory genes that may play a significant role in the natural progression of the disease. Validation of these results and further mechanistic studies would shed new light on the biology of tumor progression, and provide new tools to predict clinical outcome in children with NB.

c-Myc as a therapeutic target in cancer

c-Myc is frequently deregulated in human cancers. The c-Myc oncoprotein is a transcription factor, with many of its target genes encoding proteins that initiate and maintain the transformed state. c-Myc is also part of a dynamic network whose members interact selectively with one another and with various transcriptional coregulators and histone-modifying enzymes. This knowledge highlights several points that might be amenable to attack. This review summarizes progress in controlling the extent of c-Myc transcription, translation, interaction with other myc network members, DNA binding and transcriptional activation. Inhibition of c-Myc can be achieved with many of these approaches; however, clinical efficacy will likely require intervention at several levels, perhaps in combination with traditional chemotherapeutic drugs or agents that target other oncoproteins.

E2F-dependent histone acetylation and recruitment of the Tip60 acetyltransferase complex to chromatin in late G1

E2F proteins can either activate or repress transcription. Following mitogenic stimulation, repressive E2F4-p130-histone deacetylase complexes dissociate from, while activating species (E2F1, -2, and -3) associate with, target promoters. Histones H3 and H4 simultaneously become hyperacetylated, but it remains unclear whether this is a prerequisite or a consequence of E2F binding. Here, we show that activating E2F species are required for hyperacetylation of target chromatin in human cells. Overexpression of a dominant-negative (DN) E2F1 mutant in serum-stimulated T98G cells blocked all E2F binding, H4 acetylation, and, albeit partially, H3 acetylation. Target gene activation and S-phase entry were also blocked by DN E2F1. Conversely, ectopic activation of E2F1 rapidly induced H3 and H4 acetylation, demonstrating a direct role for E2F in these events. E2F1 was previously shown to bind the histone acetyltransferases (HATs) p300/CBP and PCAF/GCN5. In our hands, ectopically expressed E2F1 also bound the unrelated HAT Tip60 and induced recruitment of five subunits of the Tip60 complex (Tip60, TRRAP, p400, Tip48, and Tip49) to target promoters in vivo. Moreover, E2F-dependent recruitment of Tip60 to chromatin occurred in late G(1) following serum stimulation. We speculate that the activities of multiple HAT complexes account for E2F-dependent acetylation, transcription, and S-phase entry.

The ATP-dependent helicase RUVBL1/TIP49a associates with tubulin during mitosis

RUVBL1/TIP49a/Pontin52 is a recently identified multi-functional protein with 2 ATP binding (WALKER) sites, which is essential for cell proliferation. We recovered and identified RUVBL1/TIP49a as a tubulin-binding protein from Triton X-100 lysates of U937 promonocytic cells by protein affinity chromatography and tryptic peptide microsequencing. Performing co-immunoprecipitation using newly generated RUVBL1/TIP49a-specific antibodies (mAb and rabbit polyclonal Ab) and RUVBL1/TIP49a-GST fusion protein-pull down assays we demonstrate co-precipitation of alpha- and gamma tubulin with RUVBL1/TIP49a. Confocal immunoflourescence microscopy reveals that RUVBL1/TIP49a was present not only in the nucleus, as expected, but was also concentrated at the centrosome and at the mitotic spindle in colocalization with tubulin. The topology of RUVBL1/TIP49a at the mitotic spindle varied, depending on the mitotic stage. The protein was localized at the centrosome and at the polar and astral microtubules in metaphase, and was detectable at the zone of polar tubule interdigitation in anaphase B and telophase. During cytokinesis the protein reappeared at the area of decondensing chromosomes. Whereas preincubation of U937 cells with colcemid resulted in inhibition of mitotic spindle formation with subsequent loss of RUVBL1/TIP49a mitotic spindle staining, no relevant influence of colcemid on RUVBL1/TIP49a-tubulin binding was observed. An agonistic effect of RUVBL1/TIP49a on in vitro tubulin assembly is demonstrated. Our results reveal a new functional aspect of RUVBL1/TIP49a.

Structural and functional conservation of the NuA4 histone acetyltransferase complex from yeast to humans

The NuA4 histone acetyltransferase (HAT) multisubunit complex is responsible for acetylation of histone H4 and H2A N-terminal tails in yeast. Its catalytic component, Esa1, is essential for cell cycle progression, gene-specific regulation and has been implicated in DNA repair. Almost all NuA4 subunits have clear homologues in higher eukaryotes, suggesting that the complex is conserved throughout evolution to metazoans. We demonstrate here that NuA4 complexes are indeed present in human cells. Tip60 and its splice variant Tip60b/PLIP were purified as stable HAT complexes associated with identical polypeptides, with 11 of the 12 proteins being homologs of yeast NuA4 subunits. This indicates a highly conserved subunit composition and the identified human proteins underline the role of NuA4 in the control of mammalian cell proliferation. ING3, a member of the ING family of growth regulators, links NuA4 to p53 function which we confirmed in vivo. Proteins specific to the human NuA4 complexes include ruvB-like helicases and a bromodomain-containing subunit linked to ligand-dependent transcription activation by the thyroid hormone receptor. We also demonstrate that subunits MRG15 and DMAP1 are present in distinct protein complexes harboring histone deacetylase and SWI2-related ATPase activities, respectively. Finally, analogous to yeast, a recombinant trimeric complex formed by Tip60, EPC1, and ING3 is sufficient to reconstitute robust nucleosomal HAT activity in vitro. In conclusion, the NuA4 HAT complex is highly conserved in eukaryotes, in which it plays primary roles in transcription, cellular response to DNA damage, and cell cycle control.

A network of immediate early gene products propagates subtle differences in mitogen-activated protein kinase signal amplitude and duration

The strength and duration of mitogen-activated protein kinase (MAPK) signaling have been shown to regulate cell fate in different cell types. In this study, a general mechanism is described that explains how subtle differences in signaling kinetics are translated into a specific biological outcome. In fibroblasts, the expression of immediate early gene (IEG)-encoded Fos, Jun, Myc, and early growth response gene 1 (Egr-1) transcription factors is significantly extended by sustained extracellular signal-regulated kinase 1 and 2 (ERK1 and -2) signaling. Several of these proteins contain functional docking site for ERK, FXFP (DEF) domains that serve to locally concentrate the active kinase, thus showing that they can function as ERK sensors. Sustained ERK signaling regulates the posttranslational modifications of these IEG-encoded sensors, which contributes to their sustained expression during the G(1)-S transition. DEF domain-containing sensors can also interpret the small changes in ERK signal strength that arise from less than a threefold reduction in agonist concentration. As a result, downstream target gene expression and cell cycle progression are significantly changed.

Functions of myc:max in the control of cell proliferation and tumorigenesis

Deregulation and elevated expression of members of the Myc family of bHLHZip transcription factors are observed in a high percentage of tumors. This close association with human cancers has led to a tremendous effort to define their biological and biochemical activities. Although Myc family proteins have the capacity to elicit a wide range of cell behaviors, their principal function appears to be to drive cells into the cell cycle and to keep them there. However, forced expression of Myc profoundly sensitizes normal cells to apoptosis. Therefore, tumor formation caused by deregulated Myc expression requires cooperating events that disrupt pathways that mediate apoptosis. Myc-dependent tumor formation may also be impeded by a set of related bHLHZip proteins with the demonstrated potential to act as Myc antagonists in cell culture experiments. In this review, we examine the complex activities of Myc family proteins and how their actions might be regulated in the context of a network of bHLHZip proteins.

Dynamic changes in transcription factor complexes during erythroid differentiation revealed by quantitative proteomics

During erythroid differentiation, beta-globin gene expression is regulated by the locus control region (LCR). The transcription factor NF-E2p18/MafK binds within this region and is essential for beta-globin expression in murine erythroleukemia (MEL) cells. Here we use the isotope-coded affinity tag (ICAT) technique of quantitative mass spectrometry to compare proteins interacting with NF-E2p18/MafK during differentiation. Our results define MafK as a 'dual-function' molecule that shifts from a repressive to an activating mode during erythroid differentiation. The exchange of MafK dimerization partner from Bach1 to NF-E2p45 is a key step in the switch from the repressed to the active state. This shift is associated with changes in the interaction of MafK with co-repressors and co-activators. Thus, our results suggest that in addition to its role as a cis-acting activator of beta-globin gene expression in differentiated erythroid cells, the LCR also promotes an active repression of beta-globin transcription in committed cells before terminal differentiation.

Regulation of COX-2 transcription in a colon cancer cell line by Pontin52/TIP49a

Cyclooxygenase-2 (COX-2) is expressed early in colon carcinogenesis and is known to play a crucial role in the progress of the disease. Here we show that the regulation of the expression of this enzyme in a colon cancer cell line, and in patients, is associated with overexpression of the Wnt pathway-associated proteins, Pontin52/TIP49a and LEF-1. Recently shown to be essential for transformation via the c-Myc pathway, Pontin52/TIP49a promotes COX-2 expression in tissue culture and is overexpressed in colon cancer tissue, co-localizing with COX-2 expression in transformed tissue, relative to paired normal tissue.

Control of nutrient-sensitive transcription programs by the unconventional prefoldin URI

Prefoldins (PFDs) are members of a recently identified, small-molecular weight protein family able to assemble into molecular chaperone complexes. Here we describe an unusually large member of this family, termed URI, that forms complexes with other small-molecular weight PFDs and with RPB5, a shared subunit of all three RNA polymerases. Functional analysis of the yeast and human orthologs of URI revealed that both are targets of nutrient signaling and participate in gene expression controlled by the TOR kinase. Thus, URI is a component of a signaling pathway that coordinates nutrient availability with gene expression.

The ATPases: a new family for a family-based drug design approach

The rapid discovery of new drugs is greatly facilitated when a family of related proteins is targeted with a similar approach in chemistry. Few protein families have so far been investigated using this kind of 'family-based' approach. Therefore, to increase the size of our Pharmacopeia and to cure human diseases more efficiently, new druggable protein families must be identified. It is shown in this review that ATPases are very good candidates for a family-based approach. The human proteome contains many ATPases, which are involved in several diseases. All the ATPases contain a nucleotide-binding site, and it is therefore possible to target all of them with a single strategy in chemistry; the design of competitive ATP inhibitors. Moreover, because a similar approach has been conducted with the protein kinases, the compound libraries and the knowledge developed in the kinase field can be directly applied to the ATPases.

MYC recruits the TIP60 histone acetyltransferase complex to chromatin

The transcription factor MYC binds specific DNA sites in cellular chromatin and induces the acetylation of histones H3 and H4. However, the histone acetyltransferases (HATs) that are responsible for these modifications have not yet been identified. MYC associates with TRRAP, a subunit of distinct macromolecular complexes that contain the HATs GCN5/PCAF or TIP60. Although the association of MYC with GCN5 has been shown, its interaction with TIP60 has never been analysed. Here, we show that MYC associates with TIP60 and recruits it to chromatin in vivo with four other components of the TIP60 complex: TRRAP, p400, TIP48 and TIP49. Overexpression of enzymatically inactive TIP60 delays the MYC-induced acetylation of histone H4, and also reduces the level of MYC binding to chromatin. Thus, the TIP60 HAT complex is recruited to MYC-target genes and, probably with other other HATs, contributes to histone acetylation in response to mitogenic signals.

c-Myc transformation domain recruits the human STAGA complex and requires TRRAP and GCN5 acetylase activity for transcription activation

Deregulation of the c-Myc oncoprotein (Myc) is implicated in many types of cancer. Myc is a sequence-specific transcription factor that regulates transcription of genes involved in the control of cell proliferation and apoptosis via mechanisms that are still poorly understood. Cell transformation by Myc involves its association with the transformation-transactivation domain-associated protein (TRRAP) and the human histone acetyltransferase (HAT) GCN5. TRRAP and GCN5 are components of a variety of shared and distinct multiprotein HAT complexes with diverse functions. Myc induces TRRAP recruitment and histone hyperacetylation at specific Myc-activated genes in vivo. However, the identity of the HAT complexes recruited by Myc and the roles of TRRAP and GCN5 in Myc function are still unclear. Here we show that Myc co-recruits TRRAP and GCN5 via direct physical interactions of its N-terminal activation/transformation domain with the human STAGA (SPT3-TAF-GCN5 acetylase) coactivator complex. We demonstrate that GCN5 and TRRAP cooperate to enhance transcription activation by the N-terminal activation domain of Myc in vivo and that this synergy requires both the SPT3/GCN5 interaction domain of TRRAP and the HAT activity of GCN5. Thus, TRRAP might function as an adaptor within the STAGA complex, which helps recruit GCN5 HAT activity to Myc during transcription activation.

Impairment of the DNA binding activity of the TATA-binding protein renders the transcriptional function of Rvb2p/Tih2p, the yeast RuvB-like protein, essential for cell growth

In Saccharomyces cerevisiae, two highly conserved proteins, Rvb1p/Tih1p and Rvb2p/Tih2p, have been demonstrated to be major components of the chromatin-remodeling INO80 complex. The mammalian orthologues of these two proteins have been shown to physically associate with the TATA-binding protein (TBP) in vitro but not clearly in vivo. Here we show that yeast proteins interact with TBP under both conditions. To assess the functional importance of these interactions, we examined the effect of mutating both TIH2/RVB2 and SPT15, which encodes TBP, on yeast cell growth. Intriguingly, only those spt15 mutations that affected the ability of TBP to bind to the TATA box caused synthetic growth defects in a tih2-ts160 background. This suggests that Tih2p might be important in recruiting TBP to the promoter. A DNA microarray technique was used to identify genes differentially expressed in the tih2-ts160 strain grown at the restrictive temperature. Only 34 genes were significantly and reproducibly affected; some up-regulated and others down-regulated. We compared the transcription of several of these Tih2p target genes in both wild type and various mutant backgrounds. We found that the transcription of some genes depends on functions possessed by both Tih2p and TBP and that these functions are substantially impaired in the spt15/tih2-ts160 double mutants that confer synthetic growth defects.

The MYST family of histone acetyltransferases

Multiple chromatin modifying proteins and multisubunit complexes have been characterized in recent years. Histone acetyltransferase (HAT) activities have been the most thoroughly studied, both biochemically and functionally. This review sums up the current knowledge on a specific group of proteins that is extremely well conserved throughout evolution, the MYST family of histone acetyltransferases. These proteins play critical roles in various nuclear functions and the control of cell proliferation.

Loss of protooncogene c-Myc function impedes G1 phase progression both before and after the restriction point

c-myc is an important protooncogene whose misregulation is believed to causally affect the development of numerous human cancers. c-myc null rat fibroblasts are viable but display a severe (two- to threefold) retardation of proliferation. The rates of RNA and protein synthesis are reduced by approximately the same factor, whereas cell size remains unaffected. We have performed a detailed kinetic cell cycle analysis of c-myc(-/-) cells by using several labeling and synchronization methods. The majority of cells (>90%) in asynchronous, exponential phase c-myc(-/-) cultures cycle continuously with uniformly elongated cell cycles. Cell cycle elongation is due to a major lengthening of G(1) phase (four- to fivefold) and a more limited lengthening of G(2) phase (twofold), whereas S phase duration is largely unaffected. Progression from mitosis to the G1 restriction point and the subsequent progression from the restriction point into S phase are both drastically delayed. These results are best explained by a model in which c-Myc directly affects cell growth (accumulation of mass) and cell proliferation (the cell cycle machinery) by independent pathways.

The transcriptional repressor gene Mad3 is a novel target for regulation by E2F1

Mad family proteins are transcriptional repressors that antagonize the activity of the c- Myc proto-oncogene product. Mad3 is expressed specifically during the S-phase of the cell cycle in both proliferating and differentiating cells, suggesting that its biological function is probably linked to processes that occur during this period. To determine the mechanisms that regulate the cell-cycle-specific transcription of Mad3, we used reporter gene assays in stably transfected fibroblasts. We show that the activation of Mad3 at the G1-S boundary is mediated by a single E2F (E2 promoter binding factor)-binding site within the 5'-flanking region of the gene. Mutation of this element eliminated transcriptional activation at S-phase, suggesting that the positively acting E2F proteins play a role in Mad3 regulation. Using electrophoretic mobility-shift assays and chromatin immunoprecipitation, we show that E2F1 binds to the Mad3 5'-flanking region both in vitro and in vivo. We thus identify Mad3 as a novel transcriptional target of E2F1.

Mad4 is regulated by a transcriptional repressor complex that contains Miz-1 and c-Myc

Myc and Mad family proteins are central regulators of cellular proliferation and differentiation. We show that various Mad family genes have distinct patterns of expression during the chemically induced differentiation of mouse erythroleukaemia (MEL) cells, suggesting that they each serve a different function. Mad4 RNA is highly induced and persists in terminally differentiated cells, in agreement with observations in other systems. Using reporter gene assays in stably transfected MEL cells, we show that induction of Mad4 is mediated by a 49 nt core promoter region. We demonstrate that the initiator element is required for Mad4 activation, and show that induction is associated with the loss from the initiator of a complex that contains Miz-1 and c-Myc. Miz-1 activates the Mad4 promoter in transient transfection assays, and this effect is antagonized by c-Myc. We therefore identify Mad4 as a novel target of transcriptional repression by c-Myc. These data suggest that the expression of Mad4 in proliferating undifferentiated cells is suppressed by the binding of a c-Myc-Miz-1 repressor complex at the initiator, and that the activation of Mad4 during differentiation results, at least in part, from a decrease in c-Myc-mediated repression.

Nmyc upregulation by sonic hedgehog signaling promotes proliferation in developing cerebellar granule neuron precursors

Hedgehog pathway activation is required for expansion of specific neuronal precursor populations during development and is etiologic in the human cerebellar tumor, medulloblastoma. We report that sonic hedgehog (Shh) signaling upregulates expression of the proto-oncogene Nmyc in cultured cerebellar granule neuron precursors (CGNPs) in the absence of new protein synthesis. The temporal-spatial expression pattern of Nmyc, but not other Myc family members, precisely coincides with regions of hedgehog proliferative activity in the developing cerebellum and is observed in medulloblastomas of Patched (Ptch) heterozygous mice. Overexpression of Nmyc promotes cell-autonomous G(1) cyclin upregulation and CGNP proliferation independent of Shh signaling. Furthermore, Myc antagonism in vitro significantly decreases proliferative effects of Shh in cultured CGNPs. Together, these findings identify Nmyc as a direct target of the Shh pathway that functions to regulate cell cycle progression in cerebellar granule neuron precursors.

Development of mice expressing a single D-type cyclin

D-cyclins (cyclins D1, D2, and D3) are components of the core cell cycle machinery. To directly test the ability of each D-cyclin to drive development of various lineages, we generated mice expressing only cyclin D1, or only cyclin D2, or only cyclin D3. We found that these "single-cyclin" embryos develop normally until late gestation. Our analyses revealed that in single-cyclin embryos, the tissue-specific expression pattern of D-cyclins was lost. Instead, mutant embryos ubiquitously expressed the remaining D-cyclin. These findings suggest that the functions of the three D-cyclins are largely exchangeable at this stage. Later in life, single-cyclin mice displayed focused abnormalities, resulting in premature mortality. "Cyclin D1-only" mice developed severe megaloblastic anemia, "cyclin D2-only" mice presented neurological abnormalities, and "cyclin D3-only" mice lacked normal cerebella. Analyses of the affected tissues revealed that these compartments failed to sufficiently up-regulate the remaining, intact D-cyclin. In particular, we found that in cerebellar granule neuron precursors, the N-myc transcription factor communicates with the cell cycle machinery via cyclins D1 and D2, but not D3, explaining the inability of D3-only mice to up-regulate cyclin D3 in this compartment. Hence, the requirement for a particular cyclin in a given tissue is likely caused by specific transcription factors, rather than by unique properties of cyclins.

Conserved stem II of the box C/D motif is essential for nucleolar localization and is required, along with the 15.5K protein, for the hierarchical assembly of the box C/D snoRNP

The 5' stem-loop of the U4 snRNA and the box C/D motif of the box C/D snoRNAs can both be folded into a similar stem-internal loop-stem structure that binds the 15.5K protein. The homologous proteins NOP56 and NOP58 and 61K (hPrp31) associate with the box C/D snoRNPs and the U4/U6 snRNP, respectively. This raises the intriguing question of how the two homologous RNP complexes specifically assemble onto similar RNAs. Here we investigate the requirements for the specific binding of the individual snoRNP proteins to the U14 box C/D snoRNPs in vitro. This revealed that the binding of 15.5K to the box C/D motif is essential for the association of the remaining snoRNP-associated proteins, namely, NOP56, NOP58, fibrillarin, and the nucleoplasmic proteins TIP48 and TIP49. Stem II of the box C/D motif, in contrast to the U4 5' stem-loop, is highly conserved, and we show that this sequence is responsible for the binding of NOP56, NOP58, fibrillarin, TIP48, and TIP49, but not of 15.5K, to the snoRNA. Indeed, the sequence of stem II was essential for nucleolar localization of U14 snoRNA microinjected into HeLa cells. Thus, the conserved sequence of stem II determines the specific assembly of the box C/D snoRNP.