Regulation of microtubule assembly and organization in mitosis by the AAA+ ATPase Pontin

The multi-faceted roles of R2TP complex span across regulation of gene expression, translation, and protein functional assembly

Macromolecular complexes play essential roles in various cellular processes. The assembly of macromolecular assemblies within the cell must overcome barriers imposed by a crowded cellular environment which is characterized by an estimated concentration of biological macromolecules amounting to 100-450 g/L that take up approximately 5-40% of the cytoplasmic volume. The formation of the macromolecular assemblies is facilitated by molecular chaperones in cooperation with their co-chaperones. The R2TP protein complex has emerged as a co-chaperone of Hsp90 that plays an important role in macromolecular assembly. The R2TP complex is composed of a heterodimer of RPAP3:P1H1DI that is in turn complexed to members of the ATPase associated with diverse cellular activities (AAA +), RUVBL1 and RUVBL2 (R1 and R2) families. What makes the R2TP co-chaperone complex particularly important is that it is involved in a wide variety of cellular processes including gene expression, translation, co-translational complex assembly, and posttranslational protein complex formation. The functional versatility of the R2TP co-chaperone complex makes it central to cellular development; hence, it is implicated in various human diseases. In addition, their roles in the development of infectious disease agents has become of interest. In the current review, we discuss the roles of these proteins as co-chaperones regulating Hsp90 and its partnership with Hsp70. Furthermore, we highlight the structure-function features of the individual proteins within the R2TP complex and describe their roles in various cellular processes.

Unconventional roles of chromatin remodelers and long non-coding RNAs in cell division

The aim of this review article is to focus on the unconventional roles of epigenetic players (chromatin remodelers and long non-coding RNAs) in cell division, beyond their well-characterized functions in chromatin regulation during cell differentiation and development. In the last two  decades, diverse experimental evidence has shown that subunits of SRCAP and p400/TIP60 chromatin remodeling complexes in humans relocate from interphase nuclei to centrosomes, spindle or midbody, with their depletion yielding an array of aberrant outcomes of mitosis and cytokinesis. Remarkably, this behavior is shared by orthologous subunits of the Drosophila melanogaster DOM/TIP60 complex, despite fruit flies and humans diverged over 700 million years ago. In short, the available data support the view that subunits of these complexes are a new class of moonlighting proteins, in that they lead a "double life": during the interphase, they function in chromatin regulation within the nucleus, but as the cell progresses through mitosis, they interact with established mitotic factors, thus becoming integral components of the cell division apparatus.  By doing so, they contribute to ensuring the correct distribution of chromosomes in the two daughter cells and, when dysfunctional, can cause genomic instability, a condition that can trigger tumorigenesis and developmental diseases. Research over the past few years has unveiled a major contribution of long non-coding RNAs (lncRNAs) in the epigenetics regulation of gene expression which also impacts on cell division control. Here, we focus on possible structural roles of lncRNAs in the execution of cytokinesis: in particular, we suggest that specific classes of lncRNAs relocate to the midbody to form an architectural scaffold ensuring its proper assembly and function during abscission. Drawing attention to experimental evidence for non-canonical extranuclear roles of chromatin factors and lncRNAs has direct implications on important and novel aspects concerning both the epigenetic regulation and the evolutionary dynamics of cell division with a significant impact on differentiation, development, and diseases.

Knockdown of DOM/Tip60 Complex Subunits Impairs Male Meiosis of Drosophila melanogaster

ATP-dependent chromatin remodeling complexes are involved in nucleosome sliding and eviction and/or the incorporation of histone variants into chromatin to facilitate several cellular and biological processes, including DNA transcription, replication and repair. The DOM/TIP60 chromatin remodeling complex of Drosophila melanogaster contains 18 subunits, including the DOMINO (DOM), an ATPase that catalyzes the exchange of the canonical H2A with its variant (H2A.V), and TIP60, a lysine-acetyltransferase that acetylates H4, H2A and H2A.V histones. In recent decades, experimental evidence has shown that ATP-dependent chromatin remodeling factors, in addition to their role in chromatin organization, have a functional relevance in cell division. In particular, emerging studies suggested the direct roles of ATP-dependent chromatin remodeling complex subunits in controlling mitosis and cytokinesis in both humans and D. melanogaster. However, little is known about their possible involvement during meiosis. The results of this work show that the knockdown of 12 of DOM/TIP60 complex subunits generates cell division defects that, in turn, cause total/partial sterility in Drosophila males, providing new insights into the functions of chromatin remodelers in cell division control during gametogenesis.

The Green Valley of Drosophila melanogaster Constitutive Heterochromatin: Protein-Coding Genes Involved in Cell Division Control

Constitutive heterochromatin represents a significant fraction of eukaryotic genomes (10% in Arabidopsis, 20% in humans, 30% in D. melanogaster, and up to 85% in certain nematodes) and shares similar genetic and molecular properties in animal and plant species. Studies conducted over the last few years on D. melanogaster and other organisms led to the discovery of several functions associated with constitutive heterochromatin. This made it possible to revise the concept that this ubiquitous genomic territory is incompatible with gene expression. The aim of this review is to focus the attention on a group of protein-coding genes resident in D. melanogaster constitutive of heterochromatin, which are implicated in different steps of cell division.

Evolutionary conserved relocation of chromatin remodeling complexes to the mitotic apparatus

Background

ATP-dependent chromatin remodeling complexes are multi-protein machines highly conserved across eukaryotic genomes. They control sliding and displacing of the nucleosomes, modulating histone-DNA interactions and making nucleosomal DNA more accessible to specific binding proteins during replication, transcription, and DNA repair, which are processes involved in cell division. The SRCAP and p400/Tip60 chromatin remodeling complexes in humans and the related Drosophila Tip60 complex belong to the evolutionary conserved INO80 family, whose main function is promoting the exchange of canonical histone H2A with the histone variant H2A in different eukaryotic species. Some subunits of these complexes were additionally shown to relocate to the mitotic apparatus and proposed to play direct roles in cell division in human cells. However, whether this phenomenon reflects a more general function of remodeling complex components and its evolutionary conservation remains unexplored.

Results

We have combined cell biology, reverse genetics, and biochemical approaches to study the subcellular distribution of a number of subunits belonging to the SRCAP and p400/Tip60 complexes and assess their involvement during cell division progression in HeLa cells. Interestingly, beyond their canonical chromatin localization, the subunits under investigation accumulate at different sites of the mitotic apparatus (centrosomes, spindle, and midbody), with their depletion yielding an array of aberrant outcomes of mitosis and cytokinesis, thus causing genomic instability. Importantly, this behavior was conserved by the Drosophila melanogaster orthologs tested, despite the evolutionary divergence between fly and humans has been estimated at approximately 780 million years ago.

Conclusions

Overall, our results support the existence of evolutionarily conserved diverse roles of chromatin remodeling complexes, whereby subunits of the SRCAP and p400/Tip60 complexes relocate from the interphase chromatin to the mitotic apparatus, playing moonlighting functions required for proper execution of cell division.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01365-5.

MTBP and MYC: A Dynamic Duo in Proliferation, Cancer, and Aging

The oncogenic transcription factor c-MYC (MYC) is highly conserved across species and is frequently overexpressed or dysregulated in human cancers. MYC regulates a wide range of critical cellular and oncogenic activities including proliferation, metabolism, metastasis, apoptosis, and differentiation by transcriptionally activating or repressing the expression of a large number of genes. This activity of MYC is not carried out in isolation, instead relying on its association with a myriad of protein cofactors. We determined that MDM Two Binding Protein (MTBP) indirectly binds MYC and is a novel MYC transcriptional cofactor. MTBP promotes MYC-mediated transcriptional activity, proliferation, and cellular transformation by binding in a protein complex with MYC at MYC-bound promoters. This discovery provided critical context for data linking MTBP to aging as well as a rapidly expanding body of evidence demonstrating MTBP is overexpressed in many human malignancies, is often linked to poor patient outcomes, and is necessary for cancer cell survival. As such, MTBP represents a novel and potentially broad reaching oncologic drug target, particularly when MYC is dysregulated. Here we have reviewed the discovery of MTBP and the initial controversy with its function as well as its associations with proliferation, MYC, DNA replication, aging, and human cancer.

The ATPase SRCAP is associated with the mitotic apparatus, uncovering novel molecular aspects of Floating-Harbor syndrome

Background

A variety of human genetic diseases is known to be caused by mutations in genes encoding chromatin factors and epigenetic regulators, such as DNA or histone modifying enzymes and members of ATP-dependent chromatin remodeling complexes. Floating-Harbor syndrome is a rare genetic disease affecting human development caused by dominant truncating mutations in the SRCAP gene, which encodes the ATPase SRCAP, the core catalytic subunit of the homonymous chromatin-remodeling complex. The main function of the SRCAP complex is to promote the exchange of histone H2A with the H2A.Z variant. According to the canonical role played by the SRCAP protein in epigenetic regulation, the Floating-Harbor syndrome is thought to be a consequence of chromatin perturbations. However, additional potential physiological functions of SRCAP have not been sufficiently explored.

Results

We combined cell biology, reverse genetics, and biochemical approaches to study the subcellular localization of the SRCAP protein and assess its involvement in cell cycle progression in HeLa cells. Surprisingly, we found that SRCAP associates with components of the mitotic apparatus (centrosomes, spindle, midbody), interacts with a plethora of cytokinesis regulators, and positively regulates their recruitment to the midbody. Remarkably, SRCAP depletion perturbs both mitosis and cytokinesis. Similarly, DOM-A, the functional SRCAP orthologue in Drosophila melanogaster, is found at centrosomes and the midbody in Drosophila cells, and its depletion similarly affects both mitosis and cytokinesis.

Conclusions

Our findings provide first evidence suggesting that SRCAP plays previously undetected and evolutionarily conserved roles in cell division, independent of its functions in chromatin regulation. SRCAP may participate in two different steps of cell division: by ensuring proper chromosome segregation during mitosis and midbody function during cytokinesis. Moreover, our findings emphasize a surprising scenario whereby alterations in cell division produced by SRCAP mutations may contribute to the onset of Floating-Harbor syndrome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01109-x.

Assembly of the asymmetric human γ-tubulin ring complex by RUVBL1-RUVBL2 AAA ATPase

The microtubule nucleator γ-tubulin ring complex (γTuRC) is essential for the function of microtubule organizing centers such as the centrosome. Since its discovery over two decades ago, γTuRC has evaded in vitro reconstitution and thus detailed structure-function studies. Here, we show that a complex of RuvB-like protein 1 (RUVBL1) and RUVBL2 "RUVBL" controls assembly and composition of γTuRC in human cells. Likewise, RUVBL assembles γTuRC from a minimal set of core subunits in a heterologous coexpression system. RUVBL interacts with γTuRC subcomplexes but is not part of fully assembled γTuRC. Purified, reconstituted γTuRC has nucleation activity and resembles native γTuRC as revealed by its cryo-electron microscopy (cryo-EM) structure at ~4.0-Å resolution. We further use cryo-EM to identify features that determine the intricate, higher-order γTuRC architecture. Our work finds RUVBL as an assembly factor that regulates γTuRC in cells and allows production of recombinant γTuRC for future in-depth mechanistic studies.

Deregulated levels of RUVBL1 induce transcription-dependent replication stress

Chromatin regulators control transcription and replication, however if and how they might influence the coordination of these processes still is largely unknown. RUVBL1 and the related ATPase RUVBL2 participate in multiple nuclear processes and are implicated in cancer. Here, we report that both the excess and the deficit of the chromatin regulator RUVBL1 impede DNA replication as a consequence of altered transcription. Surprisingly, cells that either overexpressed or were silenced for RUVBL1 had slower replication fork rates and accumulated phosphorylated H2AX, dependent on active transcription. However, the mechanisms of transcription-dependent replication stress were different when RUVBL1 was overexpressed and when depleted. RUVBL1 overexpression led to increased c-Myc-dependent pause release of RNAPII, as evidenced by higher overall transcription, much stronger Ser2 phosphorylation of Rpb1- C-terminal domain, and enhanced colocalization of Rpb1 and c-Myc. RUVBL1 deficiency resulted in increased ubiquitination of Rpb1 and reduced mobility of an RNAP subunit, suggesting accumulation of stalled RNAPIIs on chromatin. Overall, our data show that by modulating the state of RNAPII complexes, RUVBL1 deregulation induces replication-transcription interference and compromises genome integrity during S-phase.

VPS72/YL1-Mediated H2A.Z Deposition Is Required for Nuclear Reassembly after Mitosis

The eukaryotic nucleus remodels extensively during mitosis. Upon mitotic entry, the nuclear envelope breaks down and chromosomes condense into rod-shaped bodies, which are captured by the spindle apparatus and segregated during anaphase. Through telophase, chromosomes decondense and the nuclear envelope reassembles, leading to a functional interphase nucleus. While the molecular processes occurring in early mitosis are intensively investigated, our knowledge about molecular mechanisms of nuclear reassembly is rather limited. Using cell free and cellular assays, we identify the histone variant H2A.Z and its chaperone VPS72/YL1 as important factors for reassembly of a functional nucleus after mitosis. Live-cell imaging shows that siRNA-mediated downregulation of VPS72 extends the telophase in HeLa cells. In vitro, depletion of VPS72 or H2A.Z results in malformed and nonfunctional nuclei. VPS72 is part of two chromatin-remodeling complexes, SRCAP and EP400. Dissecting the mechanism of nuclear reformation using cell-free assays, we, however, show that VPS72 functions outside of the SRCAP and EP400 remodeling complexes to deposit H2A.Z, which in turn is crucial for formation of a functional nucleus.

Drosophila Morgana is an Hsp90-interacting protein with a direct role in microtubule polymerisation

Morgana (Mora, also known as CHORD in flies) and its mammalian homologue, called CHORDC1 or CHP1, is a highly conserved cysteine and histidine-rich domain (CHORD)-containing protein that has been proposed to function as an Hsp90 co-chaperone. Morgana deregulation promotes carcinogenesis in both mice and humans while, in Drosophila, loss of mora causes lethality and a complex mitotic phenotype that is rescued by a human morgana transgene. Here, we show that Drosophila Mora localises to mitotic spindles and co-purifies with the Hsp90-R2TP-TTT supercomplex and with additional well-known Hsp90 co-chaperones. Acute inhibition of Mora function in the early embryo results in a dramatic reduction in centrosomal microtubule stability, leading to small spindles nucleated from mitotic chromatin. Purified Mora binds to microtubules directly and promotes microtubule polymerisation in vitro, suggesting that Mora directly regulates spindle dynamics independently of its Hsp90 co-chaperone role.

Liver haploinsufficiency of RuvBL1 causes hepatic insulin resistance and enhances hepatocellular carcinoma progression

RuvBL1 is an AAA+ ATPase whose expression in hepatocellular carcinoma (HCC) correlates with a poor prognosis. In vitro models suggest that targeting RuvBL1 could be an effective strategy against HCC. However, the role of RuvBL1 in the onset and progression of HCC remains unknown. To address this question, we developed a RuvBL1^hep+/- mouse model and evaluated the outcome of DEN-induced liver carcinogenesis up to 12 months of progression. We found that RuvBL1 haploinsufficiency initially delayed the onset of liver cancer, due to a reduced hepatocyte turnover in RuvBL1^hep+/- mice. However, RuvBL1^hep+/- mice eventually developed HCC nodules that, with aging, grew larger than in the control mice. Moreover, RuvBL1^hep+/- mice developed hepatic insulin resistance and impaired glucose homeostasis. We could determine that RuvBL1 regulates insulin signaling through the Akt/mTOR pathway in liver physiology in vivo as well as in normal hepatocytic and HCC cells in vitro. Whole transcriptome analysis of mice livers confirmed the major role of RuvBL1 in the regulation of hepatic glucose metabolism. Finally, RuvBL1 expression was found significantly correlated to glucose metabolism and mTOR signaling by bioinformatic analysis of human HCC sample from the publicly available TGCA database. These data uncover a role of RuvBL1 at the intersection of liver metabolism, hepatocyte proliferation and HCC development, providing a molecular rationale for its overexpression in liver cancer.

Actomyosin contraction during cellularization is regulated in part by Src64 control of Actin 5C protein levels

Src64 is required for actomyosin contraction during cellularization of the Drosophila embryonic blastoderm. The mechanism of actomyosin ring constriction is poorly understood even though a number of cytoskeletal regulators have been implicated in the assembly, organization, and contraction of these microfilament rings. How these cytoskeletal processes are regulated during development is even less well understood. To investigate the role of Src64 as an upstream regulator of actomyosin contraction, we conducted a proteomics screen to identify proteins whose expression levels are controlled by src64. Global levels of actin are reduced in src64 mutant embryos. Furthermore, we show that reduction of the actin isoform Actin 5C causes defects in actomyosin contraction during cellularization similar to those caused by src64 mutation, indicating that a relatively high level of Actin 5C is required for normal actomyosin contraction and furrow canal structure. However, reduction of Actin 5C levels only slows down actomyosin ring constriction rather than preventing it, suggesting that src64 acts not only to modulate actin levels, but also to regulate the actomyosin cytoskeleton by other means.

Quantifying post-transcriptional regulation in the development of Drosophila melanogaster

Even though proteins are produced from mRNA, the correlation between mRNA levels and protein abundances is moderate in most studies, occasionally attributed to complex post-transcriptional regulation. To address this, we generate a paired transcriptome/proteome time course dataset with 14 time points during Drosophila embryogenesis. Despite a limited mRNA-protein correlation (ρ = 0.54), mathematical models describing protein translation and degradation explain 84% of protein time-courses based on the measured mRNA dynamics without assuming complex post transcriptional regulation, and allow for classification of most proteins into four distinct regulatory scenarios. By performing an in-depth characterization of the putatively post-transcriptionally regulated genes, we postulate that the RNA-binding protein Hrb98DE is involved in post-transcriptional control of sugar metabolism in early embryogenesis and partially validate this hypothesis using Hrb98DE knockdown. In summary, we present a systems biology framework for the identification of post-transcriptional gene regulation from large-scale, time-resolved transcriptome and proteome data.

Targeted deletion of the AAA-ATPase Ruvbl1 in mice disrupts ciliary integrity and causes renal disease and hydrocephalus

Ciliopathies comprise a large number of hereditary human diseases and syndromes caused by mutations resulting in dysfunction of either primary or motile cilia. Both types of cilia share a similar architecture. While primary cilia are present on most cell types, expression of motile cilia is limited to specialized tissues utilizing ciliary motility. We characterized protein complexes of ciliopathy proteins and identified the conserved AAA-ATPase Ruvbl1 as a common novel component. Here, we demonstrate that Ruvbl1 is crucial for the development and maintenance of renal tubular epithelium in mice: both constitutive and inducible deletion in tubular epithelial cells result in renal failure with tubular dilatations and fewer ciliated cells. Moreover, inducible deletion of Ruvbl1 in cells carrying motile cilia results in hydrocephalus, suggesting functional relevance in both primary and motile cilia. Cilia of Ruvbl1-negative cells lack crucial proteins, consistent with the concept of Ruvbl1-dependent cytoplasmic pre-assembly of ciliary protein complexes.

A protein involved in building and maintaining thin protrusions from cell surfaces called cilia is implicated in “ciliopathies”, diseases in which ciliary function is disrupted. These include polycystic kidney disease and disorders collectively known as ciliary dyskinesias. “Primary cilia” perform sensory functions, detecting external chemical and physical signals and initiating responses within cells. In addition, “motile cilia” beat rhythmically to move fluids surrounding cells. Researchers in Germany and the Netherlands, led by Bernhard Schermer and Max C. Liebau at the University of Cologne, studied a protein called Ruvbl1, known to interact with DNA and other proteins. The researchers found it is crucial for the functioning of both types of cilia. Deleting the gene for Ruvbl1 in mice caused kidney failure and a build-up of fluid in the brain known as hydrocephalus. The research could help understand and ultimately treat ciliopathies.

Genome maintenance functions of the INO80 chromatin remodeller

Chromatin modification is conserved in all eukaryotes and is required to facilitate and regulate DNA-templated processes. For example, chromatin manipulation, such as histone post-translational modification and nucleosome positioning, play critical roles in genome stability pathways. The INO80 chromatin-remodelling complex, which regulates the abundance and positioning of nucleosomes, is particularly important for proper execution of inducible responses to DNA damage. This review discusses the participation and activity of the INO80 complex in DNA repair and cell cycle checkpoint pathways, with emphasis on the Saccharomyces cerevisiae model system. Furthermore, the role of ATM/ATR kinases, central regulators of DNA damage signalling, in the regulation of INO80 function will be reviewed. In addition, emerging themes of chromatin remodelling in mitotic stability pathways and chromosome segregation will be introduced. These studies are critical to understanding the dynamic chromatin landscape that is rapidly and reversibly modified to maintain the integrity of the genome.This article is part of the themed issue 'Chromatin modifiers and remodellers in DNA repair and signalling'.

Potential mechanism and drug candidates for sepsis-induced acute lung injury

The present study aimed to explore the mechanisms underlying sepsis-induced acute lung injury (ALI) and identify more effective therapeutic strategies to treat it. The gene expression data set GSE10474 was downloaded and assessed to identify differentially expressed genes (DEGs). Principal component analysis, functional enrichment analysis and differential co-expression analysis of DEGs were performed. Furthermore, potential target drugs for key DEGs were assessed. A total of 209 DEGs, including 107 upregulated and 102 downregulated genes were screened. A number of DEGs, including zinc finger and BTB domain containing 17 (ZBTB17), heat shock protein 90 kDa β, member 1 (HSP90B1) and major histocompatibility complex, class II, DR α were identified. Furthermore, gene ontology terms including antigen processing and presentation, glycerophospholipid metabolism, transcriptional misregulation in cancer, thyroid hormone synthesis and pathways associated with diseases, such as asthma were identified. In addition, a differential co-expression network containing ubiquitin-conjugating enzyme E2 D4, putative and tubulin, γ complex associated protein 3 was constructed. Furthermore, a number of gene-drug interactions, including between HSP90B1 and adenosine-5′-diphosphate and radicicol, were identified. Therefore, DEGs, including ZBTB17 and HSP90B1, may be important in the pathogenesis of sepsis-induced ALI. Furthermore, drugs including adenosine-5′-diphosphate may be novel drug candidates to treat patients with ALI.

Stress-induced Oryza sativa RuvBL1a is DNA-independent ATPase and unwinds DNA duplex in 3' to 5' direction

RuvB, a member of AAA+ (ATPases Associated with diverse cellular Activities) superfamily of proteins, is essential, highly conserved and multifunctional in nature as it is involved in DNA damage repair, mitotic assembly, switching of histone variants and assembly of telomerase core complex. RuvB family is widely studied in various systems such as Escherichia coli, yeast, human, Drosophila, Plasmodium falciparum and mouse, but not well studied in plants. We have studied the transcript level of rice homologue of RuvB gene (OsRuvBL1a) under various abiotic stress conditions, and the results suggest that it is upregulated under salinity, cold and heat stress. Therefore, the OsRuvBL1a protein was characterized using in silico and biochemical approaches. In silico study confirmed the presence of all the four characteristic motifs of AAA+ superfamily-Walker A, Walker B, Sensor I and Sensor II. Structurally, OsRuvBL1a is similar to RuvB1 from Chaetomium thermophilum. The purified recombinant OsRuvBL1a protein shows unique DNA-independent ATPase activity. Using site-directed mutagenesis, the importance of two conserved motifs (Walker B and Sensor I) in ATPase activity has been also reported with mutants D302N and N332H. The OsRuvBL1a protein unwinds the duplex DNA in the 3' to 5' direction. The presence of unique DNA-independent ATPase and DNA unwinding activities of OsRuvBL1a protein and upregulation of its transcript under abiotic stress conditions suggest its involvement in multiple cellular pathways. The first detailed characterization of plant RuvBL1a in this study may provide important contribution in exploiting the role of RuvB for developing the stress tolerant plants of agricultural importance.

The Role of Pontin and Reptin in Cellular Physiology and Cancer Etiology

Pontin (RUVBL1, TIP49, TIP49a, Rvb1) and Reptin (RUVBL2, TIP48, TIP49b, Rvb2) are highly conserved ATPases of the AAA+ (ATPases Associated with various cellular Activities) superfamily and are involved in various cellular processes that are important for oncogenesis. First identified as being upregulated in hepatocellular carcinoma and colorectal cancer, their overexpression has since been shown in multiple cancer types such as breast, lung, gastric, esophageal, pancreatic, kidney, bladder as well as lymphatic, and leukemic cancers. However, their exact functions are still quite unknown as they interact with many molecular complexes with vastly different downstream effectors. Within the nucleus, Pontin and Reptin participate in the TIP60 and INO80 complexes important for chromatin remodeling. Although not transcription factors themselves, Pontin and Reptin modulate the transcriptional activities of bona fide proto-oncogenes such as MYC and β-catenin. They associate with proteins involved in DNA damage repair such as PIKK complexes as well as with the core complex of Fanconi anemia pathway. They have also been shown to be important for cell cycle progression, being involved in assembly of telomerase, mitotic spindle, RNA polymerase II, and snoRNPs. When the two ATPases localize to the cytoplasm, they were reported to promote cancer cell invasion and metastasis. Due to their various roles in carcinogenesis, it is not surprising that Pontin and Reptin are proving to be important biomarkers for diagnosis and prognosis of various cancers. They are also current targets for the development of new therapeutic anticancer drugs.

Characterization of SKAP/kinastrin isoforms: the N-terminus defines tissue specificity and Pontin binding

Small Kinetochore-Associated Protein (SKAP)/Kinastrin is a multifunctional protein with proposed roles in mitosis, apoptosis and cell migration. Exact mechanisms underlying its activities in these cellular processes are not completely understood. SKAP is predicted to have different isoforms, however, previous studies did not differentiate between them. Since distinct molecular architectures of protein isoforms often influence their localization and functions, this study aimed to examine the expression profile and functional differences between SKAP isoforms in human and mouse. Analyses of various human tissues and cells of different origin by RT-PCR, and by Western blotting and immunocytochemistry applying newly generated anti-SKAP monoclonal antibodies revealed that human SKAP exists in two protein isoforms: ubiquitously expressed SKAP16 and testis/sperm-specific SKAP1. In mouse, SKAP1 expression is detectable in testis at 4 weeks postnatally, when the first wave of spermatogenesis in mice is complete and the elongated spermatids are present in the testes. Furthermore, we identified Pontin as a new SKAP1 interaction partner. SKAP1 and Pontin co-localized in the flagellar region of human sperm suggesting a functional relevance for SKAP1-Pontin interaction in sperm motility. Since most previous studies on SKAP were performed with the testis-specific isoform SKAP1, our findings provide a new basis for future studies on the role of SKAP in both human somatic cells and male germ cells, including studies on male fertility.

Genome-wide genetic screen identified the link between dG9a and epidermal growth factor receptor signaling pathway in vivo

G9a is one of the histone H3 Lys 9 (H3K9) specific methyltransferases first identified in mammals. Drosophila G9a (dG9a) has been reported to induce H3K9 dimethylation in vivo, and the target genes of dG9a were identified during embryonic and larval stages. Although dG9a is important for a variety of developmental processes, the link between dG9a and signaling pathways are not addressed yet. Here, by genome-wide genetic screen, taking advantage of the rough eye phenotype of flies that over-express dG9a in eye discs, we identified 16 genes that enhanced the rough eye phenotype induced by dG9a over-expression. These 16 genes included Star, anterior open, bereft and F-box and leucine-rich repeat protein 6 which are components of epidermal growth factor receptor (EGFR) signaling pathway. When dG9a over-expression was combined with mutation of Star, differentiation of R7 photoreceptors in eye imaginal discs as well as cone cells and pigment cells in pupal retinae was severely inhibited. Furthermore, the dG9a over-expression reduced the activated ERK signals in eye discs. These data demonstrate a strong genetic link between dG9a and the EGFR signaling pathway.

Specific threonine-4 phosphorylation and function of RNA polymerase II CTD during M phase progression

Dynamic phosphorylation of Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 heptad-repeats in the C-terminal domain (CTD) of the large subunit coordinates progression of RNA polymerase (Pol) II through the transcription cycle. Here, we describe an M phase-specific form of Pol II phosphorylated at Thr4, but not at Tyr1, Ser2, Ser5, and Ser7 residues. Thr4 phosphorylated Pol II binds to centrosomes and midbody and interacts with the Thr4-specific Polo-like kinase 1. Binding of Pol II to centrosomes does not require the CTD but may involve subunits of the non-canonical R2TP-Prefoldin-like complex, which bind to and co-localize with Pol II at centrosomes. CTD Thr4 mutants, but not Ser2 and Ser5 mutants, display severe mitosis and cytokinesis defects characterized by multipolar spindles and polyploid cells. We conclude that proper M phase progression of cells requires binding of Pol II to centrosomes to facilitate regulation of mitosis and cytokinesis in a CTD Thr4-P dependent manner.

A genetic interaction map of cell cycle regulators

A combination of genome-scale RNA interference screening and genetic interaction analysis using process-directed phenotypes is used to assign components to specific pathways and complexes for modulators of mitosis and cytokinesis in Drosophila S2 cells.

Cell-based RNA interference (RNAi) is a powerful approach to screen for modulators of many cellular processes. However, resulting candidate gene lists from cell-based assays comprise diverse effectors, both direct and indirect, and further dissecting their functions can be challenging. Here we screened a genome-wide RNAi library for modulators of mitosis and cytokinesis in Drosophila S2 cells. The screen identified many previously known genes as well as modulators that have previously not been connected to cell cycle control. We then characterized ∼300 candidate modifiers further by genetic interaction analysis using double RNAi and a multiparametric, imaging-based assay. We found that analyzing cell cycle–relevant phenotypes increased the sensitivity for associating novel gene function. Genetic interaction maps based on mitotic index and nuclear size grouped candidates into known regulatory complexes of mitosis or cytokinesis, respectively, and predicted previously uncharacterized components of known processes. For example, we confirmed a role for the Drosophila CCR4 mRNA processing complex component l(2)NC136 during the mitotic exit. Our results show that the combination of genome-scale RNAi screening and genetic interaction analysis using process-directed phenotypes provides a powerful two-step approach to assigning components to specific pathways and complexes.

Chromosome Missegregation Associated with RUVBL1 Deficiency

RUVBL1 (RuvB-like1) and RUVBL2 (RuvB-like 2) are integral components of multisubunit protein complexes involved in processes ranging from cellular metabolism, transcription and chromatin remodeling to DNA repair. Here, we show that although RUVBL1 and RUVBL2 are known to form heterodimeric complexes in which they stabilize each other, the subunits separate during cytokinesis. In anaphase-to-telophase transition, RUVBL1 localizes to structures of the mitotic spindle apparatus, where it partially co-localizes with polo-like kinase 1 (PLK1). The ability of PLK1 to phosphorylate RUVBL1-but not RUVBL2-in vitro and their physical association in vivo suggest that this kinase differentially regulates the function of the RuvB-like proteins during mitosis. We further show that siRNA-mediated knock-down of RuvB-like proteins causes severe defects in chromosome alignment and segregation. In addition, we show that the ATPase activity of RUVBL1 is indispensable for cell proliferation. Our data thus demonstrate that RUVBL1 is essential for efficient mitosis and proliferation.

Upregulation of alphaB-crystallin expression in the substantia nigra of patients with Parkinson's disease

Parkinson's disease (PD) is one of the most devastating neurodegenerative disorders. The underlying mechanisms of the characteristic neurodegeneration in the substantia nigra (SN) are still not fully understood. To better understand the molecular events occurring in the SN of PD brain, we used the culture-derived isotope tag-based quantitative proteomics to compare the protein expression profiles in the nigral tissue of PD patients and control subjects. We identified a total of 11 differentially expressed proteins, including alphaB-crystallin (Cryab). Both the levels and pattern of Cryab expression in the SN were validated. It was revealed that Cryab was markedly upregulated in the SN of PD brain. Cryab expression was also upregulated in reactive astrocytes and microglia in a neurotoxin-induced mouse PD model. Moreover, we showed increased expression of Cryab in cytoplasmic inclusions in a subset of glial cells in Parkinsonian brain. Thus, we identified Cryab that is highly expressed in the SN of PD brain and may be involved in the glial pathology during dopaminergic neuron degeneration in PD.

Abundance of the Fanconi anaemia core complex is regulated by the RuvBL1 and RuvBL2 AAA+ ATPases

Fanconi anaemia (FA) is a genome instability disease caused by defects in the FA DNA repair pathway that senses and repairs damage caused by DNA interstrand crosslinks. At least 8 of the 16 genes found mutated in FA encode proteins that assemble into the FA core complex, a multisubunit monoubiquitin E3 ligase. Here, we show that the RuvBL1 and RuvBL2 AAA+ ATPases co-purify with FA core complex isolated under stringent but native conditions from a vertebrate cell line. Depletion of the RuvBL1-RuvBL2 complex in human cells causes hallmark features of FA including DNA crosslinker sensitivity, chromosomal instability and defective FA pathway activation. Genetic knockout of RuvBL1 in a murine model is embryonic lethal while conditional inactivation in the haematopoietic stem cell pool confers profound aplastic anaemia. Together these findings reveal a function for RuvBL1-RuvBL2 in DNA repair through a physical and functional association with the FA core complex. Surprisingly, depletion of RuvBL1-RuvBL2 leads to co-depletion of the FA core complex in human cells. This suggests that a potential mechanism for the role of RuvBL1-RuvBL2 in maintaining genome integrity is through controlling the cellular abundance of FA core complex.

The R2TP chaperone complex: its involvement in snoRNP assembly and tumorigenesis

R2TP was originally identified in yeast Saccharomyces cerevisiae as Hsp90 interacting complex, and is composed of four different proteins: Rvb1, Rvb2, Tah1, and Pih1. This complex is well-conserved in eukaryotes, and is involved in many cellular processes such as snoRNP biogenesis, RNA polymerase assembly, PIKK signaling, and apoptosis. An increasing number of research related to R2TP suggests a linkage of its function with tumorigenesis. In this review, we provide an overview of several recent studies on R2TP that are related to cell proliferation and carcinogenesis, and propose a possible role of R2TP in tumorigenesis through regulating snoRNA/snoRNP biogenesis.

LINKIN, a new transmembrane protein necessary for cell adhesion

In epithelial collective migration, leader and follower cells migrate while maintaining cell–cell adhesion and tissue polarity. We have identified a conserved protein and interactors required for maintaining cell adhesion during a simple collective migration in the developing C. elegans male gonad. LINKIN is a previously uncharacterized, transmembrane protein conserved throughout Metazoa. We identified seven atypical FG–GAP domains in the extracellular domain, which potentially folds into a β-propeller structure resembling the α-integrin ligand-binding domain. C. elegans LNKN-1 localizes to the plasma membrane of all gonadal cells, with apical and lateral bias. We identified the LINKIN interactors RUVBL1, RUVBL2, and α-tubulin by using SILAC mass spectrometry on human HEK 293T cells and testing candidates for lnkn-1-like function in C. elegans male gonad. We propose that LINKIN promotes adhesion between neighboring cells through its extracellular domain and regulates microtubule dynamics through RUVBL proteins at its intracellular domain.

DOI:http://dx.doi.org/10.7554/eLife.04449.001

In animals, cells can move from one place to another to shape tissues, heal wounds, or defend against invading microbes. A cell may move alone or it may be attached to others and move as part of a group. One member of the group leads this ‘collective migration’, but it is not known how the cells are able to stick to each other and move together.

Collective migration takes place in the male gonad—the organ that makes sperm cells—in larvae of the nematode worm C. elegans. As the gonad matures, a group of cells form a simple chain that can move together. Kato et al. found that a protein called LINKIN must be present for this to happen.

LINKIN is found in the membrane that surrounds animal cells. One section of the protein—called the β-propeller—sits on the outside surface of the membrane. The structure of the β-propeller is similar to a section of another protein—called α-integrin—that also allows cells to attach, suggesting LINKIN may work in a similar way.

LINKIN is found in many animals, so Kato et al. searched for proteins that can interact with it in human cells. This search revealed three proteins that can interact with LINKIN and are required for the cells to move together. Two of the proteins control elements of the internal scaffolding of the cell: this scaffolding, which is known as the cytoskeleton, is involved in moving the cells.

The experiments suggest that LINKIN coordinates the process of binding together with the changes in the cytoskeleton that are needed to allow the cells to move as one. The next challenge is to understand how LINKIN changes the internal program of the cells to achieve this.

DOI:http://dx.doi.org/10.7554/eLife.04449.002

Pontin is a critical regulator for AML1-ETO-induced leukemia

The oncogenic fusion protein AML1-ETO, also known as RUNX1-RUNX1T1 is generated by the t(8;21)(q22;q22) translocation, one of the most frequent chromosomal rearrangements in acute myeloid leukemia (AML). Identifying the genes that cooperate with or are required for the oncogenic activity of this chimeric transcription factor remains a major challenge. Our previous studies showed that Drosophila provides a genuine model to study how AML1-ETO promotes leukemia. Here, using an in vivo RNA interference screen for suppressors of AML1-ETO activity, we identified pontin/RUVBL1 as a gene required for AML1-ETO-induced lethality and blood cell proliferation in Drosophila. We further show that PONTIN inhibition strongly impaired the growth of human t(8;21)(+) or AML1-ETO-expressing leukemic blood cells. Interestingly, AML1-ETO promoted the transcription of PONTIN. Moreover, transcriptome analysis in Kasumi-1 cells revealed a strong correlation between PONTIN and AML1-ETO gene signatures and demonstrated that PONTIN chiefly regulated the expression of genes implicated in cell cycle progression. Concordantly, PONTIN depletion inhibited leukemic self-renewal and caused cell cycle arrest. All together our data suggest that the upregulation of PONTIN by AML1-ETO participate in the oncogenic growth of t(8;21) cells.

Growth inhibition and differences in protein profiles in azadirachtin-treated Drosophila melanogaster larvae

Azadirachtin A is a very effective biopesticide widely used in insect pest control. It has strong antifeeding and growth inhibitory activity against most insects, however, its mode of action is still unclear. Proteomic experiments using 2DE indicate significant effects of Azadirachtin A on the amount of proteins related to growth inhibition in Drosophila melanogaster larvae. Twenty-one spots with different intensity in azadirachtin-treated larvae were identified. These proteins are involved in cytoskeletal organization, transcription and translation, hormonal regulation, and energy metabolism. Protein network analysis reveals heat shock protein 23 to be a potential target of azadirachtin. These results provide new insights into understanding the mechanism of growth inhibition in insects in response to azadirachtin.

New mitotic regulators released from chromatin

Faithful action of the mitotic spindle segregates duplicated chromosomes into daughter cells. Perturbations of this process result in chromosome mis-segregation, leading to chromosomal instability and cancer development. Chromosomes are not simply passengers segregated by spindle microtubules but rather play a major active role in spindle assembly. The GTP bound form of the Ran GTPase (RanGTP), produced around chromosomes, locally activates spindle assembly factors. Recent studies have uncovered that chromosomes organize mitosis beyond spindle formation. They distinctly regulate other mitotic events, such as spindle maintenance in anaphase, which is essential for chromosome segregation. Furthermore, the direct function of chromosomes is not only to produce RanGTP but, in addition, to release key mitotic regulators from chromatin. Chromatin-remodeling factors and nuclear pore complex proteins, which have established functions on chromatin in interphase, dissociate from mitotic chromatin and function in spindle assembly or maintenance. Thus, chromosomes actively organize their own segregation using chromatin-releasing mitotic regulators as well as RanGTP.

Chaperone-like activity of the AAA+ proteins Rvb1 and Rvb2 in the assembly of various complexes

Rvb1 and Rvb2 are highly conserved and essential eukaryotic AAA+ proteins linked to a wide range of cellular processes. AAA+ proteins are ATPases associated with diverse cellular activities and are characterized by the presence of one or more AAA+ domains. These domains have the canonical Walker A and Walker B nucleotide binding and hydrolysis motifs. Rvb1 and Rvb2 have been found to be part of critical cellular complexes: the histone acetyltransferase Tip60 complex, chromatin remodelling complexes Ino80 and SWR-C, and the telomerase complex. In addition, Rvb1 and Rvb2 are components of the R2TP complex that was identified by our group and was determined to be involved in the maturation of box C/D small nucleolar ribonucleoprotein (snoRNP) complexes. Furthermore, the Rvbs have been associated with mitotic spindle assembly, as well as phosphatidylinositol 3-kinase-related protein kinase (PIKK) signalling. This review sheds light on the potential role of the Rvbs as chaperones in the assembly and remodelling of these critical complexes.

Integrated regulation of PIKK-mediated stress responses by AAA+ proteins RUVBL1 and RUVBL2

Proteins of the phosphatidylinositol 3-kinase-related protein kinase (PIKK) family are activated by various cellular stresses, including DNA damage, premature termination codon and nutritional status, and induce appropriate cellular responses. The importance of PIKK functions in the maintenance of genome integrity, accurate gene expression and the proper control of cell growth/proliferation is established. Recently, ATPase associated diverse cellular activities (AAA+) proteins RUVBL1 and RUVBL2 (RUVBL1/2) have been shown to be common regulators of PIKKs. The RUVBL1/2 complex regulates PIKK-mediated stress responses through physical interactions with PIKKs and by controlling PIKK mRNA levels. In this review, the functions of PIKKs in stress responses are outlined and the physiological significance of the integrated regulation of PIKKs by the RUVBL1/2 complex is presented. We also discuss a putative "PIKK regulatory chaperone complex" including other PIKK regulators, Hsp90 and the Tel2 complex.

Plasmodium falciparum RuvB proteins: Emerging importance and expectations beyond cell cycle progression

The urgent requirement of next generation antimalarials has been of recent interest due to the emergence of drug-resistant parasite. The genome-wide analysis of Plasmodium falciparum helicases revealed three RuvB proteins. Due to the presence of helicase motif I and II in PfRuvBs, there is a high probability that they contain ATPase and possibly helicase activity. The Plasmodium database has homologs of several key proteins that interact with RuvBs and are most likely involved in the cell cycle progression, chromatin remodeling, and other cellular activities. Phylogenetically PfRuvBs are closely related to Saccharomyces cerevisiae RuvB, which is essential for cell cycle progression and survival of yeast. Thus PfRuvBs can serve as potential drug target if they show an essential role in the survival of parasite.

Induction of ASAP (MAP9) contributes to p53 stabilization in response to DNA damage

p53 is a key tumor suppressor that controls DNA damage response and genomic integrity. In response to genotoxic stress, p53 is stabilized and activated, resulting in controlled activation of genes involved in cell cycle arrest, DNA repair and/or apoptosis. ASAP is a centrosome- and spindle-associated protein, the deregulation of which induces severe mitotic defects. We show here that following double-strand break DNA formation, ASAP directly interacts with and stabilizes p53 by enhancing its p300-mediated acetylation and blocking its MDM2-mediated ubiquitination and degradation, leading to an increase of p53 transcriptional activity. Upon DNA damage, ASAP is transiently accumulated before being degraded upon persistent damage. This work links the p53 response with the cytoskeleton and confirms that the DNA-damaging signaling pathway is coordinated by centrosomal proteins. We reveal the existence of a new pathway through which ASAP signals the DNA damage response by regulating the p300-MDM2-p53 loop. These results point out ASAP as a possible target for the design of drugs to sensitize radio-resistant tumors.

The human Ino80 binds to microtubule via the E-hook of tubulin: implications for the role in spindle assembly

The human INO80 chromatin remodeling complex, comprising the Ino80 ATPase (hIno80) and the associated proteins such as Tip49a, has been implicated in a variety of nuclear processes other than transcription. We previously have found that hIno80 interacts with tubulin and co-localizes with the mitotic spindle and is required for spindle formation. To better understand the role of hIno80 in spindle formation, we further investigated the interaction between hIno80 and microtubule. Here, we show that the N-terminal domain, dispensable for the nucleosome remodeling activity, is important for hIno80 to interact with tubulin and co-localize with the spindle. The hIno80 N-terminal domain binds to monomeric tubulin and polymerized microtubule in vitro, and the E-hook of tubulin, involved in the polymerization of microtubule, is critical for this binding. Tip49a, which has been reported to associate with the spindle, does not bind to microtubule in vitro and dispensable for spindle formation in vivo. These results suggest that hIno80 can play a direct role in the spindle assembly independent of its chromatin remodeling activity.

The functional domain grouping of microtubule associated proteins

Microtubules (MTs), which play crucial roles in normal cell function, are regulated by MT associated proteins (MAPs). Using a combinatorial approach that includes biochemistry, proteomics and bioinformatics, we have recently identified 270 putative MAPs from Drosophila embryos and characterized some of those required for correct progression through mitosis. Here we identify functional groups of these MAPs using a reciprocal hits sequence alignment technique and assign InterPro functional domains to 28 previously uncharacterized proteins. This approach gives insight into the potential functions of MAPs and how their roles may affect MTs.

The gammaTuRC revisited: a comparative analysis of interphase and mitotic human gammaTuRC redefines the set of core components and identifies the novel subunit GCP8

The γ-tubulin complex is a multi-subunit protein complex that nucleates microtubule polymerization. γ-Tubulin complexes are present in all eukaryotes, but size and subunit composition vary. In Drosophila, Xenopus, and humans large γ-tubulin ring complexes (γTuRCs) have been described, which have a characteristic open ring-shaped structure and are composed of a similar set of subunits, named γ-tubulin, GCPs 2-6, and GCP-WD in humans. Despite the identification of these proteins, γTuRC function and regulation remain poorly understood. Here we establish a new method for the purification of native human γTuRC. Using mass spectrometry of whole protein mixtures we compared the composition of γTuRCs from nonsynchronized and mitotic human cells. Based on our analysis we can define core subunits as well as more transient interactors such as the augmin complex, which associates specifically with mitotic γTuRCs. We also identified GCP8/MOZART2 as a novel core subunit that is present in both interphase and mitotic γTuRCs. GCP8 depletion does not affect γTuRC assembly but interferes with γTuRC recruitment and microtubule nucleation at interphase centrosomes without disrupting general centrosome structure. GCP8-depleted cells do not display any obvious mitotic defects, suggesting that GCP8 specifically affects the organization of the interphase microtubule network.

Roles of human INO80 chromatin remodeling enzyme in DNA replication and chromosome segregation suppress genome instability

Although INO80 chromatin remodeling enzyme has been shown in yeast to play roles in non-transcriptional nuclear processes such as DNA replication, its cellular functions in higher eukaryotes have remained largely unexplored. Here, we provide evidence that human INO80 (hINO80) participates in both DNA replication and chromosome segregation during the normal cell division cycle. hINO80 binds to chromatin localizing at replication forks during the S-phase, and is required for efficient DNA synthesis and S-phase progression. Unexpectedly, hINO80 associates with spindle microtubule during mitosis, and its deficiency leads to defective microtubule assembly and abnormal chromosome segregation. Consistent with these results, hINO80 is critical for suppressing aneuploidy and structural chromosome abnormalities. This work therefore not only emphasizes the evolutionary importance of INO80 in DNA replication but also reveals a new role for this remodeler in chromosome segregation, both of which likely come into play in maintaining the genome integrity.

Oligomeric assembly and interactions within the human RuvB-like RuvBL1 and RuvBL2 complexes

The two closely related eukaryotic AAA+ proteins (ATPases associated with various cellular activities), RuvBL1 (RuvB-like 1) and RuvBL2, are essential components of large multi-protein complexes involved in diverse cellular processes. Although the molecular mechanisms of RuvBL1 and RuvBL2 function remain unknown, oligomerization is likely to be important for their function together or individually, and different oligomeric forms might underpin different functions. Several experimental approaches were used to investigate the molecular architecture of the RuvBL1-RuvBL2 complex and the role of the ATPase-insert domain (domain II) for its assembly and stability. Analytical ultracentrifugation showed that RuvBL1 and RuvBL2 were mainly monomeric and each monomer co-existed with small proportions of dimers, trimers and hexamers. Adenine nucleotides induced hexamerization of RuvBL2, but not RuvBL1. In contrast, the RuvBL1-RuvBL2 complexes contained single- and double-hexamers together with smaller forms. The role of domain II in complex assembly was examined by size-exclusion chromatography using deletion mutants of RuvBL1 and RuvBL2. Significantly, catalytically competent dodecameric RuvBL1-RuvBL2, complexes lacking domain II in one or both proteins could be assembled but the loss of domain II in RuvBL1 destabilized the dodecamer. The composition of the RuvBL1-RuvBL2 complex was analysed by MS. Several species of mixed RuvBL1/2 hexamers with different stoichiometries were seen in the spectra of the RuvBL1-RuvBL2 complex. A number of our results indicate that the architecture of the human RuvBL1-RuvBL2 complex does not fit the recent structural model of the yeast Rvb1-Rvb2 complex.

A membranous spindle matrix orchestrates cell division

Eukaryotic cell division uses morphologically different forms of mitosis, referred to as open, partially open and closed mitosis, for accurate chromosome segregation and proper partitioning of other cellular components such as endomembranes and cell fate determinants. Recent studies suggest that the spindle matrix provides a conserved strategy to coordinate the segregation of genetic material and the partitioning of the rest of the cellular contents in all three forms of mitosis.

Rvb1–Rvb2: essential ATP-dependent helicases for critical complexesThis paper is one of a selection of papers published in this special issue entitled 8th International Conference on AAA Proteins and has undergone the Journal's usual peer review process

Rvb1 and Rvb2 are highly conserved, essential AAA+ helicases found in a wide range of eukaryotes. The versatility of these helicases and their central role in the biology of the cell is evident from their involvement in a wide array of critical cellular complexes. Rvb1 and Rvb2 are components of the chromatin-remodeling complexes INO80, Swr-C, and BAF. They are also members of the histone acetyltransferase Tip60 complex, and the recently identified R2TP complex present in Saccharomyces cerevisiae and Homo sapiens; a complex that is involved in small nucleolar ribonucleoprotein (snoRNP) assembly. Furthermore, in humans, Rvb1 and Rvb2 have been identified in the URI prefoldin-like complex. In Drosophila, the Polycomb Repressive complex 1 contains Rvb2, but not Rvb1, and the Brahma complex contains Rvb1 and not Rvb2. Both of these complexes are involved in the regulation of growth and development genes in Drosophila. Rvbs are therefore crucial factors in various cellular processes. Their importance in chromatin remodeling, transcription regulation, DNA damage repair, telomerase assembly, mitotic spindle formation, and snoRNP biogenesis is discussed in this review.

Adenosine triphosphatase pontin is overexpressed in hepatocellular carcinoma and coregulated with reptin through a new posttranslational mechanism

Reptin and Pontin are related ATPases associated with stoichiometric amounts in several complexes involved in chromatin remodeling, transcriptional regulation, and telomerase activity. We found that Reptin was up-regulated in hepatocellular carcinoma (HCC) and that down-regulation of Reptin led to growth arrest. We show here that Pontin messenger RNA (mRNA) is also up-regulated in human HCC 3.9-fold as compared to nontumor liver (P = 0.0004). Pontin expression was a strong independent factor of poor prognosis in a multivariate analysis. As for Reptin, depletion of Pontin in HuH7 cells with small interfering RNAs (siRNAs) led to growth arrest. Remarkably, Pontin depletion led to down-regulation of Reptin as shown with western blot, and vice versa. Whereas siRNAs induced a decrease of their cognate mRNA targets, they did not affect the transcripts of the partner protein. Translation of Pontin or Reptin was not altered when the partner protein was silenced. However, pulse-chase experiments demonstrated that newly synthesized Pontin or Reptin stability was reduced in Reptin- or Pontin-depleted cells, respectively. This phenomenon was reversed upon inhibition of proteasome or ubiquitin-activating enzyme (E1). In addition, proteasome inhibition could partly restore Pontin steady-state levels in Reptin-depleted cells, as shown by western blot. This restoration was not observed when cells were also treated with cycloheximide, thus confirming that proteasomal degradation in this setting was restricted to newly synthesized Pontin.

CONCLUSION

Reptin and Pontin protein levels are strictly controlled by a posttranslational mechanism involving proteasomal degradation of newly synthesized proteins. These data demonstrate a tight regulatory and reciprocal interaction between Reptin and Pontin, which may in turn lead to the maintenance of their 1:1 stoichiometry.