A role for Ran-GTP and Crm1 in blocking re-replication

Spindle morphology changes between meiosis and mitosis driven by CK2 regulation of the Ran pathway

The transition from meiotic divisions in the oocyte to embryonic mitoses is a critical step in animal development. Despite negligible changes to cell size and shape, following fertilization the small, barrel-shaped meiotic spindle is replaced by a large zygotic spindle that nucleates abundant astral microtubules at spindle poles. To probe underlying mechanisms, we applied a drug screening approach using Ciona eggs and found that inhibition of Casein Kinase 2 (CK2) caused a shift from meiotic to mitotic-like spindle morphology with nucleation of robust astral microtubules, an effect reproduced in cytoplasmic extracts prepared from Xenopus eggs. In both species, CK2 activity decreased at fertilization. Phosphoproteomic differences between Xenopus meiotic and mitotic extracts that also accompanied CK2 inhibition pointed to RanGTP-regulated factors as potential targets. Interfering with RanGTP-driven microtubule formation suppressed astral microtubule growth caused by CK2 inhibition. These data support a model in which CK2 activity attenuation at fertilization leads to activation of RanGTP-regulated microtubule effectors that induce mitotic spindle morphology.

Latcripin-13 domain induces apoptosis and cell cycle arrest at the G1 phase in human lung carcinoma A549 cells

Latcripin-13 domain, isolated from the transcriptome of Lentinula edodes C91-3, contains a regulator of chromosome condensation (RCC1) domain/β-lactamase-inhibitor protein II (BLIP-II) and a plant homeodomain (PHD). Latcripin-13 domain has been shown to have antitumor effects. However, the underlying molecular pharmacology is largely unknown. We report here that Latcripin-13 domain induced cell cycle arrest in the G1 phase and caused the apoptosis of human lung carcinoma A549 cells via the GSK3β-cyclin D1 and caspase-8/NF-κB signaling pathways. Western blot analysis showed that Latcripin-13 domain decreased cyclin D1 and cyclin-dependent kinase 4 (CDK4), while it increased the ratio of GSK3β/phosphorylated GSK3β. Importantly, Latcripin-13 domain induced nuclear fragmentation and chromatin condensation in the A549 cells. In addition, treatment of the A549 cells with Latcripin-13 domain resulted in the loss of mitochondrial membrane potential, accompanied by an increase in the Bax/Bcl-2 ratio and activation of caspase-3, -8, and -9. Intriguingly, western blot analysis revealed that NF-κB was significantly downregulated by Latcripin-13 domain. These results demonstrated that Latcripin-13 domain induced apoptosis and cell cycle arrest at G1 phase in the A549 cells, providing a mechanism for the antitumor effects of Latcripin-13 domain.

Modulation of specific protein expression levels by PTEN: identification of AKAP121, DHFR, G3BP, Rap1, and RCC1 as potential targets of PTEN

The tumor suppressor PTEN is mutated in a high percentage of human cancers, and is implicated in pathways regulating cell growth, proliferation, survival, and migration. Despite significant advances, our understanding of its mechanisms of action remains incomplete. We have used a high-throughput proteomic immunoblotting approach to identify proteins whose expression levels are modulated by PTEN. Out of over 800 proteins screened, 22 proteins showed significant changes in expression. Five proteins that exhibited two-fold or greater changes in expression level were further characterized. AKAP121 and G3BP expression was reduced, while dihydrofolate reductase (DHFR), Rap1 and RCC1 expression was elevated in response to PTEN expression in a PTEN-null T-cell leukemia line. The phosphatase activity of PTEN was required for these effects. However, direct inhibition of PI-3 Kinase could mimic PTEN in modulating expression of DHFR, G3BP, Rap1 and RCC1, but not AKAP121. Real-time PCR showed that the effects of PTEN were primarily post-transcriptional, and would not have been revealed by mRNA-based screens. We conclude from these data that PTEN can modulate the expression level of a number of different proteins. The identified proteins have the potential to serve as previously unrecognized effectors of PTEN, and suggest the existence of additional complexity in the modes by which PTEN can regulate cellular biology.

G protein control of microtubule assembly

Microtubules are dynamic polymers required for many aspects of eukaryotic cell function. The interphase microtubule network is essential for intracellular transport, organization, and cell polarization, whereas the mitotic spindle is required for chromosome segregation and cell division. Studies in different areas such as cell migration, mitosis, and asymmetric cell division have shown that Ran, Rho, and heterotrimeric G proteins regulate many aspects of microtubule functions. This review surveys how G protein-signaling coordinates microtubule polymerization and organization with specific cellular activities.

The c-myc DNA-unwinding element-binding protein modulates the assembly of DNA replication complexes in vitro

The presence of DNA-unwinding elements (DUEs) at eukaryotic replicators has raised the question of whether these elements contribute to origin activity by their intrinsic helical instability, as protein-binding sites, or both. We used the human c-myc DUE as bait in a yeast one-hybrid screen and identified a DUE-binding protein, designated DUE-B, with a predicted mass of 23.4 kDa. Based on homology to yeast proteins, DUE-B was previously classified as an aminoacyl-tRNA synthetase; however, the human protein is approximately 60 amino acids longer than its orthologs in yeast and worms and is primarily nuclear. In vivo, chromatin-bound DUE-B localized to the c-myc DUE region. DUE-B levels were constant during the cell cycle, although the protein was preferentially phosphorylated in cells arrested early in S phase. Inhibition of DUE-B protein expression slowed HeLa cell cycle progression from G1 to S phase and induced cell death. DUE-B extracted from HeLa cells or expressed from baculovirus migrated as a dimer during gel filtration and co-purified with ATPase activity. In contrast to endogenous DUE-B, baculovirus-expressed DUE-B efficiently formed high molecular mass complexes in Xenopus egg and HeLa extracts. In Xenopus extracts, baculovirus-expressed DUE-B inhibited chromatin replication and replication protein A loading in the presence of endogenous DUE-B, suggesting that differential covalent modification of these proteins can alter their effect on replication. Recombinant DUE-B expressed in HeLa cells restored replication activity to egg extracts immunodepleted with anti-DUE-B antibody, suggesting that DUE-B plays an important role in replication in vivo.

Replication-dependent destruction of Cdt1 limits DNA replication to a single round per cell cycle in Xenopus egg extracts

In eukaryotes, prereplication complexes (pre-RCs) containing ORC, Cdc6, Cdt1, and MCM2-7 are assembled on chromatin in the G1 phase. In S phase, when DNA replication initiates, pre-RCs are disassembled, and new pre-RC assembly is restricted until the following G1 period. As a result, DNA replication is limited to a single round per cell cycle. One inhibitor of pre-RC assembly, geminin, was discovered in Xenopus, and it binds and inactivates Cdt1 in S phase. However, removal of geminin from Xenopus egg extracts is insufficient to cause rereplication, suggesting that other safeguards against rereplication exist. Here, we show that Cdt1 is completely degraded by ubiquitin-mediated proteolysis during the course of the first round of DNA replication in Xenopus egg extracts. Degradation depends on Cdk2/Cyclin E, Cdc45, RPA, and polymerase alpha, demonstrating a requirement for replication initiation. Cdt1 is ubiquitinated on chromatin, and this process also requires replication initiation. Once replication has initiated, Cdk2/Cyclin E is dispensable for Cdt1 degradation. When fresh Cdt1 is supplied after the first round of DNA replication, significant rereplication results, and rereplication is enhanced in the absence of geminin. Our results identify a replication-dependent proteolytic pathway that targets Cdt1 and that acts redundantly with geminin to inactivate Cdt1 in S phase.

The mechanism of spindle assembly: functions of Ran and its target TPX2

Recent work has provided new insights into the mechanism of spindle assembly. Growing evidence supports a model in which the small GTPase Ran plays a central role in this process. Here, we examine the evidence for the existence of a RanGTP gradient around mitotic chromosomes and some controversial data on the role that chromosomes play in spindle assembly. We review the current knowledge on the Ran downstream targets for spindle assembly and we focus on the multiple roles of TPX2, one of the targets of RanGTP during cell division.

Nuclear export of the nonenveloped parvovirus virion is directed by an unordered protein signal exposed on the capsid surface

It is uncertain whether nonenveloped karyophilic virus particles may actively traffic from the nucleus outward. The unordered amino-terminal domain of the VP2 major structural protein (2Nt) of the icosahedral parvovirus minute virus of mice (MVM) is internal in empty capsids, but it is exposed outside of the shell through the fivefold axis of symmetry in virions with an encapsidated single-stranded DNA genome, as well as in empty capsids subjected to a heat-induced structural transition. In productive infections of transformed and normal fibroblasts, mature MVM virions were found to efficiently exit from the nucleus prior to cell lysis, in contrast to the extended nuclear accumulation of empty capsids. Newly formed mutant viruses lacking the three phosphorylated serine residues of 2Nt were hampered in their exit from the human transformed NB324K nucleus, in correspondence with the capacity of 2Nt to drive microinjected phosphorylated heated capsids out of the nucleus. However, in normal mouse A9 fibroblasts, in which the MVM capsid was phosphorylated at similar sites but with a much lower rate, the nuclear exit of virions and microinjected capsids harboring exposed 2Nt required the infection process and was highly sensitive to inhibition of the exportin CRM1 in the absence of a demonstrable interaction. Thus, the MVM virion exits the nucleus by accessing nonconventional export pathways relying on cell physiology that can be intensified by infection but in which the exposure of 2Nt remains essential for transport. The flexible 2Nt nuclear transport signal may illustrate a common structural solution used by nonenveloped spherical viruses to propagate in undamaged host tissues.

Karyopherins: from nuclear-transport mediators to nuclear-function regulators

The karyopherin beta (or importin beta) family comprises soluble transport factors that mediate the movement of proteins and RNAs between the nucleus and cytoplasm. Recent studies have extended the role of karyopherins to regulating assembly of the nuclear pore complex (NPC), assembly of the nuclear envelope, mitosis and replication. New data also address how karyopherins specifically recognize and transport many distinct cargoes and traverse the NPC. These data raise the possibility that, although there might be a universal mechanism for nuclear transport, specific interactions between karyopherins and components of the NPC might function to regulate differentially the ability of the different karyopherins to cross the NPC.

Role of the tetradecapeptide repeat domain of human histone deacetylase 6 in cytoplasmic retention

Histone deacetylase 6 (HDAC6) contains tandem catalytic domains and a ubiquitin-binding zinc finger and displays deacetylase activity toward acetylated microtubules. Here we show that unlike its orthologs from Caenorhabditis elegans, Drosophila, and mouse, human HDAC6 possesses a tetradecapeptide repeat domain located between the second deacetylase domain and the C-terminal ubiquitin-binding motif. Related to this structural difference, the cytoplasmic localization of human, but not murine, HDAC6 is resistant to treatment with leptomycin B (LMB). Although it is dispensable for the deacetylase and ubiquitin binding activities of human HDAC6, the tetradecapeptide repeat domain displays acetyl-microtubule targeting ability. Moreover, it forms a unique structure and is required for the LMB-resistant cytoplasmic localization of human HDAC6. Besides the tetradecapeptide repeat domain, human HDAC6 possesses two LMB-sensitive nuclear export signals and a nuclear localization signal. These results thus indicate that the cytoplasmic localization for murine and human HDAC6 proteins is differentially regulated and suggest that the tetradecapeptide repeat domain serves as an important sequence element to stably retain human HDAC6 in the cytoplasm.

Drosophila double-parked is sufficient to induce re-replication during development and is regulated by cyclin E/CDK2

It is important that chromosomes are duplicated only once per cell cycle. Over-replication is prevented by multiple mechanisms that block the reformation of a pre-replicative complex (pre-RC) onto origins in S and G2 phase. We have investigated the developmental regulation of Double-parked (Dup) protein, the Drosophila ortholog of Cdt1, a conserved and essential pre-RC component found in human and other organisms. We find that phosphorylation and degradation of Dup protein at G1/S requires cyclin E/CDK2. The N terminus of Dup, which contains ten potential CDK phosphorylation sites, is necessary and sufficient for Dup degradation during S phase of mitotic cycles and endocycles. Mutation of these ten phosphorylation sites, however, only partially stabilizes the protein, suggesting that multiple mechanisms ensure Dup degradation. This regulation is important because increased Dup protein is sufficient to induce profound rereplication and death of developing cells. Mis-expression has different effects on genomic replication than on developmental amplification from chorion origins. The C terminus alone has no effect on genomic replication, but it is better than full-length protein at stimulating amplification. Mutation of the Dup CDK sites increases genomic re-replication, but is dominant negative for amplification. These two results suggest that phosphorylation regulates Dup activity differently during these developmentally specific types of DNA replication. Moreover, the ability of the CDK site mutant to rapidly inhibit BrdU incorporation suggests that Dup is required for fork elongation during amplification. In the context of findings from human and other cells, our results indicate that stringent regulation of Dup protein is critical to protect genome integrity.

Importin alpha/beta-mediated nuclear protein import is regulated in a cell cycle-dependent manner

Functional nuclear proteins are selectively imported into the nucleus by transport factors such as importins alpha and beta. The relationship between the efficiency of nuclear protein import and the cell cycle was measured using specific import substrates for the importin alpha/beta-mediated pathway. After the microinjection of SV40 T antigen nuclear localization signal (NLS)-containing substrates into the cytoplasm of synchronized culture cells at a certain phase of the cell cycle, the nuclear import of the substrates was measured kinetically. Cell cycle-dependent change in import efficiency, but not capacity, was found. That is, import efficiency was found low in the early S, G2/M, and M/G1 phases compared with other phases. In addition, we found that the extent of co-imunoprecipitation of importin alpha with importin beta from cell extracts was strongly associated with import efficiency. These results indicate that the importin alpha/beta-mediated nuclear import machinery is regulated in a cell cycle-dependent manner through the modulation of interaction modes between importins alpha and beta.

Structural basis for inhibition of the replication licensing factor Cdt1 by geminin

To maintain chromosome stability in eukaryotic cells, replication origins must be licensed by loading mini-chromosome maintenance (MCM2-7) complexes once and only once per cell cycle. This licensing control is achieved through the activities of geminin and cyclin-dependent kinases. Geminin binds tightly to Cdt1, an essential component of the replication licensing system, and prevents the inappropriate reinitiation of replication on an already fired origin. The inhibitory effect of geminin is thought to prevent the interaction between Cdt1 and the MCM helicase. Here we describe the crystal structure of the mouse geminin-Cdt1 complex using tGeminin (residues 79-157, truncated geminin) and tCdt1 (residues 172-368, truncated Cdt1). The amino-terminal region of a coiled-coil dimer of tGeminin interacts with both N-terminal and carboxy-terminal parts of tCdt1. The primary interface relies on the steric complementarity between the tGeminin dimer and the hydrophobic face of the two short N-terminal helices of tCdt1 and, in particular, Pro 181, Ala 182, Tyr 183, Phe 186 and Leu 189. The crystal structure, in conjunction with our biochemical data, indicates that the N-terminal region of tGeminin might be required to anchor tCdt1, and the C-terminal region of tGeminin prevents access of the MCM complex to tCdt1 through steric hindrance.

RB reversibly inhibits DNA replication via two temporally distinct mechanisms

The retinoblastoma (RB) tumor suppressor is a critical negative regulator of cellular proliferation. Repression of E2F-dependent transcription has been implicated as the mechanism through which RB inhibits cell cycle progression. However, recent data have suggested that the direct interaction of RB with replication factors or sites of DNA synthesis may contribute to its ability to inhibit S phase. Here we show that RB does not exert a cis-acting effect on DNA replication. Furthermore, the localization of RB was distinct from replication foci in proliferating cells. While RB activation strongly attenuated the RNA levels of multiple replication factors, their protein expression was not diminished coincident with cell cycle arrest. During the first 24 h of RB activation, components of the prereplication complex, initiation factors, and the clamp loader complex (replication factor C) remained tethered to chromatin. In contrast, the association of PCNA and downstream components of the processive replication machinery was specifically disrupted. This signaling from RB occurred in a manner dependent on E2F-mediated transcriptional repression. Following long-term activation of RB, we observed the attenuation of multiple replication factors, the complete cessation of DNA synthesis, and impaired replicative capacity in vitro. Therefore, functional distinctions exist between the "chronic" RB-mediated arrest state and the "acute" arrest state. Strikingly, attenuation of RB activity reversed both acute and chronic replication blocks. Thus, continued RB action is required for the maintenance of two kinetically and functionally distinct modes of replication inhibition.

Protein kinase CK2 phosphorylates the cell cycle regulatory protein Geminin

Geminin contributes to cell cycle regulation by a timely inhibition of Cdt1p, the loading factor required for the assembly of pre-replication complexes. Geminin is expressed during S and G2 phase of the HeLa cell cycle and phosphorylated soon after its synthesis. We show here that Geminin is an excellent substrate for protein kinase CK2 in vitro; and that the highly specific CK2 inhibitor tetrabromobenzotriazole (TBB) blocks the phosphorylation of Geminin in HeLa protein extracts and HeLa cells in vivo. The sites of CK2 phosphorylation are located in the carboxyterminal region of Geminin, which carries several consensus sequence motifs for CK2. We also show that a minor phosphorylating activity in protein extracts can be attributed to glycogen synthase kinase 3 (GSK3), which most likely targets a central peptide in Geminin. Treatment of HeLa cells with TBB does not interfere with the ability of Geminin to interact with the loading factor Cdt1.

Eukaryotic MCM proteins: beyond replication initiation

The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.

Perpetuating the double helix: molecular machines at eukaryotic DNA replication origins

The hardest part of replicating a genome is the beginning. The first step of DNA replication (called "initiation") mobilizes a large number of specialized proteins ("initiators") that recognize specific sequences or structural motifs in the DNA, unwind the double helix, protect the exposed ssDNA, and recruit the enzymatic activities required for DNA synthesis, such as helicases, primases and polymerases. All of these components are orderly assembled before the first nucleotide can be incorporated. On the occasion of the 50th anniversary of the discovery of the DNA structure, we review our current knowledge of the molecular mechanisms that control initiation of DNA replication in eukaryotic cells, with particular emphasis on the recent identification of novel initiator proteins. We speculate how these initiators assemble molecular machines capable of performing specific biochemical tasks, such as loading a ring-shaped helicase onto the DNA double helix.

Enigmatic variations: divergent modes of regulating eukaryotic DNA replication

Proteins involved in DNA replication are conserved from yeast to mammals, suggesting that the mechanism was established at an early stage of eukaryotic evolution. In spite of this common origin, recent findings have revealed surprising variations in how replication initiation is controlled, implying that a conserved mechanism has not necessarily resulted in regulatory conservation.

The dynamic association of RCC1 with chromatin is modulated by Ran-dependent nuclear transport

Regulator of chromosome condensation (RCC1) binding to chromatin is highly dynamic, as determined by fluorescence recovery after photobleaching analysis of GFP-RCC1 in stably transfected tsBN2 cells. Microinjection of wild-type or Q69L Ran markedly slowed the mobility of GFP-RCC1, whereas T24N Ran (defective in nucleotide loading) decreased it further still. We found significant alterations in the mobility of intranuclear GFP-RCC1 after treatment with agents that disrupt different Ran-dependent nuclear export pathways. Leptomycin B, which inhibits Crm1/RanGTP-dependent nuclear export, significantly increased the mobility of RCC1 as did high levels of actinomycin D (to inhibit RNA polymerases I, II, and III) or alpha-amanitin (to inhibit RNA polymerases II and III) as well as energy depletion. Inhibition of just mRNA transcription, however, had no affect on GFP-RCC1 mobility consistent with mRNA export being a Ran-independent process. In permeabilized cells, cytosol and GTP were required for the efficient release of GFP-RCC1 from chromatin. Recombinant Ran would not substitute for cytosol, and high levels of supplemental Ran inhibited the cytosol-stimulated release. Thus, RCC1 release from chromatin in vitro requires a factor(s) distinct from, or in addition to, Ran and seems linked in vivo to the availability of Ran-dependent transport cargo.

Cell proliferation without cyclin E-CDK2

In this issue of Cell, Geng et al. report that mice can undergo embryonic development without cyclins E1 and E2. Fibroblasts derived from the double knockout embryos proliferate normally in culture. However, E type cyclins seem essential for endoreplication and exit from quiescence.

A new role for Ran in ensuring precise duplication of chromosomal DNA

To prevent re-replication of DNA, the licensing of replication origins is inhibited in S phase and G2. New work by Yamaguchi et al. (in this issue of Cell) shows that the small GTPase Ran can directly inhibit licensing inside nuclei once CDKs are active late in the cell cycle.