Unusual centrosome cycle in Dictyostelium: correlation of dynamic behavior and structural changes

Dynamic Mitotic Localization of the Centrosomal Kinases CDK1, Plk, AurK, and Nek2 in Dictyostelium amoebae

The centrosome of the amoebozoan model Dictyostelium discoideum provides the best-established model for an acentriolar centrosome outside the Opisthokonta. Dictyostelium exhibits an unusual centrosome cycle, in which duplication is initiated only at the G2/M transition and occurs entirely during the M phase. Little is known about the role of conserved centrosomal kinases in this process. Therefore, we have generated knock-in strains for Aurora (AurK), CDK1, cyclin B, Nek2, and Plk, replacing the endogenous genes with constructs expressing the respective green fluorescent Neon fusion proteins, driven by the endogenous promoters, and studied their behavior in living cells. Our results show that CDK1 and cyclin B arrive at the centrosome first, already during G2, followed by Plk, Nek2, and AurK. Furthermore, CDK1/cyclin B and AurK were dynamically localized at kinetochores, and AurK in addition at nucleoli. The putative roles of all four kinases in centrosome duplication, mitosis, cytokinesis, and nucleolar dynamics are discussed.

Nucleolin is required for multiple centrosome-associated functions in early vertebrate mitosis

Nucleolin is a multifunctional RNA-binding protein that resides predominantly not only in the nucleolus, but also in multiple other subcellular pools in the cytoplasm in mammalian cells, and is best known for its roles in ribosome biogenesis, RNA stability, and translation. During early mitosis, nucleolin is required for equatorial mitotic chromosome alignment prior to metaphase. Using high resolution fluorescence imaging, we reveal that nucleolin is required for multiple centrosome-associated functions at the G2-prophase boundary. Nucleolin depletion led to dissociation of the centrosomes from the G2 nuclear envelope, a delay in the onset of nuclear envelope breakdown, reduced inter-centrosome separation, and longer metaphase spindles. Our results reveal novel roles for nucleolin in early mammalian mitosis, establishing multiple important functions for nucleolin during mammalian cell division.

Dictyostelium spastin is involved in nuclear envelope dynamics during semi-closed mitosis

Dictyostelium amoebae perform a semi-closed mitosis, in which the nuclear envelope is fenestrated at the insertion sites of the mitotic centrosomes and around the central spindle during karyokinesis. During late telophase the centrosome relocates to the cytoplasmic side of the nucleus, the central spindle disassembles and the nuclear fenestrae become closed. Our data indicate that Dictyostelium spastin (DdSpastin) is a microtubule-binding and severing type I membrane protein that plays a role in this process. Its mitotic localization is in agreement with a requirement for the removal of microtubules that would hinder closure of the fenestrae. Furthermore, DdSpastin interacts with the HeH/ LEM-family protein Src1 in BioID analyses as well as the inner nuclear membrane protein Sun1, and shows subcellular co-localizations with Src1, Sun1, the ESCRT component CHMP7 and the IST1-like protein filactin, suggesting that the principal pathway of mitotic nuclear envelope remodeling is conserved between animals and Dictyostelium amoebae.

The Dictyostelium Centrosome

The centrosome of Dictyostelium amoebae contains no centrioles and consists of a cylindrical layered core structure surrounded by a corona harboring microtubule-nucleating γ-tubulin complexes. It is the major centrosomal model beyond animals and yeasts. Proteomics, protein interaction studies by BioID and superresolution microscopy methods led to considerable progress in our understanding of the composition, structure and function of this centrosome type. We discuss all currently known components of the Dictyostelium centrosome in comparison to other centrosomes of animals and yeasts.

Cep192, a Novel Missing Link between the Centrosomal Core and Corona in Dictyostelium Amoebae

The Dictyostelium centrosome is a nucleus-associated body with a diameter of approx. 500 nm. It contains no centrioles but consists of a cylindrical layered core structure surrounded by a microtubule-nucleating corona. At the onset of mitosis, the corona disassembles and the core structure duplicates through growth, splitting, and reorganization of the outer core layers. During the last decades our research group has characterized the majority of the 42 known centrosomal proteins. In this work we focus on the conserved, previously uncharacterized Cep192 protein. We use superresolution expansion microscopy (ExM) to show that Cep192 is a component of the outer core layers. Furthermore, ExM with centrosomal marker proteins nicely mirrored all ultrastructurally known centrosomal substructures. Furthermore, we improved the proximity-dependent biotin identification assay (BioID) by adapting the biotinylase BioID2 for expression in Dictyostelium and applying a knock-in strategy for the expression of BioID2-tagged centrosomal fusion proteins. Thus, we were able to identify various centrosomal Cep192 interaction partners, including CDK5RAP2, which was previously allocated to the inner corona structure, and several core components. Studies employing overexpression of GFP-Cep192 as well as depletion of endogenous Cep192 revealed that Cep192 is a key protein for the recruitment of corona components during centrosome biogenesis and is required to maintain a stable corona structure.

Genetic Instability Due to Spindle Anomalies Visualized in Mutants of Dictyostelium

Aberrant centrosome activities in mutants of Dictyostelium discoideum result in anomalies of mitotic spindles that affect the reliability of chromosome segregation. Genetic instabilities caused by these deficiencies are tolerated in multinucleate cells, which can be produced by electric-pulse induced cell fusion as a source for aberrations in the mitotic apparatus of the mutant cells. Dual-color fluorescence labeling of the microtubule system and the chromosomes in live cells revealed the variability of spindle arrangements, of centrosome-nuclear interactions, and of chromosome segregation in the atypical mitoses observed.

An improved molecular tool for screening bacterial colonies using GFP expression enhanced by a Dictyostelium sequence

During molecular cloning, screening bacterial transformants is a time-consuming and labor-intensive process; however, tractable tools that can be applied to various vectors for visual confirmation of desired colonies are limited. Recently, we reported that translational enhancement by a Dictyostelium gene sequence (TED) boosted protein expression even without an expression inducer in Escherichia coli. Here, we demonstrate a generally applicable molecular tool using the expression of green fluorescent protein enhanced by TED. By inserting a module related to TED into the cloning site in advance, we effectively screened E. coli colonies harboring the desired plasmid functions in a prokaryote (Magnetospirillum gryphiswaldense) or eukaryote (Dictyostelium discoideum). Thus, our system represents a user-friendly technique for cloning.

CDK5RAP2 Is an Essential Scaffolding Protein of the Corona of the Dictyostelium Centrosome

Dictyostelium centrosomes consist of a nucleus-associated cylindrical, three-layered core structure surrounded by a corona consisting of microtubule-nucleation complexes embedded in a scaffold of large coiled-coil proteins. One of them is the conserved CDK5RAP2 protein. Here we focus on the role of Dictyostelium CDK5RAP2 for maintenance of centrosome integrity, its interaction partners and its dynamic behavior during interphase and mitosis. GFP-CDK5RAP2 is present at the centrosome during the entire cell cycle except from a short period during prophase, correlating with the normal dissociation of the corona at this stage. RNAi depletion of CDK5RAP2 results in complete disorganization of centrosomes and microtubules suggesting that CDK5RAP2 is required for organization of the corona and its association to the core structure. This is in line with the observation that overexpressed GFP-CDK5RAP2 elicited supernumerary cytosolic MTOCs. The phenotype of CDK5RAP2 depletion was very reminiscent of that observed upon depletion of CP148, another scaffolding protein of the corona. BioID interaction assays revealed an interaction of CDK5RAP2 not only with the corona markers CP148, γ-tubulin, and CP248, but also with the core components Cep192, CP75, and CP91. Furthermore, protein localization studies in both depletion strains revealed that CP148 and CDK5RAP2 cooperate in corona organization.

Centrosome Positioning in Dictyostelium: Moving beyond Microtubule Tip Dynamics

The variability in centrosome size, shape, and activity among different organisms provides an opportunity to understand both conserved and specialized actions of this intriguing organelle. Centrosomes in the model organism Dictyostelium sp. share some features with fungal systems and some with vertebrate cell lines and thus provide a particularly useful context to study their dynamics. We discuss two aspects, centrosome positioning in cells and their interactions with nuclei during division as a means to highlight evolutionary modifications to machinery that provide the most basic of cellular services.

CP39, CP75 and CP91 are major structural components of the Dictyostelium centrosome's core structure

The acentriolar Dictyostelium centrosome is a nucleus-associated body consisting of a core structure with three plaque-like layers, which are surrounded by a microtubule-nucleating corona. The core duplicates once per cell cycle at the G2/M transition, whereby its central layer disappears and the two outer layers form the mitotic spindle poles. Through proteomic analysis of isolated centrosomes, we have identified CP39 and CP75, two essential components of the core structure. Both proteins can be assigned to the central core layer as their centrosomal presence is correlated to the disappearance and reappearance of the central core layer in the course of centrosome duplication. Both proteins contain domains with centrosome-binding activity in their N- and C-terminal halves, whereby the respective N-terminal half is required for cell cycle-dependent regulation. CP39 is capable of self-interaction and GFP-CP39 overexpression elicited supernumerary microtubule-organizing centers and pre-centrosomal cytosolic clusters. Underexpression stopped cell growth and reversed the MTOC amplification phenotype. In contrast, in case of CP75 underexpression of the protein by RNAi treatment elicited supernumerary MTOCs. In addition, CP75RNAi affects correct chromosome segregation and causes co-depletion of CP39 and CP91, another central core layer component. CP39 and CP75 interact with each other directly in a yeast two-hybrid assay. Furthermore, CP39, CP75 and CP91 mutually interact in a proximity-dependent biotin identification (BioID) assay. Our data indicate that these three proteins are all required for proper centrosome biogenesis and make up the major structural components of core structure's central layer.

CP91 is a component of the Dictyostelium centrosome involved in centrosome biogenesis

The Dictyostelium centrosome is a model for acentriolar centrosomes and it consists of a three-layered core structure surrounded by a corona harboring microtubule nucleation complexes. Its core structure duplicates once per cell cycle at the G2/M transition. Through proteomic analysis of isolated centrosomes we have identified CP91, a 91-kDa coiled coil protein that was localized at the centrosomal core structure. While GFP-CP91 showed almost no mobility in FRAP experiments during interphase, both GFP-CP91 and endogenous CP91 dissociated during mitosis and were absent from spindle poles from late prophase to anaphase. Since this behavior correlates with the disappearance of the central layer upon centrosome duplication, CP91 is a putative component of this layer. When expressed as GFP-fusions, CP91 fragments corresponding to the central coiled coil domain and the preceding N-terminal part (GFP-CP91cc and GFP-CP91N, respectively) also localized to the centrosome but did not show the mitotic redistribution of the full length protein suggesting a regulatory role of the C-terminal domain. Expression of all GFP-fusion proteins suppressed expression of endogenous CP91 and elicited supernumerary centrosomes. This was also very prominent upon depletion of CP91 by RNAi. Additionally, CP91-RNAi cells exhibited heavily increased ploidy due to severe defects in chromosome segregation along with increased cell size and defects in the abscission process during cytokinesis. Our results indicate that CP91 is a central centrosomal core component required for centrosomal integrity, proper centrosome biogenesis and, independently, for abscission during cytokinesis.

The centrosomal component CEP161 of Dictyostelium discoideum interacts with the Hippo signaling pathway

CEP161 is a novel component of the Dictyostelium discoideum centrosome which was identified as binding partner of the pericentriolar component CP250. Here we show that the amino acids 1-763 of the 1381 amino acids CEP161 are sufficient for CP250 binding, centrosomal targeting and centrosome association. Analysis of AX2 cells over-expressing truncated and full length CEP161 proteins revealed defects in growth and development. By immunoprecipitation experiments we identified the Hippo related kinase SvkA (Hrk-svk) as binding partner for CEP161. Both proteins colocalize at the centrosome. In in vitro kinase assays the N-terminal domain of CEP161 (residues 1-763) inhibited the kinase activity of Hrk-svk. A comparison of D. discoideum Hippo kinase mutants with mutants overexpressing CEP161 polypeptides revealed similar defects. We propose that the centrosomal component CEP161 is a novel player in the Hippo signaling pathway and affects various cellular properties through this interaction.

Nucleocytoplasmic protein translocation during mitosis in the social amoebozoan Dictyostelium discoideum

Mitosis is a fundamental and essential life process. It underlies the duplication and survival of all cells and, as a result, all eukaryotic organisms. Since uncontrolled mitosis is a dreaded component of many cancers, a full understanding of the process is critical. Evolution has led to the existence of three types of mitosis: closed, open, and semi-open. The significance of these different mitotic species, how they can lead to a full understanding of the critical events that underlie the asexual duplication of all cells, and how they may generate new insights into controlling unregulated cell division remains to be determined. The eukaryotic microbe Dictyostelium discoideum has proved to be a valuable biomedical model organism. While it appears to utilize closed mitosis, a review of the literature suggests that it possesses a form of mitosis that lies in the middle between truly open and fully closed mitosis-it utilizes a form of semi-open mitosis. Here, the nucleocytoplasmic translocation patterns of the proteins that have been studied during mitosis in the social amoebozoan D. discoideum are detailed followed by a discussion of how some of them provide support for the hypothesis of semi-open mitosis.

Evolution of the nucleus

•

The nuclear pore complex is well conserved, with some regions of divergence.

•

The nuclear lamina appears quite variable between major supergroups.

•

Centrosomes are ancient structures, but with complex evolutionary history.

•

There is evidence for prokaryotic ancestors of some nuclear components.

•

Analysis of divergent organisms is essential to fully understand nuclear biology and its origins.

The nucleus represents a major evolutionary transition. As a consequence of separating translation from transcription many new functions arose, which likely contributed to the remarkable success of eukaryotic cells. Here we will consider what has recently emerged on the evolutionary histories of several key aspects of nuclear biology; the nuclear pore complex, the lamina, centrosomes and evidence for prokaryotic origins of relevant players.

Actin Cytoskeleton and Golgi Involvement in Barley stripe mosaic virus Movement and Cell Wall Localization of Triple Gene Block Proteins

Barley stripe mosaic virus (BSMV) induces massive actin filament thickening at the infection front of infected Nicotiana benthamiana leaves. To determine the mechanisms leading to actin remodeling, fluorescent protein fusions of the BSMV triple gene block (TGB) proteins were coexpressed in cells with the actin marker DsRed: Talin. TGB ectopic expression experiments revealed that TGB3 is a major elicitor of filament thickening, that TGB2 resulted in formation of intermediate DsRed:Talin filaments, and that TGB1 alone had no obvious effects on actin filament structure. Latrunculin B (LatB) treatments retarded BSMV cell-to-cell movement, disrupted actin filament organization, and dramatically decreased the proportion of paired TGB3 foci appearing at the cell wall (CW). BSMV infection of transgenic plants tagged with GFP-KDEL exhibited membrane proliferation and vesicle formation that were especially evident around the nucleus. Similar membrane proliferation occurred in plants expressing TGB2 and/or TGB3, and DsRed: Talin fluorescence in these plants colocalized with the ER vesicles. TGB3 also associated with the Golgi apparatus and overlapped with cortical vesicles appearing at the cell periphery. Brefeldin A treatments disrupted Golgi and also altered vesicles at the CW, but failed to interfere with TGB CW localization. Our results indicate that actin cytoskeleton interactions are important in BSMV cell-to-cell movement and for CW localization of TGB3.

Dynamics of traction stress field during cell division

We report a quantitative measurement of traction stress exerted by dividing eukaryotic cells. The stress field was highly dynamic and sequentially changed as follows: (1) strong and localized as two spots, (2) weak and broadly distributed, and (3) strong and localized as four spots. At the final stage of cytokinesis, the dividing cells exerted strong tensile force on the intercellular bridge. The asymmetry of the traction stress and the orientation of the division axis matched throughout the division process, suggesting the possible role of the mechanical force as a "store" of the orientational information.

A kinesin-mediated mechanism that couples centrosomes to nuclei

The M-type kinesin isoform, Kif9, has recently been implicated in maintaining a physical connection between the centrosome and nucleus in Dictyostelium discoideum. However, the mechanism by which Kif9 functions to link these two organelles remains obscure. Here we demonstrate that the Kif9 protein is localized to the nuclear envelope and is concentrated in the region underlying the centrosome point of attachment. Nuclear anchorage appears mediated through a specialized transmembrane domain located in the carboxyl terminus. Kif9 interacts with microtubules in in vitro binding assays and effects an endwise depolymerization of the polymer. These results suggest a model whereby Kif9 is anchored to the nucleus and generates a pulling force that reels the centrosome up against the nucleus. This is a novel activity for a kinesin motor, one important for progression of cells into mitosis and to ensure centrosome-nuclear parity in a multinuclear environment.

CP55, a novel key component of centrosomal organization in Dictyostelium

Dictyostelium centrosomes consist of a layered core structure surrounded by a microtubule-nucleating corona. At the G2/M transition, the corona dissociates and the core structure duplicates, yielding two spindle pole bodies. Finally, in telophase, the spindle poles mature into two new, complete centrosomes. CP55 was identified in a centrosomal proteome analysis. It is a component of the centrosomal core structure, and persists at the centrosome throughout the entire cell cycle. FRAP experiments revealed that during interphase the majority of centrosomal GFP-CP55 is immobile, which indicates a structural task of CP55 at the centrosome. The CP55null mutant is characterized by increased ploidy, a less structured, slightly enlarged corona, and by supernumerary, cytosolic MTOCs, containing only corona proteins and lacking a core structure. Live cell imaging showed that supernumerary MTOCs arise in telophase. Lack of CP55 also caused premature recruitment of the corona organizer CP148 to mitotic spindle poles, already in metaphase instead of telophase. Forces transmitted through astral microtubules may expel prematurely acquired or loosely attached corona fragments into the cytosol, where they act as independent MTOCs. CP55null cells were also impaired in growth, most probably due to difficulties in centrosome splitting during prophase. Furthermore, although they were still capable of phagocytosis, they appeared unable to utilize phagocytosed nutrients. This inability may be attributed to their partially disorganized Golgi apparatus.

Rules of engagement: centrosome-nuclear connections in a closed mitotic system

The assembly of a functional mitotic spindle is essential for cell reproduction and requires a precise coordination between the nuclear cycle and the centrosome. This coordination is particularly prominent in organisms that undergo closed mitosis where centrosomes must not only respond to temporal signals, but also to spatial considerations, e.g. switching from the production of cytoplasmic microtubule arrays to the generation of dynamic intra-nuclear microtubules required for spindle assembly. We utilize a gene knockout of Kif9, a Dictyostelium discoideum Kin-I kinesin, to destabilize the physical association between centrosomes and the nuclear envelope. This approach presents a unique opportunity to reveal temporal and spatial components in the regulation of centrosomal activities in a closed-mitosis organism. Here we report that centrosome–nuclear engagement is not required for the entry into mitosis. Although detached centrosomes can duplicate in the cytoplasm, neither they nor nuclei alone can produce spindle-like microtubule arrays. However, the physical association of centrosomes and the nuclear envelope is required to progress through mitosis beyond prometaphase.

Electrodynamic eigenmodes in cellular morphology

Eigenmodes of the spherical and ellipsoidal dielectric electromagnetic resonator have been analysed. The sizes and shape of the resonators have been chosen to represent the shape of the interphase and dividing animal cell. Electromagnetic modes that have shape exactly suitable for positioning of the sufficiently large organelles in cell (centrosome, nucleus) have been identified. We analysed direction and magnitude of dielectrophoretic force exerted on large organelles by electric field of the modes. We found that the TM(1m1) mode in spherical resonator acts by centripetal force which drags the large organelles which have higher permittivity than the cytosol to the center of the cell. TM-kind of mode in the ellipsoidal resonator acts by force on large polarizable organelles in a direction that corresponds to the movement of the centrosomes (also nucleus) observed during the cell division, i.e. to the foci of the ellipsoidal cell. Minimal required force (10(-16) N), gradient of squared electric field and corresponding energy (10(-16) J) of the mode have been calculated to have biological significance within the periods on the order of time required for cell division. Minimal required energy of the mode, in order to have biological significance, can be lower in the case of resonance of organelle with the field of the cellular resonator mode. In case of sufficient energy in the biologically relevant mode, electromagnetic field of the mode will act as a positioning or steering mechanism for centrosome and nucleus in the cell, thus contribute to the spatial and dynamical self-organization in biological systems.

Functional characterization of CP148, a novel key component for centrosome integrity in Dictyostelium

The Dictyostelium centrosome consists of a layered core structure surrounded by a microtubule-nucleating corona. A tight linkage through the nuclear envelope connects the cytosolic centrosome with the clustered centromeres within the nuclear matrix. At G2/M the corona dissociates, and the core structure duplicates, yielding two spindle poles. CP148 is a novel coiled coil protein of the centrosomal corona. GFP-CP148 exhibited cell cycle-dependent presence and absence at the centrosome, which correlates with dissociation of the corona in prophase and its reformation in late telophase. During telophase, GFP-CP148 formed cytosolic foci, which coalesced and joined the centrosome. This explains the hypertrophic appearance of the corona upon strong overexpression of GFP-CP148. Depletion of CP148 by RNAi caused virtual loss of the corona and disorganization of interphase microtubules. Surprisingly, formation of the mitotic spindle and astral microtubules was unaffected. Thus, microtubule nucleation complexes associate with centrosomal core components through different means during interphase and mitosis. Furthermore, CP148 RNAi caused dispersal of centromeres and altered Sun1 distribution at the nuclear envelope, suggesting a role of CP148 in the linkage between centrosomes and centromeres. Taken together, CP148 is an essential factor for the formation of the centrosomal corona, which in turn is required for centrosome/centromere linkage.

Characterization of NE81, the first lamin-like nucleoskeleton protein in a unicellular organism

Dictyostelium NE81 is the first protein found in a lower eukaryote with properties justifying its denomination as a lamin-like protein. Knockout and overexpression mutants revealed an important role for NE81 in nuclear integrity, chromatin organization, and mechanical stability of cells.

Lamins build the nuclear lamina and are required for chromatin organization, gene expression, cell cycle progression, and mechanical stabilization. Despite these universal functions, lamins have so far been found only in metazoans. We have identified protein NE81 in Dictyostelium, which has properties that justify its denomination as a lamin-like protein in a lower eukaryote. This is based on its primary structure, subcellular localization, and regulation during mitosis, and its requirement of the C-terminal CaaX box as a posttranslational processing signal for proper localization. Our knockout and overexpression mutants revealed an important role for NE81 in nuclear integrity, chromatin organization, and mechanical stability of cells. All our results are in agreement with a role for NE81 in formation of a nuclear lamina. This function is corroborated by localization of Dictyostelium NE81 at the nuclear envelope in human cells. The discovery of a lamin-like protein in a unicellular organism is not only intriguing in light of evolution, it may also provide a simple experimental platform for studies of the molecular basis of laminopathies.

Dictyostelium discoideum: a model system for ultrastructural analyses of cell motility and development

Dictyostelium occupies an interesting niche in the grand scheme of model organisms. On the one hand, it is a compact, highly motile single cell that presents numerous opportunities to investigate the fundamental mechanisms of signal transduction, cell movement, and pathogen infection. However, upon starvation, individual cells enter a developmental pathway that involves cell aggregation, cell-cell adhesion, pattern formation, and differentiation. Thus, Dictyostelium is also well known as a basic model for studying developmental processes. Electron microscopy (EM) has played a large role in both the unicellular and the multicellular life stages, for example, providing image detail for structure/function relationships of cytoskeletal proteins, the deposition of cellulose fibrils in maturing spores, and the identification of intercellular junctional complexes. Powerful combinations of robust molecular genetic tools, high-resolution light microscopy, and EM methods make this organism an attractive model for imaging dynamic cell processes. This chapter serves to highlight the past and current EM approaches that have advanced our understanding of how cells and proteins function.

CP250, a novel acidic coiled-coil protein of the Dictyostelium centrosome, affects growth, chemotaxis, and the nuclear envelope

The Dictyostelium centrosome is a nucleus associated body consisting of a box-shaped core surrounded by the corona, an amorphous matrix functionally equivalent to the pericentriolar material of animal centrosomes which is responsible for the nucleation and anchoring of microtubules. Here we describe CP250 a component of the corona, an acidic coiled coil protein that is present at the centrosome throughout interphase while disappearing during prophase and reappearing at the end of late telophase. Amino acids 756-1148 of the 2110 amino acids are sufficient for centrosomal targeting and cell cycle-dependent centrosome association. Mutant cells lacking CP250 are smaller in size, growth on bacteria is delayed, chemotaxis is altered, and development is affected, which, in general, are defects observed in cytoskeletal mutants. Furthermore, loss of CP250 affected the nuclear envelope and led to reduced amounts and altered distribution of Sun-1, a conserved nuclear envelope protein that connects the centrosome to chromatin.

Dictyostelium Sun1 is a dynamic membrane protein of both nuclear membranes and required for centrosomal association with clustered centromeres

Centrosomal attachment to nuclei is crucial for proper mitosis and nuclear positioning in various organisms, and generally involves Sun-family proteins located at the inner nuclear envelope. There is still no common scheme for the outer nuclear membrane proteins interacting with Sun1 in centrosome/nucleus attachment. Here we propose a model in which Sun1 mediates a physical link between centrosomes and clustered centromeres through both nuclear membranes in Dictyostelium. For the first time we provide a detailed microscopic analysis of the centrosomal and nuclear envelope localization of endogenous Dictyostelium Sun1 during interphase and mitosis. By immunogold electron microscopy we show that Sun1 is a resident of both nuclear membranes. Disruption of Sun1 function by overexpression of full-length GFP-Sun1 or a GFP-Sun-domain deletion construct revealed not only the established function in centrosome/nucleus attachment and maintenance of ploidy, but also a requirement of Sun1 for the association of the centromere cluster with the centrosome. Live-cell imaging visualized the occurrence of mitotic defects, and demonstrated the requirement of microtubules for dynamic distance changes between centrosomes and nuclei. FRAP analysis revealed at least two populations of Sun1, with an immobile fraction associated with the centrosome, and a mobile fraction in the nuclear envelope.

Dictyostelium discoideum CenB is a bona fide centrin essential for nuclear architecture and centrosome stability

Centrins are a family of proteins within the calcium-binding EF-hand superfamily. In addition to their archetypical role at the microtubule organizing center (MTOC), centrins have acquired multiple functionalities throughout the course of evolution. For example, centrins have been linked to different nuclear activities, including mRNA export and DNA repair. Dictyostelium discoideum centrin B is a divergent member of the centrin family. At the amino acid level, DdCenB shows 51% identity with its closest relative and only paralog, DdCenA. Phylogenetic analysis revealed that DdCenB and DdCenA form a well-supported monophyletic and divergent group within the centrin family of proteins. Interestingly, fluorescently tagged versions of DdCenB were not found at the centrosome (in whole cells or in isolated centrosomes). Instead, DdCenB localized to the nuclei of interphase cells. This localization disappeared as the cells entered mitosis, although Dictyostelium cells undergo a closed mitosis in which the nuclear envelope (NE) does not break down. DdCenB knockout cells exhibited aberrant nuclear architecture, characterized by enlarged and deformed nuclei and loss of proper centrosome-nucleus anchoring (observed as NE protrusions). At the centrosome, loss of DdCenB resulted in defects in the organization and morphology of the MTOC and supernumerary centrosomes and centrosome-related bodies. The multiple defects that the loss of DdCenB generated at the centrosome can be explained by its atypical division cycle, transitioning into the NE as it divides at mitosis. On the basis of these findings, we propose that DdCenB is required at interphase to maintain proper nuclear architecture, and before delocalizing from the nucleus, DdCenB is part of the centrosome duplication machinery.

Microtubule-nucleus interactions in Dictyostelium discoideum mediated by central motor kinesins

Kinesins are a diverse superfamily of motor proteins that drive organelles and other microtubule-based movements in eukaryotic cells. These motors play important roles in multiple events during both interphase and cell division. Dictyostelium discoideum contains 13 kinesin motors, 12 of which are grouped into nine families, plus one orphan. Functions for 11 of the 13 motors have been previously investigated; we address here the activities of the two remaining kinesins, both isoforms with central motor domains. Kif6 (of the kinesin-13 family) appears to be essential for cell viability. The partial knockdown of Kif6 with RNA interference generates mitotic defects (lagging chromosomes and aberrant spindle assemblies) that are consistent with kinesin-13 disruptions in other organisms. However, the orphan motor Kif9 participates in a completely novel kinesin activity, one that maintains a connection between the microtubule-organizing center (MTOC) and nucleus during interphase. kif9 null cell growth is impaired, and the MTOC appears to disconnect from its normally tight nuclear linkage. Mitotic spindles elongate in a normal fashion in kif9(-) cells, but we hypothesize that this kinesin is important for positioning the MTOC into the nuclear envelope during prophase. This function would be significant for the early steps of cell division and also may play a role in regulating centrosome replication.

Kinesin-5 is not essential for mitotic spindle elongation in Dictyostelium

The proper assembly and operation of the mitotic spindle is essential to ensure the accurate segregation of chromosomes and to position the cytokinetic furrow during cell division in eukaryotes. Not only are dynamic microtubules required but also the concerted actions of multiple motor proteins are necessary to effect spindle pole separation, chromosome alignment, chromatid segregation, and spindle elongation. Although a number of motor proteins are known to play a role in mitosis, there remains a limited understanding of their full range of functions and the details by which they interact with other spindle components. The kinesin-5 (BimC/Eg5) family of motors is largely considered essential to drive spindle pole separation during the initial and latter stages of mitosis. We have deleted the gene encoding the kinesin-5 member in Dictyostelium, (kif13), and find that, in sharp contrast with results found in vertebrate, fly, and yeast organisms, kif13(-) cells continue to grow at rates indistinguishable from wild type. Phenotype analysis reveals a slight increase in spindle elongation rates in the absence of Kif13. More importantly, there is a dramatic, premature separation of spindle halves in kif13(-) cells, suggesting a novel role of this motor in maintaining spindle integrity at the terminal stages of division.

Dictyostelium Aurora kinase has properties of both Aurora A and Aurora B kinases

Aurora kinases are highly conserved proteins with important roles in mitosis. Metazoans contain two kinases, Aurora A and B, which contribute distinct functions at the spindle poles and the equatorial region respectively. It is not currently known whether the specialized functions of the two kinases arose after their duplication in animal cells or were already present in their ancestral kinase. We show that Dictyostelium discoideum contains a single Aurora kinase, DdAurora, that displays characteristics of both Aurora A and B. Like Aurora A, DdAurora has an extended N-terminal domain with an A-box sequence and localizes at the spindle poles during early mitosis. Like Aurora B, DdAurora binds to its partner DdINCENP and localizes on centromeres at metaphase, the central spindle during anaphase, and the cleavage furrow at the end of cytokinesis. DdAurora also has several unusual properties. DdAurora remains associated with centromeres in anaphase, and this association does not require an interaction with DdINCENP. DdAurora then localizes at the cleavage furrow, but only at the end of cytokinesis. This localization is dependent on DdINCENP and the motor proteins Kif12 and myosin II. Thus, DdAurora may represent the ancestral kinase that gave rise to the different Aurora kinases in animals and also those in other organisms.

Spindle assembly: asters part their separate ways

Cells have developed diverse ways to separate two microtubule asters to form a mitotic spindle. Here, I focus on two mechanisms used to position asters around chromosomes during mitosis: first, aster migration around the nuclear envelope and, second, aster attachment to a contractile cortex at the plasma membrane after the nuclear envelope has broken down. Although certain cell types use one mechanism predominantly, most rely on both to ensure proper spindle assembly.

Dictyostelium LIS1 is a centrosomal protein required for microtubule/cell cortex interactions, nucleus/centrosome linkage, and actin dynamics

The widespread LIS1-proteins were originally identified as the target for sporadic mutations causing lissencephaly in humans. Dictyostelium LIS1 (DdLIS1) is a microtubule-associated protein exhibiting 53% identity to human LIS1. It colocalizes with dynein at isolated, microtubule-free centrosomes, suggesting that both are integral centrosomal components. Replacement of the DdLIS1 gene by the hypomorphic D327H allele or overexpression of an MBP-DdLIS1 fusion disrupted various dynein-associated functions. Microtubules lost contact with the cell cortex and were dragged behind an unusually motile centrosome. Previously, this phenotype was observed in cells overexpressing fragments of dynein or the XMAP215-homologue DdCP224. DdLIS1 was coprecipitated with DdCP224, suggesting that both act together in dynein-mediated cortical attachment of microtubules. Furthermore, DdLIS1-D327H mutants showed Golgi dispersal and reduced centrosome/nucleus association. Defects in DdLIS1 function also altered actin dynamics characterized by traveling waves of actin polymerization correlated with a reduced F-actin content. DdLIS1 could be involved in actin dynamics through Rho-GTPases, because DdLIS1 interacted directly with Rac1A in vitro. Our results show that DdLIS1 is required for maintenance of the microtubule cytoskeleton, Golgi apparatus and nucleus/centrosome association, and they suggest that LIS1-dependent alterations of actin dynamics could also contribute to defects in neuronal migration in lissencephaly patients.

Actin-binding proteins required for reliable chromosome segregation in mitosis

While studying mitosis in Dictyostelium mutants with deficiencies in actin-binding proteins, we found that two of these proteins, cortexillin and Aip1, are required for the precise segregation of chromosomes. Atypical spindles and nuclei with varying DNA content indicate that mutants lacking cortexillin or Aip1 are genetically unstable. These aberrations are caused by the detachment and irregular reattachment of centrosomes to the nuclear surface. Live imaging showed how coalescing mitotic complexes give rise to a multipolar spindle, and how excess centrosomes can be eliminated by mitotic cleavage between anucleate and nucleated portions of a cell. We hypothesize that mutations in regulatory proteins of the actin network might be one cause of genetic instability of malignant tumor cells.

Molecular and functional analysis of the dictyostelium centrosome

The centrosome is a nonmembranous, nucleus-associated organelle that functions not only as the main microtubule-organizing center but also as a cell cycle control unit. How the approximately 100 different proteins that make up a centrosome contribute to centrosome function is still largely unknown. Considerable progress in the understanding of centrosomal functions can be expected from comparative cell biology of morphologically different centrosomal structures fulfilling conserved functions. Dictyostelium is an alternative model organism for centrosome research in addition to yeast and animal cells. With the elucidation of morphological changes and dynamics of centrosome duplication, the establishment of a centrosome isolation protocol, and the identification of many centrosomal components, there is a solid basis for understanding the biogenesis and function of this fascinating organelle. Here we give an overview of the prospective protein inventory of the Dictyostelium centrosome based on database searches. Moreover, we focus on the comparative cell biology of known components of the Dictyostelium centrosome including the gamma-tubulin complex and the homologues of centrin, Nek2, XMAP215, and EB1.

Dynamic microtubules in Dictyostelium

The term 'microtubule dynamics' is often used to describe assembly/disassembly characteristics of this important cytoskeletal polymer. The ability to image microtubules in live Dictyostelium cells has revealed additional dynamic components, acting on the individual assembled tubules. At least two separate forces are involved, in generation of pronounced bending motions during interphase and in creating tension with the cell cortex. This review attempts to summarize what is known about conventional microtubule dynamics in Dictyostelium as well as to describe these two additional motility components. We propose that these forces are important both in maintaining the overall structure of the microtubule array and in supporting intracellular traffic.

Regulated expression of the centrosomal protein DdCP224 affects microtubule dynamics and reveals mechanisms for the control of supernumerary centrosome number

The Dictyostelium XMAP215 family member DdCP224 is involved in centrosome duplication and cytokinesis and is concentrated at the centrosome and microtubule tips. Herein, we have created a DdCP224 promoter replacement mutant that allows both over- and underexpression. Overexpression led to supernumerary microtubule-organizing centers and, independently, an increase of the number of multinuclear cells. Electron microscopy demonstrated that supernumerary microtubule-organizing centers represented bona fide centrosomes. Live cell imaging of DdCP224-green fluorescent protein mutants also expressing green fluorescent protein-histone2B as a DNA label revealed that supernumerary centrosomes were also competent of cell cycle-dependent duplication. In contrast, underexpression of DdCP224 inhibited cell growth, reduced the number and length of astral microtubules, and caused nocodazole hypersensitivity. Moreover, microtubule regrowth after nocodazole removal was dependent on DdCP224. Underexpression also resulted in a striking disappearance of supernumerary centrosomes and multinuclear cells caused by previous overexpression. We show for the first time by live cell observation that the number of supernumerary centrosomes can be reduced either by centrosome fusion (coalescence) or by the formation of cytoplasts containing supernumerary centrosomes during cytokinesis.

Dictyostelium EB1 is a genuine centrosomal component required for proper spindle formation

EB1 proteins are ubiquitous microtubule-associated proteins involved in microtubule search and capture, regulation of microtubule dynamics, cell polarity, and chromosome stability. We have cloned a complete cDNA of Dictyostelium EB1 (DdEB1), the largest known EB1 homolog (57 kDa). Immunofluorescence analysis and expression of a green fluorescent protein-DdEB1 fusion protein revealed that DdEB1 localizes along microtubules, at microtubule tips, centrosomes, and protruding pseudopods. During mitosis, it was found at the spindle, spindle poles, and kinetochores. DdEB1 is the first EB1-homolog that is also a genuine centrosomal component, because it was localized at isolated centrosomes that are free of microtubules. Furthermore, centrosomal DdEB1 distribution was unaffected by nocodazole treatment. DdEB1 colocalized with DdCP224, the XMAP215 homolog, at microtubule tips, the centrosome, and kinetochores. Furthermore, both proteins were part of the same cytosolic protein complex, suggesting that they may act together in their functions. DdEB1 deletion mutants expressed as green fluorescent protein or maltose-binding fusion proteins indicated that microtubule binding requires homo-oligomerization, which is mediated by a coiled-coil domain. A DdEB1 null mutant was viable but retarded in prometaphase progression due to a defect in spindle formation. Because spindle elongation was normal, DdEB1 seems to be required for the initiation of the outgrowth of spindle microtubules.

DdNek2, the first non-vertebrate homologue of human Nek2, is involved in the formation of microtubule-organizing centers

Dictyostelium Nek2 (DdNek2) is the first structural and functional non-vertebrate homologue of human Nek2, a NIMA-related serine/threonine kinase required for centrosome splitting in early mitosis. DdNek2 shares 43% overall amino-acid identity with its human counterpart and 54% identity within the catalytic domain. Both proteins can be subdivided in an N-terminal catalytic domain, a leucine zipper and a C-terminal domain. Kinase assays with bacterially expressed DdNek2 and C-terminal deletion mutants revealed that catalytic activity requires the presence of the leucine zipper and that autophosphorylation occurs at the C-terminus. Microscopic analyses with DdNek2 antibodies and expression of a GFP-DdNek2 fusion protein in Dictyostelium showed that DdNek2 is a permanent centrosomal resident and suggested that it is a component of the centrosomal core. The GFP-DdNek2-overexpressing mutants frequently exhibit supernumerary microtubule-organizing centers (MTOCs). This phenotype did not require catalytic activity because it was also observed in cells expressing inactive GFP-K33R. However, it was shown to be caused by overexpression of the C-terminal domain since it also occurred in GFP-mutants expressing only the C-terminus or a leucine zipper/C-terminus construct but not in those mutants expressing only the catalytic domain or a catalytic domain/leucine zipper construct. These results suggest that DdNek2 is involved in the formation of MTOCs. Furthermore, the localization of the GFP-fusion proteins revealed two independent centrosomal targeting domains of DdNek2, one within the catalytic or leucine zipper domain and one in the C-terminal domain.

Molecular analysis of the cytosolic Dictyostelium gamma-tubulin complex

gamma-Tubulin plays an essential role in microtubule nucleation and organization and occurs, besides its centrosomal localization, in the cytosol, where it forms soluble complexes with other proteins. We investigated the size and composition of gamma-tubulin complexes in Dictyostelium, using a mutant cell line in which the endogenous copy of the gamma-tubulin gene had been replaced by a tagged version. Dictyostelium gamma-tubulin complexes were generally much smaller than the large gamma-tubulin ring complexes found in higher organisms. The stability of the small Dictyostelium gamma-tubulin complexes depended strongly on the purification conditions, with a striking stabilization of the complexes under high salt conditions. Furthermore, we cloned the Dictyostelium homolog of Spc97 and an almost complete sequence of the Dictyostelium homolog of Spc98, which are both components of gamma-tubulin complexes in other organisms. Both proteins localize to the centrosome in Dictyostelium throughout the cell cycle and are also present in a cytosolic pool. We could show that the prevailing small complex present in Dictyostelium consists of DdSpc98 and gamma-tubulin, whereas DdSpc97 does not associate. Dictyostelium is thus the first organism investigated so far where the three proteins do not interact stably in the cytosol.

Dictyostelium centrin-related protein (DdCrp), the most divergent member of the centrin family, possesses only two EF hands and dissociates from the centrosome during mitosis

We have identified a Dictyostelium discoideum cDNA sequence with homology to centrins. The derived protein, Dictyostelium discoideum centrinn-related protein (DdCrp), is the most divergent member of the centrin family. Most strikingly it lacks the first two EF-hand consensus motifs, whereas a number of other centrin-specific sequence features are conserved. Southern and Northern blot analysis and the data presently available from the Dictyostelium genome and cDNA projects suggest that DdCrp is the only centrin isoform present in Dictyostelium. Immunofluorescence analysis with anti-DdCrp antibodies revealed that the protein is localized to the centrosome, to a second, centrosome-associated structure close to the nucleus and to the nucleus itself. Confocal microscopy resolved that the centrosomal label is confined to the corona surrounding the centrosome core. Unlike for other centrins the localization of DdCrp is cell cycle-dependent. Both the centrosomal and the centrosome-associated label disappear during prometaphase, most likely in concert with the dissociation of the corona at this stage. The striking differences of DdCrp to all other centrins may be related to the distinct structure and duplication mode of the Dictyostelium centrosome.

Structural and functional effects of hydrostatic pressure on centrosomes from vertebrate cells

In an attempt to better understand the role of centrioles in vertebrate centrosomes, hydrostatic pressure was applied to isolated centrosomes as a means to disassemble centriole microtubules. Treatments of the centrosomes were monitored by analyzing their protein composition, ultrastructure, their ability to nucleate microtubules from pure tubulin, and their capability to induce parthenogenetic development of Xenopus eggs. Moderate hydrostatic pressure (95 MPa) already affected the organization of centriole microtubules in isolated centrosomes, and also impaired microtubule nucleation. At higher pressure, the protein composition of the peri-centriolar matrix (PCM) was also altered and the capacity to nucleate microtubules severely impaired. Incubation of the treated centrosomes in Xenopus egg extract could restore their capacity to nucleate microtubules after treatment at 95 MPa, but not after higher pressure treatment. However, the centriole structure was in no case restored. It is noteworthy that centrosomes treated with mild pressure did not allow parthenogenetic development after injection into Xenopus eggs, even if they had recovered their capacity to nucleate microtubules. This suggested that, in agreement with previous results, centrosomes in which centriole architecture is impaired, could not direct the biogenesis of new centrioles in Xenopus eggs. Centriole structure could also be affected by applying mild hydrostatic pressure directly to living cells. Comparison of the effect of hydrostatic pressure on cells at the G1/S border or on the corresponding cytoplasts suggests that pro-centrioles are very sensitive to pressure. However, cells can regrow a centriole after pressure-induced disassembly. In that case, centrosomes eventually recover an apparently normal duplication cycle although with some delay.

Biphasic expression of rnrB in Dictyostelium discoideum suggests a direct relationship between cell cycle control and cell differentiation

Cell differentiation in Dictyostelium is strongly affected by the cell cycle. Cell cycle control is well-understood in other systems, but this has had almost no impact on the study of Dictyostelium cell differentiation, in part because the cell cycle in Dictyostelium is unusual, lacking a G1 phase. Here we describe the cell-cycle regulated expression of rnrB, which codes for the small subunit of ribonucleotide reductase and is a marker of late G1 in many systems. There appear to be two expression peaks, one in mid-G2 and the other near the G2/M transition. Using Xgal/anti-BrdU double staining, we show that cells in asynchronously growing cultures express in both phases, with a gap between them during which the gene is transcriptionally silent. Cold-synchronized cells show exclusively G2/M expression, while mid-G2 expression is seen in high-density synchronized cells and can also be inferred in cells undergoing synchronization by either method. rnrB expression occurs in other systems shortly after cells pass a point (the "restriction point" or "start") at which they commit to complete their current cell cycle. We demonstrate a similar commitment point in Dictyostelium and show that this occurs shortly before the mid-G2 rnrB expression peak. The Dictyostelium cell cycle thus appears to include a well-defined though inconspicuous event, between early and mid-G2, with some features which are normally associated with the G1/S transition. Others have described a switch from stalk to spore differentiation preference at about this time. Since Dictyostelium cells switch back from spore to stalk preference approximately at the G2/M rnrB expression maximum, cell differentiation as well as rnrB expression may be regulated directly by fundamental cell cycle control processes.

LIS1 regulates CNS lamination by interacting with mNudE, a central component of the centrosome

LIS1, a microtubule-associated protein, is required for neuronal migration, but the precise mechanism of LIS1 function is unknown. We identified a LIS1 interacting protein encoded by a mouse homolog of NUDE, a nuclear distribution gene in A. nidulans and a multicopy suppressor of the LIS1 homolog, NUDF. mNudE is located in the centrosome or microtubule organizing center (MTOC), and interacts with six different centrosomal proteins. Overexpression of mNudE dissociates gamma-tubulin from the centrosome and disrupts microtubule organization. Missense mutations that disrupt LIS1 function block LIS1-mNudE binding. Moreover, misexpression of the LIS1 binding domain of mNudE in Xenopus embryos disrupts the architecture and lamination of the CNS. Thus, LIS1-mNudE interactions may regulate neuronal migration through dynamic reorganization of the MTOC.

Microtubule organization and the effects of GFP-tubulin expression in dictyostelium discoideum

We have labeled microtubules in living Dictyostelium amoebae by incorporation of a GFP-alpha-tubulin fusion protein. The GFP-alpha-tubulin incorporates into microtubules and, as reported by others [Neujahr et al., 1998], the labeled microtubules are highly motile. Electron microscopy (EM) analysis of the distribution and organization of microtubules in the amoebae shows that some cytoplasmic microtubules form close associations. These associations could allow motor proteins attached to one microtubule to walk along an adjacent microtubule and thus generate some of the observed motility. Protein blot analysis indicates that the GFP-alpha-tubulin incorporates into microtubules at a lower efficiency than does the endogenous alpha-tubulin. EM and immunofluorescence (IF) analyses suggest that the GFP-alpha-tubulin interferes with microtubule nucleation. We have also observed an increased sensitivity of the GFP-alpha-tubulin expressing cells to blue light, as compared to wild-type cells. These results suggest that although GFP-alpha-tubulin can be used as a marker for microtubules in living cells, the use of this marker is not recommended for certain types of studies such as assembly dynamics.

Spindle pole body duplication: a model for centrosome duplication?

The yeast spindle pole body (SPB) is the functional equivalent of the centrosome and forms the two poles of the mitotic spindle. Before mitosis, both SPBs and centrosomes are present as single copies and must be duplicated to form the bipolar spindle. SPB components have been identified using a combination of biochemistry and genetics, and their role during SPB duplication has been analysed using temperature-sensitive mutants. In this article, we describe structural aspects of SPB duplication and their possible relationship to centrosome duplication.

Dictyostelium DdCP224 is a microtubule-associated protein and a permanent centrosomal resident involved in centrosome duplication

A cDNA encoding a 224-kDa Dictyostelium discoideum centrosomal protein (DdCP224) was isolated by immunoscreening. DdCP224 was detected at the centrosome and, more weakly, along microtubules throughout the entire cell cycle. Centrosomal localization does not require microtubules, suggesting that DdCP224 is a genuine centrosomal component. DdCP224 exhibits sequence identity to a weakly conserved class of microtubule-associated proteins including human TOGp and yeast Stu2p. Stu2p has a size of only approximately 100 kDa and corresponds to the N-terminal half of DdCP224. The functions of the N- and C-terminal halves of DdCP224 were investigated in the corresponding GFP-fusion mutants. Surprisingly, the N-terminal construct showed only cytosolic localization, whereas the C-terminal construct localized exclusively to the centrosome. This is unexpected because Stu2p is localized at the spindle pole body. Full-length DdCP224-GFP was present both at centrosomes and along microtubules. Furthermore, it bound to microtubules in vitro, unlike the two truncated mutants. Thus centrosome binding is determined by the C-terminal half and microtubule binding may require the interaction of the N- and C-terminal halves. Interestingly, cells expressing full-length DdCP224-GFP exhibit supernumerary centrosomes and show a cytokinesis defect, suggesting that DdCP224 plays an important role in centrosome duplication. These features are unique among the known centrosomal proteins.