Organization of microtubule assemblies in Dictyostelium syncytia depends on the microtubule crosslinker, Ase1

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.

Two modes of PRC1-mediated mechanical resistance to kinesin-driven microtubule network disruption

The proper organization of the microtubule-based spindle during cell division requires the collective activity of many different proteins. These include non-motor microtubule-associated proteins (MAPs), whose functions include crosslinking microtubules to regulate filament sliding rates and assemble microtubule arrays. One such protein is PRC1, an essential MAP that has been shown to preferentially crosslink overlapping antiparallel microtubules at the spindle midzone. PRC1 has been proposed to act as a molecular brake, but insight into the mechanism of how PRC1 molecules function cooperatively to resist motor-driven microtubule sliding and to allow for the formation of stable midzone overlaps remains unclear. Here, we employ a modified microtubule gliding assay to rupture PRC1-mediated microtubule pairs using surface-bound kinesins. We discovered that PRC1 crosslinks always reduce bundled filament sliding velocities relative to single-microtubule gliding rates and do so via two distinct emergent modes of mechanical resistance to motor-driven sliding. We term these behaviors braking and coasting, where braking events exhibit substantially slowed microtubule sliding compared to coasting events. Strikingly, braking behavior requires the formation of two distinct high-density clusters of PRC1 molecules near microtubule tips. Our results suggest a cooperative mechanism for PRC1 accumulation when under mechanical load that leads to a unique state of enhanced resistance to filament sliding and provides insight into collective protein ensemble behavior in regulating the mechanics of spindle assembly.

13 Plus 1: A 30-Year Perspective on Microtubule-Based Motility in Dictyostelium

Individual gene analyses of microtubule-based motor proteins in Dictyostelium discoideum have provided a rough draft of its machinery for cytoplasmic organization and division. This review collates their activities and looks forward to what is next. A comprehensive approach that considers the collective actions of motors, how they balance rates and directions, and how they integrate with the actin cytoskeleton will be necessary for a complete understanding of cellular dynamics.

Force balances between interphase centrosomes as revealed by laser ablation

Numerous studies have highlighted the self-centering activities of individual microtubule (MT) arrays in animal cells, but relatively few works address the behavior of multiple arrays that coexist in a common cytoplasm. In multinucleated Dictyostelium discoideum cells, each centrosome organizes a radial MT network, and these networks remain separate from one another. This feature offers an opportunity to reveal the mechanism(s) responsible for the positioning of multiple centrosomes. Using a laser microbeam to eliminate one of the two centrosomes in binucleate cells, we show that the unaltered array is rapidly repositioned at the cell center. This result demonstrates that each MT array is constantly subject to centering forces and infers a mechanism to balance the positions of multiple arrays. Our results address the limited actions of three kinesins and a cross-linking MAP that are known to have effects in maintaining MT organization and suggest a simple means used to keep the arrays separated.

Proteins of the Nucleolus of Dictyostelium discoideum: Nucleolar Compartmentalization, Targeting Sequences, Protein Translocations and Binding Partners

The nucleoli of Dictyostelium discoideum have a comparatively unique, non-canonical, localization adjacent to the inner nuclear membrane. The verified nucleolar proteins of this eukaryotic microbe are detailed while other potential proteins are introduced. Heat shock protein 32 (Hsp32), eukaryotic translation initiation factor 6 (eIF6), and tumour necrosis factor receptor-associated protein 1 (TRAP1) are essential for cell survival. NumA1, a breast cancer type 1 susceptibility protein-C Terminus domain-containing protein linked to cell cycle, functions in the regulation of nuclear number. The cell cycle checkpoint kinase 2 homologue forkhead-associated kinase A (FhkA) and BRG1-associated factor 60a homologue Snf12 are also discussed. While nucleoli appear homogeneous ultrastructurally, evidence for nucleolar subcompartments exists. Nucleolar localization sequences (NoLS) have been defined that target proteins to either the general nucleolar area or to a specific intranucleolar domain. Protein translocations during mitosis are protein-specific and support the multiple functions of the Dictyostelium nucleolus. To enrich the picture, binding partners of NumA1, the most well-characterized nucleolar protein, are examined: nucleolar Ca²⁺-binding protein 4a (CBP4a), nuclear puromycin-sensitive aminopeptidase A (PsaA) and Snf12. The role of Dictyostelium as a model for understanding the contribution of nucleolar proteins to various diseases and cellular stress is discussed throughout the review.

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.