The motor domain of the kinesin Kip2 promotes microtubule polymerization at microtubule tips.

Kinesins are microtubule-dependent motor proteins, some of which moonlight as microtubule polymerases, such as the yeast protein Kip2. Here, we show that the CLIP-170 ortholog Bik1 stabilizes Kip2 at microtubule ends where the motor domain of Kip2 promotes microtubule polymerization. Live-cell imaging and mathematical estimation of Kip2 dynamics reveal that disrupting the Kip2-Bik1 interaction aborts Kip2 dwelling at microtubule ends and abrogates its microtubule polymerization activity. Structural modeling and biochemical experiments identify a patch of positively charged residues that enables the motor domain to bind free tubulin dimers alternatively to the microtubule shaft. Neutralizing this patch abolished the ability of Kip2 to promote microtubule growth both in vivo and in vitro without affecting its ability to walk along microtubules. Our studies suggest that Kip2 utilizes Bik1 as a cofactor to track microtubule tips, where its motor domain then recruits free tubulin and catalyzes microtubule assembly.

Coupling DNA Replication and Spindle Function in Saccharomyces cerevisiae.

In the yeast Saccharomyces cerevisiae DNA replication and spindle assembly can overlap. Therefore, signaling mechanisms modulate spindle dynamics in order to ensure correct timing of chromosome segregation relative to genome duplication, especially when replication is incomplete or the DNA becomes damaged. This review focuses on the molecular mechanisms that coordinate DNA replication and spindle dynamics, as well as on the role of spindle-dependent forces in DNA repair. Understanding the coupling between genome duplication and spindle function in yeast cells can provide important insights into similar processes operating in other eukaryotic organisms, including humans.

How Does SUMO Participate in Spindle Organization?

The ubiquitin-like protein SUMO is a regulator involved in most cellular mechanisms. Recent studies have discovered new modes of function for this protein. Of particular interest is the ability of SUMO to organize proteins in larger assemblies, as well as the role of SUMO-dependent ubiquitylation in their disassembly. These mechanisms have been largely described in the context of DNA repair, transcriptional regulation, or signaling, while much less is known on how SUMO facilitates organization of microtubule-dependent processes during mitosis. Remarkably however, SUMO has been known for a long time to modify kinetochore proteins, while more recently, extensive proteomic screens have identified a large number of microtubule- and spindle-associated proteins that are SUMOylated. The aim of this review is to focus on the possible role of SUMOylation in organization of the spindle and kinetochore complexes. We summarize mitotic and microtubule/spindle-associated proteins that have been identified as SUMO conjugates and present examples regarding their regulation by SUMO. Moreover, we discuss the possible contribution of SUMOylation in organization of larger protein assemblies on the spindle, as well as the role of SUMO-targeted ubiquitylation in control of kinetochore assembly and function. Finally, we propose future directions regarding the study of SUMOylation in regulation of spindle organization and examine the potential of SUMO and SUMO-mediated degradation as target for antimitotic-based therapies.

An E2-ubiquitin thioester-driven approach to identify substrates modified with ubiquitin and ubiquitin-like molecules.

Covalent modifications of proteins with ubiquitin and ubiquitin-like molecules are instrumental to many biological processes. However, identifying the E3 ligase responsible for these modifications remains a major bottleneck in ubiquitin research. Here, we present an E2-thioester-driven identification (E2~dID) method for the targeted identification of substrates of specific E2 and E3 enzyme pairs. E2~dID exploits the central position of E2-conjugating enzymes in the ubiquitination cascade and provides in vitro generated biotinylated E2~ubiquitin thioester conjugates as the sole source for ubiquitination in extracts. This enables purification and mass spectrometry-based identification of modified proteins under stringent conditions independently of the biological source of the extract. We demonstrate the sensitivity and specificity of E2-dID by identifying and validating substrates of APC/C in human cells. Finally, we perform E2~dID with SUMO in S. cerevisiae, showing that this approach can be easily adapted to other ubiquitin-like modifiers and experimental models.

Kar9 controls the nucleocytoplasmic distribution of yeast EB1.

The precise temporal and spatial concentration of microtubule-associated proteins (MAPs) within the cell is fundamental to ensure chromosome segregation and correct spindle positioning. MAPs form an intricate web of interactions among each other and compete for binding sites on microtubules. Therefore, when assessing cellular phenotypes upon MAP up- or downregulation, it is important to consider the protein interaction network between individual MAPs. Here, we show that changes in the amounts of the spindle positioning factor Kar9 specifically affect the distribution of yeast EB1 on spindle microtubules, without influencing other microtubule-associated interacting partners of Kar9, i.e. yeast XMAP215 and CLIP-170. Alterations in the distribution of yeast EB1 explain chromosome segregation defects upon knockout, overexpression or stabilization of Kar9 and provide an example for non-linear effects on MAP behavior after perturbation of their equilibrium.

Regulation of a Spindle Positioning Factor at Kinetochores by SUMO-Targeted Ubiquitin Ligases.

Correct function of the mitotic spindle requires balanced interplay of kinetochore and astral microtubules that mediate chromosome segregation and spindle positioning, respectively. Errors therein can cause severe defects ranging from aneuploidy to developmental disorders. Here, we describe a protein degradation pathway that functionally links astral microtubules to kinetochores via regulation of a microtubule-associated factor. We show that the yeast spindle positioning protein Kar9 localizes not only to astral but also to kinetochore microtubules, where it becomes targeted for proteasomal degradation by the SUMO-targeted ubiquitin ligases (STUbLs) Slx5-Slx8. Intriguingly, this process does not depend on preceding sumoylation of Kar9 but rather requires SUMO-dependent recruitment of STUbLs to kinetochores. Failure to degrade Kar9 leads to defects in both chromosome segregation and spindle positioning. We propose that kinetochores serve as platforms to recruit STUbLs in a SUMO-dependent manner in order to ensure correct spindle function by regulating levels of microtubule-associated proteins.

Kinesin Kip2 enhances microtubule growth in vitro through length-dependent feedback on polymerization and catastrophe.

The size and position of mitotic spindles is determined by the lengths of their constituent microtubules. Regulation of microtubule length requires feedback to set the balance between growth and shrinkage. Whereas negative feedback mechanisms for microtubule length control, based on depolymerizing kinesins and severing proteins, have been studied extensively, positive feedback mechanisms are not known. Here, we report that the budding yeast kinesin Kip2 is a microtubule polymerase and catastrophe inhibitor in vitro that uses its processive motor activity as part of a feedback loop to further promote microtubule growth. Positive feedback arises because longer microtubules bind more motors, which walk to the ends where they reinforce growth and inhibit catastrophe. We propose that positive feedback, common in biochemical pathways to switch between signaling states, can also be used in a mechanical signaling pathway to switch between structural states, in this case between short and long polymers.

Yeast GSK-3 kinase regulates astral microtubule function through phosphorylation of the microtubule-stabilizing kinesin Kip2.

The S. cerevisiae kinesin Kip2 stabilises astral microtubules (MTs) and facilitates spindle positioning through transport of MT-associated proteins, such as the yeast CLIP-170 homologue Bik1, dynein and the adenomatous-polyposis-coli-related protein Kar9 to the plus ends of astral MTs. Here, we show that Kip2 associates with its processivity factor Bim1, the yeast homologue of the plus-end-tracking protein EB1. This interaction requires an EB1-binding motif in the N-terminal extension of Kip2 and is negatively regulated by phosphorylation through Mck1, the yeast glycogen synthase kinase 3. In addition, Mck1-dependent phosphorylation decreases the intrinsic MT affinity of Kip2. Reduction in Kip2 phosphorylation leads to stabilisation of astral MTs, and accumulation of Kip2, dynein and Kar9 at MT plus ends, whereas loss of Mck1 function leads to defects in spindle positioning. Furthermore, we provide evidence that a subpopulation of Mck1 at the bud-cortex phosphorylates Kip2. We propose that yeast GSK-3 spatially controls astral MT dynamics and the loading of dynein and Kar9 on astral MT plus ends by regulating Kip2 interactions with Bim1 and MTs.

An auxiliary, membrane-based mechanism for nuclear migration in budding yeast.

How nuclear shape correlates with nuclear movements during the cell cycle is poorly understood. We investigated changes in nuclear morphology during nuclear migration in budding yeast. In preanaphase cells, nuclear protrusions (nucleopodia [NP]) extend into the bud, preceding insertion of chromosomes into the bud neck. Surprisingly, formation of nucleopodia did not depend on the established nuclear migration pathways. We show that generation and maintenance of NP requires nuclear membrane expansion, actin, and the exocyst complex. Exocyst mutations cause nuclear positioning defects and display genetic interactions with mutations that deactivate astral microtubule-dependent nuclear migration. Cells that cannot perform DNA replication also fail to form nucleopodia. We propose that nuclear membrane expansion, DNA replication, and exocyst-dependent anchoring of the nuclear envelope to the bud affect nuclear morphology and facilitate correct positioning of nucleus and chromosomes relative to the cleavage apparatus.

Molecular mechanisms in spindle positioning: structures and new concepts.

Coordination of cell cleavage with respect to cell geometry, cell polarity and neighboring tissues is critical for tissue maintenance, malignant transformation and metastasis. The position of the mitotic spindle within the cell determines where cell cleavage occurs. Spindle positioning is often mediated through capture of astral microtubules by motor proteins at the cell cortex. Recently, the core dynein anchor complex has been structurally resolved. Junctional complexes were shown to provide additional capture sites for astral microtubules in proliferating tissues. Finally, latest studies show that signals from centrosomes control spindle positioning and propose novel concepts for generation of centrosome identity.

Ubiquitylation regulates interactions of astral microtubules with the cleavage apparatus.

BACKGROUND: Correct positioning of the mitotic spindle relative to the cleavage apparatus is crucial for successful mitosis. In budding yeast, the Adenomatous Polyposis Coli-related protein Kar9, yeast EB1, and Myo2, a type V myosin, mediate positioning of the mitotic spindle close to the septin-anchored cleavage apparatus at the bud neck. RESULTS: We find that Kar9 is ubiquitylated and degraded by the proteasome. Ubiquitylation requires the ubiquitin-conjugating enzymes Ubc1 and Ubc4 and phosphorylation of Kar9 by yeast Cdk1. Importantly, Kar9 ubiquitylation and degradation depend on an intact cleavage apparatus. Kar9 is stabilized in septin mutant cells or cells lacking the bud neck formin Bnr1, but not in the bud formin Bni1 or the actomyosin ring. Transport of Kar9 to the bud neck by Myo2 is also required for Kar9 degradation. Abrogation of Kar9 phosphorylation and ubiquitylation increases interactions of astral microtubules (aMTs) with the bud neck and causes spindle mispositioning. Photoconversion experiments showed that Kar9 association with aMTs is stable. CONCLUSIONS: We propose that ubiquitylation controls interactions of aMTs with the cleavage apparatus through localized disassembly of Kar9 complexes.

Arp1, an actin-related protein, in Plasmodium berghei.

Actin-related proteins (Arps) constitute a family of eukaryotic cytoskeletal proteins involved in such diverse events as cell motility, cytokinesis, vesicle transport, and chromatin remodelling. Previously, in a study of Plasmodium berghei gene expression in ookinetes and oocysts, we detected stage-specific increased expression of a gene encoding an Arp. Here we further characterize this gene and the encoded protein. We present a phylogenetic and three-dimensional modelling analysis as well as cell biological and genetic data that support classification of this gene as being an orthologue of the actin-related protein 1 (Arp1). This gene was found to be expressed in asexual stages as well as in the mosquito stages of the parasite, both on the transcript and protein level. Our attempts to delete the gene in the parasite for functional studies were unsuccessful, suggesting that it may be essential. The protein was localized apically of the nucleus in ookinetes, and in combination with the known function of Arp1 proteins, this suggests a role in vesicular transport. Expression of the gene in Saccharomyces cerevisiae resulted in toxic effects and interference with the yeast cytoskeleton.

Role of spindle asymmetry in cellular dynamics.

The mitotic spindle is mostly perceived as a symmetric structure. However, in many cell divisions, the two poles of the spindle organize asters with different dynamics, associate with different biomolecules or subcellular domains, and perform different functions. In this chapter, we describe some of the most prominent examples of spindle asymmetry. These are encountered during cell-cycle progression in budding and fission yeast and during asymmetric cell divisions of stem cells and embryos. We analyze the molecular mechanisms that lead to generation of spindle asymmetry and discuss the importance of spindle-pole differentiation for the correct outcome of cell division.

Regulation of mitotic spindle asymmetry by SUMO and the spindle-assembly checkpoint in yeast.

During mitosis, the kinetochore microtubules capture and segregate chromosomes, and the astral microtubules position the spindle within the cell. Although the spindle is symmetric, proper positioning of the spindle in asymmetrically dividing cells generally correlates with the formation of morphologically and structurally distinct asters [1]. In budding yeast, the spindle-orientation proteins Kar9 and dynein decorate only one aster of the metaphase spindle and direct it toward the bud [2, 3]. The mechanisms controlling the distribution of Kar9 and dynein remain unclear. Here, we show that SUMO regulates astral-microtubule function in at least two ways. First, Kar9 was sumoylated in vivo. Sumoylation and Cdk1-dependent phosphorylation of Kar9 independently promoted Kar9 asymmetry on the spindle. Second, proper regulation of kinetochore function by SUMO was also required for Kar9 asymmetry. Indeed, activation of the spindle-assembly checkpoint (SAC) due to SUMO and kinetochore defects promoted symmetric redistribution of Kar9 in a Mad2-dependent manner. The control of Kar9 distribution by the SAC was independent of Kar9 sumoylation and phosphorylation. Together, our data reveal that three independent mechanisms contribute to Kar9 asymmetry: Cdk1-dependent phosphorylation, sumoylation, and SAC signaling. Hence, the two seemingly independent spindle domains, kinetochores and astral microtubules, function in a tightly coordinated fashion.

Dissection of septin actin interactions using actin overexpression in Saccharomyces cerevisiae.

Although many proteins can be overexpressed several fold without much effect on cell viability and morphology, some become toxic upon a slight increase in their intracellular level. This is particularly true for cytoskeletal proteins and has proven useful in the past for studying the cytoskeleton. In yeast, actin and tubulin are examples of proteins that cannot be overexpressed without affecting cell viability. Here, we have analysed the effect of actin overexpression in Saccharomyces cerevisiae. We show that actin overexpression interferes differently with distinct aspects of actin function. For example, two- to fourfold overexpression of actin did not affect the establishment of actin polarity, whereas it abrogated its maintenance. Also, actin structures that are barely visible in wild-type cells could be observed upon actin overexpression. This allowed us to identify a new ring-like actin structure genetically distinguishable from the actomyosin contractile ring. Formation of this actin structure upon actin overexpression was dependent on the septin cytoskeleton, the poorly understood cytokinetic protein Hof1 and the Arp2/3 complex. In contrast to the actomyosin ring, the ring formed upon actin overexpression required neither Myo1 nor formins for assembly. Therefore, we propose that Hof1 acts as a linker between actin and septins. Furthermore, we found that, in the absence of actin overexpression, a novel, Hof1-dependent actin belt is formed at the bud neck of anaphase cells. The physiological role of this belt might be related to that of the similar structure observed in dividing fission yeast.

Spindle asymmetry: a compass for the cell.

Spatial coordination of the cell-division axis with cellular polarity and/or with the position of neighboring cells is crucial for embryonic development, organogenesis and tissue homeostasis. In most cell types, the position of the mitotic spindle at the onset of anaphase dictates the orientation of the division axis; in unicellular organisms, it plays an important role in chromosome segregation. Cortical factors play a key role in the orientation of the spindle. Recent data from yeast reveal that the spindle does not passively react to cortical signals but actively interprets them to find its correct position. We review the data leading to a "compass model" for spindle positioning and discuss its potential generality.

Asymmetric loading of Kar9 onto spindle poles and microtubules ensures proper spindle alignment.

Spindle alignment is the process in which the two spindle poles are directed toward preselected and opposite cell ends. In budding yeast, the APC-related molecule Kar9 is required for proper alignment of the spindle with the mother-bud axis. We find that Kar9 localizes to the prospective daughter cell spindle pole. Kar9 is transferred from the pole to cytoplasmic microtubules, which are then guided in a myosin-dependent manner to the bud. Clb4/Cdc28 kinase phosphorylates Kar9 and accumulates on the pole destined to the mother cell. Mutations that block phosphorylation at Cdc28 consensus sites result in localization of Kar9 to both poles and target them both to the bud. Thus, Clb4/Cdc28 prevents Kar9 loading on the mother bound pole. In turn, asymmetric distribution of Kar9 ensures that only one pole orients toward the bud. Our results indicate that Cdk1-dependent spindle asymmetry ensures proper alignment of the mitotic spindle with the cell division axis.