The Cik1/Kar3 motor complex is required for the proper kinetochore-microtubule interaction after stressful DNA replication

Evidence of 14-3-3 proteins contributing to kinetochore integrity and chromosome congression during mitosis

The 14-3-3 family of proteins are conserved across eukaryotes and serve myriad important regulatory functions in the cell. Homo- and hetero-dimers of these proteins mainly recognize their ligands via conserved motifs to modulate the localization and functions of those effector ligands. In most of the genetic backgrounds of Saccharomyces cerevisiae, disruption of both 14-3-3 homologs (Bmh1 and Bmh2) are either lethal or cells survive with severe growth defects, including gross chromosomal missegregation and prolonged cell cycle arrest. To elucidate their contributions to chromosome segregation, in this work, we investigated their centromere- and kinetochore-related functions of Bmh1 and Bmh2. Analysis of appropriate deletion mutants shows that Bmh isoforms have cumulative and non-shared isoform-specific contributions in maintaining the proper integrity of the kinetochore ensemble. Consequently, Bmh mutant cells exhibited perturbations in kinetochore-microtubule (KT-MT) dynamics, characterized by kinetochore declustering, mis-localization of kinetochore proteins and Mad2-mediated transient G2/M arrest. These defects also caused an asynchronous chromosome congression in bmh mutants during metaphase. In summary, this report advances the knowledge on contributions of budding yeast 14-3-3 proteins in chromosome segregation by demonstrating their roles in kinetochore integrity and chromosome congression.

Key essential oil components delocalize Candida albicans Kar3p and impact microtubule structure

BACKGROUND

Treatment of Candida albicans associated infections is often ineffective in the light of resistance, with an urgent need to discover novel antimicrobials. Fungicides require high specificity and can contribute to antifungal resistance, so inhibition of fungal virulence factors is a good strategy for developing new antifungals.

OBJECTIVES

Examine the impact of four plant-derived essential oil components (1,8-cineole, α-pinene, eugenol, and citral) on C. albicans microtubules, kinesin motor protein Kar3 and morphology.

METHODS

Microdilution assays were used to determine minimal inhibitory concentrations, microbiological assays assessed germ tube, hyphal and biofilm formation, confocal microscopy probed morphological changes and localization of tubulin and Kar3p, and computational modelling was used to examine the theoretical binding of essential oil components to tubulin and Kar3p.

RESULTS

We show for the first time that essential oil components delocalize the Kar3p, ablate microtubules, and induce psuedohyphal formation with reduced biofilm formation. Single and double deletion mutants of kar3 were resistant to 1,8-cineole, sensitive to α-pinene and eugenol, but unimpacted by citral. Strains with homozygous and heterozygous Kar3p disruption had a gene-dosage effect for all essential oil components, resulting in enhanced resistance or susceptibility patterns that were identical to that of cik1 mutants. The link between microtubule (αβ-tubulin) and Kar3p defects was further supported by computational modeling, showing preferential binding to αβ-tubulin and Kar3p adjacent to their Mg²⁺-binding sites.

CONCLUSION

This study highlights how essential oil components interfere with the localization of the kinesin motor protein complex Kar3/Cik1 and disrupt microtubules, leading to their destabilization which results in hyphal and biofilm defects.

Yeast Kinesin-5 Motor Protein CIN8 Promotes Accurate Chromosome Segregation

Accurate chromosome segregation depends on bipolar chromosome–microtubule attachment and tension generation on chromosomes. Incorrect chromosome attachment results in chromosome missegregation, which contributes to genome instability. The kinetochore is a protein complex that localizes at the centromere region of a chromosome and mediates chromosome–microtubule interaction. Incorrect chromosome attachment leads to checkpoint activation to prevent anaphase onset. Kinetochore detachment activates the spindle assembly checkpoint (SAC), while tensionless kinetochore attachment relies on both the SAC and tension checkpoint. In budding yeast Saccharomyces cerevisiae, kinesin-5 motor proteins Cin8 and Kip1 are needed to separate spindle pole bodies for spindle assembly, and deletion of CIN8 causes lethality in the absence of SAC. To study the function of Cin8 and Kip1 in chromosome segregation, we constructed an auxin-inducible degron (AID) mutant, cin8-AID. With this conditional mutant, we first confirmed that cin8-AID kip1∆ double mutants were lethal when Cin8 is depleted in the presence of auxin. These cells arrested in metaphase with unseparated spindle pole bodies and kinetochores. We further showed that the absence of either the SAC or tension checkpoint was sufficient to abolish the cell-cycle delay in cin8-AID mutants, causing chromosome missegregation and viability loss. The tension checkpoint-dependent phenotype in cells with depleted Cin8 suggests the presence of tensionless chromosome attachment. We speculate that the failed spindle pole body separation in cin8 mutants could increase the chance of tensionless syntelic chromosome attachments, which depends on functional tension checkpoint for survival.

The EB1-Kinesin-14 complex is required for efficient metaphase spindle assembly and kinetochore bi-orientation

Kinesin-14s are conserved molecular motors required for high-fidelity chromosome segregation, but their specific contributions to spindle function have not been fully defined. Here, we show that key functions of budding yeast Kinesin-14 Cik1-Kar3 are accomplished in a complex with Bim1 (yeast EB1). Genetic complementation of mitotic phenotypes identifies a novel KLTF peptide motif in the Cik1 N-terminus. We show that this motif is one element of a tripartite binding interface required to form a high-affinity Bim1–Cik1-Kar3 complex. Lack of Bim1-binding by Cik1-Kar3 delays cells in mitosis and impairs microtubule bundle organization and dynamics. Conversely, constitutive targeting of Cik1-Kar3 to microtubule plus ends induces the formation of nuclear microtubule bundles. Cells lacking the Bim1–Cik1-Kar3 complex rely on the conserved microtubule bundler Ase1/PRC1 for metaphase spindle organization, and simultaneous loss of plus-end targeted Kar3 and Ase1 is lethal. Our results reveal the contributions of an EB1–Kinesin-14 complex for spindle formation as a prerequisite for efficient kinetochore clustering and bi-orientation.

A genomic screen revealing the importance of vesicular trafficking pathways in genome maintenance and protection against genotoxic stress in diploid Saccharomyces cerevisiae cells

The ability to survive stressful conditions is important for every living cell. Certain stresses not only affect the current well-being of cells but may also have far-reaching consequences. Uncurbed oxidative stress can cause DNA damage and decrease cell survival and/or increase mutation rates, and certain substances that generate oxidative damage in the cell mainly act on DNA. Radiomimetic zeocin causes oxidative damage in DNA, predominantly by inducing single- or double-strand breaks. Such lesions can lead to chromosomal rearrangements, especially in diploid cells, in which the two sets of chromosomes facilitate excessive and deleterious recombination. In a global screen for zeocin-oversensitive mutants, we selected 133 genes whose deletion reduces the survival of zeocin-treated diploid Saccharomyces cerevisiae cells. The screen revealed numerous genes associated with stress responses, DNA repair genes, cell cycle progression genes, and chromatin remodeling genes. Notably, the screen also demonstrated the involvement of the vesicular trafficking system in cellular protection against DNA damage. The analyses indicated the importance of vesicular system integrity in various pathways of cellular protection from zeocin-dependent damage, including detoxification and a direct or transitional role in genome maintenance processes that remains unclear. The data showed that deleting genes involved in vesicular trafficking may lead to Rad52 focus accumulation and changes in total DNA content or even cell ploidy alterations, and such deletions may preclude proper DNA repair after zeocin treatment. We postulate that functional vesicular transport is crucial for sustaining an integral genome. We believe that the identification of numerous new genes implicated in genome restoration after genotoxic oxidative stress combined with the detected link between vesicular trafficking and genome integrity will reveal novel molecular processes involved in genome stability in diploid cells.

"Uno, nessuno e centomila": the different faces of the budding yeast kinetochore

"One, no one and one hundred thousand" is a masterpiece of Italian literature, written by Luigi Pirandello. The central theme is that in each individual there are multiple personalities, since one's perception of one's self differs from the view of others. As a consequence, a unique identity does not exist, but rather one hundred thousand. This concept can be very well applied to the kinetochore, one of the largest macromolecular complexes conserved in eukaryotes. The kinetochore is essential during cell division and fulfills different sophisticated functions, including linking chromosomes to spindle microtubules and delaying anaphase onset in case of incorrect bi-orientation. In order to perform these tasks, the kinetochore shapes its structure by recruiting different subunits, such as the components of the spindle assembly checkpoint (SAC) or the monopolin complex during meiosis. It also modifies its internal organization by rearranging intramolecular connections and acquiring a distinct identity at different time points of cell division. In this review, we describe recent insights into the changes in composition and configuration of the kinetochore in mitosis and meiosis, focusing on the kinetochore of Saccharomyces cerevisiae.

The current view for the silencing of the spindle assembly checkpoint

Chromosome bipolar attachment is achieved when sister kinetochores are attached by microtubules emanating from opposite spindle poles, and this process is essential for faithful chromosome segregation during anaphase. A fundamental question in cell biology is how cells ensure that chromosome segregation only occurs after bipolar attachment. It is well documented that unattached kinetochores activate the spindle assembly checkpoint (SAC) to delay chromosome segregation. Therefore, the silencing of the SAC is thought to trigger anaphase onset, but how correct chromosome attachment is coupled with SAC silencing and the subsequent anaphase onset is poorly understood. The establishment of chromosome bipolar attachment not only results in the occupancy of kinetochores by microtubules but also applies tension on sister kinetochores. A long-standing debate is whether the kinetochore attachment (occupancy) or the tension silences the SAC. Recent work in budding yeast reveals the SAC silencing network SSN that prevents SAC silencing prior to tension generation at kinetochores. Therefore, this signaling pathway ensures that SAC silencing and the subsequent anaphase onset occur only after chromosome bipolar attachment applies tension on chromosomes. This review will summarize the recent advances in the understanding of the SAC silencing process.

The yeast sphingolipid signaling landscape

Sphingolipids are recognized as signaling mediators in a growing number of pathways, and represent potential targets to address many diseases. The study of sphingolipid signaling in yeast has created a number of breakthroughs in the field, and has the potential to lead future advances. The aim of this article is to provide an inclusive view of two major frontiers in yeast sphingolipid signaling. In the first section, several key studies in the field of sphingolipidomics are consolidated to create a yeast sphingolipidome that ranks nearly all known sphingolipid species by their level in a resting yeast cell. The second section presents an overview of most known phenotypes identified for sphingolipid gene mutants, presented with the intention of illuminating not yet discovered connections outside and inside of the field.

The signaling network that silences the spindle assembly checkpoint upon the establishment of chromosome bipolar attachment

Improper kinetochore attachments activate the spindle assembly checkpoint (SAC) to prevent anaphase onset, but it is poorly understood how this checkpoint is silenced to allow anaphase onset. Chromosome bipolar attachment applies tension on sister kinetochores, and the lack of tension delays anaphase onset. In budding yeast, the delay induced by tension defects depends on the intact SAC as well as increase in ploidy (Ipl1)/Aurora kinase and a centromere-associated protein ShuGOshin (Sgo1). Here we provide evidence indicating that Ipl1-dependent phosphorylation of the kinetochore protein Duo1 and Mps1 interacting (Dam1) prevents SAC silencing when tension is absent. The nonphosphorylatable dam1 mutant cells, as well as sgo1 mutant cells, are competent in SAC activation but unable to prevent SAC silencing in response to tension defects. We further found that phosphomimetic dam1 mutants exhibited delayed anaphase onset mainly due to the failure in SAC silencing, but destabilized kinetochore attachment likely plays a minor role in this delay. Because the tension resulting from bipolar attachment triggers the dephosphorylation of Dam1 by protein phosphatase 1, this dephosphorylation likely coordinates SAC silencing with chromosome bipolar attachment. Therefore, Sgo1, Ipl1 kinase, Dam1, and protein phosphatase 1 comprise the SAC silencing network that ensures the correct timing for anaphase onset.

Identification of C18:1-phytoceramide as the candidate lipid mediator for hydroxyurea resistance in yeast

Recent studies showed that deletion of ISC1, the yeast homologue of the mammalian neutral sphingomyelinase, resulted in an increased sensitivity to hydroxyurea (HU). This raised an intriguing question as to whether sphingolipids are involved in pathways initiated by HU. In this study, we show that HU treatment led to a significant increase in Isc1 activity. Analysis of sphingolipid deletion mutants and pharmacological analysis pointed to a role for ceramide in mediating HU resistance. Lipid analysis revealed that HU induced increases in phytoceramides in WT cells but not in isc1Δ cells. To probe functions of specific ceramides, we developed an approach to supplement the medium with fatty acids. Oleate (C18:1) was the only fatty acid protecting isc1Δ cells from HU toxicity in a ceramide-dependent manner. Because phytoceramide activates protein phosphatases in yeast, we evaluated the role of CDC55, the regulatory subunit of ceramide-activated protein phosphatase PP2A. Overexpression of CDC55 overcame the sensitivity to HU in isc1Δ cells. However, addition of oleate did not protect the isc1Δ,cdc55Δ double mutant from HU toxicity. These results demonstrate that HU launches a lipid pathway mediated by a specific sphingolipid, C18:1-phytoceramide, produced by Isc1, which provides protection from HU by modulating Swe1 levels through the PP2A subunit Cdc55.

Slk19 clusters kinetochores and facilitates chromosome bipolar attachment

Yeast kinetochore protein Slk19 is required for kinetochore clustering, and nocodazole exposure to slk19 mutant cells causes impaired kinetochore capture and delayed chromosome bipolar attachment after nocodazole washout.

In all eukaryotic cells, DNA is packaged into multiple chromosomes that are linked to microtubules through a large protein complex called a kinetochore. Previous data show that the kinetochores are clustered together during most of the cell cycle, but the mechanism and the biological significance of kinetochore clustering are unknown. As a kinetochore protein in budding yeast, the role of Slk19 in the stability of the anaphase spindle has been well studied, but its function in chromosome segregation has remained elusive. Here we show that Slk19 is required for kinetochore clustering when yeast cells are treated with the microtubule-depolymerizing agent nocodazole. We further find that slk19Δ mutant cells exhibit delayed kinetochore capture and chromosome bipolar attachment after the disruption of the kinetochore–microtubule interaction by nocodazole, which is likely attributed to defective kinetochore clustering. In addition, we show that Slk19 interacts with itself, suggesting that the dimerization of Slk19 may mediate the interaction between kinetochores for clustering. Therefore Slk19 likely acts as kinetochore glue that clusters kinetochores to facilitate efficient and faithful chromosome segregation.

Loss of function of the Cik1/Kar3 motor complex results in chromosomes with syntelic attachment that are sensed by the tension checkpoint

The attachment of sister kinetochores by microtubules emanating from opposite spindle poles establishes chromosome bipolar attachment, which generates tension on chromosomes and is essential for sister-chromatid segregation. Syntelic attachment occurs when both sister kinetochores are attached by microtubules from the same spindle pole and this attachment is unable to generate tension on chromosomes, but a reliable method to induce syntelic attachments is not available in budding yeast. The spindle checkpoint can sense the lack of tension on chromosomes as well as detached kinetochores to prevent anaphase onset. In budding yeast Saccharomyces cerevisiae, tension checkpoint proteins Aurora/Ipl1 kinase and centromere-localized Sgo1 are required to sense the absence of tension but are dispensable for the checkpoint response to detached kinetochores. We have found that the loss of function of a motor protein complex Cik1/Kar3 in budding yeast leads to syntelic attachments. Inactivation of either the spindle or tension checkpoint enables premature anaphase entry in cells with dysfunctional Cik1/Kar3, resulting in co-segregation of sister chromatids. Moreover, the abolished Kar3-kinetochore interaction in cik1 mutants suggests that the Cik1/Kar3 complex mediates chromosome movement along microtubules, which could facilitate bipolar attachment. Therefore, we can induce syntelic attachments in budding yeast by inactivating the Cik1/Kar3 complex, and this approach will be very useful to study the checkpoint response to syntelic attachments.

Chromosome bipolar attachment occurs when sister chromatids are attached by microtubules emanating from opposite spindle poles and is essential for faithful sister-chromatid segregation. Chromosomes are under tension once bipolar attachment is established. The absence of tension is sensed by the tension checkpoint that prevents chromosome segregation. The attachment of sister chromatids by microtubules from the same spindle pole generates syntelic attachment, which fails to generate tension on chromosomes. However, a reliable method to induce syntelic attachment is not available. Our findings indicate that the inactivation of the motor complex, Cik1/Kar3, results in chromosomes with syntelic attachment in budding yeast. In the absence of the tension checkpoint, yeast cells with dysfunctional Cik1/Kar3 enter anaphase, resulting in co-segregation of sister chromatids. Therefore, with this method we can experimentally induce syntelic attachment in yeast and investigate how cells respond to this incorrect attachment.