The budding yeast Polo-like kinase localizes to distinct populations at centrosomes during mitosis

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

Proline-Rich Motifs Control G2-CDK Target Phosphorylation and Priming an Anchoring Protein for Polo Kinase Localization

The hydrophobic patch (hp), a docking pocket on cyclins of CDKs (cyclin-dependent kinases), has been thought to accommodate a single short linear motif (SLiM), the "RxL or Cy" docking motif. Here we show that hp can bind different motifs with high specificity. We identify a PxxPxF motif that is necessary for G2-cyclin Clb3 function in S. cerevisiae, and that mediates Clb3-Cdk1 phosphorylation of Ypr174c (proposed name: Cdc5 SPB anchor-Csa1) to regulate the localization of Polo kinase Cdc5. Similar motifs exist in other Clb3-Cdk1 targets. Our work completes the set of docking specificities for the four major cyclins: LP, RxL, PxxPxF, and LxF motifs for G1-, S-, G2-, and M-phase cyclins, respectively. Further, we show that variations in motifs can change their specificity for human cyclins. This diversity could provide complexity for the encoding of CDK thresholds to achieve ordered cell-cycle phosphorylation.

Cell-cycle phospho-regulation of the kinetochore

The kinetochore is a mega-dalton protein assembly that forms within centromeric regions of chromosomes and directs their segregation during cell division. Here we review cell cycle-mediated phosphorylation events at the kinetochore, with a focus on the budding yeast Saccharomyces cerevisiae and the insight gained from forced associations of kinases and phosphatases. The point centromeres found in the budding yeast S. cerevisiae are one of the simplest such structures found in eukaryotes. The S. cerevisiae kinetochore comprises a single nucleosome, containing a centromere-specific H3 variant Cse4^CENP-A, bound to a set of kinetochore proteins that connect to a single microtubule. Despite the simplicity of the budding yeast kinetochore, the proteins are mostly homologous with their mammalian counterparts. In some cases, human proteins can complement their yeast orthologs. Like its mammalian equivalent, the regulation of the budding yeast kinetochore is complex: integrating signals from the cell cycle, checkpoints, error correction, and stress pathways. The regulatory signals from these diverse pathways are integrated at the kinetochore by post-translational modifications, notably phosphorylation and dephosphorylation, to control chromosome segregation. Here we highlight the complex interplay between the activity of the different cell-cycle kinases and phosphatases at the kinetochore, emphasizing how much more we have to understand this essential structure.

Connecting cell polarity signals to the cytokinetic machinery in yeast and metazoan cells

Polarized growth and cytokinesis are two fundamental cellular processes that exist in virtually all cell types. Mechanisms for asymmetric distribution of materials allow for cells to grow in a polarized manner. This gives rise to a variety of cell shapes seen throughout all cell types. Following polarized growth during interphase, dividing cells assemble a cytokinetic ring containing the protein machinery to constrict and separate daughter cells. Here, we discuss how cell polarity signaling pathways act on cytokinesis, with a focus on direct regulation of the contractile actomyosin ring (CAR). Recent studies have exploited phosphoproteomics to identify new connections between cell polarity kinases and CAR proteins. Existing evidence suggests that some polarity kinases guide the local organization of CAR proteins and structures while also contributing to global organization of the division plane within a cell. We provide several examples of this regulation from budding yeast, fission yeast, and metazoan cells. In some cases, kinase-substrate connections point to conserved processes in these different organisms. We point to several examples where future work can indicate the degree of conservation and divergence in the cell division process of these different organisms.

Unifying the mechanism of mitotic exit control in a spatiotemporal logical model

The transition from mitosis into the first gap phase of the cell cycle in budding yeast is controlled by the Mitotic Exit Network (MEN). The network interprets spatiotemporal cues about the progression of mitosis and ensures that release of Cdc14 phosphatase occurs only after completion of key mitotic events. The MEN has been studied intensively; however, a unified understanding of how localisation and protein activity function together as a system is lacking. In this paper, we present a compartmental, logical model of the MEN that is capable of representing spatial aspects of regulation in parallel to control of enzymatic activity. We show that our model is capable of correctly predicting the phenotype of the majority of mutants we tested, including mutants that cause proteins to mislocalise. We use a continuous time implementation of the model to demonstrate that Cdc14 Early Anaphase Release (FEAR) ensures robust timing of anaphase, and we verify our findings in living cells. Furthermore, we show that our model can represent measured cell-cell variation in Spindle Position Checkpoint (SPoC) mutants. This work suggests a general approach to incorporate spatial effects into logical models. We anticipate that the model itself will be an important resource to experimental researchers, providing a rigorous platform to test hypotheses about regulation of mitotic exit.

The regulation of Net1/Cdc14 by the Hog1 MAPK upon osmostress unravels a new mechanism regulating mitosis

During evolution, cells have developed a plethora of mechanisms to optimize survival in a changing and unpredictable environment. In this regard, they have evolved networks that include environmental sensors, signaling transduction molecules and response mechanisms. Hog1 (yeast) and p38 (mammals) stress-activated protein kinases (SAPKs) are activated upon stress and they drive a full collection of cell adaptive responses aimed to maximize survival. SAPKs are extensively used to learn about the mechanisms through which cells adapt to changing environments. In addition to regulating gene expression and metabolism, SAPKs control cell cycle progression. In this review, we will discuss the latest findings related to the SAPK-driven regulation of mitosis upon osmostress in yeast.

Polo kinase recruitment via the constitutive centromere-associated network at the kinetochore elevates centromeric RNA

The kinetochore, a multi-protein complex assembled on centromeres, is essential to segregate chromosomes during cell division. Deficiencies in kinetochore function can lead to chromosomal instability and aneuploidy-a hallmark of cancer cells. Kinetochore function is controlled by recruitment of regulatory proteins, many of which have been documented, however their function often remains uncharacterized and many are yet to be identified. To identify candidates of kinetochore regulation we used a proteome-wide protein association strategy in budding yeast and detected many proteins that are involved in post-translational modifications such as kinases, phosphatases and histone modifiers. We focused on the Polo-like kinase, Cdc5, and interrogated which cellular components were sensitive to constitutive Cdc5 localization. The kinetochore is particularly sensitive to constitutive Cdc5 kinase activity. Targeting Cdc5 to different kinetochore subcomplexes produced diverse phenotypes, consistent with multiple distinct functions at the kinetochore. We show that targeting Cdc5 to the inner kinetochore, the constitutive centromere-associated network (CCAN), increases the levels of centromeric RNA via an SPT4 dependent mechanism.

Regulation of Mitotic Exit by Cell Cycle Checkpoints: Lessons From Saccharomyces cerevisiae

In order to preserve genome integrity and their ploidy, cells must ensure that the duplicated genome has been faithfully replicated and evenly distributed before they complete their division by mitosis. To this end, cells have developed highly elaborated checkpoints that halt mitotic progression when problems in DNA integrity or chromosome segregation arise, providing them with time to fix these issues before advancing further into the cell cycle. Remarkably, exit from mitosis constitutes a key cell cycle transition that is targeted by the main mitotic checkpoints, despite these surveillance mechanisms being activated by specific intracellular signals and acting at different stages of cell division. Focusing primarily on research carried out using Saccharomyces cerevisiae as a model organism, the aim of this review is to provide a general overview of the molecular mechanisms by which the major cell cycle checkpoints control mitotic exit and to highlight the importance of the proper regulation of this process for the maintenance of genome stability during the distribution of the duplicated chromosomes between the dividing cells.

Phospho-peptide binding domains in S. cerevisiae model organism

Protein phosphorylation is one of the main mechanisms by which signals are transmitted in eukaryotic cells, and it plays a crucial regulatory role in almost all cellular processes. In yeast, more than half of the proteins are phosphorylated in at least one site, and over 20,000 phosphopeptides have been experimentally verified. However, the functional consequences of these phosphorylation events for most of the identified phosphosites are unknown. A family of protein interaction domains selectively recognises phosphorylated motifs to recruit regulatory proteins and activate signalling pathways. Nine classes of dedicated modules are coded by the yeast genome: 14-3-3, FHA, WD40, BRCT, WW, PBD, and SH2. The recognition specificity relies on a few residues on the target protein and has coevolved with kinase specificity. In the present study, we review the current knowledge concerning yeast phospho-binding domains and their networks. We emphasise the relevance of both positive and negative amino acid selection to orchestrate the highly regulated outcomes of inter- and intra-molecular interactions. Finally, we hypothesise that only a small fraction of yeast phosphorylation events leads to the creation of a docking site on the target molecule, while many have a direct effect on the protein or, as has been proposed, have no function at all.

Adaptation in replicative senescence: a risky business

Cell proliferation is tightly regulated to avoid propagating DNA damage and mutations, which can lead to pathologies such as cancer. To ensure genome integrity, cells activate the DNA damage checkpoint in response to genotoxic lesions to block cell cycle progression. This surveillance mechanism provides time to repair the damage before resuming cell cycle with an intact genome. When the damage is not repaired, cells can, in some conditions, override the cell cycle arrest and proceed with proliferation, a phenomenon known as adaptation to DNA damage. A subpopulation of adapted cells might eventually survive, but only at the cost of extensive genome instability. How and in which context adaptation operates the trade-off between survival and genome stability is a fascinating question. After a brief review of the current knowledge on adaptation to DNA damage in budding yeast, we will discuss a new role of adaptation in the context of telomerase-negative cells and replicative senescence. We highlight the idea that, in all settings studied so far, survival through adaptation is a double-edged sword as it comes with increased genomic instability.

Budding Yeast BFA1 Has Multiple Positive Roles in Directing Late Mitotic Events

The proper regulation of cell cycle transitions is paramount to the maintenance of cellular genome integrity. In Saccharomyces cerevisiae, the mitotic exit network (MEN) is a Ras-like signaling cascade that effects the transition from M phase to G1 during the cell division cycle in budding yeast. MEN activation is tightly regulated. It occurs during anaphase and is coupled to mitotic spindle position by the spindle position checkpoint (SPoC). Bfa1 is a key component of the SPoC and functions as part of a two-component GAP complex along with Bub2 The GAP activity of Bfa1-Bub2 keeps the MEN GTPase Tem1 inactive in cells with mispositioned spindles, thereby preventing inappropriate mitotic exit and preserving genome integrity. Interestingly, a GAP-independent role for Bfa1 in mitotic exit regulation has been previously identified. However the nature of this Bub2-independent role and its biological significance are not understood. Here we show that Bfa1 also activates the MEN by promoting the localization of Tem1 primarily to the daughter spindle pole body (dSPB). We demonstrate that the overexpression of BFA1 is lethal due to defects in Tem1 localization, which is required for its activity. In addition, our studies demonstrate a Tem1-independent role for Bfa1 in promoting proper cytokinesis. Cells lacking TEM1, in which the essential mitotic exit function is bypassed, exhibit cytokinesis defects. These defects are suppressed by the overexpression of BFA1 We conclude that Bfa1 functions to both inhibit and activate late mitotic events.

Functions and regulation of the Polo-like kinase Cdc5 in the absence and presence of DNA damage

Polo-like kinases are essential cell cycle regulators that are conserved from yeast to humans. Unlike higher eukaryotes, who express multiple Polo-like kinase family members that perform many important functions, budding yeast express only a single Polo-like kinase, Cdc5, which is the homolog of mammalian cell cycle master regulator Polo-like kinase 1. Cdc5 is a fascinating multifaceted protein that is programmed to target its many substrates in a timely, sequential manner to ensure proper cell cycle progression. Over the years, many lessons about Polo-like kinase 1 have been learned by studying Cdc5 in budding yeast. Cdc5 has been well documented in regulating mitotic entry, chromosome segregation, mitotic exit, and cytokinesis. Cdc5 also plays important roles during cell division after DNA damage. Here, we briefly review the many functions of Cdc5 and its regulation in the absence and presence of DNA damage.