Telomere anchoring at the nuclear periphery requires the budding yeast Sad1-UNC-84 domain protein Mps3

Positioning of telomeres at the nuclear periphery can have dramatic effects on gene expression by establishment of heritable, transcriptionally repressive subdomains. However, little is known about the integral membrane proteins that mediate telomere tethering at the nuclear envelope. Here, we find a previously unrecognized function for the Saccharomyces cerevisiae Sad1-UNC-84 domain protein Mps3 in regulating telomere positioning in mitotic cells. Our data demonstrate that the nucleoplasmic N-terminal acidic domain of Mps3 is not essential for viability. However, this acidic domain is necessary and sufficient for telomere tethering during S phase and the silencing of reporter constructs integrated at telomeres. We show that this is caused by the role of the Mps3 acidic domain in binding and localization of the silent information regulator protein Sir4 to the nuclear periphery. Thus, Mps3 functions as an integral membrane anchor for telomeres and is a novel nuclear receptor for the Sir4 pathway of telomere tethering and gene inactivation

Boolean network model predicts cell cycle sequence of fission yeast

A Boolean network model of the cell-cycle regulatory network of fission yeast (Schizosaccharomyces Pombe) is constructed solely on the basis of the known biochemical interaction topology. Simulating the model in the computer, faithfully reproduces the known sequence of regulatory activity patterns along the cell cycle of the living cell. Contrary to existing differential equation models, no parameters enter the model except the structure of the regulatory circuitry. The dynamical properties of the model indicate that the biological dynamical sequence is robustly implemented in the regulatory network, with the biological stationary state G1 corresponding to the dominant attractor in state space, and with the biological regulatory sequence being a strongly attractive trajectory. Comparing the fission yeast cell-cycle model to a similar model of the corresponding network in S. cerevisiae, a remarkable difference in circuitry, as well as dynamics is observed. While the latter operates in a strongly damped mode, driven by external excitation, the S. pombe network represents an auto-excited system with external damping.Comment: 10 pages, 3 figure

Efficacy and safety of flexible-dose oral sildenafil citrate (Viagra((R))) in the treatment of erectile dysfunction in Brazilian and Mexican men

A 12-week, double-blind, placebo-controlled, multicenter study evaluated the efficacy and safety of flexible-dose sildenafil citrate (Viagra((R))) treatment (25, 50 or 100 mg) in Brazilian and Mexican men with erectile dysfunction (ED) of broad-spectrum etiology. Efficacy was assessed on the basis of responses to the 15-item International Index of Erectile Function (IIEF) questionnaire, completed at baseline and after 12 weeks of treatment. At end point, mean scores for all IIEF domains of sexual function (erectile function, orgasmic function, sexual desire, intercourse satisfaction and overall satisfaction) were significantly (P < 0.0001) higher in the sildenafil group (n = 109) than in the placebo group (n = 105). These findings confirm the significant increases in frequency of penetration and frequency of maintained erections reported previously. Sildenafil treatment was well tolerated. the most common adverse events were headache and flushing. in conclusion, sildenafil is a well-tolerated and effective treatment for ED of broad-spectrum etiology in Latin American men.Hosp Albert Einstein, BR-01250000 São Paulo, BrazilTorre Hosp Angeles del Pedregal, Consultorio 827, MexicoFac Ciencias Med Santa Casa, Porto Alegre, RS, BrazilHosp Clin Cuiritiba, Curitiba, Parana, BrazilUniv Estadual Campinas, Fac Ciencias Med, Campinas, SP, BrazilClin Rio Claro, Rio Claro, SP, BrazilUniv Estadual Rio de Janeiro, Fac Med, Rio de Janeiro, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, São Paulo, BrazilHosp Servidor Publ Estadual, São Paulo, BrazilUniversidade Federal de São Paulo, Escola Paulista Med, São Paulo, BrazilWeb of Scienc

Dbf2–Mob1 drives relocalization of protein phosphatase Cdc14 to the cytoplasm during exit from mitosis

Exit from mitosis is characterized by a precipitous decline in cyclin-dependent kinase (Cdk) activity, dissolution of mitotic structures, and cytokinesis. In Saccharomyces cerevisiae, mitotic exit is driven by a protein phosphatase, Cdc14, which is in part responsible for counteracting Cdk activity. Throughout interphase, Cdc14 is sequestered in the nucleolus, but successful anaphase activates the mitotic exit network (MEN), which triggers dispersal of Cdc14 throughout the cell by a mechanism that has remained unknown. In this study, we show that a MEN component, protein kinase Dbf2–Mob1, promotes transfer of Cdc14 to the cytoplasm and consequent exit from mitosis by direct phosphorylation of Cdc14 on serine and threonine residues adjacent to a nuclear localization signal (NLS), thereby abrogating its NLS activity. Our results define a mechanism by which the MEN promotes exit from mitosis

CDK-dependent nuclear localization of B-Cyclin Clb1 promotes FEAR activation during meiosis I in budding yeast

Cyclin-dependent kinases (CDK) are master regulators of the cell cycle in eukaryotes. CDK activity is regulated by the presence, post-translational modification and spatial localization of its regulatory subunit cyclin. In budding yeast, the B-cyclin Clb1 is phosphorylated and localizes to the nucleus during meiosis I. However the functional significance of Clb1's phosphorylation and nuclear localization and their mutual dependency is unknown. In this paper, we demonstrate that meiosis-specific phosphorylation of Clb1 requires its import to the nucleus but not vice versa. While Clb1 phosphorylation is dependent on activity of both CDK and polo-like kinase Cdc5, its nuclear localization requires CDK but not Cdc5 activity. Furthermore we show that increased nuclear localization of Clb1 during meiosis enhances activation of FEAR (Cdc Fourteen Early Anaphase Release) pathway. We discuss the significance of our results in relation to regulation of exit from meiosis I

A Mathematical Model of Mitotic Exit in Budding Yeast: The Role of Polo Kinase

Cell cycle progression in eukaryotes is regulated by periodic activation and inactivation of a family of cyclin–dependent kinases (Cdk's). Entry into mitosis requires phosphorylation of many proteins targeted by mitotic Cdk, and exit from mitosis requires proteolysis of mitotic cyclins and dephosphorylation of their targeted proteins. Mitotic exit in budding yeast is known to involve the interplay of mitotic kinases (Cdk and Polo kinases) and phosphatases (Cdc55/PP2A and Cdc14), as well as the action of the anaphase promoting complex (APC) in degrading specific proteins in anaphase and telophase. To understand the intricacies of this mechanism, we propose a mathematical model for the molecular events during mitotic exit in budding yeast. The model captures the dynamics of this network in wild-type yeast cells and 110 mutant strains. The model clarifies the roles of Polo-like kinase (Cdc5) in the Cdc14 early anaphase release pathway and in the G-protein regulated mitotic exit network

From START to FINISH : the influence of osmotic stress on the cell cycle

Peer reviewedPublisher PD

The SUN Protein Mps3 Is Required for Spindle Pole Body Insertion into the Nuclear Membrane and Nuclear Envelope Homeostasis

The budding yeast spindle pole body (SPB) is anchored in the nuclear envelope so that it can simultaneously nucleate both nuclear and cytoplasmic microtubules. During SPB duplication, the newly formed SPB is inserted into the nuclear membrane. The mechanism of SPB insertion is poorly understood but likely involves the action of integral membrane proteins to mediate changes in the nuclear envelope itself, such as fusion of the inner and outer nuclear membranes. Analysis of the functional domains of the budding yeast SUN protein and SPB component Mps3 revealed that most regions are not essential for growth or SPB duplication under wild-type conditions. However, a novel dominant allele in the P-loop region, MPS3-G186K, displays defects in multiple steps in SPB duplication, including SPB insertion, indicating a previously unknown role for Mps3 in this step of SPB assembly. Characterization of the MPS3-G186K mutant by electron microscopy revealed severe over-proliferation of the inner nuclear membrane, which could be rescued by altering the characteristics of the nuclear envelope using both chemical and genetic methods. Lipid profiling revealed that cells lacking MPS3 contain abnormal amounts of certain types of polar and neutral lipids, and deletion or mutation of MPS3 can suppress growth defects associated with inhibition of sterol biosynthesis, suggesting that Mps3 directly affects lipid homeostasis. Therefore, we propose that Mps3 facilitates insertion of SPBs in the nuclear membrane by modulating nuclear envelope composition

Oscillatory Dynamics of Cell Cycle Proteins in Single Yeast Cells Analyzed by Imaging Cytometry

Progression through the cell division cycle is orchestrated by a complex network of interacting genes and proteins. Some of these proteins are known to fluctuate periodically during the cell cycle, but a systematic study of the fluctuations of a broad sample of cell-cycle proteins has not been made until now. Using time-lapse fluorescence microscopy, we profiled 16 strains of budding yeast, each containing GFP fused to a single gene involved in cell cycle regulation. The dynamics of protein abundance and localization were characterized by extracting the amplitude, period, and other indicators from a series of images. Oscillations of protein abundance could clearly be identified for Cdc15, Clb2, Cln1, Cln2, Mcm1, Net1, Sic1, and Whi5. The period of oscillation of the fluorescently tagged proteins is generally in good agreement with the inter-bud time. The very strong oscillations of Net1 and Mcm1 expression are remarkable since little is known about the temporal expression of these genes. By collecting data from large samples of single cells, we quantified some aspects of cell-to-cell variability due presumably to intrinsic and extrinsic noise affecting the cell cycle