In Saccharomyces cerevisiae exit from mitosis requires the Cdc14 phosphatase to reverse CDK-mediated phosphorylation. Cdc14 is released from the nucleolus by the Cdc14 early anaphase release (FEAR) and mitotic exit network (MEN) pathways. In meiosis, the FEAR pathway is essential for exit from anaphase I. The MEN component Cdc15 is required for the formation of mature spores. To analyze the role of Cdc15 during sporulation, a conditional mutant in which CDC15 expression was controlled by the CLB2 promoter was used. Cdc15-depleted cells proceeded normally through the meiotic divisions but were unable to properly disassemble meiosis II spindles. The morphology of the prospore membrane was aberrant and failed to capture the nuclear lobes. Cdc15 was not required for Cdc14 release from the nucleoli, but it was essential to maintain Cdc14 released and for its nucleo-cytoplasmic transport. However, cells carrying a CDC14 allele with defects in nuclear export (Cdc14-DNES) were able to disassemble the spindle and to complete spore formation, suggesting that the Cdc14 nuclear export defect was not the cause of the phenotypes observed in cdc15 mutants. MEIOSIS is a specialized cell division that produces the haploid cells needed for sexual reproduction, and the completion of meiosis is normally coupled to differentiation programs that produce gametes. In Saccharomyces cerevisiae, sporulation is initiated when diploid cells are grown in the absence of nitrogen and the presence of a nonfermentable carbon source such as acetate (Esposito and Klapholtz 1981). When triggered to enter the sporulation program, cells exit the mitotic cycle from the G1 phase. This is followed by one round of S phase and two meiotic divisions that generate four haploid nuclei, which are packaged into individual spores. As a consequence of this process, four haploid gametes (spores) are formed in the cytoplasm of the mother cell (ascus) (reviewed by Neiman 2005). At the onset of meiosis II, the spindle pole body (SPB) is modified by the incorporation of several meiosis-specific proteins, which together make up the meiosis II outer plaque (Knop and Strasser 2000;Bajgier et al. 2001;Nickas et al. 2003). This change in composition converts the function of the cytoplasmic face of the SPB from microtubule nucleation to membrane nucleation. The process of spore construction requires the de novo assembly of two cellular structures, the prospore membrane (PSM) that is generated around the daughter nuclei to create prospores and a protective spore wall that surrounds the prospores. Once the meiosis II outer plaque of the SPB has been assembled, it becomes a site for the docking and coalescence of secretory vesicles to form flattened double-membrane sheets termed PSMs (Moens 1971;Neiman 1998). These nascent PSMs expand during meiosis II, and at the time of nuclear division each PSM completely engulfs the nuclear lobe to which it is anchored via the SPB. At the completion of meiosis II, each PSM fuses with itself so that each daughter nucleus (and assoc...

BackgroundIn Saccharomyces cerevisiae, nutrient limitation stimulates diploid cells to undergo DNA replication and meiosis, followed by the formation of four haploid spores. Septins are a family of proteins that assemble a ring structure at the mother-daughter neck during vegetative growth, where they control cytokinesis. In sporulating cells, the septin ring disassembles and septins relocalize to the prospore membrane.ResultsHere, we demonstrate that nutrient limitation triggers a change in the localization of at least two vegetative septins (Cdc10 and Cdc11) from the bud neck to the microtubules. The association of Cdc10 and Cdc11 with microtubules persists into meiosis, and they are found associated with the meiotic spindle until the end of meiosis II. In addition, the meiosis-specific septin Spr28 displays similar behavior, suggesting that this is a common feature of septins. Septin association to microtubules is a consequence of the nutrient limitation signal, since it is also observed when haploid cells are incubated in sporulation medium and when haploid or diploid cells are grown in medium containing non-fermentable carbon sources. Moreover, during meiosis II, when the nascent prospore membrane is formed, septins moved from the microtubules to this membrane. Proper organization of the septins on the membrane requires the sporulation-specific septins Spr3 and Spr28.ConclusionNutrient limitation in S. cerevisiae triggers the sporulation process, but it also induces the disassembly of the septin bud neck ring and relocalization of the septin subunits to the nucleus. Septins remain associated with microtubules during the meiotic divisions and later, during spore morphogenesis, they are detected associated to the nascent prospore membranes surrounding each nuclear lobe. Septin association to microtubules also occurs during growth in non-fermentable carbon sources.

SummaryWe have characterized the DBF2 gene, encoding a protein kinase of the NDR family in Candida albicans, and demonstrate that this gene is essential for cell viability. Conditional mutants were constructed by using the MET3 promoter to analyse the phenotype of cells lacking this kinase. The absence of Dbf2 resulted in cells arrested as large-budded pairs that failed to contract the actomyosin ring, a function similar to that described for its Saccharomyces cerevisiae orthologue. In addition to its role in cytokinesis, Dbf2 regulates mitotic spindle organization and nuclear segregation as Dbf2-depleted cells have abnormal microtubules and severe defects in nuclear migration to the daughter cell, which results in a cell cycle block during mitosis. Taken together, these results imply that Dbf2 performs several functions during exit from mitosis and cytokinesis. Consistent with a role in spindle organization, the protein localizes to the mitotic spindle during anaphase, and it interacts physically with tubulin, as indicated by immunoprecipitation experiments. Finally, DBF2 depletion also resulted in impaired true hyphal growth.