In vivo localisation of fission yeast cyclin-dependent kinase cdc2p and cyclin B cdc13p during mitosis and meiosis.

We investigated the in vivo localisation of fission yeast cyclin-dependent kinase cdc2p during mitosis and meiosis. Fusion to yellow fluorescent protein (YFP) revealed that cdc2-YFP is present in the cytoplasm at all stages of the cell cycle. Nuclear cdc2-YFP fluorescence oscillates with that of cdc13-YFP cyclin. At G1/S, at least one of cdc13p, cig1p or cig2p B-type cyclins is required for the accumulation of cdc2-YFP into the nucleus. Cdc2-YFP and cdc13-YFP are highly enriched on the spindle pole body of cells in late G2 or arrested at S phase. Both accumulate on the spindle pole bodies and the spindle in prophase and metaphase independently of the microtubule-associated protein dis1p. In anaphase, the cdc2p/cdc13p complex leaves the spindle prior to sister chromatid separation, and cdc13-YFP is enriched at the nuclear periphery before fluorescence disappears. If cdc13p cannot be recognized by the anaphase-promoting complex, cdc2-YFP and cdc13-YFP remain associated with the spindle. In mating cells, cdc2-YFP enters the nucleus as soon as the cells undergo fusion. During karyogamy and meiotic prophase, cdc2-YFP is highly enriched on the centromeres. In meiosis I, association of cdc2-YFP with the spindle and the spindle pole bodies shows differences to mitotic cells, suggesting different mechanisms of spindle formation. This study suggests that changes in cdc2p localisation are important for both mitosis and meiosis regulation.

Feedback regulation of the MBF transcription factor by cyclin Cig2.

The Mlu1-binding factor (MBF) from the fission yeast Schizosaccharomyces pombe contains the proteins Res1p and Res2p and binds to the Mlu1 cell-cycle box (MCB) element in DNA, activating the transcription of genes required for S phase. We report here that the cell-cycle-regulated expression of the cyclin cig2 gene is dependent on MBF. Deletion of MCB elements in the cig2 promoter perturbed the expression not only of cig2 but also of other MBF-dependent genes, indicating that Cig2p could regulate MBF activity. Cig2p can bind to Res2p, promote the phosphorylation of Res1p and inhibit MBF-dependent gene transcription. Cig2p thus forms an autoregulating feedback-inhibition loop with MBF which is important for normal regulation of the cell cycle.

Hierarchy of S-phase-promoting factors: yeast Dbf4-Cdc7 kinase requires prior S-phase cyclin-dependent kinase activation.

In all eukaryotes, the initiation of DNA synthesis requires the formation of prereplicative complexes (pre-RCs) on replication origins, followed by their activation by two S-T protein kinases, an S-phase cyclin-dependent kinase (S-CDK) and a homologue of yeast Dbf4-Cdc7 kinase (Dbf4p-dependent kinase [DDK]). Here, we show that yeast DDK activity is cell cycle regulated, though less tightly than that of the S-CDK Clb5-Cdk1, and peaks during S phase in correlation with Dbf4p levels. Dbf4p is short-lived throughout the cell cycle, but its instability is accentuated during G(1) by the anaphase-promoting complex. Downregulating DDK activity is physiologically important, as joint Cdc7p and Dbf4p overexpression is lethal. Because pre-RC formation is a highly ordered process, we asked whether S-CDK and DDK need also to function in a specific order for the firing of origins. We found that both kinases are activated independently, but we show that DDK can perform its function for DNA replication only after S-CDKs have been activated. Cdc45p, a protein needed for initiation, binds tightly to chromatin only after S-CDK activation (L. Zou and B. Stillman, Science 280:593-596, 1998). We show that Cdc45p is phosphorylated by DDK in vitro, suggesting that it might be one of DDK's critical substrates after S-CDK activation. Linking the origin-bound DDK to the tightly regulated S-CDK in a dependent sequence of events may ensure that DNA replication initiates only at the right time and place.

A yeast heat shock transcription factor (Hsf1) mutant is defective in both Hsc82/Hsp82 synthesis and spindle pole body duplication.

Cdc28 is a cyclin-dependent protein kinase of Saccharomyces cerevisiae that is required for the G1/S and G2/M transitions of the cell division cycle. All previously described cdc28 mutants aside from cdc28-1N arrest division specifically in the G1 phase. cdc28-1N arrests division in G2/mitosis. We show here that the cdc28-109 mutant exhibits a mixed cell division arrest at 37 degrees C with cells in both the G1 and G2 phases. In order to identify proteins that functionally interact with Cdc28, we isolated mutants that are colethal with cdc28-109 at its permissive temperature. We describe here our phenotypic analysis of two such mutants, hsf1-82 and ydj1-10, that affect the heat shock transcription factor and a yeast dnaj-like protein chaperone, respectively. hsf1-82 and ydj1-10 temperature-sensitive mutants arrest the cell division cycle at several stages. However, one predominant class of cells in both mutants was arrested with a large bud and a single vertex of microtubules. Electron microscopic analysis of such hsf1-82 cells showed that they contained an unduplicated spindle pole body with an enlarged half-bridge. Two-dimensional gel electrophoresis of total cell proteins revealed that the hsf1-82 cells were specifically defective in the expression of the Hsc82 and Hsp82 proteins. Furthermore, the hsf1-82 mutation was suppressed by the HSC82 gene on a multicopy plasmid that restored Hsc82 protein to high levels in these cells. These results show that Hsf1 is required for spindle pole body duplication at 37 degrees C.

The SLT2(MPK1) MAP kinase is activated during periods of polarized cell growth in yeast.

The SLT2(MPK1) mitogen-activated protein kinase signal transduction pa thway has been implicated in several biological processes in Saccharomyces cerevisiae, including the regulation of cytoskeletal and cell wall structure, polarized cell growth, and response to nutrient availability, hypo-osmotic shock and heat shock. We examined the conditions under which the SLT2 pathway is activated. We found that the SLT2 kinase is tyrosine phosphorylated and activated during periods in which yeast cells are undergoing polarized cell growth, namely during bud formation of vegetative cell division and during projection formation upon treatment with mating pheromone. BCK1(SLK1), a MEK kinase, is required for SLT2 activation in both of these situations. Upstream of BCK1(SLK1), we found that the STE20 kinase was required for SLT2 activation by mating pheromone, but was unnecessary for its activation during the vegetative cell cycle. Finally, SLT2 activation during vegetative growth was partially dependent on CDC28 in that the stimulation of SLT2 tyrosine phosphorylation was significantly reduced directly after a temperature shift in cdc28 ts mutants. Our data are consistent with a role for SLT2 in promoting polarized cell growth.