INTRODUCTION Cyclin-dependent kinases (CDKs) 1 are required for the onset of S phase and M phase of the mitotic cell cycle (reviewed by Nurse, 2000). In higher eukaryotes, distinct CDKs associate with several cyclins to control the G 1 /S transition, whereas cdk1/cyclin B is specifically required to control the G 2 /M transition (reviewed by Nigg, 1995). In the fission yeast Schizosaccharomyces pombe, cdc2p/cdc13p complex controls the G 2 /M transition of the cell cycle, whereas cig2p B-type cyclin associates with cdc2p to promote the G 1 /S transition, although this can be accomplished by the cig1p and cdc13p B- type cyclins (Fisher and Nurse, 1996; Mondesert et al., 1996). Fission yeast possesses two other cyclins, puc1p and pas1p, which are related to the Cln1-3p G 1 cyclins of Saccharomyces cerevisiae (Forsburg and Nurse, 1991; Tanaka and Okayama, 2000). Puc1p may regulate the G 1 phase progression in response to cell size (Martin-Castellanos et al., 2000), whereas pas1p associates with a second non-essential CDK, pef1p, to activate the res2p-cdc10p transcriptional complex at the START point of the cell cycle (Tanaka and Okayama, 2000). CDKs are not only required for the onset of S phase and M phase of the mitotic cell cycle but also are involved in the regulation of centrosomes and microtubule (MT) dynamics. In Xenopus egg extracts, CDKs induce a change in MT dynamics and steady-state length (Verde et al., 1990). Cytostatic factor- arrested Xenopus egg extracts (which contain active cdc2 kinase bound to cyclin), but not interphase extracts, can convert fission yeast spindle pole bodies (SPBs, the equivalent of animal centrosomes) to a nucleation competent state (Masuda et al., 1992). However, purified cdc2/cyclin B1 complex was unable to do so, suggesting that other factors besides cdc2 kinase are also required for SPB activation (Masuda et al., 1992). In multicellular eukaryotes, duplication of the centrosome requires cdk2/cyclin E activity (reviewed by Whitehead and Salisbury, 1999; Meraldi et al., 1999; Okuda et al., 2000). Cdk1 localises to spindle MTs and metazoa centrosomes via association with MT-associated proteins (MAPs), whose phosphorylation induces a change in MT dynamics at the onset of mitosis (reviewed by Andersen, 2000). In fission yeast, Alfa et al. showed by immunofluorescence that cdc2p and cdc13p are localised to the SPBs during mitosis (Alfa et al., 1990). In a more recent paper, cdc13-GFP fusion was shown to localise on the SPBs and the spindle from prophase to metaphase (Yanagida et al., 1999). S. pombe cdc2p is also required during the meiotic cell cycle for premeiotic DNA synthesis, the second division and, very likely, the first meiotic division (reviewed by Murakami and Nurse, 2000). Once karyogamy and premeiotic DNA replication have occurred, the meiotic prophase nucleus shows an elongated morphology, called a ‘horse-tail’, and oscillates back and forth between the cell poles (reviewed by Hiraoka, 1998). During this horse-tail movement, telomeres are clustered at the SPB in a bouquet-like arrangement at the 2627 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 G 1 /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 G 2 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. Key words: Schizosaccharomyces pombe, Mitosis, Meiosis, Cyclins, Cyclin-dependent kinase SUMMARY In vivo localisation of fission yeast cyclin-dependent kinase cdc2p and cyclin B cdc13p during mitosis and meiosis Anabelle Decottignies*, Patrick Zarzov and Paul Nurse Cell Cycle Laboratory, Imperial Cancer Research Fund, London, WC2A 3PX, UK *Author for correspondence (e-mail: a.decottignies@icrf.icnet.uk) Accepted 23 April 2001 Journal of Cell Science 114, 2627-2640 (2001) © The Company of Biologists Ltd RESEARCH ARTICLE