Introduction The mitotic processes by which the nucleus divides occur in an orderly manner. Mitotic spindle assembly, elongation and disassembly are co-ordinated with chromatin re-organisation. In Aspergillus nidulans, as in other eukaryotes, mitotic progression is dependent upon Cdc2 protein kinase and the anaphase promoting complex/cyclosome, or APC/C (Osmani et al., 1994; Morris, 1976a; Peters et al., 1996; Lies et al., 1998). Cdc2 must first be activated through association with mitotic cyclin and later inactivated through cyclin destruction (Blanco et al., 2000; Wasch and Cross, 2002). The APC/C, which targets proteins for destruction, confers order on mitotic events by firstly acting upon proteins involved in sister chromatid cohesion, and then on mitotic cyclin (reviewed by Uhlmann, 2003; Zachariae and Nasmyth, 1999; Gardner and Burke, 2000). When the timing of mitotic events is perturbed, or when the mitotic spindle is damaged, the spindle assembly checkpoint prevents the activation of the APC/C. Thus the checkpoint can delay sister chromatid separation and mitotic exit. The BUB and MAD genes encode checkpoint components and were isolated from budding yeast by identifying mutants that failed to arrest chromatid separation in the presence of drug-induced spindle damage (Hoyt et al., 1991; Li and Murray, 1991). The checkpoint appears to be highly conserved. In Aspergillus, mutations in BUB-related genes confer sensitivity to spindle drugs (Efimov and Morris, 1998). Loss of the checkpoint is associated with abnormal chromosome numbers (aneuploidy) in human cancer cells (Li and Benezra, 1996; Cahill et al., 1998; Jin et al., 1998; Lengauer et al., 1998; Takahashi et al., 1999; Lee et al., 1999). Thus, mutations that perturb the timing of mitotic events and uncouple the checkpoint responses are likely candidates for inactivation in cancer. A number of mutants that perturb progression through mitosis have been identified in Aspergillus (Morris, 1976b). For some of these, the corresponding genes have been isolated and shown to govern the intrinsic timing of mitotic events. The bimA (O’Donnell et al., 1991) and bimE (Osmani et al., 1988) genes encode homologues of APC/C components (Peters et al., 1996; Zachariae et al., 1996) and mutations in both genes lead to a delay in metaphase, presumably as a result of APC/C inactivation. Type 1 protein phosphatase, encoded by the bimG gene, is also required for progression through anaphase (Doonan and Morris, 1989). Mutation of the bimB gene leads to a transient mitotic delay and uncouples DNA replication from the completion of the previous mitosis (May et al., 1992). bimB-related genes in budding yeast (ESP1) and fission yeast (cut1 + ), both function as separases, which are required for sister chromatid separation in mitosis (Ciosk et al., 1998; Funabiki et al., 1996; McGrew et al., 1992). The bimC gene is the founding member of a class of motor proteins, called kinesins, that plays an essential role in spindle pole body separation in mitosis (Enos and Morris, 1990). A heat sensitive β tubulin mutation, benA33, that hyper-stabilises the mitotic spindle by blocking microtubule disassembly (Oakley and Morris, 1981), delays progression through anaphase. The activities of two protein kinases are also required for entry into mitosis in Aspergillus (Osmani et al., 1991a; Ye et al., 1995) (reviewed by Osmani and Ye, 1996), and these must be inactivated for mitotic exit. The nimX gene, isolated by reverse genetics, encodes a Cdc2 homologue (Osmani et al., 1994), and the nimA gene encodes a second protein kinase required for chromosome condensation in mitosis (DeSouza et al., 2000). Whereas fluctuations in NIMX Cdc2 activity depend, in part, upon APC/C-mediated degradation of its activating subunit, NIME cyclinB (Ye et al., 1997), NIMA is a direct target 199 Orderly progression through mitosis is essential to reduce segregation errors in the cell’s genetic material. We have used a cytological screen to identify a mutant that progresses through mitosis aberrantly and have cloned the complementing gene, nimU, which encodes a protein related to Pot1 and other telomere end-binding proteins. We show that loss of nimU function leads to premature mitotic spindle elongation, premature mitotic exit, errors in chromosome segregation, and failure to delay mitotic exit under conditions that normally evoke the mitotic spindle checkpoint response. Whereas premature mitotic exit is dependent upon anaphase promoting complex function, premature spindle elongation is not. We conclude that nimU is constitutively required for orderly mitotic progression under normal growth conditions and also required for the conditional mitotic spindle checkpoint response. Key words: Aspergillus nidulans, Telomere end binding protein, Pot1, Mitotic exit, Spindle checkpoint Summary The pot1 + homologue in Aspergillus nidulans is required for ordering mitotic events Christopher W. Pitt*, Eric Moreau, Patricia A. Lunness and John H. Doonan ‡ John Innes Centre, Colney, Norwich, NR4 7UH, UK *Present address: Telomere Biology Laboratory, Cancer Research UK, 44 Lincoln’s Inn Fields, London WC2A 3PX, UK ‡ Author for correspondence (e-mail: john.doonan@bbsrc.ac.uk) Accepted 27 August 2003 Journal of Cell Science 117, 199-209 Published by The Company of Biologists 2004 doi:10.1242/jcs.00844 Research Article JCS ePress online publication date 2 December 2003