Introduction Protein phosphorylation is central in the regulation of eukaryotic cell cycle progression. Among the several families of kinases that contribute to cell cycle control are the NIMA kinases. The founding member of the NIMA family is a serine/threonine kinase that plays an essential role in regulating the G2/M transition in Aspergillus nidulans (NIMA; never in mitosis) (Lu and Means, 1994; Morris, 1976; Osmani et al., 1991). The Aspergillus NIMA protein also promotes chromosome condensation (O’Connell et al., 1994; Osmani et al., 1988) and plays a role in the localization of cyclin B to the nucleus in late G2 (Wu et al., 1998). Studies of NIMA-related kinases (Neks) suggest that this family of proteins may serve diverse cellular functions. For example, mammalian Nek6 and Nek7 appear to mediate activation of the p70 ribosomal S6 kinase in response to insulin and growth factor stimulation (Belham et al., 2001), whereas a Nek1 mouse knock-out has a pleiotropic phenotype, including a progressive polycystic kidney disease (Upadhya et al., 2000). Whether the protein synthesis activities of Neks 6 and 7 or the PKD phenotype of the Nek1 mutant mouse ultimately relate to cell cycle control remains to be determined. Nevertheless, other members of the family are clearly involved in the regulation of cell cycle progression. Nek2 is of particular interest with respect to the present work. Expression studies have implicated vertebrate Nek2 in cell cycle progression, but there is no evidence that it is playing the same roles as NIMA in G2/M transition and chromosome condensation (Fry et al., 1995; Fry et al., 1998; Rhee and Wolgemuth, 1997; Tanaka et al., 1997). Instead, Nek2 appears to play an essential role in the progression of the centrosomal cycle. Overexpression of Nek2 results in premature separation of the replicated centrioles (Fry et al., 1998; Mayor et al., 2000). Furthermore, biochemical data demonstrates that Nek2B facilitates assembly of the Xenopus zygotic centrosome (Fry et al., 2000). Like Nek2, the FA2 gene of Chlamydomonas encodes a NIMA family kinase that has a centrosome/basal- body-associated function. The FA2 gene was discovered during a genetic screen for deflagellation-defective mutants of Chlamydomonas reinhardtii (Finst et al., 1998). Deflagellation is a highly specific process that involves a Ca 2+ signal transduction pathway originating at the plasma membrane and culminating in the severing of the nine outer-doublet axonemal microtubules at a precise site distal to the transition zone between the axoneme and the basal bodies (for a review, see Quarmby, 2000). The microtubule-severing ATPase katanin has been implicated in this severing event by its localization at the site of severing, the inhibition of calcium-induced severing by katanin p60 antibodies and its demonstrated ability to sever the complex doublet microtubules of the axoneme (Lohret et al., 1998). Our genetic screen identified three additional genes, ADF1, FA1 and FA2, which are essential for in vivo deflagellation. ADF1 plays a role in signal transduction, whereas, cells with mutations in either FA1 or FA2 fail to deflagellate because of a defect in calcium-induced axonemal microtubule severing (Finst et al., 1998; Quarmby, 1996). Fa1p is a novel 170 kDa protein with a large coiled coil domain; it localizes to the base of the flagella (Finst et al., 2000). In this work we have identified a genomic clone that rescues fa2 mutants. Sequence analysis indicates that the FA2 gene encodes a NIMA kinase. This is the first indication that this 1759 The NIMA kinases are one of several families of kinases that participate in driving the eukaryotic cell cycle. NIMA- related kinases have been implicated in G2/M progression, chromatin condensation and regulation of the centrosome cycle. Here we report the identification of a new member of this family, FA2, from Chlamydomonas reinhardtii. FA2 was originally discovered in a genetic screen for deflagellation-defective mutants. We have previously shown that FA2 is essential for basal-body/centriole-associated microtubule severing. We now report that the FA2 NIMA- related kinase also plays a role in cell cycle progression in Chlamydomonas. This is the first indication that members of the NIMA family might exert their effects through the regulation of microtubule severing. Key words: NIMA, Cell cycle, Chlamydomonas, Microtubule severing, Deflagellation Summary The FA2 gene of Chlamydomonas encodes a NIMA family kinase with roles in cell cycle progression and microtubule severing during deflagellation Moe R. Mahjoub, Ben Montpetit, Lifan Zhao, Rip J. Finst, Benjamin Goh, Apollos C. Kim and Lynne M. Quarmby* Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6 *Author for correspondence (e-mail: Quarmby@sfu.ca) Accepted 13 January 2002 Journal of Cell Science 115, 1759-1768 (2002) © The Company of Biologists Ltd Research Article