Never say never. The NIMA-related protein kinases in mitotic control Matthew J. O’Connell 1,2,4 , Michael J.E. Krien 2 and Tony Hunter 3 1 Derald H. Ruttenberg Cancer Center, Mt Sinai School of Medicine, One Gustave L. Levy Place, Box 1130, New York, NY 10029, USA 2 Trescowthick Research Laboratories, Peter MacCallum Cancer Institute, Locked Bag 1, A’Beckett St, Melbourne, VIC 8006, Australia 3 Molecular and Cell Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA 92037, USA 4 Courier address: 1425 Madison Avenue, Room 15-70, New York, NY 10029, USA Mitosis sees a massive reorganization of cellular archi- tecture. The microtubule cytoskeleton is reorganized to form a bipolar spindle between duplicated microtubule organizing centers, the chromosomes are condensed, attached to the spindle at their kinetochores, and, through the action of multiple molecular motors, the chromosomes are segregated into two daughter cells. Mitosis also sees a substantial wave of protein phos- phorylation, controlling signaling events that coordinate mitotic processes and ensure accurate chromosome seg- regation. The key switch for the onset of mitosis is the archetypal cyclin-dependent kinase, Cdc2. Under the direction of Cdc2 is an executive of protein serine/ threonine kinases that fall into three families: the Polo kinases, Aurora kinases and the NIMA-related kinases (Nrk). The latter family has proven the most enigmatic in function, although recent advances from several sources are beginning to reveal a common functional theme. Mitosis is the crucial point in the cell cycle at which one cell divides to form two. The conservation of both the general mechanisms and regulatory pathways controlling mitosis throughout the eukaryotic kingdom highlights its ancient nature and importance. It is this very conservation that has allowed the identification of a common retinue of mitotic regulators from a diverse band of eukaryotes ranging from yeasts to humans. Several key mitotic regulators are protein-serine/threonine kinases that are responsible for modulating entry into, and progression through, mitosis. Uppermost in the mitotic phosphoryl- ation cascade is the cyclin-dependent kinase Cdc2, which, upon activation late in G2 phase, induces the dramatic cellular reorganization that is associated with mitosis. Regulating the downstream events following Cdc2 acti- vation is a trio of protein kinase families: the Aurora, Polo and NIMA-related (Nrk) kinases. These are also highly conserved in evolution, although, while yeasts have a single member of each family, in humans these have grown into multi-gene families encoding related proteins. The Aurora and Polo families have been best studied to date and regulate many events, from the initiation through to the exit of mitosis (for recent reviews on these families, see [1–8]). The third family of mitotic kinases, the NIMA-related kinases (Nrk), is less well understood, and the purpose of this review is to summarize what is known about the members of this enigmatic group and discusses the implications for the regulation of mitosis. Aspergillus NIMA: discovery and early analysis The early 1970s saw the beginnings of the genetic analysis of the cell cycle, with the seminal screens of Lee Hartwell and Paul Nurse in Saccharomyces cerevisiae and Schizo- saccharomyces pombe, respectively. At around the same time, Ron Morris carried out a screen for cell cycle mutants in the multicellular filamentous fungus Aspergillus nidulans (Fig. 1). Morris screened hundreds of tempera- ture sensitive mutants, with cell cycle mutants being classified as either bim mutants, for those blocked in mitosis with condensed chromosomes and mitotic spindles, or nim mutants, for those that were never in mitosis due to an interphase arrest [9]. The bim collection eventually proved to include important mitotic genes such as spindle motors and components of the anaphase-promoting com- plex (APC). The nim mutants curiously did not include a cdc2 homolog, but did include many well-known cell cycle regulators such as a B-type cyclin, Cdc25 phosphatase and DNA polymerases. They also included four alleles of a gene denoted as nimA [10]. nimA mutants arrested late in G2, with duplicated spindle pole bodies (SPBs), the fungal equivalent of the centrosome [11]. By the mid-1980s, nimA was cloned and shown to encode a serine/threonine protein kinase designated NIMA [12,13] (see Fig. 2 for domain organiz- ation of NIMA and other Nrks). Further genetic analyses showed that the interphase arrest of nimA5 mutants was bypassed by the bimE7 mutation, with double mutants entering an aberrant mitotic state characterized by abnormalities of the nuclear envelope, which remains intact during mitosis in fungi, and the failure to form a bipolar spindle [14]. This has been attributed to attempt- ing mitosis with suboptimal NIMA function, as the bimE7 mutation does increase the level and activity of the mutant NIMA5 protein, although to only , 20% of wild-type levels [15]. Presumably, this activity is sufficient to overcome the requirement for NIMA in mitotic entry. BimE was Corresponding author: Matthew J. O’Connell (matthew.oconnell@mssm.edu). Review TRENDS in Cell Biology Vol.13 No.5 May 2003 221 http://ticb.trends.com 0962-8924/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0962-8924(03)00056-4