Introduction Cytokinesis is the final event of the cell cycle that divides the mother cell into two daughter cells, after the completion of mitosis. In contrast to animal cytokinesis, where a contractile ring tightens the cytoplasm, plant somatic cytokinesis is characterised by the formation of a transient membrane compartment called the cell plate, which matures into a new cell wall flanked by cytoplasmic membranes. The cell plate is built de novo by material mobilised from Golgi-derived vesicles and delivered by a dynamic cytokinesis-specific cytoskeletal array, the phragmoplast. The cell plate is generally initiated in the centre of the cell division plane, defined by a preprophase band (PPB) of microtubules, and fuses with the parental plasma membrane (Staehelin and Hepler, 1996). Several studies demonstrate that mitogen-activated protein kinases (MAPK) are involved in the formation of the cell plate. Two orthologous MAPK from tobacco and alfalfa (NtNTF6 and MsMMK3), as well as the tobacco MAP kinase kinase kinase (MAP3K) NtNPK1 have been shown to be activated in late mitosis. Furthermore, these proteins were shown to localise to the midplane during late anaphase and to the phragmoplast (Bögre et al., 1999; Calderini et al., 1998; Nishihama et al., 2001). Overexpression of a dominant negative form of NtNPK1 in tobacco cells inhibits expansion of both the phragmoplast and the cell plate, resulting in multinucleate cells with incomplete cell plates (Nishihama et al., 2001). Krysan et al. have shown that the three A. thaliana orthologues of NtNPK1 (ANP1, ANP2, ANP3) are also involved in the formation of the cell plate (Krysan et al., 2002). Despite recent advances in our understanding of the formation of the cell plate in plant cells, little is known about the mechanism(s) responsible for initiating cytokinesis and coordinating its timing with mitosis. The phenotype of titan and pilz mutants, affected in genes encoding tubulin-folding cofactors (Liu and Meinke, 1998; Steinborn et al., 2002), illustrate that spatial and temporal coordination between mitosis and cytokinesis is mediated in part by cytoskeletal components. The co-localisation of the cdc2 kinase and three types of microtubular structures, namely the PPB, the spindle, and the cell plate, suggests that the latter kinase also plays an important role in this coordination (Weingartner et al., 2001). The need shared by all eucaryotes to coordinate mitosis and cytokinesis, in order to ensure an even distribution of chromosomes over the daughter cells, suggests that mechanisms promoting communication between the regulation of the cell cycle and the organisation of the cytoskeleton probably involve some common features across kingdoms. The mechanisms that regulate the onset of mitosis are relatively 4265 In the fission yeast Schizosaccharomyces pombe, the onset of septum formation is signalled via the septation initiation network (SIN) involving several protein kinases and a GTPase. Arabidopsis thaliana and Brassica napus proteins homologous to fission yeast spg1p (AtSGP1, AtSGP2), cdc7p (AtMAP3Kε1, AtMAP3Kε2, BnMAP3Kε1) and sid1p (AtMAP4Kα1, AtMAP4Kα2, BnMAP4Kα2) exhibit a significant similarity. The plant proteins AtSGP1/2 and BnMAP4Kα2 are able to complement the S. pombe mutant proteins spg1-B8 and sid1-239, respectively and to induce mutisepta when overexpressed in wild-type yeast. Yeast two-hybrid assays demonstrated interactions both between plant proteins and between plant and yeast proteins of the SIN pathway. However, the primary structure of the proteins as well as the partial complementation of yeast mutants indicates that plant homologous proteins and their yeast counterparts have diverged during evolution. Real- time RT-PCR studies demonstrated plant SIN-related gene expression in all organs tested and a co-expression pattern during the cell cycle, with a higher accumulation at G2 -M. During interphase, the plant SIN-related proteins were found to co-localise predominantly in the nucleolus of the plant cells, as shown by fusions to green fluorescent protein. These data suggest the existence of a plant SIN-related pathway. Key words: Cell cycle, Cytokinesis, Kinase, Plant, Signalling Summary AtSGP1, AtSGP2 and MAP4Kα are nucleolar plant proteins that can complement fission yeast mutants lacking a functional SIN pathway Antony Champion 1, * ,¶ , Stefan Jouannic 1,‡ , Stéfanie Guillon 1 , Keithanne Mockaitis 1,§ , Andrea Krapp 2 , Alain Picaud 1 , Viesturs Simanis 2 , Martin Kreis 1 and Yves Henry 1 1 Institut de Biotechnologie des Plantes (IBP), UMR 8618, Bâtiment 630, Université de Paris-Sud, 91405 Orsay Cedex, France 2 Cell Cycle Control Laboratory, ISREC, Chemin des Boveresses 155, Epalinges, 1066, Switzerland *Present address: Institute of Biology, Leiden University, Clusius Laboratory Wassenaarseweg 64, 2333 AL Leiden, The Netherlands ‡ Present address: Institut de Recherche pour le Développement (IRD), BP 64501, 34394 Montpellier Cedex 5, France § Present address: Molecular, Cell, and Developmental Biology Section, The University of Texas, 1 University Station, A6700, Austin, Texas 78712, USA ¶ Author for correspondence (e-mail: champion@rulbim.leidenuniv.nl) Accepted 8 March 2004 Journal of Cell Science 117, 4265-4275 Published by The Company of Biologists 2004 doi:10.1242/jcs.01200 Research Article JCS ePress online publication date 3 August 2004