Research Article 4321 Cep68 and Cep215 (Cdk5rap2) are required for centrosome cohesion Susanne Graser1, York-Dieter Stierhof2 and Erich A. Nigg1,* 1Max-Planck-Institute for Biochemistry, Department of Cell Biology, Am Klopferspitz 18, D-82152 Martinsried, Germany 2Electron Microscopy Unit, Center for Plant Molecular Biology, University of Tübingen, Auf der Morgenstelle 5, D-72076 Tübingen, Germany *Author for correspondence (e-mail: [email protected]) Accepted 22 September 2007 Journal of Cell Science 120, 4321-4331 Published by The Company of Biologists 2007 doi:10.1242/jcs.020248 Summary The centrosome duplicates during the cell cycle but Cep68 is readily recruited to ectopic rootletin fibres. These functions as a single microtubule-organising centre until data suggest that Cep68 cooperates with rootletin and C- shortly before mitosis. This raises the question of how Nap1 in centrosome cohesion. By contrast, Cep215 centrosome cohesion is maintained throughout interphase. associates with centrosomes throughout the cell cycle and One dynamic model proposes that parental centrioles are does not appear to interact with Cep68, rootletin or C- held together through centriole-associated, entangling Nap1. Instead, our data suggest that Cep215 functionally filaments. Central to this model are C-Nap1, a putative interacts with pericentrin, suggesting that both proteins centriolar docking protein and rootletin, a fibrous influence centrosome cohesion through an indirect component. Here we identify two novel proteins, Cep68 and mechanism related to cytoskeletal dynamics. Cep215, as required for centrosome cohesion. Similar to rootletin, Cep68 decorates fibres emanating from the proximal ends of centrioles and dissociates from Supplementary material available online at centrosomes during mitosis. Furthermore, Cep68 and http://jcs.biologists.org/cgi/content/full/120/24/4321/DC1 rootletin depend both on each other and on C-Nap1 for centriole association. Unlike rootletin, overexpression of Key words: Cep68, Cep215, Centrosome cohesion, Rootletin, Cell Cep68 does not induce extensive fibre formation, but cycle Introduction (Bettencourt-Dias and Glover, 2007; Nigg, 2007). As a result The centrosome is the major microtubule-organising centre of this duplication, a G2 cell harbours two centrosomes, each Journal of Cell Science (MTOC) of animal cells (Bornens, 2002; Doxsey, 2001; Luders comprising two closely associated centrioles. Remarkably, the and Stearns, 2007; Nigg, 2004). The single centrosome present attachment between parent and progeny centriole within each in a mammalian G1-phase cell comprises two centrioles centrosome – termed ‘engagement’– persists throughout G2 surrounded by a fibrous protein matrix, the so-called and early M phase, until the two centrioles separate at the end pericentriolar material (PCM). The PCM is responsible for the of mitosis. This ‘disengagement’ is then thought to constitute nucleation and anchoring of microtubules (MTs) (Bornens, a necessary licensing step for a further round of duplication 2002; Job et al., 2003). It comprises several large coiled-coil (Tsou and Stearns, 2006a; Tsou and Stearns, 2006b). proteins (Doxsey et al., 2005; Ou et al., 2002) and provides At the onset of mitosis, the activation of several protein docking sites for ␥-tubulin ring complexes (Luders and kinases triggers centrosome separation, leading to the Stearns, 2007) as well as cell-cycle-regulatory proteins formation of a bipolar spindle (Berdnik and Knoblich, 2002; (Doxsey, 2001; Fry and Hames, 2004). The two centrioles Blangy et al., 1995; Fry et al., 1998a; Giet et al., 1999; Glover present within a G1 centrosome are structurally and et al., 1995; Hannak et al., 2001; Lane and Nigg, 1996; Sawin functionally distinct. Only the mature centriole carries and Mitchison, 1995). However, before late G2 phase, the two appendages at its distal end and is competent to function as a centrosomes function as a single MTOC. This raises the basal body in ciliogenesis (Sorokin, 1968). It is also this mature question of how this ‘centrosome cohesion’ is maintained centriole that anchors the bulk of the MTs, whereas the other throughout the major part of interphase. Two prominent centriole is relatively free to move (Piel et al., 2000). The models for centrosome cohesion have been proposed. The first average distance between these two centrioles is variable, cell- model emphasises the role of cytoskeletal dynamics and type dependent and influenced by cytoskeletal dynamics and proposes that the close proximity between parental centrioles physiological conditions (Buendia et al., 1990; Euteneuer and is primarily a consequence of forces exerted by the Schliwa, 1985; Holy et al., 1997; Jean et al., 1999; Rodionov cytoskeleton (Euteneuer and Schliwa, 1985; Jean et al., 1999; and Borisy, 1997; Schliwa et al., 1983; Schliwa et al., 1982; Thompson et al., 2004). The second model postulates the Sherline and Mascardo, 1982). existence of dedicated ‘linker proteins’ that function to provide During S phase, a single new centriole forms at an a dynamic connection between parental centrioles. Clearly, the orthogonal angle close to the proximal end of each parental two models are not mutually exclusive. Importantly, the centriole, suggesting that the wall of the parental centriole influence of cytoskeletal alterations on centrosome cohesion is favours the formation of exactly one progeny centriole not necessarily independent of linker structures, because 4322 Journal of Cell Science 120 (24) interference with the cytoskeleton may well affect the transport proteins previously implicated in centrosome cohesion, namely of structural or regulatory components that are important for C-Nap1, rootletin and pericentrin (Bahe et al., 2005; Jurczyk the functionality of the putative linker (Meraldi and Nigg, et al., 2004; Mayor et al., 2000; Yang et al., 2006). Centrosome 2001). One major attraction of the ‘dynamic linker model’ is splitting in response to depletion of the above proteins was also that it readily explains how cell-cycle-dependent observed in two other cell lines, A549 and hTERT-RPE1 phosphorylation can regulate centrosome cohesion (reviewed (RPE1), although effects were quantitatively reduced in RPE1, by Meraldi and Nigg, 2001). The linker model is further presumably because of lower transfection efficiency (data not supported by electron microscopy (Bornens et al., 1987; shown). The above results indicate that splitting is not an Paintrand et al., 1992), and the identification of two specific inevitable consequence of interference with overall centrosome proteins, C-Nap1 (Fry et al., 1998a; Mayor et al., 2000) and structure. Yet, the available evidence indicates that centrosome rootletin (Bahe et al., 2005; Yang et al., 2006), that are required cohesion is affected through multiple mechanisms and its for the maintenance of centrosome cohesion. Remarkably, both regulation is likely to be complex (Meraldi and Nigg, 2001). proteins dissociate from the centrosome when the activity of In this regard, two observations made during the above screen the protein kinase Nek2 exceeds that of a counteracting type 1 are noteworthy. First, depletion of ninein caused centrosome phosphatase, concomitant with centrosome separation at the splitting only in RPE1 cells, and depletion of Cep164 caused G2-to-M transition (Bahe et al., 2005; Fry et al., 1998b; Helps splitting only in serum-deprived RPE1 cells (data not shown). et al., 2000; Yang et al., 2006). Additional proteins, notably This confirms that the strength of centrosome cohesion differs pericentrin (Jurczyk et al., 2004) and dynamin-2 (Thompson et between cell types and growth conditions. Second, a al., 2004), have also been implicated in centrosome cohesion, quantitative analysis of centrosome splitting revealed that, of but these latter proteins are known to play multiple roles. all proteins examined so far, depletion of C-Nap1 and rootletin Depletion of all the above proteins causes centrosome splitting produced the most drastic phenotype, both with regard to but whether they function through a common mechanism is not penetrance – the percentage of cells showing split centrosomes presently clear. (supplementary material Fig. S1) – and the extent of separation We emphasise that the term ‘centrosome splitting’ is used between centrioles (see below). here to describe the separation of parental centrioles. From a mechanistic perspective, this event is expected to be intimately Characterisation of Cep68 and Cep215 related to centrosome separation, which occurs under The two proteins newly identified here as being involved in physiological conditions at the G2-to-M transition. It is centrosome cohesion, Cep68 and Cep215, were originally important to distinguish centrosome splitting from centriole described in the course of a proteomic characterisation of the disengagement (which is occasionally also called ‘centriole human centrosome (Andersen et al., 2003). Orthologues of splitting’), a term that describes the separation of parent and both proteins are readily detectable in other mammalian progeny centrioles (within the same centrosome) that normally genomes. Cep68 is encoded on human chromosome 2p14 and occurs at the end of mitosis (see Nigg, 2006). alternative splicing is thought to give rise to two isoforms In this study, we used a siRNA screen to search for proteins comprising 757 and 620 amino acids, respectively (Fig. 1A). Journal of Cell Science involved in centrosome cohesion. In addition to