Meeting Report

meeting report

Exploring the pole: an EMBO conference on centrosomes and spindle pole bodies

Sue L. Jaspersen and Tim Stearns

The centrosome and spindle pole body community gathered for its triennial meeting from 12–16 September, 2008 at EMBL in Heidelberg (Germany).

Sponsored by the EMBO, the conference on Centrioles are short, cylindrical structures in constituent proteins, and the identification centrosomes and spindle pole bodies was which the walls of the cylinder are made up of of those that are key functional components, organized by Trisha Davis, Susan Dutcher, nine specialized triplet microtubules. This elegant as opposed to hangers-on that use the centro- Michael Knop, Robert Palazzo, Elmar Schiebel nine-fold symmetry is absolutely conserved and some as a cellular assembly point. At the first and Kip Sluder. This was the fourth meeting gives centrioles their characteristic ‘pinwheel’ meeting twelve years ago, John Kilmartin’s in a series that started in 1996 and, as with the appearance in cross-section. Separate from their mass-spectrometry analysis of the SPB4 was previous meetings1–3, was an occasion to cel- role as a focus of PCM, centrioles also nucleate the a prescient first glimpse of the cornucopia of ebrate present accomplishments and contem- ciliary axoneme, imparting their nine-fold sym- centrosome proteins that would soon emerge plate the future. Below we summarize some of metry to this structure as well. A centriole at the from similar work on centrosomes, centrioles the major themes that emerged. base of a cilium is referred to as a basal body. and cilia. Whereas we once had the sense of The centrosome, with its pair of centrioles, having hold of only the trunk, leg or tail of the Centrosome 101 duplicates once per cell cycle at the G1/S tran- proverbial centrosomal elephant, new results Microtubules and their constellation of asso- sition so that a cell will have exactly two cen- are revealing a much more complete picture of ciated proteins and structures are strongly trosomes during mitosis. Centrioles reproduce the organelle as a whole. conserved components of all eukaryotic cells. semi-conservatively; the pairs separate and each Jens Andersen described a refinement of the One of the universal themes in the microtubule ‘mother’ centriole grows a new ‘daughter’ cen- original mass-spectrometry analysis of mamma- cytoskeleton is the use of specific structures to triole from its side. The centrosome has a mutu- lian centrosomes5, using SILAC stable isotope organize microtubules into useful arrays. The alistic relationship with the mitotic spindle, labelling technology to increase coverage and centrosome of animal cells and the spindle pole helping to form the poles of the spindle, while specificity of results from impure centrosome body of fungi are the two best characterized at the same time using that spindle to segregate material. Jean Cohen presented a compilation microtubule-organizing structures and were equally to the sister cells of a division. This equal of centriole and cilia proteomic data from across the topic of this meeting. The centrosome segregation of one centrosome per cell ensures the eukaryotic world and an associated web- contains a pair of centrioles surrounded by that each cell has the potential to grow a cilium, based analysis tool. In these and other proteomic a matrix of proteins involved in microtubule which is imparted by the mother centriole. studies, the same proteins come up repeatedly, nucleation and other centrosome functions. Most fungi have lost the capacity to make suggesting that, by analogy to genetic screens, This matrix of proteins is usually referred to as centrioles and cilia but have evolved a morpho- we are close to saturation for identification of pericentriolar material (PCM), although many logically distinct structure, known as the spindle new components. However, lest one become of the components of PCM are also found at pole body (SPB), to serve as their primary site of sanguine about this prospect, Hannah Müller’s other sites of microtubule organization in dif- microtubule nucleation. The functional orthol- proteomic analysis of centrosomes, isolated from ferentiated cell types. ogy of the SPB to the centrosome is reflected rapidly dividing Drosophila embryos, found only in the conservation of some of the important limited overlap with the list of known centro- Sue L. Jaspersen is at the Stowers Institute for Medical Research, Kansas City, MO 64110 USA components, and genetic and biochemical anal- some proteins from other systems. Perhaps and in the Department of Molecular and Integrative ysis of SPBs have provided valuable insight into this reflects differences in analysis techniques Physiology, University of Kansas Medical Center, centrosome regulation and function. or important biological changes in centrosome Kansas City, KS 66160 USA. Tim Stearns is in the Department of Biology, Stanford University, Stanford, components in the rapidly dividing embryo. CA 94305 USA and in the Department of Genetics, Centrosome parts Genetic analysis has also made an important Stanford University School of Medicine, Stanford, CA 94305 USA. e-mail: [email protected] and The rate-limiting step in understanding the contribution to identifying centrosome com- [email protected] centrosome has been the definition of its ponents and their interactions. The keynote nature cell biology volume 10 | number 12 | DECEMBER 2008 1375 © 2008 Macmillan Publishers Limited. All rights reserved.

meeting report

SZY-20

SPD-2 Mps1 ZYG-1/Plk4 APC SCF Cdk2-cyclinA/E SAS-6 SAS-6-P SAS-4/CPAP SAS-5 Bld10/Cep135

G1/S cartwheel formation centriole assembly

S M centriole Separase centriole elongation Plk1 disengagement, cytokinesis

G2 and M centriole maturation & separation

Bora Aurora-A Plk1 Recruitment of PCM, appendage, γ-TuRC, hPoc5, spindle pole proteins

Figure 1 Centriole duplication pathway. Schematic representation of the major steps in centrosome duplication, as well as structural and regulatory proteins. We have combined results from several systems, and the details may differ in specific systems (for reviews of the centriole cycle and its cell-cycle control, see refs 18, 29–31). At the end of mitosis, each of the two engaged centrioles within each pair become disengaged by the action of the separase protease and Plk1. The older of the centrioles in each pair is marked with distal and sub-distal appendages, and the two centrioles remain linked by cohesion fibres. Centriole duplication is initiated at the disengaged centrioles during G1/S by SPD-2, as well as the kinases Mps1 and Cdk2–cyclinA/E. A key regulatory step in centriole duplication is activation of the kinase ZYG–1/Plk4; this involves control of its kinase activity, localization to the centrosome and, ultimately, proteolysis by the SCF ubiquitylation complex. Active ZYG–1/Plk4 can phosphorylate SAS-6, a component of the cartwheel structure at the base of nascent centrioles. Recruitment of SAS-6, SAS-5 and Bld10/Cep135 drive formation of the cartwheel, which imparts a nine-fold symmetry on the forming centriole. During S phase, centriole duplication continues by recruitment of SAS-4/CPAP. Levels of CPAP are tightly controlled during the cell cycle by APC-mediated proteolysis, perhaps restricting centriole assembly. Daughter centrioles continue to elongate during G2 and early M phase after activation of Aurora A at the centrosome, which is regulated by Plk1 and Bora. The centrin-binding protein hPoc5 is also recruited to the centrosome. The new mother centriole matures by addition of components of distal and subdistal appendages. The cohesion link between the two mother centrioles is broken, allowing the centrosomes to move to opposite sides of the nucleus. As mitosis begins, γ-tubulin ring complex (γ-TuRC) and other PCM components are recruited to the centrosomes, along with mitotic spindle pole proteins. The two mitotic centrosomes nucleate microtubules and help to form the mitotic spindle on which both chromosomes and centrosomes will segregate to the two daughter cells. At the end of mitosis, separase and Plk1 trigger centriole disengagement, allowing the centrioles to duplicate in the G1/S phase, and completing the cycle. address from Tony Hyman stressed the power Similar RNAi screens have been performed Interestingly, hPoc5 contains Sfi1-like repeats, of RNA interference (RNAi) in Caenorhabditis in Drosophila melanogaster cultured cells8,9. which were originally discovered as Cdc31/cen- elegans and mammalian cells as a tool for iden- Naomi Stevens described three genes, ana1, trin-binding motifs in the Saccharomyces cerevi- tifying important components and determining ana2 and ana3, that are involved in centrosome siae SPB component Sfi1. Recruitment of hPoc5 their function. Combining the ability to observe duplication and whose duplication results in to the distal lumen of centrioles, where centrin early embryonic divisions of C. elegans with anastral spindles. Ana2 is structurally similar 2/3 are located, occurs late in G2 and involves whole-genome RNAi, Hyman and his worm to SAS-5, and like SAS-5 is required for centri- binding to its interacting partner, hPoc19, which ophile colleagues have discovered several of ole formation. Chad Pearson used basal body is recruited earlier in the cell cycle. the key players, including SAS-4 and SAS-6. He proteome data from Tetrahymena10 to identify described recent work revealing an unexpected Poc1 as a conserved centriole component that Centrosome pathways connection between centrosome size and spindle is also required for centrosome duplication One of the deeper mysteries of centrosome length, which was independent of microtubule and ciliogenesis in human cells. Using human biology is how the initiation of new centrioles nucleation. In addition, his group combined a centrin as the bait in a yeast two-hybrid screen, is controlled. As each centriole is potentially a mammalian RNAi screen6 with tagging of pro- Michel Bornens identified hPoc5, the human distinct centrosome, controlling initiation is teins in BACs7 to identify interaction networks orthologue of a protein originally identified in the key event in centrosome number control. among mammalian centrosome components. Chlamydomonas as a component of centrioles11. Although new centrioles typically grow from the

meeting report side of an existing centriole, Alexey Khodjakov’s size16, although the nature of the connection to results from his lab, showing that Plk1 and Bora group has shown that generic mammalian cells the RNA world is not clear. cooperate to regulate the centrosomal levels of can produce centrioles de novo12, a property Another conserved component originally Aurora A during mitotic entry in cultured cells20. they share with some single-cell organisms and identified in C. elegans is SAS-4, thought to be This is at least conceptually similar to the situation specialized mammalian cell types. However, the required to add centriolar microtubules to the in Schizosaccharomyces pombe, described by Iain presence of an existing centriole prevents this de base structure17. Susan Dutcher’s genetic and Hagan, in which recruitment of the polo kinase novo pathway. Khodjakov used laser ablation to electron microscopic analysis of basal body Plo1 to the SPB is important for mitotic entry. show that a mother centriole can only make a duplication in Chlamydomonas indicates that The results discussed above support the view new centriole after removal of its daughter. This spokes emanating from the central hub first of the centrosome as a crucial signal transduc- is consistent with experiments described by form an amorphous pinwheel before the cart- tion hub. This is perhaps most clearly true in Tim Stearns showing that in mammalian cells, wheel appears, perhaps as SAS-4 is recruited18. S. cerevisiase, where a surveillance system the protease separase and the kinase Plk1 act to Tang Tang presented evidence that levels of the known as the spindle-positioning checkpoint disengage the mother and daughter centrioles at human orthologue CPAP are tightly regulated (SPOC) monitors alignment of the mitotic the end of mitosis, and that this is required for during the cell cycle by proteolysis mediated by spindle at the bud neck and delays cell-cycle duplication in the next cell cycle. the anaphase-promoting complex (APC). Tang, progression until correct spindle orientation is A confluence of results from several systems Gönczy and Erich Nigg all noted that overex- achieved. The target of the SPOC is the mitotic has identified Polo-like kinase 4 (Plk4) as the pression of CPAP results in growth of micro- exit network (MEN), and the ultimate target of likely trigger for centriole formation in animal tubule extensions from the end of the centriole, the MEN pathway is the Cdc14 phosphatase, cells13. Monica Bettencourt-Dias and Stefan extending its length. Alex Dammermann identi- which antagonizes Cdk1–cyclinB activity. The Duensing both showed that the levels of Plk4 fied a protein, HYLS-1, that interacts with SAS-4 SPB serves as a scaffold for regulatory proteins are controlled by proteolysis in Drosophila and and found that it is required for cilium, but not and a sensor for spindle alignment. Simonetta mammalian cells, respectively. In both cell centriole, formation. In humans with hydroe- Piatti’s analysis of the E3 ubiquitin ligases Dma1 types, depletion of components of the SCF (for thalus syndrome, a mutation in HYLS-1 impairs and Dma2 suggested a new mechanism control- Skp1–Cullin–Fbox) ubiquitin-ligase complex cilia assembly, adding this disease to the known ling the SPOC, whereas Gislene Pereira focused results in more Plk4 and formation of extra cen- ciliopathies. on the dynamic association of Bub2 and Bfa1 trioles. Interestingly, Michel Bornens reported When the analysis of centriole and basal body with the SPB in cells with mis-aligned spindles centrosomal accumulation of Plk4 during mito- assembly from several different organisms is and how this is controlled by phosphorylation21. sis and described a Plk4 autophosphorylation combined, a picture of the stepwise centrosome Elmar Schiebel also discussed the important event that stimulates degradation of the protein; assembly pathway begins to emerge (Fig. 1). role that phosphorylation has in this path- however, this seems to be independent of SCF- way, through analysis of Cdk1 regulation of mediated degradation. It is still not known how Centrosomes and cell cycle signaling MEN components. Kathy Gould reported that Plk4 carries out its centriole-initiating magic, Centrosome function is intimately tied to cell- phosphorylation of Clp1, the Cdc14 ortholog but it seems likely that its level, localization and cycle progression, with characteristic changes in S. pombe, promotes binding to Rad24 and activity are tightly controlled. occurring in each phase of the cycle. Conserved cytoplasmic retention during anaphase22. At An early event in new centriole growth is cell-cycle regulatory kinases, such as the cyclin- least some MEN proteins have orthologues in formation of the cartwheel, a nine-fold sym- dependent kinases (Cdks), polo-like kinases higher eukaryotes, so an important future direc- metrical structure at the base of the centriole. (Plks) and Mps1, control the function and tion will be to elucidate their role in cell-cycle Masafumi Hirono discussed his findings on duplication of SPBs in fungi, and centrosomes progression. Also, the intimate association of mutants of bld12 and bld10, the Chlamydomonas in metazoan cells. As a start to developing a more SPBs and centrosomes with the nuclear enve- orthologues of SAS-6 and Cep135; these two complete understanding of the role of phosphor- lope seems to be important for regulating cen- proteins seem to be essential components of the ylation at the centrosome, Mark Winey described trosome function and duplication, as discussed cartwheel, probably defining its nine-fold sym- an ambitious proteomics approach to examine by Sue Jaspersen on the basis of their studies on metry14,15. SAS-6 is at the centre of the cartwheel, phosphorylation of all of the core components the evolutionarily conserved SUN proteins. and loss of SAS-6 results in centrioles with non- of the S. cerevisiase SPB, whereas Harold Fisk nine numbers of triplet microtubules. Pierre focused on Mps1 phosphorylation of centrin as Cell divisions: some more equal than Gönczy reported that SAS-6 is phosphorylated a control point for centriole duplication. others by ZYG-1 in C. elegans. ZYG-1 is a kinase related At the entry to mitosis in animal cells, more Several recent studies have highlighted the role to Plk4, and SAS-6 mutants that mimic phos- PCM is recruited to centrosomes, and this that spindle alignment and centrosome distri- phorylation at the ZYG-1 site can bypass the recruitment requires the activity of Plk1 and bution play during developmentally important requirement for the kinase, supporting the idea Aurora A. Isabelle Vernos described experiments asymmetric cell divisions, including in adult that SAS-6 is the key target of ZYG-1 for centro- in frog egg extracts to define the role of Aurora A stem cells, the germ line and the immune sys- some duplication and is a central point of regu- kinase activity at the centrosome19. Jens Lüders tem. Yukiko Yamashita examined why adult lation of centriole assembly. Kevin O’Connell talked about the connection between Plk1 and stem cells lose their ability to divide with increas- showed that a conserved RNA-binding protein, γ−tubulin recruitment through the attachment ing age during spermatogenesis in Drosophila. SZY-20 (known as PM20 in humans), acts antag- factor GCP-WD/Nedd1. Bringing some of these These cells usually divide with the older centro- onistically to ZYG-1 and regulates centrosome threads together, Erich Nigg described recent some anchored near the stem-cell niche, but she nature cell biology volume 10 | number 12 | DECEMBER 2008 1377 © 2008 Macmillan Publishers Limited. All rights reserved.

meeting report found that the number of male germ stem cells that targeting centrosome clustering mechanisms 6. Kittler, R. et al. Genome-scale RNAi profiling of cell division in human tissue culture cells. Nature Cell Biol. with mis-oriented centrosomes increases with might be a way to specifically kill cancer cells. 9, 1401–1412 (2007). time23. These cells also transiently arrested in the 7. Poser, I. et al. BAC TransgeneOmics: a high-throughput Coming full circle method for exploration of protein function in mammals. cell cycle, explaining their decreased ability to Nature Methods 5, 409–415 (2008). proliferate, and perhaps reflecting a checkpoint It is particularly satisfying to see a result that 8. Goshima, G. et al. Genes required for mitotic spindle assembly in Drosophila S2 cells. Science 316, 417– similar to that described above for yeast. clearly answers a long-standing question. 421 (2007). Correct positioning of the mitotic spindle and David Agard’s presentation on the structure of 9. Dobbelaere, J. et al. A genome-wide RNAi screen to dis- sect centriole duplication and centrosome maturation centrosomes is important to maintain cellular the γ-tubulin complex from yeast did just that, in Drosophila. PLoS Biol 6, e224 (2008). identity, and defects in this process can result in providing a molecular understanding of nuclea- 10. Kilburn, C. L. et al. New Tetrahymena basal body protein components identify basal body domain structure. uncontrolled cell division. This was described tion, the process that led most investigators to J. Cell Biol. 178, 905–912 (2007). and discussed by Cayetano Gonzalez and Jordan the centrosome in the first place. Microtubule 11. Keller, L. C., Romijn, E. P., Zamora, I., Yates, J. R., III & Marshall, W. F. Proteomic analysis of isolated Raff, who used a transplantation system to study nucleation at the centrosome involves γ-tubulin chlamydomonas centrioles reveals orthologs of ciliary- tumorigenesis in flies. It has often been observed and associated proteins. The γ-tubulin complex disease genes. Curr. Biol. 15, 1090–1098 (2005). 12. Uetake, Y. et al. Cell cycle progression and de novo centri- that cancer cells have extra centrosomes, and purified from animal cells is ring-shaped, with a ole assembly after centrosomal removal in untransformed sometimes undergo multipolar divisions, which size and diameter that suggest that it nucleates a human cells. J. Cell Biol.176, 173–182 (2007). 13. Nigg, E. A. Centrosome duplication: of rules and might lead to some of the genetic instability microtubule by directly templating it. A vexing licenses. Trends Cell Biol. 17, 215–221 (2007). observed in such cells. Indeed, this is one of problem has been that yeast cells lack some of the 14. Hiraki, M., Nakazawa, Y., Kamiya, R. & Hirono, M. Bld10p constitutes the cartwheel-spoke tip and stabi- the touchstones of the centrosome field, first γ-tubulin complex associated proteins and also lizes the 9-fold symmetry of the centriole. Curr. Biol. proposed by Theodor Boveri 100 years ago24. lack a soluble ring complex. Combining purified 17, 1778–1783 (2007). However, the results from Gonzalez and Raff yeast proteins, from a collaboration with Trisha 15. Nakazawa, Y., Hiraki, M., Kamiya, R. & Hirono, M. SAS-6 is a cartwheel protein that establishes the 9-fold symme- are most consistent with centrosome abnor- Davis’ lab, and electron microscopy, Agard try of the centriole. Curr. Biol. 17, 2169–2174 (2007). malities resulting in defects in asymmetric cell showed that the simple combination of γ-tubulin, 16. Song, M., Aravind, L., Muller-Reichert, T. & O’Connell, K. The conserved protein SZY-20 opposes the Plk-4 division and thus resulting in over-proliferation its two closest binding partners Spc97 and Spc98, related kinase ZYG-1 to limit centrosome size. Dev. of stem cells25,26. Steve Doxsey presented results and the linker protein Spc110 will form a ring Cell, advance online publication ,10.1016/j.devcel.2008.09.018 (2008). suggesting that similar mechanisms might be in vitro, with appropriate dimensions for micro- 17. Pelletier, L., O’Toole, E., Schwager, A., Hyman, A. A. & at work in vertebrates. He found that interfer- tubule nucleation. It is fitting, perhaps, that a Muller-Reichert, T. Centriole assembly in Caenorhabditis elegans. Nature 444, 619–623 (2006). ing with the function of centrosome proteins detailed understanding of γ-tubulin comes from 18. Dutcher, S. K. Finding treasures in frozen cells: new in zebrafish caused phenotypes similar to those a study of the yeast proteins, given that γ−tubulin centriole intermediates. Bioessays 29, 630–4 (2007). 19. Sardon, T., Peset, I., Petrova, B. & Vernos, I. Dissecting described for ciliary proteins. Further analysis was originally identified by Berl Oakley and col- the role of Aurora A during spindle assembly. EMBO J. in mammalian cells indicated that interfering leagues in a genetic screen in Aspergillus28. 27, 2567–79 (2008). 20. Chan, E. H., Santamaria, A., Sillje, H. H. & Nigg, E. A. with IFT88, an intraflagellar transport protein, In conclusion, our understanding of these Plk1 regulates mitotic Aurora A function through βTrCP- resulted in misoriented spindles. This led to the complex organelles, which both control microtu- dependent degradation of hBora. Chromosoma 117, 457–69 (2008). hypothesis that some of the phenotypes in cili- bule nucleation, and serve as important hubs for 21. Caydasi, A. & Pereira, G. Spindle alignment regulates opathies might be due to defective cell division cell signaling, has increased dramatically since the dynamic association of checkpoint proteins with yeast spindle pole bodies. Dev. Cell, advance online plane orientation. the first centrosome and SPB conference twelve publication, 10.1016/j.devcel.2008.10.013 (2008). Cytokinesis failure is often cited as a mecha- years ago. What was then a relative cell biological 22. Chen, C.-T. et al. The SIN kinase Sid2 regulates cytoplasmic retention of the Cdc14-like phosphatase Clp1 nism responsible for generating the many cells backwater has now become what Tony Hyman in S. pombe. Curr. Biol. 18, 1594–1599 (2008). with extra centrosomes observed in tumours. called “perhaps the most advanced organelle 23. Cheng, J. et al. Centrosome misorientation reduces stem cell division during ageing. Nature advance online However, Kip Sluder presented evidence sug- with respect to combining genomics, proteomics publication, doi: 10.1038/nature07386 (2008). gesting that cytokinesis failure is unlikely to be and cell biology”. No longer an enigma, the cen- 24. Bignold, L. P., Coghlan, B. L. & Jersmann, H. P. Hansemann, Boveri, chromosomes and the game- the culprit in this case. In a heroic effort of time- trosome is now at the center of some of the most togenesis-related theories of tumours. Cell Biol. Int. lapse imaging, the Sluder lab treated cultured important issues in biology, with the attendant 30, 640–644 (2006). 25. Basto, R. et al. Centrosome amplification can initiate mammalian cells with cytochalasin to induce burning questions about its structure, function, tumorigenesis in flies. Cell 133, 1032–1042 (2008). cytokinesis failure, then observed cells over the and duplication. Searching for the answers to 26. Castellanos, E., Dominguez, P. & Gonzalez, C. Centrosome dysfunction in Drosophila neural stem cells course of several cell cycles. Although they could these questions will keep us busy until we meet causes tumors that are not due to genome instability. frequently recover tetraploid cells, most cells did again in 2011 in Barcelona. Curr. Biol. 18, 1209–1214 (2008). 27. Kwon, M. et al. Mechanisms to suppress multipolar not contain extra centrosomes, and the tetra- divisions in cancer cells with extra centrosomes. Genes 1. Hagan, I. M. & Palazzo, R. E. Warming up at the poles. Dev. 22, 2189–203 (2008). ploid cells did not proliferate. This suggests that EMBO Rep. 7, 364–371 (2006). 28. Oakley, C. E. & Oakley, B. R. Identification of γ-tubulin, centrosome amplification in tumour cells must 2. Palazzo, R. Centrosome and spindle pole body dynam- a new member of the tubulin superfamily encoded by involve other steps, or perhaps multiple rounds ics. Cell Motil. Cytoskeleton 54, 148–194 (2003). mipA gene of Aspergillus nidulans. Nature 338, 662– 3. Stearns, T. & Winey, M. The cell center at 100. Cell 91, 664 (1989). of cleavage failure. Susana Godinho’s analysis of 303–309 (1997). 29. Strnad, P. & Gonczy, P. Mechanisms of procentriole centrosome clustering in Drosophila S2 cells and 4. Wigge, P. A. et al. Analysis of the Saccharomyces formation. Trends Cell Biol. 18, 389–396 (2008). spindle pole by matrix-assisted laser desorption/ioni- 30. Bettencourt-Dias, M. & Glover, D. M. Centrosome biogen- mammalian cancer cell lines indicates that even zation (MALDI) mass spectrometry. J. Cell Biol. 141, esis and function: centrosomics brings new understand- when extra centrosomes are present, cells cope by 967–977 (1998). ing. Nature Rev. Mol. Cell Biol. 8, 451–463 (2007). 5. Andersen, J. S. et al. Proteomic characterization of 31. Azimzadeh, J. & Bornens, M. Structure and duplica- clustering centrosomes into poles of functional the human centrosome by protein correlation profiling. tion of the centrosome. J. Cell Sci. 120, 2139–2142 bipolar spindle27. Remarkably, her results suggest Nature 426, 570–574 (2003). (2007).