Identification of centrosomal proteins in a human lymphoblastic cell line

A moment at the cell centre

I have been invited by the board of the French Society of Cell Biology (SBCF) to write a text around my presentation in the Symposium 'A day at the Cell Centre', held at the Curie Institute on May 17, 2019, and organized by four of my former students, namely Juliette Azimzadeh, Nathalie Delgehyr, Matthieu Piel and Manuel Théry. I have to thank them warmly for the quality of the science during this day. It was also a moving day for me indeed to listen to so many figures in the field.

Quantitative analysis of human centrosome architecture by targeted proteomics and fluorescence imaging

Centrioles are essential for the formation of centrosomes and cilia. While numerical and/or structural centrosomes aberrations are implicated in cancer, mutations in centriolar and centrosomal proteins are genetically linked to ciliopathies, microcephaly, and dwarfism. The evolutionarily conserved mechanisms underlying centrosome biogenesis are centered on a set of key proteins, including Plk4, Sas-6, and STIL, whose exact levels are critical to ensure accurate reproduction of centrioles during cell cycle progression. However, neither the intracellular levels of centrosomal proteins nor their stoichiometry within centrosomes is presently known. Here, we have used two complementary approaches, targeted proteomics and EGFP-tagging of centrosomal proteins at endogenous loci, to measure protein abundance in cultured human cells and purified centrosomes. Our results provide a first assessment of the absolute and relative amounts of major components of the human centrosome. Specifically, they predict that human centriolar cartwheels comprise up to 16 stacked hubs and 1 molecule of STIL for every dimer of Sas-6. This type of quantitative information will help guide future studies of the molecular basis of centrosome assembly and function.

Pericentriolar material structure and dynamics

A centrosome consists of two barrel-shaped centrioles embedded in a matrix of proteins known as the pericentriolar material (PCM). The PCM serves as a platform for protein complexes that regulate organelle trafficking, protein degradation and spindle assembly. Perhaps most important for cell division, the PCM concentrates tubulin and serves as the primary organizing centre for microtubules in metazoan somatic cells. Thus, similar to other well-described organelles, such as the nucleus and mitochondria, the cell has compartmentalized a multitude of vital biochemical reactions in the PCM. However, unlike these other organelles, the PCM is not membrane bound, but rather a dynamic collection of protein complexes and nucleic acids that constitute the organelle's interior and determine its boundary. How is the complex biochemical machinery necessary for the myriad centrosome functions concentrated and maintained in the PCM? Recent advances in proteomics and RNAi screening have unveiled most of the key PCM components and hinted at their molecular interactions ( table 1). Now we must understand how the interactions between these molecules contribute to the mesoscale organization and the assembly of the centrosome. Among outstanding questions are the intrinsic mechanisms that determine PCM shape and size, and how it functions as a biochemical reaction hub.

Centrioles resist forces applied on centrosomes during G2/M transition

BACKGROUND INFORMATION

Centrosome movements at the onset of mitosis result from a balance between the pulling and pushing forces mediated by microtubules. The structural stability of the centrosome core structure, the centriole pair, is correlated with a heavy polyglutamylation of centriole tubulin.

RESULTS

Using HeLa cells stably expressing centrin-green fluorescent protein as a centriole marker, we monitored the effect of microinjecting an anti-(polyglutamylated tubulin) monoclonal antibody, GT335, in G1/S or G2 cells. In contrast with the slow effect of the monoclonal antibody GT335 during interphase, a dramatic and rapid centrosome fragmentation occurred in cells microinjected in G2 that was both Eg5- and dynein-dependent. Inhibition of either one of these two motors significantly decreased the scattering of centrosome fragments, and inhibition of centrosome segregation by impairing microtubule dynamics abolished centrosome fragmentation.

CONCLUSIONS

Our results demonstrate that the compact structure of the mitotic centrosome is capable of absorbing most of the pulling and pushing forces during G2/M transition and suggest that centrosomes could act as mechanosensors integrating tensions during cell division.

Hec1 contributes to mitotic centrosomal microtubule growth for proper spindle assembly through interaction with Hice1

Previous studies have stipulated Hec1 as a conserved kinetochore component critical for mitotic control in part by directly binding to kinetochore fibers of the mitotic spindle and by recruiting spindle assembly checkpoint proteins Mad1 and Mad2. Hec1 has also been reported to localize to centrosomes, but its function there has yet to be elucidated. Here, we show that Hec1 specifically colocalizes with Hice1, a previously characterized centrosomal microtubule-binding protein, at the spindle pole region during mitosis. In addition, the C-terminal region of Hec1 directly binds to the coiled-coil domain 1 of Hice1. Depletion of Hice1 by small interfering RNA (siRNA) reduced levels of Hec1 in the cell, preferentially at centrosomes and spindle pole vicinity. Reduction of de novo microtubule nucleation from mitotic centrosomes can be observed in cells treated with Hec1 or Hice1 siRNA. Consistently, neutralization of Hec1 or Hice1 by specific antibodies impaired microtubule aster formation from purified mitotic centrosomes in vitro. Last, disruption of the Hec1/Hice1 interaction by overexpressing Hice1DeltaCoil1, a mutant defective in Hec1 interaction, elicited abnormal spindle morphology often detected in Hec1 and Hice1 deficient cells. Together, the results suggest that Hec1, through cooperation with Hice1, contributes to centrosome-directed microtubule growth to facilitate establishing a proper mitotic spindle.

Interaction between Poly(ADP-ribose) and NuMA contributes to mitotic spindle pole assembly

Poly(ADP-ribose) (pADPr), made by PARP-5a/tankyrase-1, localizes to the poles of mitotic spindles and is required for bipolar spindle assembly, but its molecular function in the spindle is poorly understood. To investigate this, we localized pADPr at spindle poles by immuno-EM. We then developed a concentrated mitotic lysate system from HeLa cells to probe spindle pole assembly in vitro. Microtubule asters assembled in response to centrosomes and Ran-GTP in this system. Magnetic beads coated with pADPr, extended from PARP-5a, also triggered aster assembly, suggesting a functional role of the pADPr in spindle pole assembly. We found that PARP-5a is much more active in mitosis than interphase. We used mitotic PARP-5a, self-modified with pADPr chains, to capture mitosis-specific pADPr-binding proteins. Candidate binding proteins included the spindle pole protein NuMA previously shown to bind to PARP-5a directly. The rod domain of NuMA, expressed in bacteria, bound directly to pADPr. We propose that pADPr provides a dynamic cross-linking function at spindle poles by extending from covalent modification sites on PARP-5a and NuMA and binding noncovalently to NuMA and that this function helps promote assembly of exactly two poles.

Centrosomal CAP350 protein stabilises microtubules associated with the Golgi complex

A comprehensive model of how the centrosome organises the microtubule network in animal cells has not yet been elucidated. Here we show that the centrosomal large CAP-Gly protein CAP350 is not only present at the centrosome, but is also present as numerous dots in the pericentrosomal area. Using in vitro and in vivo expression of partial constructs, we demonstrated that CAP350 binds microtubules through an N-terminal basic region rather than through its CAP-Gly domain. CAP-Gly-containing domains of CAP350 are targeted not only to the centrosome but also to a Golgi-like network. Interestingly, full-length GFP-tagged CAP350 bound preferentially to microtubules in the pericentrosomal area. These results indicate that the large CAP350 protein has a dual binding ability. Overexpression of CAP350 promoted an increase in the stability of the whole microtubule network, as judged by a significant decrease in the number of EB1 comets and by an enhanced microtubule resistance to Nocodazole treatment. In support of this, CAP350 depletion decreased microtubule stability. Moreover, both depletion and overexpression of CAP350 induced specific fragmentation of the Golgi complex while maintaining a juxtanuclear localisation. We propose that CAP350 specifically stabilises Golgi-associated microtubules and in this way participates in the maintenance of a continuous pericentrosomal Golgi ribbon.

Molecular components of the centrosome

The centrosome organizes microtubules during both interphase and mitosis and therefore governs fundamental processes in the life of a eukaryotic cell. The past few years have seen a substantial increase in the identification of potential components localized at the centrosome. Although we are still far from achieving a coherent picture of the workings of the centrosome, these recent discoveries are promising first steps towards an understanding of centrosomal functions at the molecular level.

Glutamylation of centriole and cytoplasmic tubulin in proliferating non-neuronal cells

We have examined the distribution of glutamylated tubulin in non-neuronal cell lines. A major part of centriole tubulin is highly modified on both the alpha- and beta-tubulin subunits, whereas a minor part of the cytoplasmic tubulin is slightly modified, on the beta-tubulin only. Furthermore, we observed that tubulin glutamylation varies during the cell cycle: an increase occurs during mitosis on both centriole and spindle microtubules. In the spindle, this increase appears more obvious on the pole-to-pole and kinetochore microtubules than on the astral microtubules. The cellular pattern and the temporal variation of this post-translational modification contrast with other previously described tubulin modifications. The functional significance of this distribution is discussed.

AKAP350, a multiply spliced protein kinase A-anchoring protein associated with centrosomes

Protein kinase A-anchoring proteins (AKAPs) localize the second messenger response to particular subcellular domains by sequestration of the type II protein kinase A. Previously, AKAP120 was identified from a rabbit gastric parietal cell cDNA library; however, a monoclonal antibody raised against AKAP120 labeled a 350-kDa band in Western blots of parietal cell cytosol. Recloning has now revealed that AKAP120 is a segment of a larger protein, AKAP350. We have now obtained a complete sequence of human gastric AKAP350 as well as partial cDNA sequences from human lung and rabbit parietal cells. The genomic region containing AKAP350 is found on chromosome 7q21 and is multiply spliced, producing at least three distinct AKAP350 isoforms as well as yotiao, a protein associated with the N-methyl-D-aspartate receptor. Rabbit parietal cell AKAP350 is missing a sequence corresponding to a single exon in the middle of the molecule located just after the yotiao homology region. Two carboxyl-terminal splice variants were also identified. Both of the major splice variants showed tissue- and cell-specific expression patterns. Immunofluorescence microscopy demonstrated that AKAP350 was associated with centrosomes in many cell types. In polarized Madin-Darby canine kidney cells, AKAP350 localized asymmetrically to one pole of the centrosome, and nocodazole did not alter its localization. During the cell cycle, AKAP350 was associated with the centrosomes as well as with the cleavage furrow during anaphase and telophase. Several epithelial cell types also demonstrated noncentrosomal pools of AKAP350, especially parietal cells, which contained multiple cytosolic immunoreactive foci throughout the cells. The localization of AKAP350 suggests that it may regulate centrosomal and noncentrosomal cytoskeletal systems in many different cell types.

Centriole disassembly in vivo and its effect on centrosome structure and function in vertebrate cells

Glutamylation is the major posttranslational modification of neuronal and axonemal tubulin and is restricted predominantly to centrioles in nonneuronal cells (Bobinnec, Y., M. Moudjou, J.P. Fouquet, E. Desbruyères, B. Eddé, and M. Bornens. 1998. Cell Motil. Cytoskel. 39:223-232). To investigate a possible relationship between the exceptional stability of centriole microtubules and the compartmentalization of glutamylated isoforms, we loaded HeLa cells with the monoclonal antibody GT335, which specifically reacts with polyglutamylated tubulin. The total disappearance of the centriole pair was observed after 12 h, as judged both by immunofluorescence labeling with specific antibodies and electron microscopic observation of cells after complete thick serial sectioning. Strikingly, we also observed a scattering of the pericentriolar material (PCM) within the cytoplasm and a parallel disappearance of the centrosome as a defined organelle. However, centriole disappearance was transient, as centrioles and discrete centrosomes ultimately reappeared in the cell population. During the acentriolar period, a large proportion of monopolar half-spindles or of bipolar spindles with abnormal distribution of PCM and NuMA were observed. However, as judged by a quasinormal increase in cell number, these cells likely were not blocked in mitosis. Our results suggest that a posttranslational modification of tubulin is critical for long-term stability of centriolar microtubules. They further demonstrate that in animal cells, centrioles are instrumental in organizing centrosomal components into a structurally stable organelle.

MAP kinase inactivation is required only for G2-M phase transition in early embryogenesis cell cycles of the starfishes Marthasterias glacialis and Astropecten aranciacus

Downregulation of MAP kinase is a universal consequence of fertilization in the animal kingdom. Here we show that oocytes of the starfishes Astropecten aranciacus and Marthasterias glacialis complete meiotic maturation and form a pronucleus when treated with 1-methyladenine and then complete DNA replication and arrest at G2 if not fertilized. Release of G2 by fertilization or a variety of parthenogenetic treatments is associated with inactivation of MAP kinase. Prevention of MAP kinase inactivation by microinjection of Ste11-DeltaN, a constitutively active budding yeast MAP kinase kinase kinase, arrests fertilized eggs at G2 in either the first or the second mitotic cell cycle, in a dose-dependent manner. G1 arrest is never observed. Conversely, inactivation of MAP kinase by microinjection of the MAP kinase-specific phosphatase Pyst-1 releases mature starfish oocytes from G2 arrest. The role of MAP kinase in arresting cell cycle at various stages in oocytes of different animal species is discussed.

Organisation and functional regulation of the centrosome in animal cells

Molecular characterisation of centrosomal components is slowly progressing. Recent results indicate that the major aspects of centrosome-mediated microtubule nucleation may soon be understood at the molecular level. In contrast, centrosome reproduction, which is an important aspect of animal cell division, remains terra incognita. The most challenging issue for the future is to understand the molecular mechanisms which control centrosome biogenesis. There is a urgent need to identify with certainty proteins implicated in this process. Comparison between organisms with structurally different centrosomes might be critical for a better understanding of centrosome duplication if a general mechanism has been conserved throughout evolution.

The human p167 gene encodes a unique structural protein that contains centrosomin A homology and associates with a multicomponent complex

The characterization of novel cytoplasmic, structural, and enzymatic proteins has been enhanced by a panel of monoclonal antibodies specific for protein substrates of transforming and nontransforming c-Src mutants. These protein substrates have included the focal adhesion kinase (FAK), cortactin, AFAP-110, p120CAS, and p130CAS. The monoclonal antibody 4G8 was generated as part of this panel of antibodies and was used to isolate the human gene for a 167-kD polypeptide. The cDNA sequence is 5,238 nucleotides in length with a predicted open reading frame consisting of 1,382 amino acids. The polypeptide is largely hydrophilic and highly charged. The central region of p167 has 88% identity with the entire 278-amino-acid encoded sequence of the murine centrosomin A gene. The carboxyl third of p167 contains a unique cluster of 10 amino acid repeats with the consensus sequence (A/M)DDDRGPRRG. The p167 protein was found primarily in the cytoplasm of lymphocytes and is part of a multicomponent protein complex with prominent members of 167, 120, 64, 45, 40, 38, and 25 kD. Finally, we illustrate the conservation of p167 and its associated complex, and demonstrate its expression in different human tissues and cell types. The data suggest that p167 is novel and has an important cellular function as a cytoplasmic structural protein.

Molecular characterisation of ninein, a new coiled-coil protein of the centrosome

We describe the cDNA cloning of ninein, a novel component of centrosomes. In the mouse, ninein is predicted to be an acidic protein (calculated pI of 4.8) with alternatively spliced forms of 245 kDa and 249 kDa that contain extensive regions of coiled-coil structure flanked by non-coiled ends. Other interesting features of this protein include an EF-hand-like domain, a potential GTP binding site and four leucine zipper domains. Specific polyclonal antisera were raised to two non-overlapping recombinant fragments of the protein and used to characterise the cellular distribution of ninein. Immunofluorescence and immunoelectron microscopy experiments with macrophage-like cells, Mm1, showed that ninein is localised specifically in the pericentriolar matrix of the centrosome. Studies with NIH3T3 fibroblasts demonstrated that ninein is associated with the centrosome throughout the cell cycle and can also be detected within nuclei at interphase. At mitosis ninein was also observed in association with the mitotic spindle. Immunocytochemical staining of mouse tissues showed that ninein was expressed in a heterogeneous fashion. Staining, if present, was always consistent with a centrosomal localisation and was never associated with nuclei. Ninein provides a new molecular tool for analysing the structure and function of the centrosome.

gamma-Tubulin redistribution in taxol-treated mitotic cells probed by monoclonal antibodies

Monoclonal antibodies were prepared against conserved synthetic peptide from the C-terminus of the gamma-tubulin and their specificity was confirmed by immunoblotting, competitive enzyme-linked immunosorbent assay (ELISA) and immunofluorescence. The antibodies decorated interphase centrosomes as well as half-spindles and midbodies in mitotic cells of various origin. The prepared antibodies were used to study the gamma-tubulin distribution in nocodazole and taxol-treated cells. In the cells recovering from the nocodazole treatment, gamma-tubulin was found in centers of all microtubule asters. Examination of relative location of gamma-tubulin and microtubule asters in taxol-treated mitotic cells 3T3, HeLa and PtK2 revealed that the number of taxol-induced microtubule asters exceeded the number of gamma-tubulin-positive spots. The gamma-tubulin was often found in the periphery of microtubule asters. Centrosomal phosphoprotein epitope detected by MPM-2 antibody colocalized with gamma-tubulin in taxol-treated mitotic cells. The presented data suggest that taxol-induced microtubule asters are in vivo nucleated independently of gamma-tubulin, and other minus-end nucleator(s) are necessary for formation of such asters. Alternatively, gamma-tubulin is present in subthreshold amounts undetectable by immunofluorescence.

The 190 kDa centrosome-associated protein of Drosophila melanogaster contains four zinc finger motifs and binds to specific sites on polytene chromosomes

Microinjection of a bacterially expressed, TRITC labelled fragment of the centrosome-associated protein CP190 of Drosophila melanogaster, into syncytial Drosophila embryos, shows it to associate with the centrosomes during mitosis, and to relocate to chromatin during interphase. Indirect immunofluorescence staining of salivary gland chromosomes of third instar Drosophila larvae, with antibodies specific to CP190, indicate that the protein is associated with a large number of loci on these interphase polytene chromosomes. The 190 kDa CP190 protein is encoded by a 4.1 kb transcript with a single, long open reading frame specifying a polypeptide of 1,096 amino acids, with a molecular mass of 120 kDa, and an isoelectric point of 4.5. The central region of the predicted amino acid sequence of the CP190 protein contains four CysX2CysX12HisX4His zinc-finger motifs which are similar to those described for several well characterised DNA binding proteins. The data suggest that the function of CP190 involves cell cycle dependent associations with both the centrosome, and with specific chromosomal loci.

Identification of an HSP70-related protein associated with the centrosome from dinoflagellates to human cells

The monoclonal antibody CTR210 raised against isolated human centrosomes strongly decorates the centrosome and more weakly a domain congruent with the Golgi apparatus in several animal cells (HeLa, 3T3, CHO, PtK2). Both decorations resist Triton extraction in conditions which totally extract the Golgi apparatus, as judged by galactosyltransferase decoration. A 67 kDa centrosomal antigen can be demonstrated in human cells with this antibody. CTR210 also decorates the centrosome or associated structures in several systems, including unicellular eukaryotes such as dinoflagellates or ciliates. A 72 kDa antigen has been identified and purified from the dinoflagellate C. cohnii and its NH2-terminal sequence partially established. It shows a close homology with HSP70 proteins. The possibility that the 72 kDa antigen belongs to this chaperone family was further supported using a mAb reacting, in most species, with HSP70. A polyclonal antibody raised against the 72 kDa antigen from C. cohnii decorates the centrosome in human cells and reacts with the CTR210 centrosomal 67 kDa antigen. These results suggest that specific chaperone proteins are associated with the centrosome in eukaryotic cells. The centrosomal chaperones could participate in the microtubule nucleation reaction or in the process of centrosome assembly.

Association of vimentin intermediate filaments with the centrosome

SW-13 cells that lack cytoplasmic intermediate filaments (IFs) were stably transfected with a human vimentin cDNA expression vector. Isolated subclones displayed two prevalent patterns of vimentin distribution as observed by indirect immuno-localization: (1) cytoplasmic filaments characteristic of a vimentin IF network; and (2) a distinct, juxtanuclear focus with limited filamentous extensions. Comparative analysis of two subclones that uniquely segregated these patterns of vimentin organization indicated that vimentin accumulated as a perinuclear focus in cells that expressed a 4-fold lower level of the protein. The observed variation in cellular organization was not due to detectable differences in vimentin protein modification, as determined by two-dimensional gel analysis. Increasing the amount of vimentin in a low expressing clone by a secondary transfection with human or mouse vimentin cDNA resulted in well-dispersed, cytoplasmic filaments, suggesting that the distinct juxtanuclear organization of vimentin arose due to lower cellular vimentin levels. Employing anti-gamma-tubulin and anti-vimentin antibodies, dual immunofluorescence together with confocal microscopy revealed that the juxtanuclear focus of vimentin was located in the centrosomal region. Electron microscopy showed a spheroidal, filamentous structure with at least some filaments closely associated with the pericentriolar material (PCM). Because vimentin IF organization is at least partially dependent on microtubules, the effects of nocodazole and taxol on perinuclear vimentin foci were examined. Neither drug affected the juxtanuclear localization of foci, although taxol (10 microM, 5 hours) caused a release of pericentriolar gamma-tubulin from the nuclear region in 50–60% of the cells. These studies indicate that lower, in vivo, levels of vimentin fail to form extended IFs but rather are organized as a perinuclear aggregate. Moreover, the PCM of the centrosome appears to possess attachment sites for vimentin IFs.

A protein related to brain microtubule-associated protein MAP1B is a component of the mammalian centrosome

The centrosome is the main microtubule organizing center of mammalian cells. Structurally, it is composed of a pair of centrioles surrounded by a fibro-granular material (the pericentriolar material) from which microtubules are nucleated. However, the nature of centrosomal molecules involved in microtubules nucleation is still obscure. Since brain microtubule-associated proteins (MAPs) lower the critical tubulin concentration required for microtubule nucleation in tubulin solution in vitro, we have examined their possible association with centrosomes. By immunofluorescence, monoclonal and polyclonal antibodies raised against MAP1B stain the centrosome in cultured cells as well as purified centrosomes, whereas antibodies raised against MAP2 give a completely negative reaction. The MAP1B-related antigen is localized to the pericentriolar material as revealed by immunoelectron microscopy. In preparations of purified centrosomes analyzed on poly-acrylamide gels, a protein that migrates as brain MAP1B is present. After blotting on nitrocellulose, it is decorated by anti-MAP1B antibodies and the amino acid sequence of proteolytic fragments of this protein is similar to brain MAP1B. Moreover, brain MAP1B and its centrosomal counterpart share the same phosphorylation features and have similar peptide maps. These data strongly suggest that a protein homologue to MAP1B is present in centrosomes and it is a good candidate for being involved in the nucleating activity of the pericentriolar material.

Identification of a 102 kDa protein (cytocentrin) immunologically related to keratin 19, which is a cytoplasmically derived component of the mitotic spindle pole

The mAb RK7, previously shown to recognize keratin 19, was also found to cross-react with a biologically unrelated 102 kDa protein, which becomes associated with the poles of the mitotic apparatus. This newly identified protein, called cytocentrin, is a stable cellular component, may be at least in part phosphorylated, and displays a cell cycle-dependent cellular localization. In interphase cells, it is diffusely distributed in the cytosol and shows no affinity for cytoplasmic microtubules. It becomes localized to the centrosome in early prophase, prior to nuclear envelope breakdown, separation of replicated centrosomes, and nucleation of mitotic apparatus microtubules. During metaphase, cytocentrin is located predominately at the mitotic poles, often appearing as an aggregate of small globular sub-components; it also associates with some polar microtubules. In late anaphase/early telophase cytocentrin dissociates entirely from the mitotic apparatus and becomes temporarily localized with microtubules in the midbody, from which it disappears by late telophase. In taxol-treated cells cytocentrin was associated with the center of the miniasters but also showed affinity for some cytoplasmic microtubules. Studies employing G2-synchronized cells and nocodazole demonstrated that cytocentrin can become associated with mitotic centrosomes independently of tubulin polymerization and that microtubules regrow from antigen-containing foci. We interpret these results to suggest that cytocentrin is a cytoplasmic protein that becomes specifically activated or modified at the onset of mitosis so that it can affiliate with the mitotic poles where it may provide a link between the pericentriolar material and other components of the mitotic apparatus.

Centractin is an actin homologue associated with the centrosome

Actin is one of the most ubiquitous, abundant and well-conserved proteins of eukaryotes, participating in many crucial cellular processes including the maintenance of cell shape, motility and cell division. Actins from the most divergent sources still share amino-acid identities in excess of 70% (ref. 3). This may well explain why low-abundance homologues of actin have been difficult to isolate. Genes encoding distant relatives of actin in budding and fisson yeast have now been cloned. We report here the discovery of a vertebrate actin-like protein, which we name centractin. A full-length complementary DNA clone was isolated whose sequence reveals amino-acid identities with actin of over 50%, increasing to more than 70% when conservative amino-acid changes are considered. Northern analysis and western blotting indicate a ubiquitous tissue and species distribution. Morphological and biochemical criteria show that centractin is associated with centrosomes.

Human centrosomal epitope is shared specifically with human lactate dehydrogenase-B isozyme

A rabbit serum (0013) used to identify pericentriolar proteins from isolated centrosomes (Gosti-Testu, F., Marty, M.C., Berges, J., Maunoury, R. and Bornens, M. (1986) EMBO J. 5, 2545-2550) was shown also to react through the same epitope with several non-centrosomal proteins including a major 36 kDa cytosolic antigen. This protein was identified to be human lactate dehydrogenase and the co-distribution of 0013 epitope on the centrosomal protein and on lactate dehydrogenase (LDH) was shown to be specific for human cells (Gosti, F., Marty, M.C., Courvalin, J.C., Maunoury, R. and Bornens, M. (1987) Proc. Natl. Acad. Sci. USA 84, 1000-1004). Human hepatic cells constitute, so far, the only exception to this co-distribution rule. By using this cell type which expresses only the LDH-A4 isozyme, we demonstrate that 0013 epitope is specific for the human LDH-B subunit, making serum 0013 the strongest anti-LDH-B available so far. The evolutionary and physiological significance of this situation is discussed.

Centrosome organization and centriole architecture: their sensitivity to divalent cations

The centrosome plays a major role in the spatial organization of the microtubular network and has a controlled cycle of duplication, the two duplicated centrosomes functioning as mitotic poles during subsequent cell division. However, a comprehensive description of the overall organization of the centrosome in animal cells is lacking. In order to integrate the various pieces contributing to the centrosome structure and to optimize the quality of the data, we have undertaken an extensive ultrastructural study of centrosomes isolated from human lymphoblasts, which involved (i) orientation of centrosomes by sedimentation before embedding and sectioning, (ii) ultrathin serial sectioning, (iii) digitalization of micrographs to obtain quantitative data, and finally, (iv) comparison between two methods of isolation, which differ by the presence or absence of EDTA. Using this strategy, we have unambiguously described the pericentriolar organization of two distinct sets of appendages (distal and subdistal) about the so-called parental centriole. New structures have been also observed in association with the microtubule sets in this study: (i) external columns, which are dense structures localized at the basis of the subdistal appendages and (ii) internal columns, which are made of globular subunits integrated in a more luminal and probably helical structure. We have also observed that removal of divalent cations by the EDTA during the isolation procedure could affect the centrosomal structure at different levels (subdistal appendages, internal and external columns, pericentriolar matrix), including a significant variation in centriole diameter. A scheme of the overall organization of the centrosome from animal cells and of its modulation by divalent cations can be drawn from this study. Our data gives a view of the centrosome as an organelle displaying a complex and possibly dynamic structural organization.

A high molecular weight centrosomal protein of mammalian cells is antigenically related to myosin II

Available data on the molecular composition of the centrosome, the typical microtubule-organizing center of animal cells, are still fragmentary. To address this important issue we have taken advantage of centrosome isolation from a human lymphoblastic cell line (KE37) to generate a monoclonal antibody (mAb) library. Here we present the characterization of one of these mAbs (CTR56). On the basis of both its immunofluorescence staining pattern and its reactivity with a major 200 kD antigen on immunoblots, CTR56 has been tentatively classified as an anticellular myosin heavy chain. In light of cytological and biochemical data obtained in parallel with two other well-characterized myosin antibodies, it appears that myosin cannot be considered as a genuine centrosomal protein. We have resolved the paradoxical results with CTR56 by showing that in addition to the cellular myosin heavy chain, this antibody also recognizes a high molecular weight protein specifically enriched in centrosomal fractions. The possible biological significance of this finding is discussed in structural and functional terms.

The cortical actomyosin system of cytochalasin D-treated lymphoblasts

Global cytoskeleton dynamics is likely to exist in animal cells and some experimental evidence for this has recently been obtained in cells from the human lymphoblastic cell line KE37. We have further investigated the dramatic and reversible microtubule-dependent cell elongation which occurs upon treatment of KE37 cells with cytochalasin D. This phenomenon results in a non-locomotory cell with definite polarity. It involves a sustained equatorial myosin II-dependent contraction of cortical, most of the myosin II being accumulated on segments of the main cellular extension. We report here that such a cell lengthening is energy-dependent and can be inhibited, or suppressed, by surface ligands such as wheat germ agglutinin but not by concanavalin A. Suppression of the cytochalasin D effect by wheat germ agglutinin is rapid and appears to be collapse of the cell extension and relocalization of the contracted actomyosin as a whole. It suggests that the binding of the wheat germ agglutinin to the cell surface results in the transient disassembly of microtubules, a possibility also raised by the potent antagonist effect of taxol on wheat germ agglutinin action. Taken together, the data are consistent with a specific role of microtubules in the control of the activity of the cortical actomyosin system.

The identification of mammalian centrosomal antigens using human autoimmune anticentrosome antisera

Human autoimmune sera were screened for the presence of anticentrosome autoantibodies. Two high titer sera were identified that reacted with HeLa, CHO, and PtK2 centrosomes by immunofluorescence, although the fluorescent patterns that were obtained using the two antisera were separate and distinct. Serum obtained from patient IJ contained antibodies that reacted with epitopes present only in mitotic centrosomes; staining of interphase centrosomes was never detected uing IJ antiserum. Immunoblot analysis demonstrated that antibodies present in IJ antiserum reacted with a 190 kD spindle pole antigen. Immunofluorescent staining of cultured mammalian cells demonstrated that antibodies present in serum obtained from patient SPJ reacted with both interphase and mitotic centrosomes. Characterization of SPJ antiserum by immunoblotting demonstrated that antibodies present in the SPJ serum recognized proteins of Mrs of 39, 185, and 220 kD, although the possibility that the 185 kD polypeptide was a proteolytic breakdown product of the 220 kD protein has not been eliminated. Neither antiserum was able to inhibit microtubule nucleation from centrosomes in a lysed cell system in which pure 6S tubulin was added to permeabilized cells following pretreatment of the cells with either SPJ or IJ antiserum. These antisera should be useful probes for studying the biochemistry of the mammalian centrosome.

Formation of the astral mitotic spindle: ultrastructural basis for the centrosome-kinetochore interaction

The formation of the astral mitotic spindle is initiated at the time of nuclear envelope breakdown from an interaction between the replicated spindle poles (i.e. centrosomes) and the chromosomes. As a result of this interaction bundles of microtubules are generated which firmly attach the kinetochores on each chromosome to opposite spindle poles. Since these kinetochore fibers are also involved in moving the chromosomes, the mechanism by which they are formed is of paramount importance to understanding the etiology of force production within the spindle. As a prelude to outlining such a mechanism, the dynamics of spindle formation and chromosome behavior are examined in the living cell. Next, the properties of centrosomes and kinetochores are reviewed with particular emphasis on the structural and functional changes that occur within these organelles as the cell transits from interphase to mitosis. Finally, a number of recent observations relevant to the mechanism by which these organelles interact are detailed and discussed. From these diverse data it can be concluded that kinetochore fiber microtubules are derived from dynamically unstable astral microtubules that grow into, or grow by and then interact laterally with, the kinetochore. Moreover, the data clearly demonstrate that the interaction of a single astral microtubule with one of the kinetochores on an unattached chromosome is sufficient to attach the chromosome to the spindle, orient it towards a pole, and initiate poleward motion. As the chromosomes move into the region of the forming spindle more astral microtubules become incorporated into the nascent kinetochore fibers and chromosome velocity decreases dramatically. During this time the distribution of spindle microtubules changes from two overlapping radial arrays to the fusiform array characteristic of metaphase cells.

p34cdc2 is located in both nucleus and cytoplasm; part is centrosomally associated at G2/M and enters vesicles at anaphase

The cdc2+ gene product p34cdc2 is located immunocytochemically in both the nucleus and cytoplasm of human cells. It is uniformly distributed throughout the cytoplasm and is irregularly distributed in the nucleus. Part of p34cdc2 is associated with the centrosome and centrosomal staining increases late in the cell cycle and at the onset of mitosis. This distribution is corroborated by cell fractionation which also indicates that slower migrating forms of p34cdc2 are found in isolated centrosomes and in Triton-insoluble fractions. We propose that one role of the p34cdc2 protein kinase is to modify the centrosome bringing about formation of the mitotic spindle. At anaphase p34cdc2 becomes associated with vesicles in the middle of the cell between the reforming nuclei. A similar location is found for p13suc1 and we suggest that the vesicular localization plays a role in p34cdc2 kinase inactivation at the end of mitosis.

Structural and chemical characterization of isolated centrosomes

A procedure adapted from that described by Mitchison and Kirschner [Nature 312:232-237, 1984] was used to isolate centrosomes from human lymphoid cells. High yields of homogeneous centrosomes (60% of the theoretical total, assuming one centrosome per cell) were obtained. Centrosomes were isolated as pairs of centrioles, plus their associated pericentriolar material. Ultrastructural investigation revealed: 1) a link between both centrioles in a centrosome formed by the gathering in of a unique bundle of thin filaments surrounding each centriole; 2) a stereotypic organization of the pericentriolar material, including a rim of constant width at the proximal end of each centriole and a disc of nine satellite arms organized according to a ninefold symmetry at the distal end and; 3) an axial hub in the lumen of each centriole at the distal end surrounded by some ill-defined material. The total protein content was 2 to 3 X 10(-2) pg per isolated centrosome, a figure that suggests that the preparations were close to homogeneity. The protein composition was complex but specific, showing proteins ranging from 180 to 300 kD, one prominent band at 130 kD, and a group of proteins between 50 and 65 kD. Actin was also present in centrosome preparations. Functional studies demonstrated that the isolated centrosomes were competent to nucleate microtubules in vitro from purified tubulin in conditions in which spontaneous assembly could not occur. They were also very effective at inducing cleavage when microinjected into unfertilized Xenopus eggs.

A protein of Mr 80,000 is associated with the nucleolus organizer of human cell lines

A rabbit serum which had previously been reported to have an immunological affinity for centrosomes of human cell lines was shown also to be specific for the nucleus. Optical and ultrastructural immunolocalization in HeLa cells showed that this specificity is restricted to the fibrillar centre of nucleoli either in untreated or actinomycin D treated interphase cells. In mitotic cells discrete labelling was observed on chromosomes and shown to correspond, on spread metaphase plates, to the short arms of acrocentric chromosomes, i.e. to the nucleolar organizer regions (NORs). Using independent cell fractionation procedures in the human T-lymphoblastic KE 37 cell line and purification of immunoglobulins by affinity to antigens detected by electrophoresis and blotting, a strict correlation between immunoreactive proteins and cytological staining was established. The nucleolar specificity was shown to correspond to a protein with an Mr of 80,000 while the centrosomal specificity corresponded principally to a protein doublet of 60,000-65,000. These antigens share common epitopes as shown by the staining of both NOR and centrosome by immunoglobulins purified by affinity to either type of protein.