Mammalian p55CDC mediates association of the spindle checkpoint protein Mad2 with the cyclosome/anaphase-promoting complex, and is involved in regulating anaphase onset and late mitotic events

The spindle checkpoint: two transitions, two pathways

The spindle checkpoint is an evolutionarily conserved mitotic regulatory mechanism that ensures that anaphase is not attempted until chromosomes are properly aligned on the spindle. Two different cell-cycle transitions must be inhibited by the spindle checkpoint to arrest cells at metaphase and prevent mitotic exit. The checkpoint proteins interact in ways that are more complex than was originally envisioned. This review summarizes the evidence for two pathways of spindle-checkpoint regulation in budding yeast. We describe how the proteins are involved in these pathways and discuss the ways in which the spindle checkpoint inhibits the cell-cycle machinery.

Mitotic misregulation and human aging

Messenger RNA levels were measured in actively dividing fibroblasts isolated from young, middle-age, and old-age humans and humans with progeria, a rare genetic disorder characterized by accelerated aging. Genes whose expression is associated with age-related phenotypes and diseases were identified. The data also suggest that an underlying mechanism of the aging process involves increasing errors in the mitotic machinery of dividing cells in the postreproductive stage of life. We propose that this dysfunction leads to chromosomal pathologies that result in misregulation of genes involved in the aging process.

Cdk1 is essential for mammalian cyclosome/APC regulation

The cyclosome/APC (anaphase-promoting complex), the major component of cell-cycle-specific ubiquitin-mediated proteolysis of mitotic cyclins and of other cell cycle proteins, is essential for sister chromatid separation and for exit from mitosis. Cyclosome activity and substrate specificity are modulated by phosphorylation and by transient interactions with Fizzy/cdc20 (Fzy) and Fizzy-related/Hct1/Cdh1 (Fzr). This regulation has been studied so far in Drosophila embryos, in yeast, and in cell-free extracts in vitro. Studying cyclosome regulation in mammalian cells in vivo we found that both Fzr overexpression and Cdk1 inhibition can override the prometaphase checkpoint. We further show that Fzr activation of the cyclosome is negatively regulated by Cdk1. Finally, we show that the mammalian cdc14 phosphatase, like its budding yeast homologue, plays a role in cyclosome pathway regulation. These results suggest that Cdk1 is essential for coupling various activities of the cyclosome and in particular for preventing Fzr from short-circuiting the spindle pole checkpoint. Cdk1-cyclin B is thus an inhibitor, activator, and substrate of the cyclosome.

MAD3 encodes a novel component of the spindle checkpoint which interacts with Bub3p, Cdc20p, and Mad2p

We show that MAD3 encodes a novel 58-kD nuclear protein which is not essential for viability, but is an integral component of the spindle checkpoint in budding yeast. Sequence analysis reveals two regions of Mad3p that are 46 and 47% identical to sequences in the NH(2)-terminal region of the budding yeast Bub1 protein kinase. Bub1p is known to bind Bub3p (Roberts et al. 1994) and we use two-hybrid assays and coimmunoprecipitation experiments to show that Mad3p can also bind to Bub3p. In addition, we find that Mad3p interacts with Mad2p and the cell cycle regulator Cdc20p. We show that the two regions of homology between Mad3p and Bub1p are crucial for these interactions and identify loss of function mutations within each domain of Mad3p. We discuss roles for Mad3p and its interactions with other spindle checkpoint proteins and with Cdc20p, the target of the checkpoint.

Two different modes of cyclin clb2 proteolysis during mitosis in Saccharomyces cerevisiae

Sister chromatid separation and mitotic exit are triggered by the anaphase-promoting complex (APC/C) which is a multi-subunit ubiquitin ligase required for proteolytic degradation of various target proteins. Cdc20 and Cdh1 are substrate-specific activators of the APC/C. It was previously proposed that Cdh1 is essential for proteolysis of the yeast mitotic cyclin Clb2. We show that Clb2 proteolysis is triggered by two different modes during mitosis. A fraction of Clb2 is degraded during anaphase in the absence of Cdh1. However, a second fraction of Clb2 remains stable during anaphase and is degraded in a Cdh1-dependent manner as cells exit from mitosis. Most of cyclin Clb3 is degraded independently of Cdh1. Our data imply that degradation of mitotic cyclins is initiated by a Cdh1-independent mechanism.

Human p55(CDC)/Cdc20 associates with cyclin A and is phosphorylated by the cyclin A-Cdk2 complex

The initiation of anaphase and exit from mitosis depend on the activation of the anaphase-promoting complex/cyclosome (APC/C), a multicomponent, ubiquitin-protein ligase. The WD-repeat protein called p55(CDC)(Cdc20) directly binds to and activates APC/C. By using yeast two-hybrid screening, we found that cyclin A, a critical cell cycle regulator in the S and G2/M phases, specifically interacts with p55(CDC). Ectopically expressed p55(CDC) and cyclin A form a stable protein complex in mammalian cells. The p55(CDC)-cyclin A interaction occurs through the region containing the WD repeats of p55(CDC) and the region between the destruction box and the cyclin box of cyclin A. In addition to the physical interaction, p55(CDC) is phosphorylated by cyclin A-associated kinase. These findings suggest that the function of p55(CDC) is mediated or regulated by its complex formation with cyclin A.

A comprehensive analysis of protein-protein interactions in Saccharomyces cerevisiae

Two large-scale yeast two-hybrid screens were undertaken to identify protein-protein interactions between full-length open reading frames predicted from the Saccharomyces cerevisiae genome sequence. In one approach, we constructed a protein array of about 6,000 yeast transformants, with each transformant expressing one of the open reading frames as a fusion to an activation domain. This array was screened by a simple and automated procedure for 192 yeast proteins, with positive responses identified by their positions in the array. In a second approach, we pooled cells expressing one of about 6,000 activation domain fusions to generate a library. We used a high-throughput screening procedure to screen nearly all of the 6,000 predicted yeast proteins, expressed as Gal4 DNA-binding domain fusion proteins, against the library, and characterized positives by sequence analysis. These approaches resulted in the detection of 957 putative interactions involving 1,004 S. cerevisiae proteins. These data reveal interactions that place functionally unclassified proteins in a biological context, interactions between proteins involved in the same biological function, and interactions that link biological functions together into larger cellular processes. The results of these screens are shown here.

Controlling the end of the cell cycle

The past year has seen significant advances in our understanding of how the events which occur at the end of mitosis, such as cytokinesis and the inactivation of mitotic cyclin dependent kinases are triggered, and also how they are prevented from occurring prematurely or inappropriately. This control is achieved through a combination of temporally ordered proteolytic events and changes in the subcellular localisation of proteins. These studies have also revealed that the nucleolus and spindle pole bodies play a key role in this regulation.

Complexity in the spindle checkpoint

Cell viability requires accurate chromosome segregation at mitosis. The spindle checkpoint ensures that anaphase is not attempted until the sister chromatids of each chromosome are attached to spindle microtubules from opposite poles. The checkpoint mechanism involves a signal transduction cascade that is more complex than was originally envisioned.

Hypomorphic bimA(APC3) alleles cause errors in chromosome metabolism that activate the DNA damage checkpoint blocking cytokinesis in Aspergillus nidulans

The Aspergillus nidulans sepI(+) gene has been implicated in the coordination of septation with nuclear division and cell growth. We find that the temperature-sensitive (ts) sepI1 mutation represents a novel allele of bimA(APC3), which encodes a conserved component of the anaphase-promoting complex/cyclosome (APC/C). We have characterized the septation, nuclear division, cell-cycle checkpoint defects, and DNA sequence alterations of sepI1 (renamed bimA10) and two other ts lethal bimA(APC3) alleles, bimA1 and bimA9. Our observations that bimA9 and bimA10 strains had morphologically abnormal nuclei, chromosome segregation defects, synthetic phenotypes with mutations in the DNA damage checkpoint genes uvsB(MEC1/rad3) or uvsD(+), and enhanced sensitivity to hydroxyurea strongly suggest that these strains accumulate errors in DNA metabolism. We found that the aseptate phenotype of bimA9 and bimA10 strains was substantially relieved by mutations in uvsB(MEC1/rad3) or uvsD(+), suggesting that the presence of a functional DNA damage checkpoint inhibits septation in these bimA(APC3) strains. Our results demonstrate that mutations in bimA(APC3) lead to errors in DNA metabolism that indirectly block septation.

The mammalian Fizzy and Fizzy-related genes are regulated at the transcriptional and post-transcriptional levels

The cyclosome pathway of ubiquitin-mediated proteolysis plays an essential role in cell cycle control. The multisubunit cyclosome is regulated by transient interactions with Fizzy (Fzy) and Fizzy-related (Fzr) genes. We report here that both Fzy and Fzr are transcribed in a cell cycle specific but distinct manner. Fzy transcription starts after the restriction point in late G1 and ceases upon cell division. Fzr transcription also ceases upon cell division but resumes already in mid G1, before the restriction point, and takes place also in G0. Fzr has further a striking cell cycle specific pattern of mRNA stability. During most of the cell cycle its message is fairly stable, however upon exit from mitosis it is rapidly degraded. This result is puzzling because Fzr is essential for cyclosome activity in G1, and points to a complex pattern of Fzr regulation.

Protein dynamics at the kinetochore: cell cycle regulation of the metaphase to anaphase transition

The spindle checkpoint blocks the initiation of anaphase in mitosis and meiosis if chromosomes are not aligned at the metaphase plate. The checkpoint functions by preventing a ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC/C) from ubiquitinylating proteins whose destruction is required for anaphase onset. The spindle checkpoint signal originates at the kinetochores of unaligned chromosomes and is broadcast to the rest of the cell. Although the spindle checkpoint is not understood in detail, several components of the checkpoint-signaling pathway have been identified. Many of these components associate transiently with the kinetochores of unaligned chromosomes. We propose a model in which kinetochores that lack stable attachments to the spindle microtubules serve as catalytic staging areas for the assembly of inhibitor complexes. These inhibitor complexes then leave the kinetochores and block activity of the APC/C throughout the cell. We suggest that microtubule occupancy at kinetochores or physical tension induced by microtubule capture turns off the capability of the kinetochore to produce the APC/C inhibitor. Subsequently, the inhibitor concentration in the cell wanes and anaphase initiates.

Components of the spindle-assembly checkpoint are essential in Caenorhabditis elegans

The spindle-assembly checkpoint ensures that, during mitosis and meiosis, chromosomes do not segregate until they are properly attached to the microtubules of the spindle. Here we show that mdf-1 and mdf-2 are components of the spindle-assembly checkpoint in Caenorhabditis elegans, and are essential for the long-term survival and fertility of this organism. Loss of function of either of these genes leads to the accumulation of a variety of defects, including chromosome abnormalities, X-chromosome non-disjunction or loss, problems in gonad development, and embryonic lethality. Antibodies that recognize the MDF-2 protein localize to nuclei of the cleaving embryo in a cell-cycle-dependent manner. mdf-1, a gene encoding a product that interacts with MDF-2, is required for cell-cycle arrest and proper chromosome segregation in premeiotic germ cells treated with nocodazole, a microtubule-depolymerizing agent. In the absence of mdf gene products, errors in chromosome segregation arise and accumulate, ultimately leading to genetic lethality.

The rough deal protein is a new kinetochore component required for accurate chromosome segregation in Drosophila

Mutations in the rough deal (rod) gene of Drosophila greatly increase the missegregation of sister chromatids during mitosis, suggesting a role for this gene product in spindle or kinetochore function. The activity provided by rod also appears to be necessary for the recruitment of two known kinetochore components, Zw10 and cytoplasmic dynein. In this paper we describe the cloning of rough deal and an initial cytological characterization of its product. The Rod protein shares no identifiable structural motif with other known proteins, although apparent homologs exist in the genomes of nematode and man. By immunocytochemistry we show that Rod displays a dynamic intracellular staining pattern, localizing first to kinetochores in prometaphase, but moving to kinetochore microtubules at metaphase. Early in anaphase the protein is once again restricted to the kinetochores, where it persists until the end of telophase. This behavior is in all respects similar to that described for Zw10, and suggests that the proteins function together.

Evidence for an interaction of the metalloprotease-disintegrin tumour necrosis factor alpha convertase (TACE) with mitotic arrest deficient 2 (MAD2), and of the metalloprotease-disintegrin MDC9 with a novel MAD2-related protein, MAD2beta

Metalloprotease-disintegrins are a family of transmembrane glycoproteins that have a role in fertilization, sperm migration, myoblast fusion, neural development and ectodomain shedding. In the present study we used the yeast two-hybrid system to search for proteins that interact with the cytoplasmic domain of two metalloprotease-disintegrins, tumour necrosis factor alpha convertase (TACE; ADAM17) and MDC9 (ADAM9; meltrin gamma). We have identified mitotic arrest deficient 2 (MAD2) as a binding partner of the TACE cytoplasmic domain, and a novel MAD2-related protein, MAD2beta, as a binding partner of the MDC9 cytoplasmic domain. MAD2beta has 23% sequence identity with MAD2, which is a component of the spindle assembly (or mitotic) checkpoint mechanism. Northern blot analysis of human tissues indicates that MAD2beta mRNA is expressed ubiquitously. The interaction of the TACE and MDC9 cytoplasmic domains with their binding partners has been confirmed biochemically. The independent identification of MAD2 and MAD2beta as potential interacting partners of distinct metalloprotease-disintegrins raises the possibility of a link between metalloprotease-disintegrins and the cell cycle, or of functions for MAD2 and MAD2beta that are not related to cell cycle control.

Cell cycle-regulated proteolysis of mitotic target proteins

The ubiquitin-dependent proteolysis of mitotic cyclin B, which is catalyzed by the anaphase-promoting complex/cyclosome (APC/C) and ubiquitin-conjugating enzyme H10 (UbcH10), begins around the time of the metaphase-anaphase transition and continues through G1 phase of the next cell cycle. We have used cell-free systems from mammalian somatic cells collected at different cell cycle stages (G0, G1, S, G2, and M) to investigate the regulated degradation of four targets of the mitotic destruction machinery: cyclins A and B, geminin H (an inhibitor of S phase identified in Xenopus), and Cut2p (an inhibitor of anaphase onset identified in fission yeast). All four are degraded by G1 extracts but not by extracts of S phase cells. Maintenance of destruction during G1 requires the activity of a PP2A-like phosphatase. Destruction of each target is dependent on the presence of an N-terminal destruction box motif, is accelerated by additional wild-type UbcH10 and is blocked by dominant negative UbcH10. Destruction of each is terminated by a dominant activity that appears in nuclei near the start of S phase. Previous work indicates that the APC/C-dependent destruction of anaphase inhibitors is activated after chromosome alignment at the metaphase plate. In support of this, we show that addition of dominant negative UbcH10 to G1 extracts blocks destruction of the yeast anaphase inhibitor Cut2p in vitro, and injection of dominant negative UbcH10 blocks anaphase onset in vivo. Finally, we report that injection of dominant negative Ubc3/Cdc34, whose role in G1-S control is well established and has been implicated in kinetochore function during mitosis in yeast, dramatically interferes with congression of chromosomes to the metaphase plate. These results demonstrate that the regulated ubiquitination and destruction of critical mitotic proteins is highly conserved from yeast to humans.

Control of mitotic transitions by the anaphase-promoting complex

Proteolysis controls key transitions at several points in the cell cycle. In mitosis, the activation of a large ubiquitin-protein ligase, the anaphase-promoting complex (APC), is required for anaphase initiation and for exit from mitosis. We show that APC is under complex control by a network of regulatory factors, CDC20, CDH1 and MAD2. CDC20 and CDH1 are activators of APC; they bind directly to APC and activate its cyclin ubiquitination activity. CDC20 activates APC at the onset of anaphase in a destruction box (DB)-dependent manner, while CDH1 activates APC from late anaphase through G1 with apparently a much relaxed specificity for the DB. Therefore, CDC20 and CDH1 control both the temporal order of activation and the substrate specificity of APC, and hence regulate different events during mitosis and G1. Counteracting the effect of CDC20, the checkpoint protein MAD2 acts as an inhibitor of APC. When the spindle-assembly checkpoint is activated, MAD2 forms a ternary complex with CDC20 and APC to prevent activation of APC, and thereby arrests cells at prometaphase. Thus, a combination of positive and negative regulators establishes a regulatory circuit of APC, ensuring an ordered progression of events through cell division.

The kinetochore of higher eucaryotes: a molecular view

This review summarizes results concerning the molecular nature of the higher eucaryotic kinetochore. The first major section of this review includes kinetochore proteins whose general functions remain to be determined, precluding their entry into a discrete functional category. Many of the proteins in this section, however, are likely to be involved in kinetochore formation or structure. The second major section is concerned with how microtubule motor proteins function to cause chromosome movement. The microtubule motors dynein, CENP-E, and MCAK have all been observed at the kinetochore. While their precise functions are not well understood, all three are implicated in chromosome movement during mitosis. Finally, the last section deals with kinetochore components that play a role in the spindle checkpoint; a checkpoint that delays mitosis until all kinetochores have attached to the mitotic spindle. Brief reviews of kinetochore morphology and of an important technical breakthrough that enabled the molecular dissection of the kinetochore are also included.

Characterization of a novel kinetochore protein, CENP-H

Macromolecular centromere-kinetochore complex plays a critical role in sister chromatid separation, but its complete protein composition as well as its precise dynamic function during mitosis has not yet been clearly determined. Here we report the isolation of a novel mouse kinetochore protein, CENP-H. The CENP-H, with an apparent molecular mass of 33 kDa, was found to contain a coiled-coil structure and a nuclear localization signal. The CENP-H transcripts were relatively scarce but were detectable in most tissues and embryos at various stages of development. Immunofluorescence stainings of mouse fibroblast cells with anti-CENP-H-specific antibody demonstrated that the CENP-H is specifically and constitutively localized in kinetochores throughout the cell cycle; this was also confirmed by stainings with anti-centromere-specific antibody. Thus the newly isolated CENP-H may play a role in kinetochore organization and function throughout the cell cycle.

Identification of human APC10/Doc1 as a subunit of anaphase promoting complex

Anaphase-promoting complex or cyclosome (APC) is a ubiquitin ligase which specifically targets mitotic regulatory factors such as Pds1/Cut2 and cyclin B. Identification of the subunits of multiprotein complex APC in several species revealed the highly conserved composition of APC from yeast to human. It has been reported, however, that vertebrate APC is composed of at least eight subunits, APC1 to APC8, while budding yeast APC is constituted of at least 12 components, Apc1 to Apc13. It has not yet been clearly understood whether additional components found in budding yeast, Apc9 to Apc13, are actually composed of mammalian APC. Here we isolated and characterized human APC10/Doc1, and found that APC10/Doc1 binds to APC core subunits throughout the cell cycle. Further, it was found that APC10/Doc1 is localized in centrosomes and mitotic spindles throughout mitosis, while it is also localized in kinetochores from prophase to anaphase and in midbody in telophase and cytokinesis. These results strongly support the notion that human APC10/Doc1 may be one of the APC core subunits rather than the transiently associated regulatory factor.

Human BUBR1 is a mitotic checkpoint kinase that monitors CENP-E functions at kinetochores and binds the cyclosome/APC

Human cells express two kinases that are related to the yeast mitotic checkpoint kinase BUB1. hBUB1 and hBUBR1 bind to kinetochores where they are postulated to be components of the mitotic checkpoint that monitors kinetochore activities to determine if chromosomes have achieved alignment at the spindle equator (Jablonski, S.A., G.K.T. Chan, C.A. Cooke, W.C. Earnshaw, and T.J. Yen. 1998. Chromosoma. 107:386-396). In support of this, hBUB1 and the homologous mouse BUB1 have been shown to be important for the mitotic checkpoint (Cahill, D.P., C. Lengauer, J. Yu, G.J. Riggins, J.K. Willson, S.D. Markowitz, K.W. Kinzler, and B. Vogelstein. 1998. Nature. 392:300-303; Taylor, S.S., and F. McKeon. 1997. Cell. 89:727-735). We now demonstrate that hBUBR1 is also an essential component of the mitotic checkpoint. hBUBR1 is required by cells that are exposed to microtubule inhibitors to arrest in mitosis. Additionally, hBUBR1 is essential for normal mitotic progression as it prevents cells from prematurely entering anaphase. We establish that one of hBUBR1's checkpoint functions is to monitor kinetochore activities that depend on the kinetochore motor CENP-E. hBUBR1 is expressed throughout the cell cycle, but its kinase activity is detected after cells have entered mitosis. hBUBR1 kinase activity was rapidly stimulated when the spindle was disrupted in mitotic cells. Finally, hBUBR1 was associated with the cyclosome/anaphase-promoting complex (APC) in mitotically arrested cells but not in interphase cells. The combined data indicate that hBUBR1 can potentially provide two checkpoint functions by monitoring CENP-E-dependent activities at the kinetochore and regulating cyclosome/APC activity.

Regulation of APC activity by phosphorylation and regulatory factors

Ubiquitin-dependent proteolysis of Cut2/Pds1 and Cyclin B is required for sister chromatid separation and exit from mitosis, respectively. Anaphase-promoting complex/cyclosome (APC) specifically ubiquitinates Cut2/Pds1 at metaphase-anaphase transition, and ubiquitinates Cyclin B in late mitosis and G1 phase. However, the exact regulatory mechanism of substrate-specific activation of mammalian APC with the right timing remains to be elucidated. We found that not only the binding of the activators Cdc20 and Cdh1 and the inhibitor Mad2 to APC, but also the phosphorylation of Cdc20 and Cdh1 by Cdc2-Cyclin B and that of APC by Polo-like kinase and cAMP-dependent protein kinase, regulate APC activity. The cooperation of the phosphorylation/dephosphorylation and the regulatory factors in regulation of APC activity may thus control the precise progression of mitosis.

Analysis with flow cytometry of green fluorescent protein expression in leukemic cells

BACKGROUND

The measurement of DNA content with propidium iodide (PI) in cells transfected with expression vectors encoding the green fluorescent protein (GFP) is a useful tool in studying a variety of biological functions of proteins within cells. The purpose of this study was to determine conditions of formaldehyde fixation that permit intracellular GFP fluorescence and adequate DNA histograms to be generated following transient transfection of cells with a GFP-encoding plasmid. Cell cycle analysis was also performed in GFP-positive cells.

METHODS

The murine myeloid leukemic cell line, 32Dcl3, was used as the model system. Cells were transfected with a GFP-encoding plasmid (pEGFPC1). Following fixation in different formaldehyde concentrations and permeabilization with 70% ethanol, cells were stained with PI and analyzed by flow cytometry for GFP fluorescence and DNA content. Transfected cells were also analyzed for GFP fluorescence and DNA content following release from nocodazole block.

RESULTS

Fixing cells in 0.51-1.75% formaldehyde concentrations prior to ethanol permeabilization resulted in 14-19% of transfected cells being GFP-positive, with acceptable coefficients of variation on the G(1) peak of DNA histograms. Analysis of cells synchronized to and released from the G(2)-M phase by nocodazole suggested that GFP-positive cells, when compared to GFP-negative cells, did not appear to progress out of G(2)-M following release from nocodazole block. Simultaneous detection of GFP fluorescence and DNA content by PI staining is possible following transient transfection of cells with a single expression vector encoding GFP. Our results demonstrate that GFP expression can be detected, using flow cytometry to perform cell cycle analysis in murine leukemic cells.

Mutations in the essential spindle checkpoint gene bub1 cause chromosome missegregation and fail to block apoptosis in Drosophila

We have characterized the Drosophila mitotic checkpoint control protein Bub1 and obtained mutations in the bub1 gene. Drosophila Bub1 localizes strongly to the centromere/kinetochore of mitotic and meiotic chromosomes that have not yet reached the metaphase plate. Animals homozygous for P-element-induced, near-null mutations of bub1 die during late larval/pupal stages due to severe mitotic abnormalities indicative of a bypass of checkpoint function. These abnormalities include accelerated exit from metaphase and chromosome missegregation and fragmentation. Chromosome fragmentation possibly leads to the significantly elevated levels of apoptosis seen in mutants. We have also investigated the relationship between Bub1 and other kinetochore components. We show that Bub1 kinase activity is not required for phosphorylation of 3F3/2 epitopes at prophase/prometaphase, but is needed for 3F3/2 dephosphorylation at metaphase. Neither 3F3/2 dephosphorylation nor loss of Bub1 from the kinetochore is a prerequisite for anaphase entry. Bub1's localization to the kinetochore does not depend on the products of the genes zw10, rod, polo, or fizzy, indicating that the kinetochore is constructed from several independent subassemblies.

Four-dimensional control of the cell cycle

The cell-division cycle has to be regulated in both time and space. In the time dimension, the cell ensures that mitosis does not begin until DNA replication is completed and any damaged DNA is repaired, and that DNA replication normally follows mitosis. This is achieved by the synthesis and destruction of specific cell-cycle regulators at the right time in the cell cycle. In the spatial dimension, the cell coordinates dramatic reorganizations of the subcellular architecture at the entrance to and exit from mitosis, largely through the actions of protein kinases and phosphatases that are often localized to specific subcellular structures. Evidence is now accumulating to suggest that the spatial organization of cell-cycle regulators is also important in the temporal control of the cell cycle. Here I will focus on how the locations of the main components of the cell-cycle machinery are regulated as part of the mechanism by which the cell controls when and how it replicates and divides.

The metaphase to anaphase transition: a case of productive destruction

The metaphase to anaphase transition is a point of no return; the duplicated sister chromatids segregate to the future daughter cells, and any mistake in this process may be deleterious to both progeny. At the heart of this process lies the anaphase inhibitor, which must be degraded in order for this transition to take place. The degradation of the anaphase inhibitor occurs via the ubiquitin-degradation pathway, and it involves the activity of the cyclosome/anaphase promoting complex (APC). The fidelity of the metaphase to anaphase transition is ensured by several different regulatory mechanisms that modulate the activity of the cyclosome/APC. Great advancements have been made in this field in the past few years, but many questions still remain to be answered.

Cdc20 associates with the kinase aurora2/Aik

Cdc20/fizzy family proteins are involved in activation of the anaphase-promoting complex/cyclosome, which catalyzes the ubiquitin-dependent proteolysis of cell cycle regulatory proteins such as anaphase inhibitors and mitotic cyclins, leading to chromosome segregation and exit from mitosis. Previous work has shown that human Cdc20 (hCdc20/p55CDC) associates with one or more kinases. We report here that Cdc20-associated myelin basic protein kinase activity peaks sharply in early M phase (embryonic cells) or in G2 phase (somatic cells). In HeLa cells, Cdc20 is associated with the kinase aurora2/Aik. Aurora2/Aik is a member of the aurora/Ipl1 family of kinases that, like Cdc20, previously has been shown to be localized at mitotic spindle poles and is involved in regulating chromosome segregation and maintaining genomic stability. The demonstration that Cdc20 is associated with aurora2/Aik suggests that some function of Cdc20 is carried out or regulated through its association with aurora2/Aik.

Mad2 binding by phosphorylated kinetochores links error detection and checkpoint action in mitosis

The spindle checkpoint must detect the presence of unattached or improperly attached kinetochores and must then inhibit progression through the cell cycle until the offending condition is resolved. Detection probably involves attachment-sensitive kinetochore phosphorylation (reviewed in [1,2]). A key player in the checkpoint's response is the Mad2 protein, which prevents activation of the anaphase-promoting complex (APC) by the Cdc20 protein [3-8]. Microinjection of Mad2 antibodies results in premature anaphase onset [9,10], and excess Mad2 protein causes arrest in mitosis [5,11]. We have previously shown that Mad2 localizes to unattached kinetochores in vertebrate cells, and that this localization ceases as kinetochores accumulate microtubules [10,12,13]. But how is Mad2 binding limited to unattached kinetochores? Here, we used lysed PtK1 cells to study kinetochore phosphorylation and Mad2 binding. We found that Mad2 binds to phosphorylated kinetochores, but not to unphosphorylated ones. Our data suggest that it is kinetochore protein phosphorylation that promotes Mad2 binding to unattached kinetochores. Thus, we have identified a probable molecular link between attachment-sensitive kinetochore phosphorylation and the inhibition of anaphase. The complete pathway for error control in mitosis can now be outlined.

Regulation of the APC and the exit from mitosis

The events of late mitosis, from sister-chromatid separation to cytokinesis, are governed by the anaphase-promoting complex (APC), a multisubunit assembly that triggers the ubiquitin-dependent proteloysis of key regulatory proteins. An intricate regulatory network governs APC activity and helps to ensure that late mitotic events are properly timed and coordinated.

Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae

The spindle checkpoint arrests cells in mitosis in response to defects in the assembly of the mitotic spindle or errors in chromosome alignment. We determined which spindle defects the checkpoint can detect by examining the interaction of mutations that compromise the checkpoint (mad1, mad2, and mad3) with those that damage various structural components of the spindle. Defects in microtubule polymerization, spindle pole body duplication, microtubule motors, and kinetochore components all activate the MAD-dependent checkpoint. In contrast, the cell cycle arrest caused by mutations that induce DNA damage (cdc13), inactivate the cyclin proteolysis machinery (cdc16 and cdc23), or arrest cells in anaphase (cdc15) is independent of the spindle checkpoint.

Characterization of MAD2B and other mitotic spindle checkpoint genes

Aneuploidy is a characteristic of the majority of human cancers, and recent work has suggested that mitotic checkpoint defects play a role in its development. To further explore this issue, we isolated a novel human gene, MAD2B (MAD2L2), which is homologous to the spindle checkpoint gene MAD2 (MAD2L1). We determined the chromosomal localization of it and other spindle checkpoint genes, including MAD1L1, MAD2, BUB3, TTK (MPS1L1), and CDC20. In addition, we resolved the genomic intron-exon structure of the human BUB1 gene. We then searched for mutations in these genes in a panel of 19 aneuploid colorectal tumors. No new mutations were identified, suggesting that genes yet to be discovered are responsible for most of the checkpoint defects observed in aneuploid cancers.

Budding yeast Bub2 is localized at spindle pole bodies and activates the mitotic checkpoint via a different pathway from Mad2

The mitotic checkpoint blocks cell cycle progression before anaphase in case of mistakes in the alignment of chromosomes on the mitotic spindle. In budding yeast, the Mad1, 2, 3, and Bub1, 2, 3 proteins mediate this arrest. Vertebrate homologues of Mad1, 2, 3, and Bub1, 3 bind to unattached kinetochores and prevent progression through mitosis by inhibiting Cdc20/APC-mediated proteolysis of anaphase inhibitors, like Pds1 and B-type cyclins. We investigated the role of Bub2 in budding yeast mitotic checkpoint. The following observations indicate that Bub2 and Mad1, 2 probably activate the checkpoint via different pathways: (a) unlike the other Mad and Bub proteins, Bub2 localizes at the spindle pole body (SPB) throughout the cell cycle; (b) the effect of concomitant lack of Mad1 or Mad2 and Bub2 is additive, since nocodazole-treated mad1 bub2 and mad2 bub2 double mutants rereplicate DNA more rapidly and efficiently than either single mutant; (c) cell cycle progression of bub2 cells in the presence of nocodazole requires the Cdc26 APC subunit, which, conversely, is not required for mad2 cells in the same conditions. Altogether, our data suggest that activation of the mitotic checkpoint blocks progression through mitosis by independent and partially redundant mechanisms.

Characterization of the DOC1/APC10 subunit of the yeast and the human anaphase-promoting complex

The anaphase-promoting complex/cyclosome (APC) is a ubiquitin-protein ligase whose activity is essential for progression through mitosis. The vertebrate APC is thought to be composed of 8 subunits, whereas in budding yeast several additional APC-associated proteins have been identified, including a 33-kDa protein called Doc1 or Apc10. Here, we show that Doc1/Apc10 is a subunit of the yeast APC throughout the cell cycle. Mutation of Doc1/Apc10 inactivates the APC without destabilizing the complex. An ortholog of Doc1/Apc10, which we call APC10, is associated with the APC in different vertebrates, including humans and frogs. Biochemical fractionation experiments and mass spectrometric analysis of a component of the purified human APC show that APC10 is a genuine APC subunit whose cellular levels or association with the APC are not cell cycle-regulated. We have further identified an APC10 homology region, which we propose to call the DOC domain, in several protein sequences that also contain either cullin or HECT domains. Cullins are present in several ubiquitination complexes including the APC, whereas HECT domains represent the catalytic core of a different type of ubiquitin-protein ligase. DOC domains may therefore be important for reactions catalyzed by several types of ubiquitin-protein ligases.

The maize homologue of the cell cycle checkpoint protein MAD2 reveals kinetochore substructure and contrasting mitotic and meiotic localization patterns

We have identified a maize homologue of yeast MAD2, an essential component in the spindle checkpoint pathway that ensures metaphase is complete before anaphase begins. Combined immunolocalization of MAD2 and a recently cloned maize CENPC homologue indicates that MAD2 localizes to an outer domain of the prometaphase kinetochore. MAD2 staining was primarily observed on mitotic kinetochores that lacked attached microtubules; i.e., at prometaphase or when the microtubules were depolymerized with oryzalin. In contrast, the loss of MAD2 staining in meiosis was not correlated with initial microtubule attachment but was correlated with a measure of tension: the distance between homologous or sister kinetochores (in meiosis I and II, respectively). Further, the tension-sensitive 3F3/2 phosphoepitope colocalized, and was lost concomitantly, with MAD2 staining at the meiotic kinetochore. The mechanism of spindle assembly (discussed here with respect to maize mitosis and meiosis) is likely to affect the relative contributions of attachment and tension. We support the idea that MAD2 is attachment-sensitive and that tension stabilizes microtubule attachments.

Subunits and substrates of the anaphase-promoting complex

The initiation of anaphase and exit from mitosis depend on a ubiquitination complex called the anaphase-promoting complex (APC) or cyclosome. The APC is composed of more than 10 constitutive subunits and associates with additional regulatory factors in mitosis and during the G1 phase of the cell cycle. At the metaphase-anaphase transition the APC ubiquitinates proteins such as Pds1 in budding yeast and Cut2 in fission yeast whose subsequent degradation by the 26S proteasome is essential for the initiation of sister chromatid separation. Later in anaphase and telophase the APC promotes the inactivation of the mitotic cyclin-dependent protein kinase 1 by ubiquitinating its activating subunit cyclin B. The APC also mediates the ubiquitin-dependent proteolysis of several other mitotic regulators, including other protein kinases, APC activators, spindle-associated proteins, and inhibitors of DNA replication.

The disappearance of cyclin B at the end of mitosis is regulated spatially in Drosophila cells

We have followed the behaviour of a cyclin B-green fluorescent protein (GFP) fusion protein in living Drosophila embryos in order to study how the localization and destruction of cyclin B is regulated in space and time. We show that the fusion protein accumulates at centrosomes in interphase, in the nucleus in prophase, on the mitotic spindle in prometaphase and on the microtubules that overlap in the middle of the spindle in metaphase. In cellularized embryos, toward the end of metaphase, the spindle-associated cyclin B-GFP disappears from the spindle in a wave that starts at the spindle poles and spreads to the spindle equator; when the cyclin B-GFP on the spindle is almost undetectable, the chromosomes enter anaphase, and any remaining cytoplasmic cyclin B-GFP then disappears over the next few minutes. The endogenous cyclin B protein appears to behave in a similar manner. These findings suggest that the inactivation of cyclin B is regulated spatially in Drosophila cells. We show that the anaphase-promoting complex/cyclosome (APC/C) specifically interacts with microtubules in embryo extracts, but it is not confined to the spindle in mitosis, suggesting that the spatially regulated disappearance of cyclin B may reflect the spatially regulated activation of the APC/C.

Centromere proteins and chromosome inheritance: a complex affair

Centromeres and the associated kinetochores are involved in essential aspects of chromosome transmission. Recent advances have included the identification and understanding of proteins that have a pivotal role in centromere structure, kinetochore formation, and the coordination of chromosome inheritance with the cell cycle in several organisms. A picture is beginning to emerge of the centromere-kinetechore as a complex and dynamic structure with conservation of function at the protein level across diverse species.

Inhibitory phosphorylation of the APC regulator Hct1 is controlled by the kinase Cdc28 and the phosphatase Cdc14

BACKGROUND

Exit from mitosis requires inactivation of mitotic cyclin-dependent kinases (CDKs). A key mechanism of CDK inactivation is ubiquitin-mediated cyclin proteolysis, which is triggered by the late mitotic activation of a ubiquitin ligase known as the anaphase-promoting complex (APC). Activation of the APC requires its association with substoichiometric activating subunits termed Cdc20 and Hct1 (also known as Cdh1). Here, we explore the molecular function and regulation of the APC regulatory subunit Hct1 in Saccharomyces cerevisiae.

RESULTS

Recombinant Hct1 activated the cyclin-ubiquitin ligase activity of APC isolated from multiple cell cycle stages. APC isolated from cells arrested in G1, or in late mitosis due to the cdc14-1 mutation, was more responsive to Hct1 than APC isolated from other stages. We found that Hct1 was phosphorylated in vivo at multiple CDK consensus sites during cell cycle stages when activity of the cyclin-dependent kinase Cdc28 is high and APC activity is low. Purified Hct1 was phosphorylated in vitro at these sites by purified Cdc28-cyclin complexes, and phosphorylation abolished the ability of Hct1 to activate the APC in vitro. The phosphatase Cdc14, which is known to be required for APC activation in vivo, was able to reverse the effects of Cdc28 by catalyzing Hct1 dephosphorylation and activation.

CONCLUSIONS

We conclude that Hct1 phosphorylation is a key regulatory mechanism in the control of cyclin destruction. Phosphorylation of Hct1 provides a mechanism by which Cdc28 blocks its own inactivation during S phase and early mitosis. Following anaphase, dephosphorylation of Hct1 by Cdc14 may help initiate cyclin destruction.

Separating sister chromatids

Loss of cohesion between sister chromatids triggers their segregation during anaphase. Recent work has identified both a cohesin complex that holds sisters together and a sister-separating protein, separin, that destroys cohesion. Separins are bound by inhibitory proteins whose proteolysis at the metaphase-anaphase transition is mediated by the anaphase-promoting complex and its activator protein CDC20 (APCCDC20). When chromosomes are misaligned, a surveillance mechanism (checkpoint) blocks sister separation by inhibiting APCCDC20. Defects in this apparatus are implicated in causing aneuploidy in human cells.

Kinetochores and the checkpoint mechanism that monitors for defects in the chromosome segregation machinery

Whether we consider the division of the simplest unicellular organisms into two daughter cells or the generation of haploid gametes by the most complex eukaryotes, no two processes secure the continuance of life more than the proper replication and segregation of the genetic material. The cell cycle, marked in part by the periodic rise and fall of cyclin-dependent kinase (CDK) activities, is the means by which these two processes are separated. DNA damage and mistakes in chromosome segregation are costly, so nature has further devised elaborate checkpoint mechanisms that halt cell cycle progression, allowing time for repairs or corrections. In this article, we review the mitotic checkpoint mechanism that responds to defects in the chromosome segregation machinery and arrests cells in mitosis prior to anaphase onset. At opposite ends of this pathway are the kinetochore, where many checkpoint proteins reside, and the anaphase-promoting complex (APC), the metaphase-to-interphase transition regulator. Throughout this review we focus on budding yeast but reference parallel processes found in other organisms.

The spindle checkpoint

Prior to sister-chromatid separation, the spindle checkpoint inhibits cell-cycle progression in response to a signal generated by mitotic spindle damage or by chromosomes that have not attached to microtubules. Recent work has shown that the spindle checkpoint inhibits cell-cycle progression by direct binding of components of the spindle checkpoint pathway to components of a specialized ubiquitin-conjugating system that is responsible for triggering sister-chromatid separation.

Mammalian centromeres: DNA sequence, protein composition, and role in cell cycle progression

The centromere is a specialized region of the eukaryotic chromosome that is responsible for directing chromosome movements in mitosis and for coordinating the progression of mitotic events at the crucial transition between metaphase and anaphase. In this review, we will focus on recent advances in the understanding of centromere composition at the protein and DNA level and of the role of centromeres in sister-chromatid cohesion and mitotic checkpoint control.

Fission yeast bub1 is a mitotic centromere protein essential for the spindle checkpoint and the preservation of correct ploidy through mitosis

The spindle checkpoint ensures proper chromosome segregation by delaying anaphase until all chromosomes are correctly attached to the mitotic spindle. We investigated the role of the fission yeast bub1 gene in spindle checkpoint function and in unperturbed mitoses. We find that bub1(+) is essential for the fission yeast spindle checkpoint response to spindle damage and to defects in centromere function. Activation of the checkpoint results in the recruitment of Bub1 to centromeres and a delay in the completion of mitosis. We show that Bub1 also has a crucial role in normal, unperturbed mitoses. Loss of bub1 function causes chromosomes to lag on the anaphase spindle and an increased frequency of chromosome loss. Such genomic instability is even more dramatic in Deltabub1 diploids, leading to massive chromosome missegregation events and loss of the diploid state, demonstrating that bub1(+ )function is essential to maintain correct ploidy through mitosis. As in larger eukaryotes, Bub1 is recruited to kinetochores during the early stages of mitosis. However, unlike its vertebrate counterpart, a pool of Bub1 remains centromere-associated at metaphase and even until telophase. We discuss the possibility of a role for the Bub1 kinase after the metaphase-anaphase transition.

SCF and APC: the Yin and Yang of cell cycle regulated proteolysis

Progression through the cell cycle requires the activity of two ubiquitination complexes, the Skp1-cullin-F-box-protein complex (SCF) and the anaphase-promoting complex/cyclosome (APC). Observations in the past year have revealed unexpected similarities between the SCF and the APC and have allowed detailed insight into the regulation of their activities. Both complexes are now known to exist in different forms that target different substrates for ubiquitin-dependent proteolysis.

Activation of the human anaphase-promoting complex by proteins of the CDC20/Fizzy family

The initiation of anaphase and exit from mitosis depend on the activation of the cyclosome/anaphase-promoting complex (APC) that ubiquitinates regulatory proteins such as anaphase inhibitors and mitotic cyclins [1-4]. Genetic experiments have demonstrated that two related WD40-repeat proteins--called Cdc20p and Hct1p/Cdh1p in budding yeast and Fizzy and Fizzy-related in Drosophila--are essential for APC--dependent proteolysis [5-11]. Human orthologs of these proteins--hCDC20/p55CDC [12] and hCDH1--have recently been found to associate with APC in a cell-cycle-dependent manner [13,14]. Here, we show that the amount of hCDC20 and hCDH1 bound to APC correlates with a high ubiquitination activity of APC and that binding of recombinant hCDC20 and hCDH1 can activate APC in vitro. Our results suggest that the association between hCDH1 and APC is regulated by post-translational mechanisms, whereas the amount of hCDC20 bound to APC may in addition be controlled by hCDC20 synthesis and destruction [15]. The temporally distinct association of hCDC20 and hCDH1 with APC suggests that these proteins are, respectively, mitosis-specific and G1-specific activating subunits of APC.

Spindle checkpoint protein Xmad1 recruits Xmad2 to unattached kinetochores

The spindle checkpoint prevents the metaphase to anaphase transition in cells containing defects in the mitotic spindle or in chromosome attachment to the spindle. When the checkpoint protein Xmad2 is depleted from Xenopus egg extracts, adding Xmad2 to its endogenous concentration fails to restore the checkpoint, suggesting that other checkpoint component(s) were depleted from the extract through their association with Xmad2. Mass spectrometry provided peptide sequences from an 85-kD protein that coimmunoprecipitates with Xmad2 from egg extracts. This information was used to clone XMAD1, which encodes a homologue of the budding yeast (Saccharomyces cerevisiae) checkpoint protein Mad1. Xmad1 is essential for establishing and maintaining the spindle checkpoint in egg extracts. Like Xmad2, Xmad1 localizes to the nuclear envelope and the nucleus during interphase, and to those kinetochores that are not bound to spindle microtubules during mitosis. Adding an anti-Xmad1 antibody to egg extracts inactivates the checkpoint and prevents Xmad2 from localizing to unbound kinetochores. In the presence of excess Xmad2, neither chromosomes nor Xmad1 are required to activate the spindle checkpoint, suggesting that the physiological role of Xmad1 is to recruit Xmad2 to kinetochores that have not bound microtubules.

Characterization of the kinetochore binding domain of CENP-E reveals interactions with the kinetochore proteins CENP-F and hBUBR1

We have identified a 350-amino acid domain in the kinetochore motor CENP-E that specifies kinetochore binding in mitosis but not during interphase. The kinetochore binding domain was used in a yeast two-hybrid screen to isolate interacting proteins that included the kinetochore proteins CENP-E, CENP-F, and hBUBR1, a BUB1-related kinase that was found to be mutated in some colorectal carcinomas (Cahill, D.P., C. Lengauer, J. Yu, G.J. Riggins, J.K. Wilson, S.D. Markowitz, K.W. Kinzler, and B. Vogelstein. 1998. Nature. 392:300-303). CENP-F, hBUBR1, and CENP-E assembled onto kinetochores in sequential order during late stages of the cell cycle. These proteins therefore define discrete steps along the kinetochore assembly pathway. Kinetochores of unaligned chromosome exhibited stronger hBUBR1 and CENP-E staining than those of aligned chromosomes. CENP-E and hBUBR1 remain colocalized at kinetochores until mid-anaphase when hBUBR1 localized to portions of the spindle midzone that did not overlap with CENP-E. As CENP-E and hBUBR1 can coimmunoprecipitate with each other from HeLa cells, they may function as a motor-kinase complex at kinetochores. However, the complex distribution pattern of hBUBR1 suggests that it may regulate multiple functions that include the kinetochore and the spindle midzone.

Mad2 transiently associates with an APC/p55Cdc complex during mitosis

Activation of the mitotic checkpoint pathway in response to mitotic spindle damage in eukaryotic cells delays the exit from mitosis in an attempt to prevent chromosome missegregation. One component of this pathway, hsMad2, has been shown in mammalian cells to physically associate with components of a ubiquitin ligase activity (termed the anaphase promoting complex or APC) when the checkpoint is activated, thereby preventing the degradation of inhibitors of the mitotic exit machinery. In the present report, we demonstrate that the inhibitory association between Mad2 and the APC component Cdc27 also takes place transiently during the early stages of a normal mitosis and is lost before mitotic exit. We also show that Mad2 associates with the APC regulatory protein p55Cdc in mammalian cells as has been reported in yeast. In contrast, however, this complex is present only in nocodazole-arrested or early mitotic cells and is associated with the APC as a Mad2/p55Cdc/Cdc27 ternary complex. Evidence for a Mad2/Cdc27 complex that forms independent of p55Cdc also is presented. These results suggest a model for the regulation of the APC by Mad2 and may explain how the spindle assembly checkpoint apparatus controls the timing of mitosis under normal growth conditions.

The vertebrate cell kinetochore and its roles during mitosis

A replicated chromosome possesses two discrete, complex, dynamic, macromolecular assemblies, known as kinetochores, that are positioned on opposite sides of the primary constriction of the chromosome. Here, the authors review how kinetochores control chromosome segregation during mitosis in vertebrates. They attach the chromosome to the opposing spindle poles by trapping the dynamic plus-ends of microtubules growing from the poles. They then produce much of the force for chromosome poleward motion, regulate when this force is applied, and act as a site for microtubule assembly and disassembly. Finally, they control the metaphase-anaphase transition by inhibiting chromatid separation until the chromatids are properly attached.