Abstract SY14-02: From disease genetics to diagnostics to functional biology: High-throughput proteomics and protein interaction network building guides discovery of novel disease genes in ciliopathies, cystic disease, and cancer

With the advent of deep sequencing, genome-wide association studies, and rapid advances in human genetics, extensive genomic data linking human gene variants to specific diseases, including cancer, neurological disease, and metabolic syndromes, is rapidly accumulating. These disease associations, especially mutations, provide a definitive link between protein function and disease state, and can provide critical diagnostics for the disease. However, in many cases, the disease links are unclear and fail to 1) connect individual mutations to specific biological pathways, 2) provide functional hypotheses for understanding disease mechanism, or 3) identify targets for therapeutic intervention. Bioinformatic analyses can provide key clues to specific experimental tests, but a more extensible methodology is required to build from genomic hits to functional links. We have developed and implemented automated, a recombination-driven “tandem-affinity” proteomic probes, called the g-LAP (Gateway-Location and Affinity Purification) system. This approach is amenable to high-throughput and allows us to purify proteins associated with disease-related “bait” proteins and thus identify protein complexes and networks with high confidence. We have also established similarly high-throughput tools for studying protein localization, protein-protein interaction both in vivo and in vitro, and for testing interactions among biochemical complexes and activities.

To date, we have tagged over 100 new human genetic disease genes, which have lent insight into a number of important pathways, notably in ciliary diseases or ciliopathies, neurological disease, and for novel tumor suppressors. Within the ciliopathies, the identification of assembly and regulatory complexes has provided new insight into the cell biology and signaling pathways associated with primary cilia. In the case of Bardet-Biedl syndrome, patients present with retinal degeneration, olfactory and hearing loss, polydactyly, neurological problems, and obesity linked to hyperphagia. Purifying proteins associated with LAP-tagged BBS proteins, we discovered seven of fourteen known BBS disease genes were present in the highly conserved BBSome complex, important to activate Rab8 vesicles for ciliary membrane formation1. In the tubby/Tubby-like (TULP) family of proteins, we recently purified the Hedgehog regulatory TULP3 protein and found the protein forms a complex together with the complete, six-component intraflagellar transport A (IFT-A) complex, which function together at the base of the cilia to regulate entry of G-protein coupled receptors into cilia2.

A link between cilia and cancer was demonstrated by the link between cilia, Hedgehog signaling, and specific receptor tyrosine kinases, including the PDGF receptor α. Among cancers and proliferative diseases, specific ciliary pathways function in the kidney to limit polycystic kidney disease and renal cell carcinoma. Notably, the Von Hippel-Lindau (VHL) tumor suppressor protein and autosomal dominant polycystic kidney disease genes PKD1 and PKD2 proteins are present in primary cilia. Among ciliopathies, several may control the polarity and function of ductal structures of the kidney, but also of the pancreas, and liver. Do ciliary genes play an important role in cancer progression in these tissues? The molecular mechanism suppressing formation of cysts is poorly understood, but may include changes in the apical-basal polarity of cells, cell cycle, or secretory behavior of cells. Specific changes in ciliary signaling mechanisms may be important for cyst suppression.

Among cystic regulators, nephronophthisis (NPHP), Joubert (JBTS), and Meckel-Gruber (MKS) syndromes are autosomal-recessive ciliopathies presenting with cystic kidneys, retinal degeneration, and cerebellar/neural tube malformation. Nephronophthisis (NPHP) is a rare pediatric autosomal cystic kidney disease caused by mutations of nine known genes, NPHP1-9. NPHP patients also show cerebellar defects, situs inversus, and retinitis pigmentosa (RP). The pediatric Joubert (JBST) and Meckel-Gruber (MKS) syndromes also present nephronophthisis and RP, but are characterized by cerebellar malformation or severe neural tube defects. There are nine JBST and seven MKS genes, with considerable allelism among NPHP, JBST, and MKS genes. We used high-confidence proteomics to identify over 200 interactors copurifying with nine NPHP/JBTS/MKS proteins and discovered three connected modules functioning at the apical cell surface and ciliary “transition zone” (NPHP1/4/8), at the basal body (NPHP5/6), or in a novel Hedgehog pathway (Mks1/Mks6/Tectonic). We employed assays for ciliogenesis and cell polarity defects using 3-D renal cultures to demonstrate that renal cystic disease links primarily to apical organization defects, whereas ciliary and Hedgehog pathway defects appear central to neural malformations. Using 38 interactors as candidates, linkage and sequencing analysis of 250 NPHP patients identified TCTN2 as a new Joubert syndrome disease gene, plus two other novel NPHP human. Our Tctn2 mouse knockout showed neural tube and Hedgehog signaling defects. Our studies demonstrate the power of linking proteomic networks and human genetics to uncover critical disease pathways.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr SY14-02. doi:10.1158/1538-7445.AM2011-SY14-02

A novel acetylation of β-tubulin by San modulates microtubule polymerization via down-regulating tubulin incorporation

We report that San, an acetyltransferase required for sister chromatid cohesion, also acetylates β-tubulin at lysine 252. The acetylation happens only on free tubulin heterodimers, and it delays the incorporation of modified tubulins into microtubules in vivo.

Dynamic instability is a critical property of microtubules (MTs). By regulating the rate of tubulin polymerization and depolymerization, cells organize the MT cytoskeleton to accommodate their specific functions. Among many processes, posttranslational modifications of tubulin are implicated in regulating MT functions. Here we report a novel tubulin acetylation catalyzed by acetyltransferase San at lysine 252 (K252) of β-tubulin. This acetylation, which is also detected in vivo, is added to soluble tubulin heterodimers but not tubulins in MTs. The acetylation-mimicking K252A/Q mutants were incorporated into the MT cytoskeleton in HeLa cells without causing any obvious MT defect. However, after cold-induced catastrophe, MT regrowth is accelerated in San-siRNA cells while the incorporation of acetylation-mimicking mutant tubulins is severely impeded. K252 of β-tubulin localizes at the interface of α-/β-tubulins and interacts with the phosphate group of the α-tubulin-bound GTP. We propose that the acetylation slows down tubulin incorporation into MTs by neutralizing the positive charge on K252 and allowing tubulin heterodimers to adopt a conformation that disfavors tubulin incorporation.

APC/C(Cdc20) targets E2F1 for degradation in prometaphase

The mechanisms that control E2F-1 activity are complex. We previously showed that Chk1 and Chk2 are required for E2F1 stabilization and p73 target gene induction following DNA damage. To gain further insight into the processes regulating E2F1 protein stability, we focused our investigation on the mechanisms responsible for regulating E2F1 turnover. Here we show that E2F1 is a substrate of the anaphase promoting complex or cyclosome (APC/C), a ubiquitin ligase that plays an important role in cell cycle progression. Ectopic expression of the APC/C activators Cdh1 and Cdc20 reduced the levels of co-expressed E2F-1 protein. Co-expression of DP1 with E2F1 blocked APC/C-induced E2F1 degradation, suggesting that the E2F1/DP1 heterodimer is protected from APC/C regulation. Following Cdc20 knockdown, E2F1 levels increased and remained stable in extracts over a time course, indicating that APC/C(Cdc20) is a primary regulator of E2F1 stability in vivo. Moreover, cell synchronization experiments showed that siRNA directed against Cdc20 induced an accumulation of E2F1 protein in prometaphase cells. These data suggest that APC/C(Cdc20) specifically targets E2F1 for degradation in early mitosis and reveal a novel mechanism for limiting free E2F1 levels in cells, failure of which may compromise cell survival and/or homeostasis.

TULP3 bridges the IFT-A complex and membrane phosphoinositides to promote trafficking of G protein-coupled receptors into primary cilia

Primary cilia function as a sensory signaling compartment in processes ranging from mammalian Hedgehog signaling to neuronal control of obesity. Intraflagellar transport (IFT) is an ancient, conserved mechanism required to assemble cilia and for trafficking within cilia. The link between IFT, sensory signaling, and obesity is not clearly defined, but some novel monogenic obesity disorders may be linked to ciliary defects. The tubby mouse, which presents with adult-onset obesity, arises from mutation in the Tub gene. The tubby-like proteins comprise a related family of poorly understood proteins with roles in neural development and function. We find that specific Tubby family proteins, notably Tubby-like protein 3 (TULP3), bind to the IFT-A complex. IFT-A is linked to retrograde ciliary transport, but, surprisingly, we find that the IFT-A complex has a second role directing ciliary entry of TULP3. TULP3 and IFT-A, in turn, promote trafficking of a subset of G protein-coupled receptors (GPCRs), but not Smoothened, to cilia. Both IFT-A and membrane phosphoinositide-binding properties of TULP3 are required for ciliary GPCR localization. TULP3 and IFT-A proteins both negatively regulate Hedgehog signaling in the mouse embryo, and the TULP3-IFT-A interaction suggests how these proteins cooperate during neural tube patterning.

A chemosensitization screen identifies TP53RK, a kinase that restrains apoptosis after mitotic stress

Taxanes are very effective at causing mitotic arrest; however, there is variability among cancer cells in the apoptotic response to mitotic arrest. The variability in clinical efficacy of taxane-based therapy is likely a reflection of this variability in apoptotic response, thus elucidation of the molecular mechanism of the apoptotic response to mitotic stress could lead to improved clinical strategies. To identify genes whose expression influences the rate and extent of apoptosis after mitotic arrest, we screened a kinase-enriched small interfering RNA library for effects on caspase activation in response to maximally effective doses of paclitaxel, a PLK1 inhibitor, or cisplatin. Small interfering RNA oligonucleotides directed against an atypical protein kinase, TP53RK, caused the greatest increase in caspase-3/7 activation in response to antimitotic agents. Time-lapse microscopy revealed that cells entered mitosis with normal kinetics, but died after entry into mitosis in the presence of paclitaxel more rapidly when TP53RK was depleted. Because expression levels of TP53RK vary in cancers, TP53RK levels could provide a molecular marker to predict response to antimitotic agents. TP53RK inhibition may also sensitize cancers to taxanes.

A specific form of phospho protein phosphatase 2 regulates anaphase-promoting complex/cyclosome association with spindle poles

In early mitosis, the END (Emi1/NuMA/Dynein-dynactin) network anchors the anaphase-promoting complex/cyclosome (APC/C) to the mitotic spindle and poles. Spindle anchoring restricts APC/C activity, thereby limiting the destruction of spindle-associated cyclin B and ensuring maintenance of spindle integrity. Emi1 binds directly to hypophosphorylated APC/C, linking the APC/C to the spindle via NuMA. However, whether the phosphorylation state of the APC/C is important for its association with the spindle and what kinases and phosphatases are necessary for regulating this event remain unknown. Here, we describe the regulation of APC/C-mitotic spindle pole association by phosphorylation. We find that only hypophosphorylated APC/C associates with microtubule asters, suggesting that phosphatases are important. Indeed, a specific form of PPP2 (CA/R1A/R2B) binds APC/C, and PPP2 activity is necessary for Cdc27 dephosphorylation. Screening by RNA interference, we find that inactivation of CA, R1A, or R2B leads to delocalization of APC/C from spindle poles, early mitotic spindle defects, a failure to congress chromosomes, and decreased levels of cyclin B on the spindle. Consistently, inhibition of cyclin B/Cdk1 activity increased APC/C binding to microtubules. Thus, cyclin B/Cdk1 and PPP2 regulate the dynamic association of APC/C with spindle poles in early mitosis, a step necessary for proper spindle formation.

Individuals with mutations in XPNPEP3, which encodes a mitochondrial protein, develop a nephronophthisis-like nephropathy

The autosomal recessive kidney disease nephronophthisis (NPHP) constitutes the most frequent genetic cause of terminal renal failure in the first 3 decades of life. Ten causative genes (NPHP1-NPHP9 and NPHP11), whose products localize to the primary cilia-centrosome complex, support the unifying concept that cystic kidney diseases are "ciliopathies". Using genome-wide homozygosity mapping, we report here what we believe to be a new locus (NPHP-like 1 [NPHPL1]) for an NPHP-like nephropathy. In 2 families with an NPHP-like phenotype, we detected homozygous frameshift and splice-site mutations, respectively, in the X-prolyl aminopeptidase 3 (XPNPEP3) gene. In contrast to all known NPHP proteins, XPNPEP3 localizes to mitochondria of renal cells. However, in vivo analyses also revealed a likely cilia-related function; suppression of zebrafish xpnpep3 phenocopied the developmental phenotypes of ciliopathy morphants, and this effect was rescued by human XPNPEP3 that was devoid of a mitochondrial localization signal. Consistent with a role for XPNPEP3 in ciliary function, several ciliary cystogenic proteins were found to be XPNPEP3 substrates, for which resistance to N-terminal proline cleavage resulted in attenuated protein function in vivo in zebrafish. Our data highlight an emerging link between mitochondria and ciliary dysfunction, and suggest that further understanding the enzymatic activity and substrates of XPNPEP3 will illuminate novel cystogenic pathways.

High-throughput generation of tagged stable cell lines for proteomic analysis

We present an optimized system for rapid generation of localization and affinity purification-tagged mammalian stable cell lines that facilitates complex purification and interacting protein identification. The improved components of this method, including the flexibility of inducible expression, circumvent issues associated with toxicity, clonal selection, sample yields and time to data acquisition. We have applied this method to the study of cell-cycle regulators and novel microtubule-associated proteins.

A BBSome subunit links ciliogenesis, microtubule stability, and acetylation

Primary cilium dysfunction affects the development and homeostasis of many organs in Bardet-Biedl syndrome (BBS). We recently showed that seven highly conserved BBS proteins form a stable complex, the BBSome, that functions in membrane trafficking to and inside the primary cilium. We have now discovered a BBSome subunit that we named BBIP10. Similar to other BBSome subunits, BBIP10 localizes to the primary cilium, BBIP10 is present exclusively in ciliated organisms, and depletion of BBIP10 yields characteristic BBS phenotypes in zebrafish. Unexpectedly, BBIP10 is required for cytoplasmic microtubule polymerization and acetylation, two functions not shared with any other BBSome subunits. Strikingly, inhibition of the tubulin deacetylase HDAC6 restores microtubule acetylation in BBIP10-depleted cells, and BBIP10 physically interacts with HDAC6. BBSome-bound BBIP10 may therefore function to couple acetylation of axonemal microtubules and ciliary membrane growth.

Biochemical analysis of the Anaphase Promoting Complex: activities of E2 enzymes and substrate competitive (pseudosubstrate) inhibitors

The Anaphase Promoting Complex (APC) ubiquitin ligase is critical for multiple processes including cell cycle, development, meiosis, and senescence. The importance of regulation of the APC by substrate competitive (pseudosubstrate) inhibitors, such as Emi1 and BubR1, has recently been demonstrated. Substrate competitive inhibitors typically bind to enzymes via the same site as substrates, but by having any combination of increased enzyme affinity and low turnover numbers, are able to "clog" the ability of the enzyme to bind and turnover substrates. For the APC, these pseudosubstrates can both position and block the APC and have been well validated as critical regulators for the APC enzymes.We have found that the substrate competitive mechanism of inhibition is sensitive to the E2 activity driving APC catalyzed ubiquitination events. This chapter provides detailed protocols for multiple in vitro ubiquitination assays of increasing complexity and the use of pseudosubstrate inhibitors in these assays. These assays are instrumental in examining the use of E2 enzymes by the APC and the intimate relationship this has with pseudosubstrate inhibition.

The hunt for cyclin

It is 25 years since Tim Hunt discovered cyclin, the oscillating protein that drives activation of cyclin-dependent kinases and entry into mitosis (Evans et al., 1983).

Cdc2 and Mos regulate Emi2 stability to promote the meiosis I-meiosis II transition

The transition of oocytes from meiosis I (MI) to meiosis II (MII) requires partial cyclin B degradation to allow MI exit without S phase entry. Rapid reaccumulation of cyclin B allows direct progression into MII, producing a cytostatic factor (CSF)-arrested egg. It has been reported that dampened translation of the anaphase-promoting complex (APC) inhibitor Emi2 at MI allows partial APC activation and MI exit. We have detected active Emi2 translation at MI and show that Emi2 levels in MI are mainly controlled by regulated degradation. Emi2 degradation in MI depends not on Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), but on Cdc2-mediated phosphorylation of multiple sites within Emi2. As in MII, this phosphorylation is antagonized by Mos-mediated recruitment of PP2A to Emi2. Higher Cdc2 kinase activity in MI than MII allows sufficient Emi2 phosphorylation to destabilize Emi2 in MI. At MI anaphase, APC-mediated degradation of cyclin B decreases Cdc2 activity, enabling Cdc2-mediated Emi2 phosphorylation to be successfully antagonized by Mos-mediated PP2A recruitment. These data suggest a model of APC autoinhibition mediated by stabilization of Emi2; Emi2 proteins accumulate at MI exit and inhibit APC activity sufficiently to prevent complete degradation of cyclin B, allowing MI exit while preventing interphase before MII entry.

Cyclin E overexpression impairs progression through mitosis by inhibiting APC(Cdh1)

Overexpression of cyclin E, an activator of cyclin-dependent kinase 2, has been linked to human cancer. In cell culture models, the forced expression of cyclin E leads to aneuploidy and polyploidy, which is consistent with a direct role of cyclin E overexpression in tumorigenesis. In this study, we show that the overexpression of cyclin E has a direct effect on progression through the latter stages of mitotic prometaphase before the complete alignment of chromosomes at the metaphase plate. In some cases, such cells fail to divide chromosomes, resulting in polyploidy. In others, cells proceed to anaphase without the complete alignment of chromosomes. These phenotypes can be explained by an ability of overexpressed cyclin E to inhibit residual anaphase-promoting complex (APC^Cdh1) activity that persists as cells progress up to and through the early stages of mitosis, resulting in the abnormal accumulation of APC^Cdh1 substrates as cells enter mitosis. We further show that the accumulation of securin and cyclin B1 can account for the cyclin E–mediated mitotic phenotype.

A bacterial effector targets Mad2L2, an APC inhibitor, to modulate host cell cycling

The gut epithelium self-renews every several days, providing an important innate defense system that limits bacterial colonization. Nevertheless, many bacterial pathogens, including Shigella, efficiently colonize the intestinal epithelium. Here, we show that the Shigella effector IpaB, when delivered into epithelial cells, causes cell-cycle arrest by targeting Mad2L2, an anaphase-promoting complex/cyclosome (APC) inhibitor. Cyclin B1 ubiquitination assays revealed that APC undergoes unscheduled activation due to IpaB interaction with the APC inhibitor Mad2L2. Synchronized HeLa cells infected with Shigella failed to accumulate Cyclin B1, Cdc20, and Plk1, causing cell-cycle arrest at the G2/M phase in an IpaB/Mad2L2-dependent manner. IpaB/Mad2L2-dependent cell-cycle arrest by Shigella infection was also demonstrated in rabbit intestinal crypt progenitors, and the IpaB-mediated arrest contributed to efficient colonization of the host cells. These results strongly indicate that Shigella employ special tactics to influence epithelial renewal in order to promote bacterial colonization of intestinal epithelium.

Control of Emi2 activity and stability through Mos-mediated recruitment of PP2A

Before fertilization, vertebrate eggs are arrested in meiosis II by cytostatic factor (CSF), which holds the anaphase-promoting complex (APC) in an inactive state. It was recently reported that Mos, an integral component of CSF, acts in part by promoting the Rsk-mediated phosphorylation of the APC inhibitor Emi2/Erp1. We report here that Rsk phosphorylation of Emi2 promotes its interaction with the protein phosphatase PP2A. Emi2 residues adjacent to the Rsk phosphorylation site were important for PP2A binding. An Emi2 mutant that retained Rsk phosphorylation but lacked PP2A binding could not be modulated by Mos. PP2A bound to Emi2 acted on two distinct clusters of sites phosphorylated by Cdc2, one responsible for modulating its stability during CSF arrest and one that controls binding to the APC. These findings provide a molecular mechanism for Mos action in promoting CSF arrest and also define an unusual mechanism, whereby protein phosphorylation recruits a phosphatase for dephosphorylation of distinct sites phosphorylated by another kinase.

Loss of Emi1-dependent anaphase-promoting complex/cyclosome inhibition deregulates E2F target expression and elicits DNA damage-induced senescence

Expression of the anaphase-promoting complex/cyclosome (APC/C) inhibitor Emi1 is required for the accumulation of APC/C substrates crucial for DNA synthesis and mitotic entry. We show that in vivo Emi1 expression correlates with the proliferative status of the cellular compartment and that cells lacking Emi1 undergo cellular senescence. Emi1 depletion leads to strong decreases in E2F target mRNA and APC/C substrate protein abundances. However, cyclin E mRNA and cyclin E protein levels and associated kinase activities are increased. Cells lacking Emi1 undergo DNA damage, likely explained by replication stress upon deregulated cyclin E- and A-associated kinase activities. Inhibition of ATM kinase prevents induction of senescence, implying that senescence is a consequence of DNA damage. Surprisingly, no senescence or no extensive amount of senescence is evident upon depletion of the Emi1-stabilizing factor Evi5 or Pin1, respectively. Our data suggest that maintenance of a protein stabilization/mRNA expression positive-feedback circuit fueled by Emi1 is required for accurate cell cycle progression, maintenance of DNA integrity, and prevention of cellular senescence.

A core complex of BBS proteins cooperates with the GTPase Rab8 to promote ciliary membrane biogenesis

Primary cilium dysfunction underlies the pathogenesis of Bardet-Biedl syndrome (BBS), a genetic disorder whose symptoms include obesity, retinal degeneration, and nephropathy. However, despite the identification of 12 BBS genes, the molecular basis of BBS remains elusive. Here we identify a complex composed of seven highly conserved BBS proteins. This complex, the BBSome, localizes to nonmembranous centriolar satellites in the cytoplasm but also to the membrane of the cilium. Interestingly, the BBSome is required for ciliogenesis but is dispensable for centriolar satellite function. This ciliogenic function is mediated in part by the Rab8 GDP/GTP exchange factor, which localizes to the basal body and contacts the BBSome. Strikingly, Rab8(GTP) enters the primary cilium and promotes extension of the ciliary membrane. Conversely, preventing Rab8(GTP) production blocks ciliation in cells and yields characteristic BBS phenotypes in zebrafish. Our data reveal that BBS may be caused by defects in vesicular transport to the cilium.

The END network couples spindle pole assembly to inhibition of the anaphase-promoting complex/cyclosome in early mitosis

Cyclin-dependent kinase 1 (Cdk1) initiates mitosis and later activates the anaphase-promoting complex/cyclosome (APC/C) to destroy cyclins. Kinetochore-derived checkpoint signaling delays APC/C-dependent cyclin B destruction, and checkpoint-independent mechanisms cooperate to limit APC/C activity when kinetochores lack checkpoint components in early mitosis. The APC/C and cyclin B localize to the spindle and poles, but the significance and regulation of these populations remain unclear. Here we describe a critical spindle pole-associated mechanism, called the END (Emi1/NuMA/dynein-dynactin) network, that spatially restricts APC/C activity in early mitosis. The APC/C inhibitor Emi1 binds the spindle-organizing NuMA/dynein-dynactin complex to anchor and inhibit the APC/C at spindle poles, and thereby limits destruction of spindle-associated cyclin B. Cyclin B/Cdk1 activity recruits the END network and establishes a positive feedback loop to stabilize spindle-associated cyclin B critical for spindle assembly. The organization of the APC/C on the spindle also provides a framework for understanding microtubule-dependent organization of protein destruction.

A role for Cdc2- and PP2A-mediated regulation of Emi2 in the maintenance of CSF arrest

BACKGROUND

Vertebrate oocytes are arrested in metaphase II of meiosis prior to fertilization by cytostatic factor (CSF). CSF enforces a cell-cycle arrest by inhibiting the anaphase-promoting complex (APC), an E3 ubiquitin ligase that targets Cyclin B for degradation. Although Cyclin B synthesis is ongoing during CSF arrest, constant Cyclin B levels are maintained. To achieve this, oocytes allow continuous slow Cyclin B degradation, without eliminating the bulk of Cyclin B, which would induce release from CSF arrest. However, the mechanism that controls this continuous degradation is not understood.

RESULTS

We report here the molecular details of a negative feedback loop wherein Cyclin B promotes its own destruction through Cdc2/Cyclin B-mediated phosphorylation and inhibition of the APC inhibitor Emi2. Emi2 bound to the core APC, and this binding was disrupted by Cdc2/Cyclin B, without affecting Emi2 protein stability. Cdc2-mediated phosphorylation of Emi2 was antagonized by PP2A, which could bind to Emi2 and promote Emi2-APC interactions.

CONCLUSIONS

Constant Cyclin B levels are maintained during a CSF arrest through the regulation of Emi2 activity. A balance between Cdc2 and PP2A controls Emi2 phosphorylation, which in turn controls the ability of Emi2 to bind to and inhibit the APC. This balance allows proper maintenance of Cyclin B levels and Cdc2 kinase activity during CSF arrest.