A nonproteolytic function of the proteasome is required for the dissociation of Cdc2 and cyclin B at the end of M phase

Proteasomal subunit depletions differentially affect germline integrity in C. elegans

The 26S proteasome is a multi-subunit protein complex that is canonically known for its ability to degrade proteins in cells and maintain protein homeostasis. Non-canonical or non-proteolytic roles of proteasomal subunits exist but remain less well studied. We provide characterization of germline-specific functions of different 19S proteasome regulatory particle (RP) subunits in C. elegans using RNAi specifically from the L4 stage and through generation of endogenously tagged 19S RP lid subunit strains. We show functions for the 19S RP in regulation of proliferation and maintenance of integrity of mitotic zone nuclei, in polymerization of the synaptonemal complex (SC) onto meiotic chromosomes and in the timing of SC subunit redistribution to the short arm of the bivalent, and in turnover of XND-1 proteins at late pachytene. Furthermore, we report that certain 19S RP subunits are required for proper germ line localization of WEE-1.3, a major meiotic kinase. Additionally, endogenous fluorescent labeling revealed that the two isoforms of the essential 19S RP proteasome subunit RPN-6.1 are expressed in a tissue-specific manner in the hermaphrodite. Also, we demonstrate that the 19S RP subunits RPN-6.1 and RPN-7 are crucial for the nuclear localization of the lid subunits RPN-8 and RPN-9 in oocytes, further supporting the ability to utilize the C. elegans germ line as a model to study proteasome assembly real-time. Collectively, our data support the premise that certain 19S RP proteasome subunits are playing tissue-specific roles, especially in the germ line. We propose C. elegans as a versatile multicellular model to study the diverse proteolytic and non-proteolytic roles that proteasome subunits play in vivo.

Activation of bovine oocytes by protein synthesis inhibitors: new findings on the role of MPF/MAPKs†

The present study evaluated the mechanism by which protein synthesis inhibitors activate bovine oocytes. The aim was to analyze the dynamics of MPF and MAPKs. MII oocytes were activated with ionomycin (Io), ionomycin+anisomycin (ANY) and ionomycin+cycloheximide (CHX) and by in vitro fertilization (IVF). The expression of cyclin B1, p-CDK1, p-ERK1/2, p-JNK, and p-P38 were evaluated by immunodetection and the kinase activity of ERK1/2 was measured by enzyme assay. Evaluations at 1, 4, and 15 hours postactivation (hpa) showed that the expression of cyclin B1 was not modified by the treatments. ANY inactivated MPF by p-CDK1Thr14-Tyr15 at 4 hpa (P < 0.05), CHX increased pre-MPF (p-CDK1Thr161 and p-CDK1Thr14-Tyr15) at 1 hpa and IVF increased p-CDK1Thr14-Tyr15 at 17 hours postfertilization (hpf) (P < 0.05). ANY and CHX reduced the levels of p-ERK1/2 at 4 hpa (P < 0.05) and its activity at 4 and 1 hpa, respectively (P < 0.05). Meanwhile, IVF increased p-ERK1/2 at 6 hpf (P < 0.05); however, its kinase activity decreased at 6 hpf (P < 0.05). p-JNK in ANY, CHX, and IVF oocytes decreased at 4 hpa (P < 0.05). p-P38 was only observed at 1 hpa, with no differences between treatments. In conclusion, activation of bovine oocytes by ANY, CHX, and IVF inactivates MPF by CDK1-dependent specific phosphorylation without cyclin B1 degradation. ANY or CHX promoted this inactivation, which seemed to be more delayed in the physiological activation (IVF). Both inhibitors modulated MPF activity via an ERK1/2-independent pathway, whereas IVF activated the bovine oocytes via an ERK1/2-dependent pathway. Finally, ANY does not activate the JNK and P38 kinase pathways.

The Caenorhabditis elegans proteasome subunit RPN-12 is required for hermaphrodite germline sex determination and oocyte quality

BACKGROUND

The proteasome is a multi-subunit complex and a major proteolytic machinery in cells. Most subunits are essential for proteasome function, and depletion of individual subunits normally results in lethality. RPN-12/Rpn12/PSMD8 is a lid subunit of the 19S regulatory particle (RP) of the 26S proteasome. Studies in Caenorhabditis elegans demonstrated that RNAi depletion of RPN-12 does not result in lethality. RPN-12 has not been well studied in higher eukaryotes. In this study, we investigate the biological significance of RPN-12 in C. elegans.

RESULTS

We found that the null mutant rpn-12(av93) did not cause major impairment of the proteolytic activity of the proteasome. Most rpn-12(av93) hermaphrodites lack sperm leading to feminization of the germ line that can be partially rescued by mating to males. The lack of sperm phenotype can be suppressed by downregulation of TRA-1, a player in the hermaphrodite germline sex determination pathway. Also, rpn-12(av93) animals show significant nuclear accumulation of the meiotic kinase WEE-1.3, a protein predominantly localized to the perinuclear region. Interestingly, chemical inhibition of the proteasome did not cause nuclear accumulation of WEE-1.3.

CONCLUSIONS

RPN-12 plays a previously unknown role in oogenesis and the germline sex determination pathway in C. elegans hermaphrodites.

Effects of Ferrocenyl 4-(Imino)-1,4-Dihydro-quinolines on Xenopus laevis Prophase I - Arrested Oocytes: Survival and Hormonal-Induced M-Phase Entry

Xenopus oocytes were used as cellular and molecular sentinels to assess the effects of a new class of organometallic compounds called ferrocenyl dihydroquinolines that have been developed as potential anti-cancer agents. One ferrocenyl dihydroquinoline compound exerted deleterious effects on oocyte survival after 48 h of incubation at 100 μM. Two ferrocenyl dihydroquinoline compounds had an inhibitory effect on the resumption of progesterone induced oocyte meiosis, compared to controls without ferrocenyl groups. In these inhibited oocytes, no MPF (Cdk1/cyclin B) activity was detected by western blot analysis as shown by the lack of phosphorylation of histone H3. The dephosphorylation of the inhibitory Y15 residue of Cdk1 occurred but cyclin B was degraded. Moreover, two apoptotic death markers, the active caspase 3 and the phosphorylated histone H2, were detected. Only 7-chloro-1-ferrocenylmethyl-4-(phenylylimino)-1,4-dihydroquinoline (8) did not show any toxicity and allowed the assembly of a histologically normal metaphase II meiotic spindle while inhibiting the proliferation of cancer cell lines with a low IC₅₀, suggesting that this compound appears suitable as an antimitotic agent.

MPF-based meiotic cell cycle control: Half a century of lessons from starfish oocytes

In metazoans that undergo sexual reproduction, genomic inheritance is ensured by two distinct types of cell cycle, mitosis and meiosis. Mitosis maintains the genomic ploidy in somatic cells reproducing within a generation, whereas meiosis reduces by half the ploidy in germ cells to prepare for successive generations. The meiotic cell cycle is believed to be a derived form of the mitotic cell cycle; however, the molecular mechanisms underlying both of these processes remain elusive. My laboratory has long studied the meiotic cell cycle in starfish oocytes, particularly the control of meiotic M-phase by maturation- or M phase-promoting factor (MPF) and the kinase cyclin B-associated Cdk1 (cyclin B-Cdk1). Using this system, we have unraveled the molecular principles conserved in metazoans that modify M-phase progression from the mitotic type to the meiotic type needed to produce a haploid genome. Furthermore, we have solved a long-standing enigma concerning the molecular identity of MPF, a universal inducer of M-phase both in mitosis and meiosis of eukaryotic cells.

Keeping Order in Anaphase

The critical components of chromosome segregation machinery are well established, but how they orchestrate the relative order of events during mitosis remains unclear. Kamenz et al. (2015) now report in Molecular Cell quantitative data suggesting competing networks and adaptive thresholds in the control of mitotic exit by the anaphase-promoting complex.

RO-3306 prevents postovulatory aging-mediated spontaneous exit from M-II arrest in rat eggs cultured in vitro

BACKGROUND

Postovulatory aging-mediated spontaneous exit from metaphase-II (M-II) arrest deteriorates egg quality and limits assisted reproductive technologies outcome (ART) outcome. Present study was aimed to find out whether RO-3306, specific cyclin dependent kinase 1 (Cdk1) inhibitor could protect against postovulatory aging-mediated spontaneous exit from M-II arrest in rat eggs cultured in vitro.

METHODS

Freshly ovulated M-II arrested eggs were exposed to various concentrations of RO-3306 for 3h in vitro. The morphological changes, percentage of spontaneous exit from M-II arrest, total and specific phosphorylation status of Cdk1, cyclin B1 level and Cdk1 activity were analyzed.

RESULTS

Data suggest that RO-3306 protected postovulatory aging-mediated spontaneous exit from M-II arrest in a concentration-dependent manner. Postovulatory aging increased Thr14/Tyr15 phosphorylated Cdk1 level, decreased Thr161 phosphorylated Cdk1 as well as cyclin B1 levels and increased Cdk1 activity in aged eggs cultured in vitro. On the other hand, RO-3306 protected postovulatory aging-induced changes in specific phosphorylation of Cdk1, cyclin B1 level, inhibited the kinase activity and prevented spontaneous exit from M-II arrest.

CONCLUSIONS

Our results suggest that postovulatory aging destabilizes MPF by modulating specific phosphorylation of Cdk1 and cyclin B1 level. RO-3306 prevented these changes and maintained M-II arrest in rat eggs cultured in vitro. Hence, maintenance of M-II arrest in ovulated eggs using RO-3306 could be beneficial to increase the number of eggs available for various ART programs.

Cell Cycle Synchronization in Xenopus Egg Extracts

Many important discoveries in cell cycle research have been made using cell-free extracts prepared from the eggs of the South African clawed frog Xenopus laevis. These extracts efficiently support the key nuclear functions of the eukaryotic cell cycle in vitro under apparently the same controls that exist in vivo. The Xenopus cell-free system is therefore uniquely suited to the study of the mechanisms, dynamics and integration of cell cycle regulated processes at a biochemical level. Here, we describe methods currently in use in our laboratory for the preparation of Xenopus egg extracts and demembranated sperm nuclei. We detail how these extracts can be used to study the key transitions of the eukaryotic cell cycle and describe conditions under which these transitions can be manipulated by addition of drugs that either retard or advance passage. In addition, we describe in detail essential techniques that provide a practical starting point for investigating the function of proteins involved in the operation of the eukaryotic cell cycle.

Robust Ordering of Anaphase Events by Adaptive Thresholds and Competing Degradation Pathways

The splitting of chromosomes in anaphase and their delivery into the daughter cells needs to be accurately executed to maintain genome stability. Chromosome splitting requires the degradation of securin, whereas the distribution of the chromosomes into the daughter cells requires the degradation of cyclin B. We show that cells encounter and tolerate variations in the abundance of securin or cyclin B. This makes the concurrent onset of securin and cyclin B degradation insufficient to guarantee that early anaphase events occur in the correct order. We uncover that the timing of chromosome splitting is not determined by reaching a fixed securin level, but that this level adapts to the securin degradation kinetics. In conjunction with securin and cyclin B competing for degradation during anaphase, this provides robustness to the temporal order of anaphase events. Our work reveals how parallel cell-cycle pathways can be temporally coordinated despite variability in protein concentrations.

Functions of the proteasome on chromatin

The proteasome is a large self-compartmentalized protease complex that recognizes, unfolds, and destroys ubiquitylated substrates. Proteasome activities are required for a host of cellular functions, and it has become clear in recent years that one set of critical actions of the proteasome occur on chromatin. In this review, we discuss some of the ways in which proteasomes directly regulate the structure and function of chromatin and chromatin regulatory proteins, and how this influences gene transcription. We discuss lingering controversies in the field, the relative importance of proteolytic versus non-proteolytic proteasome activities in this process, and highlight areas that require further investigation. Our intention is to show that proteasomes are involved in major steps controlling the expression of the genetic information, that proteasomes use both proteolytic mechanisms and ATP-dependent protein remodeling to accomplish this task, and that much is yet to be learned about the full spectrum of ways that proteasomes influence the genome.

Calcium signaling and meiotic exit at fertilization in Xenopus egg

Calcium is a universal messenger that mediates egg activation at fertilization in all sexually reproducing species studied. However, signaling pathways leading to calcium generation and the mechanisms of calcium-induced exit from meiotic arrest vary substantially among species. Here, we review the pathways of calcium signaling and the mechanisms of meiotic exit at fertilization in the eggs of the established developmental model, African clawed frog, Xenopus laevis. We also discuss calcium involvement in the early fertilization-induced events in Xenopus egg, such as membrane depolarization, the increase in intracellular pH, cortical granule exocytosis, cortical contraction, contraction wave, cortical rotation, reformation of the nuclear envelope, sperm chromatin decondensation and sister chromatid segregation.

Regulated protein turnover: snapshots of the proteasome in action

The ubiquitin proteasome system (UPS) is the main ATP-dependent protein degradation pathway in the cytosol and nucleus of eukaryotic cells. At its centre is the 26S proteasome, which degrades regulatory proteins and misfolded or damaged proteins. In a major breakthrough, several groups have determined high-resolution structures of the entire 26S proteasome particle in different nucleotide conditions and with and without substrate using cryo-electron microscopy combined with other techniques. These structures provide some surprising insights into the functional mechanism of the proteasome and will give invaluable guidance for genetic and biochemical studies of this key regulatory system.

High-resolution live-cell imaging reveals novel cyclin A2 degradation foci involving autophagy

Cyclin A2 is a key player in the regulation of the cell cycle. Its degradation in mid-mitosis relies on the ubiquitin-proteasome system (UPS). Using high-resolution microscopic imaging, we find that cyclin A2 persists beyond metaphase. Indeed, we identify a novel cyclin-A2-containing compartment that forms dynamic foci. Förster (or fluorescence) resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) analyses show that cyclin A2 ubiquitylation takes place predominantly in these foci before spreading throughout the cell. Moreover, inhibition of autophagy in proliferating cells induces the stabilisation of a subset of cyclin A2, whereas induction of autophagy accelerates the degradation of cyclin A2, thus showing that autophagy is a novel regulator of cyclin A2 degradation.

Cdc25B phosphatase participates in maintaining metaphase II arrest in mouse oocytes

Cdc25B is an essential regulator for meiotic resumption in mouse oocytes. However, the role of this phosphatase during the later stage of the meiotic cell cycle is not known. In this study, we investigated the role of Cdc25B during metaphase II (MII) arrest in mouse oocytes. Cdc25B was extensively phosphorylated during MII arrest with an increase in the phosphatase activity toward Cdk1. Downregulation of Cdc25B by antibody injection induced the formation of a pronucleus-like structure. Conversely, overexpression of Cdc25B inhibited Ca(2+)-mediated release from MII arrest. Moreover, Cdc25B was immediately dephosphorylated and hence inactivated during MII exit, suggesting that Cdk1 phosphorylation is required to exit from MII arrest. Interestingly, this inactivation occurred prior to cyclin B degradation. Taken together, our data demonstrate that MII arrest in mouse oocytes is tightly regulated not only by the proteolytic degradation of cyclin B but also by dynamic phosphorylation of Cdk1.

Cdc25A activity is required for the metaphase II arrest in mouse oocytes

Mammalian oocytes are arrested in metaphase of second meiosis (MII) until fertilization. This arrest is enforced by the cytostatic factor (CSF), which maintains the M-phase promoting factor (MPF) in a highly active state. Although the continuous synthesis and degradation of cyclin B to maintain the CSF-mediated MII arrest is well established, it is unknown whether cyclin-dependent kinase 1 (Cdk1) phosphorylations are involved in this arrest in mouse oocytes. Here, we show that a dynamic equilibrium of Cdk1 phosphorylation is required to maintain MII arrest. When the Cdc25A phosphatase is downregulated, mouse oocytes are released from MII arrest and MPF becomes inactivated. This inactivation occurs in the absence of cyclin B degradation and is dependent on Wee1B-mediated phosphorylation of Cdk1. Thus, our data demonstrate that Cdk1 activity is maintained during MII arrest not only by cyclin turnover but also by steady state phosphorylation.

Similar temporal and spatial recruitment of native 19S and 20S proteasome subunits to transcriptionally active chromatin

It has recently become clear that components of the proteasome are recruited to sites of gene transcription. Prevailing evidence suggests that the transcriptionally relevant form of the proteasome is a subcomplex of 19S base proteins, which functions as an ATP-dependent chaperone that influences transcriptional processes. Despite this notion, compelling evidence for a transcription-dedicated 19S base complex is lacking, and 20S proteasome subunits have been shown to associate with chromatin in some contexts. To gain insight into the form of the proteasome that is recruited to chromatin, we assembled a panel of highly specific antibodies that recognize native yeast proteasome subunits in chromatin immunoprecipitation assays. Using these reagents, we show that components from the three major subassemblies of the proteasome--19S lid, 19S base, and 20S core--associate with the activated GAL10 gene in yeast in a virtually indistinguishable manner. We find that proteasome subunits Rpt1, Rpt4, Rpn8, Rpn12, Pre6, and Pre10 are recruited to GAL10 rapidly upon galactose induction. These subunits associate with the entire transcribed portion of GAL10, display near-identical patterns of distribution, and dissociate from chromatin rapidly once transcription is shut down. We also find that proteasome subunits are enriched at telomeres and at genes transcribed by RNA polymerase III. Our data suggest that the transcriptionally relevant form of the proteasome is the canonical 26S complex.

Ubiquitin and proteasomes in transcription

Regulation of gene transcription is vitally important for the maintenance of normal cellular homeostasis. Failure to correctly regulate gene expression, or to deal with problems that arise during the transcription process, can lead to cellular catastrophe and disease. One of the ways cells cope with the challenges of transcription is by making extensive use of the proteolytic and nonproteolytic activities of the ubiquitin-proteasome system (UPS). Here, we review recent evidence showing deep mechanistic connections between the transcription and ubiquitin-proteasome systems. Our goal is to leave the reader with a sense that just about every step in transcription-from transcription initiation through to export of mRNA from the nucleus-is influenced by the UPS and that all major arms of the system--from the first step in ubiquitin (Ub) conjugation through to the proteasome-are recruited into transcriptional processes to provide regulation, directionality, and deconstructive power.

Polarity and asymmetry during mouse oogenesis and oocyte maturation

Cell polarity and asymmetry play a fundamental role in embryo development. The unequal segregation of determinants, cues, and activities is the major event in the differentiation of cell fate and function in all multicellular organisms. In oocytes, polarity and asymmetry in the distribution of different molecules are prerequisites for the progression and proper outcome of embryonic development. The mouse oocyte, like the oocytes of other mammals, seems to apply a less stringent strategy of polarization than other vertebrates. The mouse embryo undergoes a regulative type of development, which permits the full rectification of development even if the embryo loses up to half of its cells or its size is experimentally doubled during the early stages of embryogenesis. Such pliability is strongly related to the proper oocyte polarization before fertilization. Thus, the molecular mechanisms leading to the development and maintenance of oocyte polarity must be included in any fundamental understanding of the principles of embryo development. In this chapter, we provide an overview of current knowledge regarding the development and maintenance of polarity and asymmetry in the distribution of organelles and molecules in the mouse oocyte. Curiously, the mouse oocyte becomes polarized at least twice during ontogenesis; the question of how this phenomenon is achieved and what role it might play is addressed in this chapter.

Proteomics reveals a switch in CDK1-associated proteins upon M-phase exit during the Xenopus laevis oocyte to embryo transition

Cyclin-dependent kinase 1 (CDK1) is a major M-phase kinase which requires the binding to a regulatory protein, Cyclin B, to be active. CDK1/Cyclin B complex is called M-phase promoting factor (MPF) for its key role in controlling both meiotic and mitotic M-phase of the cell cycle. CDK1 inactivation is necessary for oocyte activation and initiation of embryo development. This complex process requires both Cyclin B polyubiquitination and proteosomal degradation via the ubiquitin-conjugation pathway, followed by the dephosphorylation of the monomeric CDK1 on Thr161. Previous proteomic analyses revealed a number of CDK1-associated proteins in human HeLa cells. It is, however, unknown whether specific partners are involved in CDK1 inactivation upon M-phase exit. To better understand CDK1 regulation during MII-arrest and oocyte activation, we immunoprecipitated (IPed) CDK1 together with its associated proteins from M-phase-arrested and M-phase-exiting Xenopus laevis oocytes. A mass spectrometry (MS) analysis revealed a number of new putative CDK1 partners. Most importantly, the composition of the CDK1-associated complex changed rapidly during M-phase exit. Additionally, an analysis of CDK1 complexes precipitated with beads covered with p9 protein, a fission yeast suc1 homologue well known for its high affinity for CDKs, was performed to identify the most abundant proteins associated with CDK1. The screen was auto-validated by identification of: (i) two forms of CDK1: Cdc2A and B, (ii) a set of Cyclins B with clearly diminishing number of peptides identified upon M-phase exit, (iii) a number of known CDK1 substrates (e.g. peroxiredoxine) and partners (e.g. HSPA8, a member of the HSP70 family) both in IP and in p9 precipitated pellets. In IP samples we also identified chaperones, which can modulate CDK1 three-dimensional structure, as well as calcineurin, a protein necessary for successful oocyte activation. These results shed a new light on CDK1 regulation via a dynamic change in the composition of the protein complex upon M-phase exit and the oocyte to embryo transition.

Regulatory pathways coordinating cell cycle progression in early Xenopus development

The African clawed frog, Xenopus laevis, is used extensively as a model organism for studying both cell development and cell cycle regulation. For over 20 years now, this model organism has contributed to answering fundamental questions concerning the mechanisms that underlie cell cycle transitions--the cellular components that synthesize, modify, repair, and degrade nucleic acids and proteins, the signaling pathways that allow cells to communicate, and the regulatory pathways that lead to selective expression of subsets of genes. In addition, the remarkable simplicity of the Xenopus early cell cycle allows for tractable manipulation and dissection of the basic components driving each transition. In this organism, early cell divisions are characterized by rapid cycles alternating phases of DNA synthesis and division. The post-blastula stages incorporate gap phases, lengthening progression, and allowing more time for DNA repair. Various cyclin/Cdk complexes are differentially expressed during the early cycles with orderly progression being driven by both the combined action of cyclin synthesis and degradation and the appropriate selection of specific substrates by their Cdk components. Like other multicellular organisms, chief developmental events in early Xenopus embryogenesis coincide with profound remodeling of the cell cycle, suggesting that cell proliferation and differentiation events are linked and coordinated through crosstalk mechanisms acting on signaling pathways involving the expression of cell cycle control genes.

Canonical and Alternative Pathways in Cyclin-Dependent Kinase 1/Cyclin B Inactivation upon M-Phase Exit in Xenopus laevis Cell-Free Extracts

Cyclin-Dependent Kinase 1 (CDK1) is the major M-phase kinase known also as the M-phase Promoting Factor or MPF. Studies performed during the last decade have shown many details of how CDK1 is regulated and also how it regulates the cell cycle progression. Xenopus laevis cell-free extracts were widely used to elucidate the details and to obtain a global view of the role of CDK1 in M-phase control. CDK1 inactivation upon M-phase exit is a primordial process leading to the M-phase/interphase transition during the cell cycle. Here we discuss two closely related aspects of CDK1 regulation in Xenopus laevis cell-free extracts: firstly, how CDK1 becomes inactivated and secondly, how other actors, like kinases and phosphatases network and/or specific inhibitors, cooperate with CDK1 inactivation to assure timely exit from the M-phase.

The APC/C recruits cyclin B1-Cdk1-Cks in prometaphase before D box recognition to control mitotic exit

Prior associations with the APC/C complex during prometaphase makes cyclin B1 a better substrate for the cell cycle–regulating ubiquitin ligase in metaphase (see also a related paper by Di Fiore et al. in this issue).

The ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) is activated at prometaphase by mitotic phosphorylation and binding of its activator, Cdc20. This initiates cyclin A degradation, whereas cyclin B1 is stabilized by the spindle checkpoint. Upon checkpoint release, the RXXL destruction box (D box) was proposed to direct cyclin B1 to core APC/C or Cdc20. In this study, we report that endogenous cyclin B1–Cdk1 is recruited to checkpoint-inhibited, phosphorylated APC/C in prometaphase independently of Cdc20 or the cyclin B1 D box. Like cyclin A, cyclin B1 binds the APC/C by the Cdk cofactor Cks and the APC3 subunit. Prior binding to APC/C^Cdc20 makes cyclin B1 a better APC/C substrate in metaphase, driving mitotic exit and cytokinesis. We conclude that in prometaphase, the phosphorylated APC/C can recruit both cyclin A and cyclin B1 in a Cks-dependent manner. This suggests that the spindle checkpoint blocks D box recognition of APC/C-bound cyclin B1, whereas distinctive complexes between the N terminus of cyclin A and Cdc20 evade checkpoint control.

The Mycobacterium tuberculosis proteasome active site threonine is essential for persistence yet dispensable for replication and resistance to nitric oxide

Previous work revealed that conditional depletion of the core proteasome subunits PrcB and PrcA impaired growth of Mycobacterium tuberculosis in vitro and in mouse lungs, caused hypersusceptibility to nitric oxide (NO) and impaired persistence of the bacilli during chronic mouse infections. Here, we show that genetic deletion of prcBA led to similar phenotypes. Surprisingly, however, an active site mutant proteasome complemented the in vitro and in vivo growth defects of the prcBA knockout (Delta prcBA) as well as its NO hypersensitivity. In contrast, long-term survival of M. tuberculosis in stationary phase and during starvation in vitro and in the chronic phase of mouse infection required a proteolytically active proteasome. Inhibition of inducible nitric oxide synthase did not rescue survival of Delta prcBA, revealing a function beyond NO defense, by which the proteasome contributes to M. tuberculosis fitness during chronic mouse infections. These findings suggest that proteasomal proteolysis facilitates mycobacterial persistence, that M. tuberculosis faces starvation during chronic mouse infections and that the proteasome serves a proteolysis-independent function.

Gamma-tubulin regulates the anaphase-promoting complex/cyclosome during interphase

Activation of the APC/C requires microtubule-nucleating independent aspects of γ-tubulin function.

A cold-sensitive γ-tubulin allele of Aspergillus nidulans, mipAD159, causes defects in mitotic and cell cycle regulation at restrictive temperatures that are apparently independent of microtubule nucleation defects. Time-lapse microscopy of fluorescently tagged mitotic regulatory proteins reveals that cyclin B, cyclin-dependent kinase 1, and the Ancdc14 phosphatase fail to accumulate in a subset of nuclei at restrictive temperatures. These nuclei are permanently removed from the cell cycle, whereas other nuclei, in the same multinucleate cell, cycle normally, accumulating and degrading these proteins. After each mitosis, additional daughter nuclei fail to accumulate these proteins, resulting in an increase in noncycling nuclei over time and consequent inhibition of growth. Extensive analyses reveal that these noncycling nuclei result from a nuclear autonomous, microtubule-independent failure of inactivation of the anaphase-promoting complex/cyclosome. Thus, γ-tubulin functions to regulate this key mitotic and cell cycle regulatory complex.

G(alpha)12/13 inhibition enhances the anticancer effect of bortezomib through PSMB5 downregulation

Bortezomib is a proteasome inhibitor approved for anticancer therapy. However, variable sensitivity of tumor cells exists in this therapy probably due to differences in the expression of proteasome subunits. G(alpha)(12/13) serves modulators or signal transducers in diverse pathways. This study investigated whether cancer cells display differential sensitivity to bortezomib with reference to G(alpha)(12/13) expression, and if so, whether G(alpha)(12/13) affects the expression of proteasome subunits and their activities. Bortezomib treatment exhibited greater sensitivities in Huh7 and SNU886 cells (epithelial type) than SK-Hep1 and SNU449 cells (mesenchymal type) that exhibited higher levels of G(alpha)(12/13). Overexpression of an active mutant of G(alpha)(12) (Galpha(12)QL) or G(alpha)(13) (G(alpha)(13)QL) diminished the ability of bortezomib to induce cytotoxicity in Huh7 cells. Moreover, transfection with the minigene that disturbs G protein-coupled receptor-G protein coupling (CT12 or CT13) increased it in SK-Hep1 cells. Consistently, MiaPaCa2 cells transfected with CT12 or CT13 exhibited a greater sensitivity to bortezomib. Evidence of G(alpha)(12/13)'s antagonism on the anticancer effect of bortezomib was verified in the reversal by G(alpha)(12)QL or G(alpha)(13)QL of the minigenes' enhancement of cytotoxity. Real-time polymerase chain reaction assay enabled us to identify PSMB5, multicatalytic endopeptidase complex-like-1, and proteasome activator subunit-1 repression by CT12 or CT13. Furthermore, G(alpha)(12/13) inhibition enhanced the ability of bortezomib to repress PSMB5, as shown by immunoblotting and proteasome activity assay. Moreover, this inhibitory effect on PSMB5 was attenuated by G(alpha)G(alpha)(12)QL or G(alpha)(13)QL. In conclusion, the inhibition of G(alpha)(12/13) activities may enhance the anticancer effect of bortezomib through PSMB5 repression, providing insight into the G(alpha)(12/13) pathway for the regulation of proteasomal activity.

Analysis of signal transduction in cell-free extracts and rafts of Xenopus eggs

Intracellular signaling during egg activation/fertilization has been extensively studied using intact eggs, which can be manipulated by microinjection of different mRNAs, proteins, or chemical drugs. Furthermore, egg extracts, which retain high CSF activity (CSF-arrested extracts), were developed for studying fertilization/activation signal transduction, which have significant advantages as a model system. The addition of calcium to CSF-arrested extracts initiates a plethora of signaling events that take place during egg activation. Hence, the signaling downstream of calcium mobilization has been successfully studied in the egg extracts. Moreover, despite disruption of membrane-associated signaling compartments and ordered compartmentalization during extract preparation, CSF-arrested extracts can be successfully used to study early signaling events, which occur upstream of calcium release during egg activation/fertilization. In combination with the CSF-arrested extracts, activated egg rafts can reproduce some events of egg activation, including PLCgamma activation, IP3 production, transient calcium release, MAPK inactivation, and meiotic exit. This becomes possible due to complementation of the sperm-induced egg activation signaling machinery present in the rafts with the components of signal transduction system localized in the extracts. Herein, we describe protocols for studying molecular mechanisms of egg fertilization/activation using cell-free extracts and membrane rafts prepared from metaphase-arrested Xenopus eggs.

Targeting proteins for degradation

Protein degradation plays a central role in many cellular functions. Misfolded and damaged proteins are removed from the cell to avoid toxicity. The concentrations of regulatory proteins are adjusted by degradation at the appropriate time. Both foreign and native proteins are digested into small peptides as part of the adaptive immune response. In eukaryotic cells, an ATP-dependent protease called the proteasome is responsible for much of this proteolysis. Proteins are targeted for proteasomal degradation by a two-part degron, which consists of a proteasome binding signal and a degradation initiation site. Here we describe how both components contribute to the specificity of degradation.

Metaphase I arrest in LT/Sv mouse oocytes involves the spindle assembly checkpoint

During meiotic maturation, the majority of oocytes from LT/Sv mice arrest at metaphase I. However, anaphase may be induced through parthenogenetic activation. If this happens within the ovary, it often results in the development of ovarian teratomas. Here, we show that the induction of first meiotic anaphase in LT/Sv oocytes results in incorrect chromosome segregation. In search of the molecular basis of this complex phenotype, we analyzed the localization/destruction of cohesins, as well as the function of the components of the spindle assembly checkpoint (SAC). Both localization and removal of meiotic cohesin REC8 from chromosomes are unperturbed. In contrast, there is prolonged localization of SAC proteins BUB1 and MAD2L1 (MAD2) at the metaphase I kinetochores in mutant oocytes compared with the wild-type. Interfering with BUB1 function through expression of a dominant-negative mutant protein resulted in the increase of the number of LT/Sv oocytes completing the first meiosis, which indicates SAC involvement in metaphase I arrest. These data show for the first time that there is a direct link between the SAC function and the heritable meiotic incompetence of a mammalian oocyte.

Fertilization differently affects the levels of cyclin B1 and M-phase promoting factor activity in maturing and metaphase II mouse oocytes

Fertilization affects levels of cyclin B1 and M-phase promoting factor (MPF) activity in maturing and metaphase II mouse oocytes in two distinct ways. In metaphase II oocytes, it leads to a Ca2+-dependent, continuous degradation of cyclin B1 and inactivation of cyclin dependent kinase (CDC2A)–cyclin B1 complex (MPF). In this paper, we show that neither mono- nor polyspermic fertilization of prometaphase I and metaphase I oocytes triggered degradation of cyclin B1. However, polyspermic fertilization of prometaphase I oocytes led to a transient decrease in MPF activity that lasted for 2 h. The inactivation of MPF in polyspermic prometaphase I oocytes did not depend on the fertilization-induced increase in the cytoplasmic concentration of free Ca2+ions, but was caused, at least in part, by dephosphorylation of CDC2A at threonine 161 (Thr161). We found that polyspermic fertilization did not affect glutathione levels in prometaphase I oocytes, and concluded that the decrease in MPF activity and dephosphorylation of CDC2A at Thr161 in polyspermic prometaphase I oocytes were not caused by a change in the redox status of the cell induced by an introduction of excessive amount of sperm protamines. Instead, we propose that inactivation of MPF activity in polyspermic maturing oocytes is caused by a change in nucleo-cytoplasmic ratio that leads to a ‘titration’ of kinases and phosphatases responsible for keeping MPF in an active state. This idea is supported by the finding that oocytes fused with thymocytes rather than spermatozoa also showed a transient decrease in MPF activity.

Substrate selection by the proteasome during degradation of protein complexes

The proteasome controls the turnover of most cellular proteins. Two structural features are typically required for proteins to be degraded: covalently attached ubiquitin polypeptides that allow binding to the proteasome, and an unstructured region in the targeted protein that initiates proteolysis. Here, we have tested the degradation of model proteins to further explore how the proteasome selects its substrates. Using purified yeast proteasome and mammalian proteasome in cell lysate, we have demonstrated that the two structural features can act in trans when separated onto different proteins in a multi-subunit complex. In such complexes, the location of the unstructured initiation site and its chemical properties determine which subunit is degraded. Thus, our findings reveal the molecular basis of subunit specificity in the degradation of protein complexes. In addition, our data provide a plausible explanation for how adaptor proteins can bind to otherwise stable proteins and target them for degradation.

Temporal regulation of embryonic M-phases

Temporal regulation of M-phases of the cell cycle requires precise molecular mechanisms that differ among different cells. This variable regulation is particularly clear during embryonic divisions. The first embryonic mitosis in the mouse lasts twice as long as the second one. In other species studied so far (C. elegans, Sphaerechinus granularis, Xenopus laevis), the first mitosis is also longer than the second, yet the prolongation is less pronounced than in the mouse. We have found recently that the mechanisms prolonging the first embryonic M-phase differ in the mouse and in Xenopus embryos. In the mouse, the metaphase of the first mitosis is specifically prolonged by the unknown mechanism acting similarly to the CSF present in oocytes arrested in the second meiotic division. In Xenopus, higher levels of cyclins B participate in the M-phase prolongation, however, without any cell cycle arrest. In Xenopus embryo cell-free extracts, the inactivation of the major M-phase factor, MPF, depends directly on dissociation of cyclin B from CDK1 subunit and not on cyclin B degradation as was thought before. In search for other mitotic proteins behaving in a similar way as cyclins B we made two complementary proteomic screens dedicated to identifying proteins ubiquitinated and degraded by the proteasome upon the first embryonic mitosis in Xenopus laevis. The first screen yielded 175 proteins. To validate our strategy we are verifying now which of them are really ubiquitinated. In the second one, we identified 9 novel proteins potentially degraded via the proteasome. Among them, TCTP (Translationally Controlled Tumor Protein), a 23-kDa protein, was shown to be partially degraded during mitosis (as well as during meiotic exit). We characterized the expression and the role of this protein in Xenopus, mouse and human somatic cells, Xenopus and mouse oocytes and embryos. TCTP is a mitotic spindle protein positively regulating cellular proliferation. Analysis of other candidates is in progress.

Temporal regulation of the first mitosis in Xenopus and mouse embryos

Cell cycle regulation in Eukaryotes is based on common molecular actors and mechanisms. However, the canonical cell cycle is modified in certain cells. Such modifications play a key role in oocyte maturation and embryonic development. They can be achieved either by introduction of new components, pathways, substrates, changed interactions between them, or by elimination of some factors inherited by the cells from previous developmental stages. Here we discuss a particular temporal regulation of the first embryonic M-phase of Xenopus and mouse embryos. These two examples help to understand better the general regulation of M-phase of the cell cycle.

Histone deacetylase inhibitors induce premature sister chromatid separation and override the mitotic spindle assembly checkpoint

Histone deacetylase inhibitors (HDACI) are powerful antiproliferative drugs, and are currently undergoing clinical trials as antitumor agents. It would be valuable for both cancer therapy and our knowledge of basic cellular processes to understand the mechanisms by which HDACIs block cell proliferation. Most current models postulate that HDACIs allow the reexpression of tumor suppressor genes silenced in cancer cells. However, other mechanisms, distinct from transcription regulation, may participate in HDACI antiproliferative properties. We report that HDACI treatment induces premature sister chromatid separation in cells in which the mitotic spindle assembly checkpoint (SAC) has already been activated. This effect was transcription-independent. In addition, HDACI-treated mitotic cells displayed SAC inactivation characteristics, including anaphase-promoting complex/cyclosome target degradation, cyclin-dependent kinase 1 inactivation, histone H3 dephosphorylation, and loss of the SAC component MAD2 from the kinetochore. Thus, HDAC inhibition renders the SAC ineffective. Our findings help elucidate the molecular mechanisms of proliferative cell death induced by HDACI treatment and may allow new HDACI-based preclinical and clinical trial protocols to be redesigned so as to target mitosis.

Geldanamycin induces G2 arrest in U87MG glioblastoma cells through downregulation of Cdc2 and cyclin B1

Cell cycle progression requires precise expression and activation of several cyclins and cyclin-dependent kinases. Geldanamycin (GA) affects cell cycle progression in various kinds of cells. We analyzed GA-induced cell cycle regulation in glioblastoma cells. GA-induced G2 or M arrest in glioblastoma cells in a cell line-dependent manner. GA decreased the expression of Cdc2 and cyclin B1 in U87MG cells. And phosphorylated Cdc2 decreased along with Cdc2 in the GA-treated cells. This cell line showed G2 arrest after GA treatment. In contrast, GA failed to down-regulate these cell cycle regulators in U251MG cells. In U251MG cells, the cell cycle was arrested at M phase in addition to G2 by GA. Next, we analyzed the mechanism of the GA-induced regulation of Cdc2 and cyclin B1 in U87MG cells. Cdc2 and cyclin B1 were ubiquitinated by GA. MG132 abrogated the GA-induced decrease of Cdc2 and cyclin B1 indicating that these proteins were degraded by proteasomes. In conclusion, GA controls the stability of Cdc2 and cyclin B1 in glioblastomas cell species-dependently. Cdc2 and cyclin B1 might be responsible for the different responses of glioblastoma cell lines to GA.

Isolation, characterization, and genetic complementation of a cellular mutant resistant to retroviral infection

By using a genetic screen, we have isolated a mammalian cell line that is resistant to infection by retroviruses that are derived from the murine leukemia virus, human immunodeficiency virus type 1, and feline immunodeficiency virus. We demonstrate that the cell line is genetically recessive for the resistance, and hence it is lacking a factor enabling infection by retroviruses. The block to infection is early in the life cycle, at the poorly understood uncoating stage. We implicate the proteasome at uncoating by completely rescuing the resistant phenotype with the proteasomal inhibitor MG-132. We further report on the complementation cloning of a gene (MRI, modulator of retrovirus infection) that can also act to reverse the inhibition of infection in the mutant cell line. These data implicate a role for the proteasome during uncoating, and they suggest that MRI is a regulator of this activity. Finally, we reconcile our findings and other published data to suggest a model for the involvement of the proteasome in the early phase of the retroviral life cycle.

Comparative proteome analysis of changes in the 26S proteasome during oocyte maturation in goldfish

Proteasomes are large, multi-subunit particles that act as the proteolytic machinery for most of the regulated intracellular protein degradation in eukaryotic cells. An alteration of proteasome function may be important for the regulation of the meiotic cell cycle. To study the change at the subunit level of the 26S proteasome during meiotic maturation, we purified 26S proteasomes from immature and mature oocytes of goldfish. Two-dimensional polyacrylamide gel electrophoresis was used to separate proteins. For differential analysis, whole spots of the 26S proteasome from goldfish oocytes were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and database analysis. Four spots that were different (only detected in mature oocyte 265 proteasomes and not in immature ones) and four protein spots that were up- or down-regulated were identified unambiguously. The mature-specific spots were not 26S proteasome components but rather their interacting proteins, and were identified as chaperonin-containing TCP-1 subunits and myosin light chain. Minor spots of three subunits of the 20S core particle and one of the 19S regulatory particle showed meiotic cell cycle-dependent changes. These results demonstrate that modifications of proteasomal subunits and cell cycle phase-dependent interactions of proteins with proteasomes occur during oocyte maturation in goldfish.

Cell-cycle-dependent Xenopus TRF1 recruitment to telomere chromatin regulated by Polo-like kinase

Telomeres are regulated by a homeostatic mechanism that includes telomerase and telomeric repeat binding proteins, TRF1 and TRF2. Recently, it has been hypothesized that telomeres assume distinct configurations in a cell-cycle-dependent manner, although direct biochemical evidence is lacking. Here we demonstrated that Xenopus TRF1 (xTRF1) associates with telomere chromatin specifically in mitotic Xenopus egg extracts, and dissociates from it upon mitotic exit. Both the N-terminal TRF-homology (TRFH) domain and the linker region connecting the TRFH domain and the C-terminal Myb domain are required for this cell-cycle-dependent association of xTRF1 with chromatin. In contrast, Xenopus TRF2 (xTRF2) associates with chromatin throughout the cell cycle. We showed that Polo-like kinase (Plx1) phosphorylates xTRF1 in vitro. Moreover, the mitotic xTRF1-chromatin association was significantly impaired when Plx1 was immunodepleted from the extracts. Finally, high telomerase activities were detected in association with replicating interphase chromatin compared with mitotic chromatin. These results indicate that telomere chromatin is actively regulated by cell-cycle-dependent processes, and provide an insight for understanding how telomeres undergo DNA metabolisms during the cell cycle.

Proteasomes from Structure to Function: Perspectives from Archaea

Insight into the world of proteolysis has expanded considerably over the past decade. Energy-dependent proteases, such as the proteasome, are no longer viewed as nonspecific degradative enzymes associated solely with protein catabolism but are intimately involved in controlling biological processes that span life to death. The proteasome maintains this exquisite control by catalyzing the precisely timed and rapid turnover of key regulatory proteins. Proteasomes also interplay with chaperones to ensure protein quality and to readjust the composition of the proteome following stress. Archaea encode proteasomes that are highly related to those of eukaryotes in basic structure and function. Investigations of archaeal proteasomes coupled with those of eukaryotes has greatly facilitated our understanding of the molecular mechanisms that govern regulated protein degradation by this elaborate nanocompartmentalized machine.

Unequal cell division regulated by the contents of germinal vesicles

Fertilization occurs during meiosis in many animals, when maternal centrosomes participate in the formation of spindles at the animal pole, which results in polar body formation. Paternal centrosomes do not participate in cell division during oocyte maturation. After meiosis, they form the spindles while the maternal centrosomes are discarded. It is unknown why paternal centrosomes do not form spindles during meiosis. Here, we show that the artificial incorporation of sperm at the animal pole of immature starfish oocytes causes unequal cell division and the formation of polar body-like fragments. The removal of germinal vesicles from the animal pole blocks the formation of polar body-like fragments. Furthermore, translocation of germinal vesicles to the vegetal pole by centrifugation induces the extrusion of polar body-like fragments from the vegetal pole, where sperm penetration is prerequisite. After germinal vesicle breakdown, cyclin B is localized in the maternal and paternal asters and spindles near the germinal vesicle. These results suggest that germinal vesicle components such as the cdc2-cyclin B complex interact with asters and spindles and can induce unequal cell division. During normal fertilization, paternal centrosomes are likely kept away from the germinal vesicle components, resulting in the inhibition of unequal paternal centrosome-dependent cell division.

Degradation of Polyubiquitinated Cyclin B Is Blocked by the MAPK Pathway at the Metaphase I Arrest in Starfish Oocytes

In the starfish ovary, maturing oocytes stimulated by 1-methyladenine undergo synchronous germinal vesicle breakdown and then arrest in metaphase of the first meiotic division (metaphase I). Immediately after spawning, an increase of intracellular pH (pH(i)) from approximately 7.0 to approximately 7.3 is induced by Na(+)/H(+) antiporter in oocytes, and meiosis reinitiation occurs. Here we show that an endogenous substrate of the proteasome, polyubiquitinated cyclin B, was stable at pH 7.0, whereas it was degraded at pH 7.3. When the MAPK pathway was blocked by MEK inhibitor U0126, degradation of polyubiquitinated cyclin B occurred even at pH 7.0 without an increase of the peptidase activity of the proteasome. These results indicate that the proteasome activity at pH 7.0 is sufficient for degradation of polyubiquitinated cyclin B and that the MAPK pathway blocks the degradation of polyubiquitinated cyclin B in the maturing oocytes in the ovary. Immediately after spawning, the increase in pH(i) mediated by Na(+)/H(+) antiporter cancels the inhibitory effects of the MAPK pathway, resulting in the degradation of polyubiquitinated cyclin B and the release of the arrest. Thus, the key step of metaphase I arrest in starfish oocytes occurs after the polyubiqutination of cyclin B but before cyclin B proteolysis by the proteasome.

Negative regulation of geminin by CDK-dependent ubiquitination controls replication licensing

The replication licensing system ensures the precise duplication of chromosomal DNA in each cell cycle. In metazoans, a small protein called geminin plays a central role in negatively regulating licensing late in the cell cycle. Recent work using Xenopus egg extracts shows how geminin activity is downregulated on exit from metaphase in a process that requires mitotic cyclin-dependent kinases (CDKs). Geminin is polyubiquitinated by the Anaphase Promoting Complex, but instead of being proteolysed-the normal fate of polyubiquitinated proteins-much of the geminin is deubiquitinated, leaving it inactive. These results suggest a simple model for how precise chromosome duplication is ensured in the Xenopus model system.

Non-proteolytic inactivation of geminin requires CDK-dependent ubiquitination

In late mitosis and G1, a complex of the essential initiation proteins Mcm2-7 are assembled onto replication origins to 'license' them for initiation. At other times licensing is inhibited by cyclin-dependent kinases (CDKs) and geminin, thus ensuring that origins fire only once per cell cycle. Here we show that, paradoxically, CDKs are also required to inactivate geminin and activate the licensing system. On exit from metaphase in Xenopus laevis egg extracts, CDK-dependent activation of the anaphase-promoting complex (APC/C) results in the transient polyubiquitination of geminin. This ubiquitination triggers geminin inactivation without requiring ubiquitin-dependent proteolysis, and is essential for replication origins to become licensed. This reveals an unexpected role for CDKs and ubiquitination in activating chromosomal DNA replication.

Degradation of proliferating cell nuclear antigen by 26S proteasome in rice (Oryza sativa L.)

To study whether metabolic control of proliferating cell nuclear antigen (PCNA) during the cell cycle is similar to that of associated protein factors, two-hybrid analysis with PCNA from rice (Oryza sativa L. cv. Nipponbare) was performed. PCNA interacted with rice Rpt6, which is the ATPase subunit of 26S proteasome, both in vitro and in vivo, and the degradation of PCNA was disrupted by the proteasome in vivo. The tissue-specific expression pattern of the transcripts of Rpt6 and PCNA suggested that the rice proteasome played important roles in DNA replication involving PCNA. These findings indicate a proteasome-dependent degradation of PCNA.

Diverse roles for ubiquitin-dependent proteolysis in transcriptional activation

A growing literature points to a fundamental role for the ubiquitin-proteasome degradation system (UPS) in transcription. Four recent publications add significant insight to our understanding of the connections between these processes. Each provides evidence that some aspect of the UPS can stimulate the activity of transcriptional activators. UPS might promote transcription by several mechanisms, and in some cases, even the final step of the UPS - proteolysis - might enhance activator function.

Regulated interaction between polypeptide chain elongation factor-1 complex with the 26S proteasome during Xenopus oocyte maturation

BACKGROUND

During Xenopus oocyte maturation, the amount of a 48 kDa protein detected in the 26S proteasome fraction (p48) decreased markedly during oocyte maturation to the low levels seen in unfertilized eggs. The results indicate that the interaction of at least one protein with the 26S proteasome changes during oocyte maturation and early development. An alteration in proteasome function may be important for the regulation of developmental events, such as the rapid cell cycle, in the early embryo. In this study, we identified p48.

RESULTS

p48 was purified by conventional column chromatography. The resulting purified fraction contained two other proteins with molecular masses of 30 (p30) and 37 (p37) kDa. cDNAs encode elongation factor-1gamma and delta were obtained by an immuno-screening method using polyclonal antibodies against purified p48 complex, which recognized p48 and p37. N-terminal amino acid sequence analysis of p30 revealed that it was identical to EF-1beta. To identify the p48 complex bound to the 26S proteasome as EF-1betagammadelta, antibodies were raised against the components of purified p48 complex. Recombinant EF-1 beta,gamma and delta were expressed in Escherichia coli, and an antibody was raised against purified recombinant EF-1gamma. Cross-reactivity of the antibodies toward the p48 complex and recombinant proteins showed it to be specific for each component. These results indicate that the p48 complex bound to the 26S proteasome is the EF-1 complex. MPF phosphorylated EF-1gamma was shown to bind to the 26S proteasome. When EF-1gamma is phosphorylated by MPF, the association is stabilized.

CONCLUSION

p48 bound to the 26S proteasome is identified as the EF-1gamma. EF-1 complex is associated with the 26S proteasome in Xenopus oocytes and the interaction is stabilized by MPF-mediated phosphorylation.

Proteasome activity is critical for the cAMP-induced differentiation of neuroblastoma cells

1. The ubiquitin-proteasome pathway is involved in a variety of cellular functions in mammalian cells. The role of proteasome, however, in the course of cell differentiation is not well characterized. We hypothesized that proteasome activity might be essential during neuronal cell differentiation. 2. To investigate the role of proteasome during neuronal differentiation, we made use of a murine neuroblastoma cell line (NBP2) that terminally differentiates into mature neurons upon elevation of the intracellular level of adenosine 3',5'-cyclic monophosphate (cAMP). To monitor proteasome activity in NBP2 cells, we integrated an expression cassette for a short-lived green fluorescent protein (d2EGFP) into these cells, which were designated as NBP2-PN25. When NBP2-PN25 cells were treated with a proteasome inhibitor, lactacystin or MG132, a dose-dependent increase in the constitutive levels of d2EGFP expression was detected. 3. We also found that proteasome inhibition by lactacystin during the cAMP-induced differentiation of NBP2-PN25 cells triggered cell death. Both lactacystin and cAMP induction reduced the expression of mRNA for the differentiation-associated genes, such as N-myc and cyclin B1. While cAMP-inducing agents decreased the level of N-myc and cyclin B1 proteins, lactacystin increased the level of these proteins. 4. Our data suggest that a reduced level of N-myc and cyclin B1 proteins is critical to commence differentiation, and this can be blocked by a proteasome inhibitor, leading to cell death. Concomitant induction of differentiation and proteasome inhibition, may, therefore, be potentially useful for the treatment of human neuroblastomas.