Dominant-negative cyclin-selective ubiquitin carrier protein E2-C/UbcH10 blocks cells in metaphase

Cyclin A is destroyed in prometaphase and can delay chromosome alignment and anaphase

Mitosis is controlled by the specific and timely degradation of key regulatory proteins, notably the mitotic cyclins that bind and activate the cyclin-dependent kinases (Cdks). In animal cells, cyclin A is always degraded before cyclin B, but the exact timing and the mechanism underlying this are not known. Here we use live cell imaging to show that cyclin A begins to be degraded just after nuclear envelope breakdown. This degradation requires the 26S proteasome, but is not affected by the spindle checkpoint. Neither deletion of its destruction box nor disrupting Cdk binding prevents cyclin A proteolysis, but Cdk binding is necessary for degradation at the correct time. We also show that increasing the levels of cyclin A delays chromosome alignment and sister chromatid segregation. This delay depends on the proteolysis of cyclin A and is not caused by a lag in the bipolar attachment of chromosomes to the mitotic spindle, nor is it mediated via the spindle checkpoint. Thus, proteolysis that is not under the control of the spindle checkpoint is required for chromosome alignment and anaphase.

Securin degradation is mediated by fzy and fzr, and is required for complete chromatid separation but not for cytokinesis

We have studied the ubiquitination and degradation patterns of the human securin/PTTG protein. We show that, in contrast to budding yeast pds1, securin degradation is catalyzed by both fzy (fizzy/cdc20) and fzr (fizzy-related/cdh1/hct1). Both fzy and fzr also induce the APC/C to ubiquitinate securin in vitro. Securin degradation is mediated by an RXXL destruction box and a KEN box, and is inhibited only when both sequences are mutated. Interestingly, the non-degradable securin mutant is also partially ubiquitinated by fzy and fzr in vitro. Expressing the non-degradable securin mutant in cells frequently resulted in incomplete chromatid separation and gave rise to daughter cells connected by a thin chromatin fiber, presumably of chromosomes that failed to split completely. Strikingly, the mutant securin did not prevent the majority of sister chromatids from separating completely, nor did it prevent mitotic cyclin degradation and cytokinesis. This phenotype, reminiscent of the fission yeast cut (cells untimely torn) phenotype, is reported here for the first time in mammals.

Constitutive instability of muscle regulatory factor Myf5 is distinct from its mitosis-specific disappearance, which requires a D-box-like motif overlapping the basic domain

Transcription factors Myf5 and MyoD play critical roles in controlling myoblast identity and differentiation. In the myogenic cell line C2, we have found that Myf5 expression, unlike that of MyoD, is restricted to cycling cells and regulated by proteolysis at mitosis. In the present study, we have examined Myf5 proteolysis through stable transfection of myogenically convertible U20S cells with Myf5 derivatives under the control of a tetracycline-sensitive promoter. A motif within the basic helix-loop-helix domain of Myf5 (R93 to Q101) resembles the "destruction box" characteristic of substrates of mitotic proteolysis and thought to be recognized by the anaphase-promoting complex or cyclosome (APC). Mutation of this motif in Myf5 stabilizes the protein at mitosis but does not affect its constitutive turnover. Conversely, mutation of a serine residue (S158) stabilizes Myf5 in nonsynchronized cultures but not at mitosis. Thus, at least two proteolytic pathways control Myf5 levels in cycling cells. The mitotic proteolysis of Myf5 is unlike that which has been described for other destruction box-dependent substrates: down-regulation of Myf5 at mitosis appears to precede that of known targets of the APC and is not affected by a dominant-negative version of the ubiquitin carrier protein UbcH10, implicated in the APC-mediated pathway. Finally, we find that induction of Myf5 perturbs the passage of cells through mitosis, suggesting that regulation of Myf5 levels at mitosis may influence cell cycle progression of Myf5-expressing muscle precursor cells.

Cell cycle-dependent expression of mammalian E2-C regulated by the anaphase-promoting complex/cyclosome

Progression through mitosis requires the precisely timed ubiquitin-dependent degradation of specific substrates. E2-C is a ubiquitin-conjugating enzyme that plays a critical role with anaphase-promoting complex/cyclosome (APC/C) in progression of and exit from M phase. Here we report that mammalian E2-C is expressed in late G(2)/M phase and is degraded as cells exit from M phase. The mammalian E2-C shows an autoubiquitinating activity leading to covalent conjugation to itself with several ubiquitins. The ubiquitination of E2-C is strongly enhanced by APC/C, resulting in the formation of a polyubiquitin chain. The polyubiquitination of mammalian E2-C occurs only when cells exit from M phase. Furthermore, mammalian E2-C contains two putative destruction boxes that are believed to act as recognition motifs for APC/C. The mutation of this motif reduced the polyubiquitination of mammalian E2-C, resulting in its stabilization. These results suggest that mammalian E2-C is itself a substrate of the APC/C-dependent proteolysis machinery, and that the periodic expression of mammalian E2-C may be a novel autoregulatory system for the control of the APC/C activity and its substrate specificity.

The anaphase-promoting complex: new subunits and regulators

Ubiquitin-mediated proteolysis of cell cycle regulators is a crucial process during the cell cycle. The anaphase-promoting complex (APC) is a large, multiprotein complex whose E3-ubiquitin ligase activity is required for the ubiquitination of mitotic cyclins and other regulatory proteins that are targeted for destruction during cell division. The recent identification of new APC subunits and regulatory proteins has begun to reveal some of the intricate mechanisms that govern APC regulation. One mechanism is the use of specificity factors to impose temporal control over substrate degradation. A second mechanism is the APC-mediated proteolysis of specific APC regulators. Finally, components of both the APC and the SCF E3 ubiquitin-ligase complex contain several conserved sequence motifs, including WD-40 repeats and cullin homology domains, which suggest that both complexes may use a similar mechanism for substrate ubiquitination.

Molecular cloning of cDNA encoding a ubiquitin-activating enzyme (E1) from goldfish (Carassius auratus) and expression analysis of the cloned gene

Destruction of cyclin B is required to the mitotic and meiotic cycles. A cyclin-specific ubiquitinating system, including ubiquitin-activating enzyme (E1), is thought to be responsible for cyclin B destruction. Here we present the cloning, sequencing and expression analysis of goldfish, Carassius auratus, E1 from goldfish ovary. The cloned cDNA is 4069 bp long and encodes 1059 amino acids. The deduced amino acid sequence is highly homologous to E1 from other species. Recombinant goldfish E1 could transfer ubiquitin to cyclin-selective ubiquitin-conjugating enzyme. Tissue distribution revealed a single 4.0-kb message ubiquitous among tissues.

Degradation of human Aurora2 protein kinase by the anaphase-promoting complex-ubiquitin-proteasome pathway

Human Aurora2 was originally identified by its close homology to yeast IPL1 and fly aurora, which are key regulators of chromosome segregation and a family of serine/threonine kinases. Here we demonstrate that the Aurora2 protein is degraded rapidly after G2/M phase release in mammalian cells. Aurora2 protein has a rapid turnover rate with a half-life of approximately 2 h. In eukaryotic cells, the ubiquitin-proteasome pathway is the major mechanism for the targeted degradation of unstable proteins. The treatment of mammalian cells with proteasome inhibitors blocks Aurora2 degradation. Furthermore, Aurora2 is polyubiquitinated in vivo and in vitro using anaphase-promoting complex (APC). These results demonstrate that Aurora2 protein is turned over through the APC-ubiquitin-proteasome pathway. Oncogene (2000) 19, 2812 - 2819

A Nedd8 conjugation pathway is essential for proteolytic targeting of p27Kip1 by ubiquitination

Temporal control of p27(Kip1) (p27) degradation imposes periodicity in its activity during cell cycle progression and its accumulation during cell cycle exit. Degradation of p27 is initiated by phosphorylation of p27 at Thr-187, which marks the protein for ubiquitination by SCF(Skp2) and subsequent proteolysis by the 26S proteasome. Here we show that the p27 ubiquitination activity in cell extracts depends on the presence of the ubiquitin-like protein Nedd8 and enzymes that catalyze Nedd8 conjugation to proteins. Moreover, we show that reconstitution of the p27 ubiquitination activity of recombinant SCF(Skp2) also requires Nedd8 conjugation pathway components. Inactivation of the Nedd8 conjugation pathway by a dominant negative mutant of the Nedd8-conjugating enzyme Nce1/Ubc12 blocks the ubiquitination and degradation of p27 in cell extracts. Consistent with a role in cell-cycle progression, Nedd8 is expressed in proliferating cells and is itself down-regulated upon cellular differentiation. These results suggest that the Nedd8 conjugation pathway may regulate the turnover of p27(Kip1), independently of p27 phosphorylation, and further establishes the identity of protein components involved in p27 ubiquitination. Finally, these findings provide a direct demonstration of a function for Nedd8 in a biological process.

Nedd8 modification of cul-1 activates SCF(beta(TrCP))-dependent ubiquitination of IkappaBalpha

Regulation of NF-kappaB occurs through phosphorylation-dependent ubiquitination of IkappaBalpha, which is degraded by the 26S proteasome. Recent studies have shown that ubiquitination of IkappaBalpha is carried out by a ubiquitin-ligase enzyme complex called SCF(beta(TrCP)). Here we show that Nedd8 modification of the Cul-1 component of SCF(beta(TrCP)) is important for function of SCF(beta(TrCP)) in ubiquitination of IkappaBalpha. In cells, Nedd8-conjugated Cul-1 was complexed with two substrates of SCF(beta(TrCP)), phosphorylated IkappaBalpha and beta-catenin, indicating that Nedd8-Cul-1 conjugates are part of SCF(beta(TrCP)) in vivo. Although only a minute fraction of total cellular Cul-1 is modified by Nedd8, the Cul-1 associated with ectopically expressed betaTrCP was highly enriched for the Nedd8-conjugated form. Moreover, optimal ubiquitination of IkappaBalpha required Nedd8 and the Nedd8-conjugating enzyme, Ubc12. The site of Nedd8 ligation to Cul-1 is essential, as SCF(beta(TrCP)) containing a K720R mutant of Cul-1 only weakly supported IkappaBalpha ubiquitination compared to SCF(beta(TrCP)) containing WT Cul-1, suggesting that the Nedd8 ligation of Cul-1 affects the ubiquitination activity of SCF(beta(TrCP)). These observations provide a functional link between the highly related ubiquitin and Nedd8 pathways of protein modification and show how they operate together to selectively target the signal-dependent degradation of IkappaBalpha.

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.

Ubiquitin conjugation by the N-end rule pathway and mRNAs for its components increase in muscles of diabetic rats

Insulin deficiency (e.g., in acute diabetes or fasting) is associated with enhanced protein breakdown in skeletal muscle leading to muscle wasting. Because recent studies have suggested that this increased proteolysis is due to activation of the ubiquitin-proteasome (Ub-proteasome) pathway, we investigated whether diabetes is associated with an increased rate of Ub conjugation to muscle protein. Muscle extracts from streptozotocin-induced insulin-deficient rats contained greater amounts of Ub-conjugated proteins than extracts from control animals and also 40-50% greater rates of conjugation of (125)I-Ub to endogenous muscle proteins. This enhanced Ub-conjugation occurred mainly through the N-end rule pathway that involves E2(14k) and E3alpha. A specific substrate of this pathway, alpha-lactalbumin, was ubiquitinated faster in the diabetic extracts, and a dominant negative form of E2(14k) inhibited this increase in ubiquitination rates. Both E2(14k) and E3alpha were shown to be rate-limiting for Ub conjugation because adding small amounts of either to extracts stimulated Ub conjugation. Furthermore, mRNA for E2(14k) and E3alpha (but not E1) were elevated 2-fold in muscles from diabetic rats, although no significant increase in E2(14k) and E3alpha content could be detected by immunoblot or activity assays. The simplest interpretation of these results is that small increases in both E2(14k) and E3alpha in muscles of insulin-deficient animals together accelerate Ub conjugation and protein degradation by the N-end rule pathway, the same pathway activated in cancer cachexia, sepsis, and hyperthyroidism.

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.

Molecular cloning of cDNA encoding a cyclin-selective ubiquitin carrier protein (E2-C) from Carassius auratus (goldfish) and expression analysis of the cloned gene

Destruction of cyclin B is required for exit from mitosis and meiosis. A cyclin-specific ubiquitinating system, including cyclin-selective ubiquitin carrier protein (E2-C), is thought to be responsible for cyclin B destruction. Here we present the cloning, sequencing and expression analysis of goldfish, Carassius auratus, E2-C which encodes the cyclin-selective ubiquitin carrier protein from goldfish ovary. The cloned cDNA is 677 bp long and encodes 172 amino acids. The deduced amino acid sequence is highly homologous to E2-C from other species. Recombinant goldfish E2-C possesses ubiquitinating activity against cyclin B. The expression of mRNA for E2-C was similar to that of mRNA for cyclin B, occurring at very high level in the ovary. The similarity of the expression pattern of E2-C and cyclin B suggests that E2-C mediates a cyclin-specific ubiquitination.

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.

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 ubiquitin-homology protein, DAP-1, associates with tumor necrosis factor receptor (p60) death domain and induces apoptosis

The tumor necrosis factor receptor, p60 (TNF-R1), transduces death signals via the association of its cytoplasmic domain with several intracellular proteins. By screening a mammalian cDNA library using the yeast two-hybrid cloning technique, we isolated a ubiquitin-homology protein, DAP-1, which specifically interacts with the cytoplasmic death domain of TNF-R1. Sequence analysis reveals that DAP-1 shares striking sequence homology with the yeast SMT3 protein that is essential for the maintenance of chromosome integrity during mitosis (Meluh, P. B., and Koshland, D. (1995) Mol. Biol. Cell 6, 793-807). DAP-1 is nearly identical to PIC1, a protein that interacts with the PML tumor suppressor implicated in acute promyelocytic leukemia (Boddy, M. N., Howe, K., Etkin, L. D., Solomon, E., and Freemont, P. S. (1996) Oncogene 13, 971-982), and the sentrin protein, which associates with the Fas death receptor (Okura, T., Gong, L., Kamitani, T., Wada, T., Okura, I., Wei, C. F., Chang, H. M., and Yeh, E. T. (1996) J. Immunol. 157, 4277-4281). The in vivo interaction between DAP-1 and TNF-R1 was further confirmed in mammalian cells. In transient transfection assays, overexpression of DAP-1 suppresses NF-kappaB/Rel activity in 293T cells, a human kidney embryonic carcinoma cell line. Overexpression of either DAP-1 or sentrin causes apoptosis of TNF-sensitive L929 fibroblast cell line, as well as TNF-resistant osteosarcoma cell line, U2OS. Furthermore, the dominant negative Fas-associated death domain protein (FADD) protein blocks the cell death induced by either DAP-1 or FADD. Collectively, these observations highly suggest a role for DAP-1 in mediating TNF-induced cell death signaling pathways, presumably through the recruitment of FADD death effector.

Role of Suc1 in the activation of the cyclosome by protein kinase Cdk1/cyclin B

A large complex, called the cyclosome or anaphase-promoting complex, has specific and regulated protein-ubiquitin ligase activity that targets mitotic regulators (such as cyclin B) for degradation at the end of mitosis. In early embryonic cell cycles the cyclosome is inactive in the interphase, but is subsequently converted by protein kinase Cdk1/cyclin B to an active, phosphorylated form, in a process that includes an initial lag period. This time lag may be important to prevent premature self-inactivation of Cdk1/cyclin B before the end of mitosis. We have previously observed that the phosphorylated form of the cyclosome binds to Suc1, a protein that associates with Cdk1 and with phosphate-containing compounds. We now report that low, physiological concentrations of Suc1 stimulate the activation of the interphase form of the cyclosome by the protein kinase. When Suc1 was present from the beginning of the incubation together with protein kinase Cdk1/cyclin B, activation of the cyclosome took place with the normal lag kinetics. However, when interphase cyclosome was first incubated with protein kinase Cdk1/cyclin B without Suc1, the subsequent addition of Suc1 caused a rapid burst of cyclosome activation and the lag was completely abolished. These findings are consistent with the interpretation that following initial slow phosphorylations of the cyclosome by the protein kinase, Suc1 accelerates multiple phosphorylations that culminate in the full activation of the cyclosome. In support of this interpretation, we find that Suc1 stimulates the phosphorylation of several proteins in the preparation of interphase cyclosome and that the effect of Suc1 on phosphorylation was augmented by prior incubation of interphase cyclosome with protein kinase Cdk1/cyclin B.

Expression of a new isoform of the tumor susceptibility TSG101 protein lacking a leucine zipper domain in Burkitt lymphoma cell lines

The tumor susceptibility gene, TSG101, has been identified as a candidate tumor suppressor gene. We have examined the expression of TSG101 in Burkitt lymphoma cell lines. Several aberrant messages were detected in all cell lines. Aberrant splice donor sites are located within exon 1 at positions 132, 154, 172 and 284. Splice acceptors are located at positions 847 and 1054 within exon 5. The aberrant messages are coexpressed with a normal message and could be the result of additional splicing reactions of the mature message that behaves as an intermediate. The normal message codes for 46 kDa protein (TSG101A). One aberrant message joins in frame nucleotides 283-1055 and codes for a protein isoform of 17 kDa (TSG101B), as demonstrated by in vitro translation assays. The TSG101B isoform lacks the leucine zipper near the C-terminus, a transcriptional repressor domain, and retains most of the N-terminal region which has homology to E2 ubiquitin regulatory enzymes and the CROC-1 transcriptional regulator. The TSG101B isoform was detected in sixteen out of twenty-two (72%) BL cell lines, but not in normal lymphoid populations. The presence of two TSG101 isoforms with different dimerization potential opens up a new level of regulation of the TSG101 proteins possibly affecting cell cycle regulation.

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.

The ubiquitin system

The selective degradation of many short-lived proteins in eukaryotic cells is carried out by the ubiquitin system. In this pathway, proteins are targeted for degradation by covalent ligation to ubiquitin, a highly conserved small protein. Ubiquitin-mediated degradation of regulatory proteins plays important roles in the control of numerous processes, including cell-cycle progression, signal transduction, transcriptional regulation, receptor down-regulation, and endocytosis. The ubiquitin system has been implicated in the immune response, development, and programmed cell death. Abnormalities in ubiquitin-mediated processes have been shown to cause pathological conditions, including malignant transformation. In this review we discuss recent information on functions and mechanisms of the ubiquitin system. Since the selectivity of protein degradation is determined mainly at the stage of ligation to ubiquitin, special attention is focused on what we know, and would like to know, about the mode of action of ubiquitin-protein ligation systems and about signals in proteins recognized by these systems.

The N-end rule pathway catalyzes a major fraction of the protein degradation in skeletal muscle

In skeletal muscle, overall protein degradation involves the ubiquitin-proteasome system. One property of a protein that leads to rapid ubiquitin-dependent degradation is the presence of a basic, acidic, or bulky hydrophobic residue at its N terminus. However, in normal cells, substrates for this N-end rule pathway, which involves ubiquitin carrier protein (E2) E214k and ubiquitin-protein ligase (E3) E3alpha, have remained unclear. Surprisingly, in soluble extracts of rabbit muscle, we found that competitive inhibitors of E3alpha markedly inhibited the 125I-ubiquitin conjugation and ATP-dependent degradation of endogenous proteins. These inhibitors appear to selectively inhibit E3alpha, since they blocked degradation of 125I-lysozyme, a model N-end rule substrate, but did not affect the degradation of proteins whose ubiquitination involved other E3s. The addition of several E2s or E3alpha to the muscle extracts stimulated overall proteolysis and ubiquitination, but only the stimulation by E3alpha or E214k was sensitive to these inhibitors. A similar general inhibition of ubiquitin conjugation to endogenous proteins was observed with a dominant negative inhibitor of E214k. Certain substrates of the N-end rule pathway are degraded after their tRNA-dependent arginylation. We found that adding RNase A to muscle extracts reduced the ATP-dependent proteolysis of endogenous proteins, and supplying tRNA partially restored this process. Finally, although in muscle extracts the N-end rule pathway catalyzes most ubiquitin conjugation, it makes only a minor contribution to overall protein ubiquitination in HeLa cell extracts.

Functional analysis of the Saccharomyces cerevisiae UBC11 gene

UBC11 is the Saccharomyces cerevisiae gene that is most similar in sequence to E2-C, a ubiquitin carrier protein required for the destruction of mitotic cyclins and proteins that maintain sister chromatid cohesion in animal cells and in Schizosaccharomyces pombe. We have disrupted the UBC11 gene and found it is not essential for yeast cell viability even when combined with deletion of UBC4, a gene that has also been implicated in mitotic cyclin destruction. Ubc11p does not ubiquitinate cyclin B in clam cell-free extracts in vitro and the destruction of Clb2p is not impaired in extracts prepared from delta ubc11 or delta ubc4 delta ubc11 cells. These results suggest Ubc4p and Ubc11p together are not essential for mitotic cyclin destruction in S. cerevisiae and we can find no evidence to suggest that Ubc11p is the true functional homologue of E2-C.

PKA and MPF-activated polo-like kinase regulate anaphase-promoting complex activity and mitosis progression

Ubiquitin-mediated proteolysis is the key to cell cycle control. Anaphase-promoting complex/cyclosome (APC) is a ubiquitin ligase that targets cyclin B and factors regulating sister chromatid separation for proteolysis by the proteasome and, consequently, regulates metaphase-anaphase transition and exit from mitosis. Here we report that Cdc2-cyclin B-activated Polo-like kinase (Plk) specifically phosphorylates at least three components of APC and activates APC to ubiquitinate cyclin B in the in vitro-reconstituted system. Conversely, protein kinase A (PKA) phosphorylates two subunits of APC but suppresses APC activity. PKA is superior to Plk in its regulation of APC, and Plk activity peaks whereas PKA activity is falling at metaphase. These results indicate that Plk and PKA regulate mitosis progression by controlling APC activity.

Ubiquitin-dependent degradation of cyclin B is accelerated in polyploid megakaryocytes

During the endomitotic cell cycle of megakaryocytic cell lines, the levels of cyclin B1 and the activity of cyclin B1-dependent Cdc2 kinase, although detectable, are reduced as compared with megakaryocytes undergoing a mitotic cell cycle. The levels of cyclin A, however, are comparable during both cell cycles. The expression of cyclin B1 mRNA is also equivalent in proliferating and polyploidizing cells. In the current study, we found that the rate of cyclin B1 protein degradation is enhanced in polyploidizing megakaryocytes. This finding has led us to further investigate whether the ubiquitin-proteosome pathway responsible for cyclin B degradation is accelerated in these cells. Our data indicate that polyploidizing megakaryocytic cell lines nad primary bone marrow cells treated with the megakaryocyte proliferation- and ploidy-promoting factor, the c-Mpl ligand, display increased activities of the ubiquitin-proteosome pathway, which degrades cyclin B, as compared with proliferating megakaryocytic cell lines or diploid bone marrow cells, respectively. This degradation has all the hallmarks of a ubiquitin pathway, including the dependence on ATP, the appearance of high molecular weight conjugated forms of cyclin B, and inhibition of the proteolytic process by a mutated form of the ubiquitin-conjugating enzyme Ubc4. Our studies also indicate that the ability to degrade cyclin A is equivalent in both the mitotic and endomitotic cell cycles. The increased potential of polyploid megakaryocytes to degrade cyclin B may be part of the cellular programming that leads to aborted mitosis.

Oncoprotein MDM2 is a ubiquitin ligase E3 for tumor suppressor p53

The tumor suppressor p53 is degraded by the ubiquitin-proteasome system. p53 was polyubiquitinated in the presence of E1, UbcH5 as E2 and MDM2 oncoprotein. A ubiquitin molecule bound MDM2 through sulfhydroxy bond which is characteristic of ubiquitin ligase (E3)-ubiquitin binding. The cysteine residue in the carboxyl terminus of MDM2 was essential for the activity. These data suggest that the MDM2 protein, which is induced by p53, functions as a ubiquitin ligase, E3, in human papillomavirus-uninfected cells which do not have E6 protein.

Roles of ubiquitin-mediated proteolysis in cell cycle control

Selective degradation of cyclins, inhibitors of cyclin-dependent kinases and anaphase inhibitors is responsible for several major cell cycle transitions. The degradation of these cell cycle regulators is controlled by the action of ubiquitin-protein-ligase complexes, which target the regulators for degradation by the 26S proteasome. Recent results indicate that two types of multisubunit ubiquitin ligase complexes, which are connected to the protein kinase regulatory network of the cell cycle in different ways, are responsible for the specific and programmed degradation of many cell cycle regulators.