Yeast Chfr homologs retard cell cycle at G1 and G2/M via Ubc4 and Ubc13/Mms2-dependent ubiquitination

A novel role for Mms2 in the control of spontaneous mutagenesis and Pol3 abundance

Mms2 is a ubiquitin E2-variant protein with a very well-documented function in the tolerance pathway that protects both human and yeast cells from the lethal and mutagenic effects of DNA damage. Interestingly, a high expression level of human MMS2 is associated with poor survival prognosis in different cancer diseases. Here we have analyzed the physiological effects of Mms2 overproduction in yeast cells. We show that an increased level of this protein causes a spontaneous mutator effect independent of Ubc13, a cognate partner of Mms2 in the PCNA-polyubiquitinating complex responsible for the template switch. Instead, this new promutagenic role of Mms2 requires Ubc4 (E2) and two ubiquitin ligases of HECT and RING families, Rsp5 and Not4, respectively. We have established that the promutagenic activity of Mms2 is dependent on the activities of error-prone DNA polymerase ζ and Rev1. Additionally, it requires the ubiquitination of K164 in PCNA which facilitates recruitment of these translesion polymerases to the replication complex. Importantly, we have established also that the cellular abundance of Mms2 influences the cellular level of Pol3, the catalytic subunit of replicative DNA polymerase δ. Lack of Mms2 increases the Pol3 abundance, whereas in response to Mms2 overproduction the Pol3 level decreases. We hypothesize that increased levels of spontaneous mutagenesis may result from the Mms2-induced reduction in Pol3 accumulation leading to increased participation of error-prone polymerase ζ in the replication complex.

Spatiotemporal regulation of the Dma1-mediated mitotic checkpoint coordinates mitosis with cytokinesis

During cell division, the timing of mitosis and cytokinesis must be ordered to ensure that each daughter cell receives a complete, undamaged copy of the genome. In fission yeast, the septation initiation network (SIN) is responsible for this coordination, and a mitotic checkpoint dependent on the E3 ubiquitin ligase Dma1 and the protein kinase CK1 controls SIN signaling to delay cytokinesis when there are errors in mitosis. The participation of kinases and ubiquitin ligases in cell cycle checkpoints that maintain genome integrity is conserved from yeast to human, making fission yeast an excellent model system in which to study checkpoint mechanisms. In this review, we highlight recent advances and remaining questions related to checkpoint regulation, which requires the synchronized modulation of protein ubiquitination, phosphorylation, and subcellular localization.

Ubc13: the Lys63 ubiquitin chain building machine

Ubc13 is an ubiquitin E2 conjugating enzyme that participates with many different E3 ligases to form lysine 63-linked (Lys63) ubiquitin chains that are critical to signaling in inflammatory and DNA damage response pathways. Recent studies have suggested Ubc13 as a potential therapeutic target for intervention in various human diseases including several different cancers, alleviation of anti-cancer drug resistance, chronic inflammation, and viral infections. Understanding a potential therapeutic target from different angles is important to assess its usefulness and potential pitfalls. Here we present a global review of Ubc13 from its structure, function, and cellular activities, to its natural and chemical inhibition. The aim of this article is to review the literature that directly implicates Ubc13 in a biological function, and to integrate structural and mechanistic insights into the larger role of this critical E2 enzyme. We discuss observations of multiple Ubc13 structures that suggest a novel mechanism for activation of Ubc13 that involves conformational change of the active site loop.

Control of Formin Distribution and Actin Cable Assembly by the E3 Ubiquitin Ligases Dma1 and Dma2

Formins are widespread actin-polymerizing proteins that play pivotal roles in a number of processes, such as cell polarity, morphogenesis, cytokinesis, and cell migration. In agreement with their crucial function, formins are prone to a variety of regulatory mechanisms that include autoinhibition, post-translational modifications, and interaction with formin modulators. Furthermore, activation and function of formins is intimately linked to their ability to interact with membranes. In the budding yeast Saccharomyces cerevisiae, the two formins Bni1 and Bnr1 play both separate and overlapping functions in the organization of the actin cytoskeleton. In addition, they are controlled by both common and different regulatory mechanisms. Here we show that proper localization of both formins requires the redundant E3 ubiquitin ligases Dma1 and Dma2, which were previously involved in spindle positioning and septin organization. In dma1 dma2 double mutants, formin distribution at polarity sites is impaired, thus causing defects in the organization of the actin cable network and hypersensitivity to the actin depolymerizer latrunculin B. Expression of a hyperactive variant of Bni1 (Bni1-V360D) rescues these defects and partially restores proper spindle positioning in the mutant, suggesting that the failure of dma1 dma2 mutant cells to position the spindle is partly due to faulty formin activity. Strikingly, Dma1/2 interact physically with both formins, while their ubiquitin-ligase activity is required for formin function and polarized localization. Thus, ubiquitylation of formin or a formin interactor(s) could promote formin binding to membrane and its ability to nucleate actin. Altogether, our data highlight a novel level of formin regulation that further expands our knowledge of the complex and multilayered controls of these key cytoskeleton organizers.

Bioinformatics analysis identifies several intrinsically disordered human E3 ubiquitin-protein ligases

The ubiquitin-proteasome system targets misfolded proteins for degradation. Since the accumulation of such proteins is potentially harmful for the cell, their prompt removal is important. E3 ubiquitin-protein ligases mediate substrate ubiquitination by bringing together the substrate with an E2 ubiquitin-conjugating enzyme, which transfers ubiquitin to the substrate. For misfolded proteins, substrate recognition is generally delegated to molecular chaperones that subsequently interact with specific E3 ligases. An important exception is San1, a yeast E3 ligase. San1 harbors extensive regions of intrinsic disorder, which provide both conformational flexibility and sites for direct recognition of misfolded targets of vastly different conformations. So far, no mammalian ortholog of San1 is known, nor is it clear whether other E3 ligases utilize disordered regions for substrate recognition. Here, we conduct a bioinformatics analysis to examine >600 human and S. cerevisiae E3 ligases to identify enzymes that are similar to San1 in terms of function and/or mechanism of substrate recognition. An initial sequence-based database search was found to detect candidates primarily based on the homology of their ordered regions, and did not capture the unique disorder patterns that encode the functional mechanism of San1. However, by searching specifically for key features of the San1 sequence, such as long regions of intrinsic disorder embedded with short stretches predicted to be suitable for substrate interaction, we identified several E3 ligases with these characteristics. Our initial analysis revealed that another remarkable trait of San1 is shared with several candidate E3 ligases: long stretches of complete lysine suppression, which in San1 limits auto-ubiquitination. We encode these characteristic features into a San1 similarity-score, and present a set of proteins that are plausible candidates as San1 counterparts in humans. In conclusion, our work indicates that San1 is not a unique case, and that several other yeast and human E3 ligases have sequence properties that may allow them to recognize substrates by a similar mechanism as San1.

UbiSite: incorporating two-layered machine learning method with substrate motifs to predict ubiquitin-conjugation site on lysines

Background

The conjugation of ubiquitin to a substrate protein (protein ubiquitylation), which involves a sequential process – E1 activation, E2 conjugation and E3 ligation, is crucial to the regulation of protein function and activity in eukaryotes. This ubiquitin-conjugation process typically binds the last amino acid of ubiquitin (glycine 76) to a lysine residue of a target protein. The high-throughput of mass spectrometry-based proteomics has stimulated a large-scale identification of ubiquitin-conjugated peptides. Hence, a new web resource, UbiSite, was developed to identify ubiquitin-conjugation site on lysines based on large-scale proteome dataset.

Results

Given a total of 37,647 ubiquitin-conjugated proteins, including 128026 ubiquitylated peptides, obtained from various resources, this study carries out a large-scale investigation on ubiquitin-conjugation sites based on sequenced and structural characteristics. A TwoSampleLogo reveals that a significant depletion of histidine (H), arginine (R) and cysteine (C) residues around ubiquitylation sites may impact the conjugation of ubiquitins in closed three-dimensional environments. Based on the large-scale ubiquitylation dataset, a motif discovery tool, MDDLogo, has been adopted to characterize the potential substrate motifs for ubiquitin conjugation. Not only are single features such as amino acid composition (AAC), positional weighted matrix (PWM), position-specific scoring matrix (PSSM) and solvent-accessible surface area (SASA) considered, but also the effectiveness of incorporating MDDLogo-identified substrate motifs into a two-layered prediction model is taken into account. Evaluation by five-fold cross-validation showed that PSSM is the best feature in discriminating between ubiquitylation and non-ubiquitylation sites, based on support vector machine (SVM). Additionally, the two-layered SVM model integrating MDDLogo-identified substrate motifs could obtain a promising accuracy and the Matthews Correlation Coefficient (MCC) at 81.06 % and 0.586, respectively. Furthermore, the independent testing showed that the two-layered SVM model could outperform other prediction tools, reaching at 85.10 % sensitivity, 69.69 % specificity, 73.69 % accuracy and the 0.483 of MCC value.

Conclusion

The independent testing result indicated the effectiveness of incorporating MDDLogo-identified motifs into the prediction of ubiquitylation sites. In order to provide meaningful assistance to researchers interested in large-scale ubiquitinome data, the two-layered SVM model has been implemented onto a web-based system (UbiSite), which is freely available at http://csb.cse.yzu.edu.tw/UbiSite/. Two cases given in the UbiSite provide a demonstration of effective identification of ubiquitylation sites with reference to substrate motifs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0246-z) contains supplementary material, which is available to authorized users.

Release from myosin V via regulated recruitment of an E3 ubiquitin ligase controls organelle localization

Molecular motors transport organelles to specific subcellular locations. Upon arrival at their correct locations, motors release organelles via unknown mechanisms. The yeast myosin V, Myo2, binds the vacuole-specific adaptor Vac17 to transport the vacuole from the mother cell to the bud. Here, we show that vacuole detachment from Myo2 occurs in multiple regulated steps along the entire pathway of vacuole transport. Detachment initiates in the mother cell with the phosphorylation of Vac17 that recruits the E3 ligase Dma1 to the vacuole. However, Dma1 recruitment also requires the assembly of the vacuole transport complex and is first observed after the vacuole enters the bud. Dma1 remains on the vacuole until the bud and mother vacuoles separate. Subsequently, Dma1 targets Vac17 for proteasomal degradation. Notably, we find that the termination of peroxisome transport also requires Dma1. We predict that this is a general mechanism that detaches myosin V from select cargoes.

Saccharomyces cerevisiae Dma proteins participate in cytokinesis by controlling two different pathways

Cytokinesis completion in the budding yeast S. cerevisiae is driven by tightly regulated pathways, leading to actomyosin ring contraction coupled to plasma membrane constriction and to centripetal growth of the primary septum, respectively. These pathways can partially substitute for each other, but their concomitant inactivation leads to cytokinesis block and cell death. Here we show that both the lack of the functionally redundant FHA-RING ubiquitin ligases Dma1 and Dma2 and moderate Dma2 overproduction affect actomyosin ring contraction as well as primary septum deposition, although they do not apparently alter cell cycle progression of otherwise wild-type cells. In addition, overproduction of Dma2 impairs the interaction between Tem1 and Iqg1, which is thought to be required for AMR contraction, and causes asymmetric primary septum deposition as well as mislocalization of the Cyk3-positive regulator of this process. In agreement with these multiple inhibitory effects, a Dma2 excess that does not cause any apparent defect in wild-type cells leads to lethal cytokinesis block in cells lacking the Hof1 protein, which is essential for primary septum formation in the absence of Cyk3. Altogether, these findings suggest that the Dma proteins act as negative regulators of cytokinesis.

Dma/RNF8 proteins are evolutionarily conserved E3 ubiquitin ligases that target septins

The budding yeast proteins Dma1 and Dma2 are members of the unique FHA-RING domain protein family and are linked to mitotic regulation and septin organization by ill-defined mechanisms. We show that Dma2 has ubiquitin ligase activity, and that septins Shs1 and Cdc11 are likely direct in vivo targets. We further propose that human RNF8, rather than Chfr, is the mammalian Dma homolog. As in yeast, RNF8 localizes to the centrosomes and cell division sites and promotes ubiquitylation of the septin SEPT7, whose depletion increases cell division anomalies. Together, these findings reveal evolutionary and functional conservation of Dma proteins, and suggest that RNF8 maintains genome stability through independent, yet analogous, nuclear and cytoplasmic ubiquitylation activities.

Defective in mitotic arrest 1 (Dma1) ubiquitin ligase controls G1 cyclin degradation

Progression through the G(1) phase of the cell cycle is controlled by diverse cyclin-dependent kinases (CDKs) that might be associated to numerous cyclin isoforms. Given such complexity, regulation of cyclin degradation should be crucial for coordinating progression through the cell cycle. In Saccharomyces cerevisiae, SCF is the only E3 ligase known to date to be involved in G(1) cyclin degradation. Here, we report the design of a genetic screening that uncovered Dma1 as another E3 ligase that targets G(1) cyclins in yeast. We show that the cyclin Pcl1 is ubiquitinated in vitro and in vivo by Dma1, and accordingly, is stabilized in dma1 mutants. We demonstrate that Pcl1 must be phosphorylated by its own CDK to efficiently interact with Dma1 and undergo degradation. A nonphosphorylatable version of Pcl1 accumulates throughout the cell cycle, demonstrating the physiological relevance of the proposed mechanism. Finally, we present evidence that the levels of Pcl1 and Cln2 are independently controlled in response to nutrient availability. This new previously unknown mechanism for G(1) cyclin degradation that we report here could help elucidate the specific roles of the redundant CDK-cyclin complexes in G(1).

Dnt1 acts as a mitotic inhibitor of the spindle checkpoint protein dma1 in fission yeast

The interaction between Dma1 and Dnt1 in fission yeast is characterized. The results show that, similar to its homologue Chfr in higher eukaryotes, Dma1 in fission yeast can also affect factors required for microtubule nucleation and spindle formation at early mitosis.

The Schizosaccharomyces pombe checkpoint protein Dma1 couples mitotic progression with cytokinesis and is important in delaying mitotic exit and cytokinesis when kinetochores are not properly attached to the mitotic spindle. Dma1 is a ubiquitin ligase and potential functional relative of the human tumor suppressor Chfr. Dma1 delays mitotic exit and cytokinesis by ubiquitinating a scaffold protein (Sid4) of the septation initiation network, which, in turn, antagonizes the ability of the Polo-like kinase Plo1 to promote cell division. Here we identify Dnt1 as a Dma1-binding protein. Several lines of evidence indicate that Dnt1 inhibits Dma1 function during metaphase. First, Dnt1 interacts preferentially with Dma1 during metaphase. Second, Dma1 ubiquitin ligase activity and Sid4 ubiquitination are elevated in dnt1∆ cells. Third, the enhanced mitotic defects in dnt1Δ plo1 double mutants are partially rescued by deletion of dma1⁺, suggesting that the defects in dnt1∆ plo1 double mutants are attributable to excess Dma1 activity. Taken together, these data show that Dnt1 acts to restrain Dma1 activity in early mitosis to allow normal mitotic progression.

Budding yeast dma proteins control septin dynamics and the spindle position checkpoint by promoting the recruitment of the Elm1 kinase to the bud neck

The first step towards cytokinesis in budding yeast is the assembly of a septin ring at the future site of bud emergence. Integrity of this ring is crucial for cytokinesis, proper spindle positioning, and the spindle position checkpoint (SPOC). This checkpoint delays mitotic exit and cytokinesis as long as the anaphase spindle does not properly align with the division axis. SPOC signalling requires the Kin4 protein kinase and the Kin4-regulating Elm1 kinase, which also controls septin dynamics. Here, we show that the two redundant ubiquitin-ligases Dma1 and Dma2 control septin dynamics and the SPOC by promoting the efficient recruitment of Elm1 to the bud neck. Indeed, dma1 dma2 mutant cells show reduced levels of Elm1 at the bud neck and Elm1-dependent activation of Kin4. Artificial recruitment of Elm1 to the bud neck of the same cells is sufficient to re-establish a normal septin ring, proper spindle positioning, and a proficient SPOC response in dma1 dma2 cells. Altogether, our data indicate that septin dynamics and SPOC function are intimately linked and support the idea that integrity of the bud neck is crucial for SPOC signalling.

CHFR suppression by hypermethylation sensitizes endometrial cancer cells to paclitaxel

OBJECTIVE

Impairment of a cell cycle checkpoint is often associated with sensitivity to chemotherapeutic drugs. Here, we studied the correlations between the checkpoint with forkhead-associated and ring finger (CHFR) gene expression and responses to paclitaxel in endometrial cancer cells.

METHODS

We cultured 6 endometrial cancer cell lines exposed to paclitaxel, studied the cell cytotoxicity, cell cycle distribution, CHFR expression, and methylation status before and after a demethylation agent (5-aza) treatment. CHFR was silenced by small interfering RNA (siRNA). Then we examined tumor growth and CHFR expression with paclitaxel alone or combined with 5-aza pretreatment in vivo.

RESULTS

We found that HEC-1B, RL-952, and AN3CA cells were sensitive to paclitaxel. Moreover, CHFR was weakly expressed in these cells, whereas paclitaxel-resistant cells (ISH, HEC-1A, and KLE) had high CHFR expression. Then we found that restored expression of CHFR by demethylation decreased the sensitivity to paclitaxel in AN3CA cells. In addition, cells with CHFR demethylation resulted in G2/M phase arrest that induced to paclitaxel resistance. These results were confirmed again in small interfering RNA-transfected HEC-1A cells. Furthermore, in nude mice model, restored expression of CHFR by demethylation inhibited tumor growth and decreased sensitivity to paclitaxel.

CONCLUSION

Our data suggest that CHFR suppression regulated by hypermethylation may sensitize endometrial cancer cells to paclitaxel, and CHFR may be a promising marker to predict the response of endometrial cancer to paclitaxel.

Substrate analysis of the Pneumocystis carinii protein kinases PcCbk1 and PcSte20 using yeast proteome microarrays provides a novel method for Pneumocystis signalling biology

Pneumocystis carinii (Pc) undergoes morphological transitions between cysts and trophic forms. We have previously described two Pc serine/threonine kinases, termed PcCbk1 and PcSte20, with PcSte20 belonging to a family of kinases involved in yeast mating, while PcCbk1 is a member of a group of protein kinases involved in regulation of cell cycle, shape, and proliferation. As Pc remains genetically intractable, knowledge on specific substrates phosphorylated by these kinases remains limited. Utilizing the phylogenetic relatedness of Pc to Saccharomyces cerevisiae, we interrogated a yeast proteome microarray containing >4000 purified protein based peptides, leading to the identification of 18 potential PcCbk1 and 15 PcSte20 substrates (Z-score > 3.0). A number of these potential protein substrates are involved in bud site selection, polarized growth, and response to mating α factor and pseudohyphal and invasive growth. Full-length open reading frames suggested by the PcCbk1 and PcSte20 protoarrays were amplified and expressed. These five proteins were used as substrates for PcCbk1 or PcSte20, with each being highly phosphorylated by the respective kinase. Finally, to demonstrate the utility of this method to identify novel PcCbk1 and PcSte20 substrates, we analysed DNA sequence data from the partially complete Pc genome database and detected partial sequence information of potential PcCbk1 kinase substrates PcPxl1 and PcInt1. We additionally identified the potential PcSte20 kinase substrate PcBdf2. Full-length Pc substrates were cloned and expressed in yeast, and shown to be phosphorylated by the respective Pc kinases. In conclusion, the yeast protein microarray represents a novel crossover technique for identifying unique potential Pc kinase substrates.

The auto-ubiquitylation of E3 ubiquitin-protein ligase Chfr at G2 phase is required for accumulation of polo-like kinase 1 and mitotic entry in mammalian cells

The E3 ubiquitin-protein ligase Chfr is a mitotic stress checkpoint protein that delays mitotic entry in response to microtubule damage; however, the molecular mechanism by which Chfr accomplishes this remains elusive. Here, we show that Chfr levels are elevated in response to microtubule-damaging stress. Moreover, G(2)/M transition is associated with cell cycle-dependent turnover of Chfr accompanied by high autoubiquitylation activity, suggesting that regulation of Chfr levels and auto-ubiquitylation activity are functionally significant. To test this, we generated Chfr mutants Chfr-K2A and Chfr-K5A in which putative lysine target sites of auto-ubiquitylation were replaced with alanine. Chfr-K2A did not undergo cell cycle-dependent degradation, and its levels remained high during G(2)/M phase. The elevated levels of Chfr-K2A caused a significant reduction in phosphohistone H3 levels and cyclinB1/Cdk1 kinase activities, leading to mitotic entry delay. Notably, polo-like kinase 1 levels at G(2) phase, but not at S phase, were ∼2-3-fold lower in cells expressing Chfr-K2A than in wild-type Chfr-expressing cells. Consistent with this, ubiquitylation of Plk1 at G(2) phase was accelerated in Chfr-K2A-expressing cells. In contrast, Aurora A levels remained constant, indicating that Plk1 is a major target of Chfr in controlling the timing of mitotic entry. Indeed, overexpression of Plk1 in Chfr-K2A-expressing cells restored cyclin B1/Cdk1 kinase activity and promoted mitotic entry. Collectively, these data indicate that Chfr auto-ubiquitylation is required to allow Plk1 to accumulate to levels necessary for activation of cyclin B1/Cdk1 kinase and mitotic entry. Our results provide the first evidence that Chfr auto-ubiquitylation and degradation are important for the G(2)/M transition.

Budding yeast Dma1 and Dma2 participate in regulation of Swe1 levels and localization

Timely down-regulation of the evolutionarily conserved protein kinase Swe1 plays an important role in cell cycle control, as Swe1 can block nuclear division through inhibitory phosphorylation of the catalytic subunit of cyclin-dependent kinase. In particular, Swe1 degradation is important for budding yeast cell survival in case of DNA replication stress, whereas it is inhibited by the morphogenesis checkpoint in response to alterations in actin cytoskeleton or septin structure. We show that the lack of the Dma1 and Dma2 ubiquitin ligases, which moderately affects Swe1 localization and degradation during an unperturbed cell cycle with no apparent phenotypic effects, is toxic for cells that are partially defective in Swe1 down-regulation. Moreover, Swe1 is stabilized, restrained at the bud neck, and hyperphosphorylated in dma1Δ dma2Δ cells subjected to DNA replication stress, indicating that the mechanism stabilizing Swe1 under these conditions is different from the one triggered by the morphogenesis checkpoint. Finally, the Dma proteins are required for proper Swe1 ubiquitylation. Taken together, the data highlight a previously unknown role of these proteins in the complex regulation of Swe1 and suggest that they might contribute to control, directly or indirectly, Swe1 ubiquitylation.

Dma1 ubiquitinates the SIN scaffold, Sid4, to impede the mitotic localization of Plo1 kinase

Proper cell division requires strict coordination between mitotic exit and cytokinesis. In the event of a mitotic error, cytokinesis must be inhibited to ensure equal partitioning of genetic material. In the fission yeast, Schizosaccharomyces pombe, the checkpoint protein and E3 ubiquitin ligase, Dma1, delays cytokinesis by inhibiting the septation initiation network (SIN) when chromosomes are not attached to the mitotic spindle. To elucidate the mechanism by which Dma1 inhibits the SIN, we screened all SIN components as potential Dma1 substrates and found that the SIN scaffold protein, Sid4, is ubiquitinated in vivo in a Dma1-dependent manner. To investigate the role of Sid4 ubiquitination in checkpoint function, a ubiquitination deficient sid4 allele was generated and our data indicate that Sid4 ubiquitination by Dma1 is required to prevent cytokinesis during a mitotic checkpoint arrest. Furthermore, Sid4 ubiquitination delays recruitment of the Polo-like kinase and SIN activator, Plo1, to spindle pole bodies (SPBs), while at the same time prolonging residence of the SIN inhibitor, Byr4, providing a mechanistic link between Dma1 activity and cytokinesis inhibition.

Degradation of the Saccharomyces cerevisiae mating-type regulator alpha1: genetic dissection of cis-determinants and trans-acting pathways

Mating phenotype in the yeast Saccharomyces cerevisiae is a dynamic trait, and efficient transitions between alternate haploid cell types allow the organism to access the advantageous diploid form. Mating identity is determined by cell type-specific transcriptional regulators, but these factors must be rapidly removed upon mating-type switching to allow the master regulators of the alternate state to establish a new gene expression program. Targeted proteolysis by the ubiquitin-proteasome system is a commonly employed strategy to quickly disassemble regulatory networks, and yeast use this approach to evoke efficient switching from the alpha to the a phenotype by ensuring the rapid removal of the alpha2 transcriptional repressor. Transition to the a cell phenotype, however, also requires the inactivation of the alpha1 transcriptional activator, but the mechanism by which this occurs is currently unknown. Here, we report a central role for the ubiquitin-proteasome system in alpha1 inactivation. The alpha1 protein is constitutively short lived and targeted for rapid turnover by multiple ubiquitin-conjugation pathways. Intriguingly, the alpha-domain, a conserved region of unknown function, acts as a degradation signal for a pathway defined by the SUMO-targeted ligase Slx5-Slx8, which has also been implicated in the rapid destruction of alpha2. Our observations suggest coordinate regulation in the turnover of two master regulatory transcription factors ensures a rapid mating-type switch.

TRAF-interacting protein with a forkhead-associated domain B (TIFAB) is a negative regulator of the TRAF6-induced cellular functions

Tumour necrosis factor receptor-associated factor (TRAF)-interacting protein with a forkhead-associated domain (TIFA) activates TRAF6 to induce NF-kappaB activation. TIFA-related protein, TIFAB, is highly expressed in the spleen and inhibits TIFA-mediated TRAF6 activation. However, little is known about cell types that express TIFAB and its function in those cells. Here, we show that TIFAB is mainly expressed in B cells rather than T cells in the spleen and that the expression level was much higher in dendritic cells (DCs) and macrophages than that in splenic lymphocytes. TIFAB expression was downregulated when B cells, DCs or macrophages were stimulated by TRAF6-mediated proliferative or maturation signals including those emanating from CD40, sIgM and TLRs. Furthermore, microinjection experiments using NIH3T3 cells revealed that TIFAB inhibited entry into the S phase of the cell cycle. Our results suggest that TIFAB could act as a negative regulator of the TRAF6-induced cellular function such as B cell proliferation and maturation of DCs and macrophages.

FHA-RING ubiquitin ligases in cell division cycle control

Despite the common occurrence of forkhead associated (FHA) phosphopeptide-binding domains and really interesting new gene (RING) E3 ubiquitin ligase domains, gene products containing both an N-terminal FHA domain and C-terminal RING domain constitute a highly distinctive intersection. Characterized FHA-RING ligases include the two vertebrate proteins, Checkpoint with FHA and RING (Chfr) and RING finger 8 (Rnf8), as well as three fungal proteins, Defective in mitosis (Dma1), Chf1 and Chf2. These FHA-RING ligases play roles in negative regulation of the cell division cycle, apparently by coupling protein phosphorylation events to specific ubiquitylation of target proteins. Here, the available data on upstream and downstream regulation of and by FHA-RING ligases are reviewed.

Chfr interacts and colocalizes with TCTP to the mitotic spindle

Chfr is a checkpoint protein that plays an important function in cell cycle progression and tumor suppression, although its exact role and regulation are unclear. Previous studies have utilized overexpression of Chfr to determine the signaling pathway of this protein in vivo. In this study, we demonstrate, by using three different antibodies against Chfr, that the endogenous and highly overexpressed ectopic Chfr protein is localized and regulated differently in cells. Endogenous and lowly expressed ectopic Chfr are cytoplasmic and localize to the spindle during mitosis. Higher expression of ectopic Chfr correlates with a shift in the localization of this protein to the nucleus/PML bodies, and with a block of cell proliferation. In addition, endogenous and lowly expressed ectopic Chfr is stable throughout the cell cycle, whereas when highly expressed, ectopic Chfr is actively degraded during S-G2/M phases in an autoubiquitination and proteasome-dependent manner. A two-hybrid screen identified TCTP as a possible Chfr-interacting partner. Biochemical analysis with the endogenous proteins confirmed this interaction and identified beta-tubulin as an additional partner for Chfr, supporting the mitotic spindle localization of Chfr. The Chfr-TCTP interaction was stable throughout the cell cycle, but it could be diminished by the complete depolymerization of the microtubules, providing a possible mechanism where Chfr could be the sensor that detects microtubule disruption and then activates the prophase checkpoint.