A novel component involved in ubiquitination is required for development of Dictyostelium discoideum

Arabidopsis thaliana ubiquitin-associated protein 2 (AtUAP2) functions as an E4 ubiquitin factor and negatively modulates dehydration stress response

E4, a ubiquitin (Ub) chain assembly factor and post-translational modification protein, plays a key role in the regulation of multiple cellular functions in plants during biotic or abiotic stress. We have more recently reported that E4 factor AtUAP1 is a negative regulator of the osmotic stress response and enhances the multi-Ub chain assembly of E3 ligase Arabidopsis thaliana RING Zinc Finger 1 (AtRZF1). To further investigate the function of other E4 Ub factors in osmotic stress, we isolated AtUAP2, an AtUAP1 homolog, which interacted with AtRZF1, using pull-down assay and bimolecular fluorescence complementation analysis. AtUAP2, a Ub-associated motif-containing protein, interacts with oligo-Ub5, -Ub6, and -Ub7 chains. The yeast functional complementation experiment revealed that AtUAP2 functions as an E4 Ub factor. In addition, AtUAP2 is localized in the cytoplasm, different from AtUAP1. The activity of AtUAP2 was relatively strongly induced in the leaf tissue of AtUAP2 promoter-β-glucuronidase transgenic plants by abscisic acid, dehydration, and oxidative stress. atuap2 RNAi lines were more insensitive to osmotic stress condition than wild-type during the early growth of seedlings, whereas the AtUAP2-overexpressing line exhibited relatively more sensitive responses. Analyses of molecular and physiological experiments showed that AtUAP2 could negatively mediate the osmotic stress-induced signaling. Genetic studies showed that AtRZF1 mutation could suppress the dehydration-induced sensitive phenotype of the AtUAP2-overexpressing line, suggesting that AtRZF1 acts genetically downstream of AtUAP2 during osmotic stress. Taken together, our findings show that the AtRZF1-AtUAP2 complex may play important roles in the ubiquitination pathway, which controls the osmotic stress response in Arabidopsis.

The SWIB/MDM2 motif of UBE4B activates the p53 pathway

The tumor suppressor p53 plays a critical role in cancer pathogenesis, and regulation of p53 expression is essential for maintaining normal cell growth. UBE4B is an E3/E4 ubiquitin ligase involved in a negative-feedback loop with p53. UBE4B is required for Hdm2-mediated p53 polyubiquitination and degradation. Thus, targeting the p53-UBE4B interactions is a promising anticancer strategy for cancer therapy. In this study, we confirm that while the UBE4B U box does not bind to p53, it is essential for the degradation of p53 and acts in a dominant-negative manner, thereby stabilizing p53. C-terminal UBE4B mutants lose their ability to degrade p53. Notably, we identified one SWIB/Hdm2 motif of UBE4B that is vital for p53 binding. Furthermore, the novel UBE4B peptide activates p53 functions, including p53-dependent transactivation and growth inhibition, by blocking the p53-UBE4B interactions. Our findings indicate that targeting the p53-UBE4B interaction presents a novel approach for p53 activation therapy in cancer.

PTBP3 modulates P53 expression and promotes colorectal cancer cell proliferation by maintaining UBE4A mRNA stability

The RNA binding protein PTBP3 was recently reported to play a critical role in multiple cancers, and the molecular mechanisms involved RNA splicing, 3′ end processing and translation. However, the role of PTBP3 in colorectal cancer (CRC) remains poorly explored. Herein, PTBP3 was upregulated in CRC and associated with a poor prognosis. PTBP3 knockdown in colorectal cancer cell lines restricted CRC proliferative capacities in vitro and in vivo. Mechanistically, PTBP3 regulated the expression of the E3 ubiquitin ligase UBE4A by binding the 3′ UTR of its mRNA, preventing its degradation. UBE4A participated in P53 degradation, and PTBP3 knockdown in colorectal cancer cell lines showed increased P53 expression. UBE4A overexpression rescued PTBP3 knockdown-induced inhibition of CRC cell proliferation and P53 expression. Our results demonstrated that PTBP3 plays an essential role in CRC cell proliferation by stabilizing UBE4A to regulate P53 expression and may serve as a new prognostic biomarker and effective therapeutic target for CRC.

The Downregulation of Both Giant HERCs, HERC1 and HERC2, Is an Unambiguous Feature of Chronic Myeloid Leukemia, and HERC1 Levels Are Associated with Leukemic Cell Differentiation

Large HERC E3 ubiquitin ligase family members, HERC1 and HERC2, are staggeringly complex proteins that can intervene in a wide range of biological processes, such as cell proliferation, DNA repair, neurodevelopment, and inflammation. Therefore, mutations or dysregulation of large HERCs is associated with neurological disorders, DNA repair defects, and cancer. Though their role in solid tumors started to be investigated some years ago, our knowledge about HERCs in non-solid neoplasm is greatly lagging behind. Chronic Myeloid Leukemia (CML) is a model onco-hematological disorder because of its unique and unambiguous relation between genotype and phenotype due to a single genetic alteration. In the present study, we ascertained that the presence of the BCR-ABL fusion gene was inversely associated with the expression of the HERC1 and HERC2 genes. Upon the achievement of remission, both HERC1 and HERC2 mRNAs raised again to levels comparable to those of the healthy donors. Additionally, our survey unveiled that their gene expression is sensitive to different Tyrosine Kinases Inhibitors (TKIs) in a time-dependent fashion. Interestingly, for the first time, we also observed a differential HERC1 expression when the leukemic cell lines were induced to differentiate towards different lineages revealing that HERC1 protein expression is associated with the differentiation process in a lineage-specific manner. Taken together, our findings suggest that HERC1 might act as a novel potential player in blood cell differentiation. Overall, we believe that our results are beneficial to initiate exploring the role/s of large HERCs in non-solid neoplasms.

The wheat E3 ligase TaPUB26 is a negative regulator in response to salt stress in transgenic Brachypodium distachyon

Various abiotic stresses, including high salinity, affect the growth and yield of crop plants. We isolated a gene, TaPUB26, from wheat that encodes a protein containing a U-box domain and armadillo (ARM) repeats. The TaPUB26 transcript levels were upregulated by high salinity, temperature, drought and phytohormones, suggesting the involvement of TaPUB26 in abiotic stress responses. An in vitro ubiquitination assay revealed that TaPUB26 is an E3 ubiquitin ligase. We overexpressed TaPUB26 in Brachypodium distachyon to evaluate TaPUB26 regulation of salt stress tolerance. Compared with the wild type (WT) line, the overexpression lines showed higher salt stress sensitivity under salt stress conditions, but lower chlorophyll (Chl) content, lower photosynthetic levels and overall reduced salt stress tolerance. Additionally, the transgenic plants showed more severe membrane damage, lower antioxidant enzyme activity and more reactive oxygen species (ROS) accumulation than WT plants under salt stress, which might be related to the changes in the expression levels of some antioxidant genes. In addition, the transgenic plants also had higher Na⁺ and lower K⁺ contents, thus maintaining a higher cytosolic Na⁺/K⁺ ratio in leaves and roots than that in WT plants. Further analysis of the molecular mechanisms showed that TaPUB26 interacted with TaRPT2a, an ATPase subunit of the 26S proteasome complex in wheat. We speculated that TaPUB26 negatively regulates salt stress tolerance by interacting with other proteins, such as TaRPT2a, and that this mechanism involves altered antioxidant competition and cytosolic Na⁺/K⁺ equilibrium.

An E4 ligase facilitates polyubiquitination of plant immune receptor resistance proteins in Arabidopsis

Proteins with nucleotide binding and leucine-rich repeat domains (NLRs) serve as immune receptors in animals and plants that recognize pathogens and activate downstream defense responses. As high accumulation of NLRs can result in unwarranted autoimmune responses, their cellular concentrations must be tightly regulated. However, the molecular mechanisms of this process are poorly detailed. The F-box protein Constitutive expressor of PR genes 1 (CPR1) was previously identified as a component of a Skp1, Cullin1, F-box protein E3 complex that targets NLRs, including Suppressor of NPR1, Constitutive 1 (SNC1) and Resistance to Pseudomonas syringae 2 (RPS2), for ubiquitination and further protein degradation. From a forward genetic screen, we identified Mutant, snc1-enhancing 3 (MUSE3), an E4 ubiquitin ligase involved in polyubiquitination of its protein targets. Knocking out MUSE3 in Arabidopsis thaliana results in increased levels of NLRs, including SNC1 and RPS2, whereas overexpressing MUSE3 together with CPR1 enhances polyubiquitination and protein degradation of these immune receptors. This report on the functional role of an E4 ligase in plants provides insight into the scarcely understood NLR degradation pathway.

Phosphorylations of Sds23/Psp1/Moc1 by stress-activated kinase and cAMP-dependent kinase are essential for regulating cell viability in prolonged stationary phase

Under nutritional deprivation caused by prolonged culture, actively growing cells prepare to enter stationary phase. We showed here that Sds23/Psp1/Moc1 was phosphorylated by both cAMP-dependent kinase and stress-activated MAP kinase Sty1 upon entry into stationary phase. Overexpression of the phosphorylation-defective mutant Sds23/Psp1/Moc1 induced cell death in prolonged culture and blocked re-entry into the cell division cycle. These phosphorylations are likely to be required for cell survival during stationary phase and for binding of Ufd2, a Schizosaccharomyces pombe homologue of multi-ubiquitin chain assembly factor E4. Deletion of the Ufd2 gene and overexpression of Sds23/Psp1/Moc1 increased cell viability in prolonged stationary phase. These results suggested that Ufd2 induces cell death in prolonged nutrient deprivation, that Sds23/Psp1/Moc1 may be a target protein of the ubiquitin-fusion degradation pathway for regulation of cell viability under this stress condition, and that Sty1 and PKA activity in stationary phase is essential for interaction between Sds23/Psp1/Moc1 and Ufd2.

UBE4B, a ubiquitin chain assembly factor, is required for MDM2-mediated p53 polyubiquitination and degradation

Although MDM2 is known to be a critical negative regulator of p53, MDM2 only catalyzes p53 mono- or multiple monoubiquitination in vitro and in vivo, which is insufficient for the initiation of proteasomal degradation. MDM2 does not polyubiquitinate p53 in vitro, however, which indicates that the activity of other ubiquitin ligase(s) or cofactor(s) is required for MDM2-mediated p53 polyubiquitination and degradation. In our recent study, we demonstrated that UBE4B, an E3 and E4 ubiquitin ligase with a U-box domain, interacts physically with both p53 and MDM2. Our findings revealed that UBE4B negatively regulates the level of p53 and inhibits p53-dependent transactivation and apoptosis. We propose that inhibition of MDM2 binding to UBE4B may provide another approach to inhibit MDM2 E3 ligase activity for tumor suppressor p53. It could lead to novel anticancer therapies, with the possibility of reducing the public health burden from cancer.

Type VI secretion system translocates a phage tail spike-like protein into target cells where it cross-links actin

Genes encoding type VI secretion systems (T6SS) are widely distributed in pathogenic Gram-negative bacterial species. In Vibrio cholerae, T6SS have been found to secrete three related proteins extracellularly, VgrG-1, VgrG-2, and VgrG-3. VgrG-1 can covalently cross-link actin in vitro, and this activity was used to demonstrate that V. cholerae can translocate VgrG-1 into macrophages by a T6SS-dependent mechanism. Protein structure search algorithms predict that VgrG-related proteins likely assemble into a trimeric complex that is analogous to that formed by the two trimeric proteins gp27 and gp5 that make up the baseplate "tail spike" of Escherichia coli bacteriophage T4. VgrG-1 was shown to interact with itself, VgrG-2, and VgrG-3, suggesting that such a complex does form. Because the phage tail spike protein complex acts as a membrane-penetrating structure as well as a conduit for the passage of DNA into phage-infected cells, we propose that the VgrG components of the T6SS apparatus may assemble a "cell-puncturing device" analogous to phage tail spikes to deliver effector protein domains through membranes of target host cells.

Multiubiquitylation by E4 enzymes: ‘one size’ doesn't fit all

Selective protein degradation by the 26S proteasome requires the covalent attachment of several ubiquitin molecules in the form of a multiubiquitin chain. Ubiquitylation usually involves three classes of enzymes: a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2) and a ubiquitin ligase (E3). However, in some cases, multiubiquitylation requires the additional activity of certain ubiquitin-chain elongation factors. Yeast UFD2 (ubiquitin fusion degradation), for example, binds to oligoubiquitylated substrates (proteins modified by only a few ubiquitin molecules) and catalyses multiubiquitin-chain assembly in collaboration with E1, E2 and E3. Enzymes possessing this specific activity have been proposed to be termed 'E4 enzymes'. Recent studies have provided accumulating evidence that has led some researchers in the field to conclude that E4, indeed, represents a distinct and novel class of enzymes.

A user's guide to restriction enzyme-mediated integration in Dictyostelium

Restriction enzyme-mediated integration (REMI) has been used to study a number of cellular and developmental processes in Dictyostelium discoideum. In this paper we review the basics of this powerful method of introducing random mutations in Dictyostelium. Here we discuss several mutation screens that have been devised and some of the genes that have been discovered through this approach to mutagenesis. Included in this discussion is how one goes about isolating a gene that has been disrupted by REMI, and how one confirms that this disruption is actually responsible for the observed phenotype. Finally, we describe how REMI can be used as an effective teaching tool in undergraduate cell biology laboratory courses.

The human homologue of the yeast polyubiquitination factor Ufd2p is cleaved by caspase 6 and granzyme B during apoptosis

In the present study, we demonstrate that a human homologue of Ufd2p (a yeast protein that catalyses the formation of long polyubiquitin chains, and is implicated in responses to environmental stress), UFD2 (ubiquitin fusion degradation protein-2), is cleaved during apoptosis induced by multiple stimuli, including UVB irradiation, Fas ligation, staurosporine treatment and cytotoxic lymphocyte granule-induced death. Caspase 6 and granzyme B efficiently cleave UFD2 [k(cat)/K(m)=(4-5) x 10(4) M(-1) x s(-1)] at Asp(123), whereas caspases 3 and 7 cleave UFD2 approx. 10-fold less efficiently immediately upstream at Asp(109). Thus UFD2 is added to the growing list of proteins with closely spaced caspase and granzyme B cleavage sites, suggesting the presence of a previously unrecognized, conserved motif. Both cleavage sites are contained and conserved within a novel 300-amino-acid N-terminal domain present in apparent UFD2 orthologues in mice and zebrafish, but absent in all UFD2 family members in lower eukaryotes. Full-length recombinant UFD2 exhibited ubiquitin-protein ligase ('E3')-like ubiquitination activity in vitro, but this activity was abolished in recombinant UFD2 truncated at the granzyme B/caspase 6 cleavage site. Cleavage of UFD2 by caspases or granzyme B within this putative regulatory N-terminal domain might have important functional consequences within the apoptotic cascade.

CulB, a putative ubiquitin ligase subunit, regulates prestalk cell differentiation and morphogenesis in Dictyostelium spp

Dictyostelium amoebae accomplish a starvation-induced developmental process by aggregating into a mound and forming a single fruiting body with terminally differentiated spores and stalk cells. culB was identified as the gene disrupted in a developmental mutant with an aberrant prestalk cell differentiation phenotype. The culB gene product appears to be a homolog of the cullin family of proteins that are known to be involved in ubiquitin-mediated protein degradation. The culB mutants form supernumerary prestalk tips atop each developing mound that result in the formation of multiple small fruiting bodies. The prestalk-specific gene ecmA is expressed precociously in culB mutants, suggesting that prestalk cell differentiation occurs earlier than normal. In addition, when culB mutant cells are mixed with wild-type cells, they display a cell-autonomous propensity to form stalk cells. Thus, CulB appears to ensure that the proper number of prestalk cells differentiate at the appropriate time in development. Activation of cyclic AMP-dependent protein kinase (PKA) by disruption of the regulatory subunit gene (pkaR) or by overexpression of the catalytic subunit gene (pkaC) enhances the prestalk/stalk cell differentiation phenotype of the culB mutant. For example, culB- pkaR- cells form stalk cells without obvious multicellular morphogenesis and are more sensitive to the prestalk O (pstO) cell inducer DIF-1. The sensitized condition of PKA activation reveals that CulB may govern prestalk cell differentiation in Dictyostelium, in part by controlling the sensitivity of cells to DIF-1, possibly by regulating the levels of one or more proteins that are rate limiting for prestalk differentiation.

A new gun in town: the U box is a ubiquitin ligase domain

Ubiquitin ligases determine protein stability in a highly regulated manner by coordinating the addition of polyubiquitin chains to proteins that are then targeted to the proteasome for degradation. Ubiquitin ligases have generally been separated into two groups--those containing HECT domains and those with RING finger domains. Recently, a third group of ubiquitin ligases has emerged: those containing a U-box domain. Patterson discusses what is known about the few U-box-containing proteins that have been characterized, although the general properties of U-box proteins that distinguish them from other ubiquitin ligases are still a matter of speculation.

CHIP is a U-box-dependent E3 ubiquitin ligase: identification of Hsc70 as a target for ubiquitylation

Proper folding of proteins (either newly synthesized or damaged in response to a stressful event) occurs in a highly regulated fashion. Cytosolic chaperones such as Hsc/Hsp70 are assisted by cofactors that modulate the folding machinery in a positive or negative manner. CHIP (carboxyl terminus of Hsc70-interacting protein) is such a cofactor that interacts with Hsc70 and, in general, attenuates its most well characterized functions. In addition, CHIP accelerates ubiquitin-dependent degradation of chaperone substrates. Using an in vitro ubiquitylation assay with recombinant proteins, we demonstrate that CHIP possesses intrinsic E3 ubiquitin ligase activity and promotes ubiquitylation. This activity is dependent on the carboxyl-terminal U-box. CHIP interacts functionally and physically with the stress-responsive ubiquitin-conjugating enzyme family UBCH5. Surprisingly, a major target of the ubiquitin ligase activity of CHIP is Hsc70 itself. CHIP ubiquitylates Hsc70, primarily with short, noncanonical multiubiquitin chains but has no appreciable effect on steady-state levels or half-life of this protein. This effect may have heretofore unanticipated consequences with regard to the chaperoning activities of Hsc70 or its ability to deliver substrates to the proteasome. These studies demonstrate that CHIP is a bona fide ubiquitin ligase and indicate that U-box-containing proteins may comprise a new family of E3s.

Gibberellins signal nuclear import of PHOR1, a photoperiod-responsive protein with homology to Drosophila armadillo

S. tuberosum ssp. andigena potato plants require short days (SD) for tuberization. We have isolated PHOR1 (photoperiod-responsive 1), which shows upregulated expression in induced leaves (SD). PHOR1 encodes an arm repeat protein with homology to the Drosophila segment polarity protein armadillo. Antisense inhibition of PHOR1 produces a semidwarf phenotype similar to that of GA-deficient plants, and the antisense lines show reduced GA responsiveness combined with a higher endogenous GA content than wild-type plants. Feedback regulation of GA biosynthetic genes is also altered in these lines. Conversely, transgenic lines overexpressing PHOR1 show an enhanced response to GA. GA application induces rapid migration of PHOR1-GFP protein to the nucleus. Thus, PHOR1 appears to be a general component of GA signaling pathways that relocalizes to the nucleus in the presence of GA.

The U-box protein family in plants

The U-box is a highly conserved domain recently identified at the C terminus of yeast UFD2, an E4 ubiquitination factor. In yeast, UFD2 is the only U-box-containing protein, but there are two UFD2 homologs and several other proteins containing a U-box domain in humans. Intriguingly, a database search revealed 37 predicted proteins containing a U-box in Arabidopsis. The plant U-box (PUB) proteins form five distinct subclasses, suggesting that they play diverse roles. The ARC1 gene from Brassica, required for self-incompatibility, is currently the only PUB gene functionally characterized. Here, we discuss the characteristics and possible functions of the PUB gene family.

Interaction of the ring finger-related U-box motif of a nuclear dot protein with ubiquitin-conjugating enzymes

The U-box domain has been suggested to be a modified RING finger motif where the metal-coordinating cysteines and histidines have been replaced with other amino acids. Known U-box-containing proteins have been implicated in the ubiquitin/proteasome system. In a search for proteins interacting with the ubiquitin-conjugating enzyme UbcM4/UbcH7, we have identified a novel U-box containing protein, termed UIP5, that is exclusively found in the nucleus as part of a nuclear dot-like structure. Interaction between UbcM4 and UIP5 was observed in vivo and in vitro with bacterially expressed proteins. In addition to UbcM4, several other ubiquitin-conjugating enzymes (E2s) that share the same sequence within the L1 loop bind to UIP5. Mutational analysis showed that the U-box, like the RING finger in other proteins, forms the physical basis for the interaction with E2 enzymes. Further support for the structural similarity between U-box and RING finger comes from the observation that, in both cases, the same regions within the UbcM4 molecule are required for interaction. Our results establish at the molecular level a link between the U-box and the ubiquitin conjugating system and strongly suggest that proteins containing U-box domains are functionally closely related to RING finger proteins.

The Hsc70 co-chaperone CHIP targets immature CFTR for proteasomal degradation

The folding of both wild-type and mutant forms of the cystic-fibrosis transmembrane-conductance regulator (CFTR), a plasma-membrane chloride-ion channel, is inefficient. Most nascent CFTR is retained in the endoplasmic reticulum and degraded by the ubiquitin proteasome pathway. Aberrant folding and defective trafficking of CFTRDeltaF508 is the principal cause of cystic fibrosis, but how the endoplasmic-reticulum quality-control system targets CFTR for degradation remains unknown. CHIP is a cytosolic U-box protein that interacts with Hsc70 through a set of tetratricorepeat motifs. The U-box represents a modified form of the ring-finger motif that is found in ubiquitin ligases and that defines the E4 family of polyubiquitination factors. Here we show that CHIP functions with Hsc70 to sense the folded state of CFTR and targets aberrant forms for proteasomal degradation by promoting their ubiquitination. The U-box appeared essential for this process because overexpresion of CHIPDeltaU-box inhibited the action of endogenous CHIP and blocked CFTR ubiquitination and degradation. CHIP is a co-chaperone that converts Hsc70 from a protein-folding machine into a degradation factor that functions in endoplasmic-reticulum quality control.

A genetic interaction between a ubiquitin-like protein and ubiquitin-mediated proteolysis in Dictyostelium discoideum(1)

A ubiquitination factor, NosA, is essential for cellular differentiation in Dictyostelium discoideum. In the absence of nosA, development is blocked, resulting in a developmental arrest at the tight-aggregate stage, when cells differentiate into two precursor cell types, prespore and prestalk cells. Development is restored when a second gene, encoding the ubiquitin-like protein SonA, is inactivated in nosA-mutant cells. SonA has homology over its entire length to Dsk2 from Saccharomyces cerevisiae, a ubiquitin-like protein that is involved in the assembly of the spindle pole body. Dsk2 and SonA are both stable proteins that do not seem to be subjected to degradation via the ubiquitin pathway. SonA does not become ubiquitinated and the intracellular levels of SonA are not affected by the absence of NosA. The high degree of suppression suggests that SonA rescues most or all of the defects caused by the absence of nosA. We propose that NosA and SonA act in concert to control the activity of a developmental regulator that must be deactivated for cells to cross a developmental boundary.

Identification and characterization of a 500-kb homozygously deleted region at 1p36.2-p36.3 in a neuroblastoma cell line

Loss of heterozygosity of the distal region of chromosome 1p where tumor suppressor gene(s) might harbor is frequently observed in many human cancers including neuroblastoma (NBL) with MYCN amplification and poor prognosis. We have identified for the first time a homozygously deleted region at the marker D1S244 within the smallest region of overlap at 1p36.2-p36.3 in two NBL cell lines, NB-1 and NB-C201 (MASS-NB-SCH1), although our genotyping has suggested the possibility that both lines are derived from the same origin. The 800-kb PAC contig covering the entire region of homozygous deletion was made and partially sequenced (about 60%). The estimated length of the deleted region was 500 kb. We have, thus far, identified six genes within the region which include three known genes (DFF45, PGD, and CORT) as well as three other genes which have been reported during processing our present project for the last 3(1/2) years (HDNB1/UFD2, KIAA0591F/KIF1B-beta, and PEX14). They include the genes related to apoptosis, glucose metabolism, ubiquitin-proteasome pathway, a neuronal microtubule-associated motor molecule and biogenesis of peroxisome. At least three genes (HDNB1/UFD2, KIAA0591F/KIF1B-beta, and PEX14) were differentially expressed at high levels in favorable and at low levels in unfavorable subsets of primary neuroblastoma. Since the 1p distal region is reported to be imprinted, those differentially expressed genes could be the new members of the candidate NBL suppressor, although RT-PCR-SSCP analysis has demonstrated infrequent mutation of the genes so far identified. Full-sequencing and gene prediction for the region of homozygous deletion would elucidate more detailed structure of this region and might lead to discovery of additional candidate genes. Oncogene (2000) 19, 4302 - 4307

An F-Box/WD40 repeat-containing protein important for Dictyostelium cell-type proportioning, slug behaviour, and culmination

FbxA is a novel member of a family of proteins that contain an F-box and WD40 repeats and that target specific proteins for degradation via proteasomes. In fruiting bodies formed from cells where the fbxA gene is disrupted (fbxA(-) cells), the spore mass fails to fully ascend the stalk. In addition, fbxA(-) slugs continue to migrate under environmental conditions where the parental strain immediately forms fruiting bodies. Consistent with this latter behaviour, the development of fbxA(-) cells is hypersensitive to ammonia, the signaling molecule that regulates the transition from the slug stage to terminal differentiation. The slug comprises an anterior prestalk region and a posterior prespore region and the fbxA mRNA is highly enriched in the prestalk cells. The prestalk zone of the slug is further subdivided into an anterior pstA region and a posterior pstO region. In fbxA(-) slugs the pstO region is reduced in size and the prespore region is proportionately expanded. Our results indicate that FbxA is part of a regulatory pathway that controls cell fate decisions and spatial patterning via regulated protein degradation.

Dictyostelium amoebae lacking an F-box protein form spores rather than stalk in chimeras with wild type

Using a selection for Dictyostelium mutants that preferentially form spores, we have recovered a mutant called CheaterA. In chimeras with isogenic wild-type cells, the CheaterA mutant preferentially forms viable spores rather than inviable stalk cells. The mutant causes wild-type cells that have begun to express spore-specific genes to accumulate in the prestalk compartment of the developing organism. In the wild-type cells, the chtA transcript is absent in growing cells and appears early in development. No transcript was detected in the mutant by Northern blot. The chtA gene codes for a protein with an F-box and WD40 domains. This class of protein usually forms part of an Skp1, cullin, F-box (SCF) complex that targets specific protein substrates for ubiquitination and degradation.

Dictyostelium amoebae lacking an F-box protein form spores rather than stalk in chimeras with wild type

Using a selection for Dictyostelium mutants that preferentially form spores, we have recovered a mutant called CheaterA. In chimeras with isogenic wild-type cells, the CheaterA mutant preferentially forms viable spores rather than inviable stalk cells. The mutant causes wild-type cells that have begun to express spore-specific genes to accumulate in the prestalk compartment of the developing organism. In the wild-type cells, the chtA transcript is absent in growing cells and appears early in development. No transcript was detected in the mutant by Northern blot. The chtA gene codes for a protein with an F-box and WD40 domains. This class of protein usually forms part of an Skp1, cullin, F-box (SCF) complex that targets specific protein substrates for ubiquitination and degradation.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Identification of CHIP, a novel tetratricopeptide repeat-containing protein that interacts with heat shock proteins and negatively regulates chaperone functions

The chaperone function of the mammalian 70-kDa heat shock proteins Hsc70 and Hsp70 is modulated by physical interactions with four previously identified chaperone cofactors: Hsp40, BAG-1, the Hsc70-interacting protein Hip, and the Hsc70-Hsp90-organizing protein Hop. Hip and Hop interact with Hsc70 via a tetratricopeptide repeat domain. In a search for additional tetratricopeptide repeat-containing proteins, we have identified a novel 35-kDa cytoplasmic protein, carboxyl terminus of Hsc70-interacting protein (CHIP). CHIP is highly expressed in adult striated muscle in vivo and is expressed broadly in vitro in tissue culture. Hsc70 and Hsp70 were identified as potential interaction partners for this protein in a yeast two-hybrid screen. In vitro binding assays demonstrated direct interactions between CHIP and both Hsc70 and Hsp70, and complexes containing CHIP and Hsc70 were identified in immunoprecipitates of human skeletal muscle cells in vivo. Using glutathione S-transferase fusions, we found that CHIP interacted with the carboxy-terminal residues 540 to 650 of Hsc70, whereas Hsc70 interacted with the amino-terminal residues 1 to 197 (containing the tetratricopeptide domain and an adjacent charged domain) of CHIP. Recombinant CHIP inhibited Hsp40-stimulated ATPase activity of Hsc70 and Hsp70, suggesting that CHIP blocks the forward reaction of the Hsc70-Hsp70 substrate-binding cycle. Consistent with this observation, both luciferase refolding and substrate binding in the presence of Hsp40 and Hsp70 were inhibited by CHIP. Taken together, these results indicate that CHIP decreases net ATPase activity and reduces chaperone efficiency, and they implicate CHIP in the negative regulation of the forward reaction of the Hsc70-Hsp70 substrate-binding cycle.

A novel ubiquitination factor, E4, is involved in multiubiquitin chain assembly

Proteins modified by multiubiquitin chains are the preferred substrates of the proteasome. Ubiquitination involves a ubiquitin-activating enzyme, E1, a ubiquitin-conjugating enzyme, E2, and often a substrate-specific ubiquitin-protein ligase, E3. Here we show that efficient multiubiquitination needed for proteasomal targeting of a model substrate requires an additional conjugation factor, named E4. This protein, previously known as UFD2 in yeast, binds to the ubiquitin moieties of preformed conjugates and catalyzes ubiquitin chain assembly in conjunction with E1, E2, and E3. Intriguingly, E4 defines a novel protein family that includes two human members and the regulatory protein NOSA from Dictyostelium required for fruiting body development. In yeast, E4 activity is linked to cell survival under stress conditions, indicating that eukaryotes utilize E4-dependent proteolysis pathways for multiple cellular functions.