cDNA cloning of a novel human ubiquitin carrier protein. An antigenic domain specifically recognized by endemic pemphigus foliaceus autoantibodies is encoded in a secondary reading frame of this human epidermal transcript

E2-EPF UCP Possesses E3 Ubiquitin Ligase Activity via Its Cysteine 118 Residue

Here, we show that E2-EPF ubiquitin carrier protein (UCP) elongated E3-independent polyubiquitin chains on the lysine residues of von Hippel-Lindau protein (pVHL) and its own lysine residues both in vitro and in vivo. The initiation of the ubiquitin reaction depended on not only Lys11 linkage but also the Lys6, Lys48 and Lys63 residues of ubiquitin, which were involved in polyubiquitin chain formation on UCP itself. UCP self-association occurred through the UBC domain, which also contributed to the interaction with pVHL. The polyubiquitin chains appeared on the N-terminus of UCP in vivo, which indicated that the N-terminus of UCP contains target lysines for polyubiquitination. The Lys76 residue of UCP was the most critical site for auto-ubiquitination, whereas the polyubiquitin chain formation on pVHL occurred on all three of its lysines (Lys159, Lys171 and Lys196). A UCP mutant in which Cys118 was changed to alanine (UCP^C118A) did not form a polyubiquitin chain but did strongly accumulate mono- and di-ubiquitin via auto-ubiquitination. Polyubiquitin chain formation required the coordination of Cys95 and Cys118 between two interacting molecules. The mechanism of the polyubiquitin chain reaction of UCP may involve the transfer of ubiquitin from Cys95 to Cys118 by trans-thiolation, with polyubiquitin chains forming at Cys118 by reversible thioester bonding. The polyubiquitin chains are then moved to the lysine residues of the substrate by irreversible isopeptide bonding. During the elongation of the ubiquitin chain, an active Cys118 residue is required in both parts of UCP, namely, the catalytic enzyme and the substrate. In conclusion, UCP possesses not only E2 ubiquitin conjugating enzyme activity but also E3 ubiquitin ligase activity, and Cys118 is critical for polyubiquitin chain formation.

E2-EPF UCP regulates stability and functions of missense mutant pVHL via ubiquitin mediated proteolysis

Background

Missense mutation of VHL gene is frequently detected in type 2 VHL diseases and linked to a wide range of pVHL functions and stability. Certain mutant pVHLs retain ability to regulate HIFs but lose their function by instability. In this case, regulating of degradation of mutant pVHLs, can be postulated as therapeutic method.

Method

The stability and cellular function of missense mutant pVHLs were determine in HEK293T transient expressing cell and 786-O stable cell line. Ubiquitination assay of mutant VHL proteins was performed in vitro system. Anticacner effect of adenovirus mediated shUCP expressing was evaluated using ex vivo mouse xenograft assay.

Results

Three VHL missense mutants (V155A, L158Q, and Q164R) are directly ubiquitinated by E2-EPF UCP (UCP) in vitro. Mutant pVHLs are more unstable than wild type in cell. Missense mutant pVHLs interact with UCP directly in both in vitro and cellular systems. Lacking all of lysine residues of pVHL result in resistance to ubiquitination thereby increase its stability. Missense mutant pVHLs maintained the function of E3 ligase to ubiquitinate HIF-1α in vitro. In cells expressing mutant pVHLs, Glut-1 and VEGF were relatively upregulated compared to their levels in cells expressing wild-type. Depletion of UCP restored missense mutant pVHLs levels and inhibited cell growth. Adenovirus-mediated shUCP RNA delivery inhibited tumor growth in ex vivo mouse xenograft model.

Conclusion

These data suggest that targeting of UCP can be one of therapeutic method in type 2 VHL disease caused by unstable but functional missense mutant pVHL.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1786-8) contains supplementary material, which is available to authorized users.

Neutrophil elastase cleaves the murine hemidesmosomal protein BP180/type XVII collagen and generates degradation products that modulate experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease associated with autoantibodies against the hemidesmosomal proteins BP180 and BP230. In the IgG passive transfer model of BP, blister formation is triggered by anti-BP180 IgG and depends on complement activation, mast cell degranulation, and neutrophil recruitment. Mice lacking neutrophil elastase (NE) do not develop experimental BP. Here, we demonstrated that NE degrades recombinant mouse BP180 within the immunodominant extracellular domain at amino acid positions 506 and 561, generating peptide p561 and peptide p506. Peptide p561 is chemotactic for neutrophils both in vitro and in vivo. Local injection of NE into B6 mice recruits neutrophils to the skin, and neutrophil infiltration is completely blocked by co-injection with the NE inhibitor α1-proteinase inhibitor. More importantly, NE directly cleaves BP180 in mouse and human skin, as well as the native human BP180 trimer molecule. These results demonstrate that (i) NE directly damages the extracellular matrix and (ii) NE degradation of mouse BP180 generates neutrophil chemotactic peptides that amplify disease severity at the early stage of the disease.

Dual targets for mouse mast cell protease-4 in mediating tissue damage in experimental bullous pemphigoid

Mouse mast cell protease-4 (mMCP-4) has been linked to autoimmune and inflammatory diseases, although the exact mechanisms underlying its role in these pathological conditions remain unclear. Here, we have found that mMCP-4 is critical in a mouse model of the autoimmune skin blistering disease bullous pemphigoid (BP). Mice lacking mMCP-4 were resistant to experimental BP. Complement activation, mast cell (MC) degranulation, and the early phase of neutrophil (PMN) recruitment occurred comparably in mMCP-4(-/-) and WT mice. However, without mMCP-4, activation of matrix metalloproteinase (MMP)-9 was impaired in cultured mMCP-4(-/-) MCs and in the skin of pathogenic IgG-injected mMCP-4(-/-) mice. MMP-9 activation was not fully restored by local reconstitution with WT or mMCP-4(-/-) PMNs. Local reconstitution with mMCP-4(+/+) MCs, but not with mMCP-4(-/-) MCs, restored blistering, MMP-9 activation, and PMN recruitment in mMCP-4(-/-) mice. mMCP-4 also degraded the hemidesmosomal transmembrane protein BP180 both in the skin and in vitro. These results demonstrate that mMCP-4 plays two different roles in the pathogenesis of experimental BP, by both activating MMP-9 and by cleaving BP180, leading to injury of the hemidesmosomes and extracellular matrix of the basement membrane zone.

The C5a receptor on mast cells is critical for the autoimmune skin-blistering disease bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin-blistering disease characterized by the presence of autoantibodies against the hemidesmosomal proteins BP230 and BP180. In the IgG passive transfer mouse model of BP, subepidermal blistering is triggered by anti-BP180 antibodies and depends on the complement system, mast cell (MC) degranulation, and neutrophil infiltration. In this study, we have identified the signaling events that connect the activation of the complement system and MC degranulation. We found that mice deficient in MCs or the C5a receptor (C5aR) injected with pathogenic anti-BP180 IgG failed to develop subepidermal blisters and exhibited a drastic reduction in p38 MAPK phosphorylation compared with WT mice. Local reconstitution with MCs from WT but not C5aR-deficient mice restored high levels of p38 MAPK phosphorylation and subepidermal blistering in MC-deficient mice. Local injection of recombinant C5a induced phosphorylation of p38 MAPK in WT but not MC-deficient mice. Cultured mouse MCs treated with recombinant C5a exhibited a significant increase in p38 MAPK phosphorylation and MC degranulation. Taken together, these data demonstrate that C5a interacts with C5aR on MCs and that this C5a-C5aR interaction triggers activation of the p38 MAPK pathway, subsequent MC degranulation, and ultimately BP blistering.

UBE2S drives elongation of K11-linked ubiquitin chains by the anaphase-promoting complex

The Anaphase-Promoting Complex (APC) is an E3 ubiquitin ligase that regulates mitosis and G1 by sequentially targeting cell-cycle regulators for ubiquitination and proteasomal degradation. The mechanism of ubiquitin chain formation by APC and the resultant chain topology remains controversial. By using a single-lysine APC substrate to dissect the topology of ubiquitinated substrates, we find that APC-catalyzed ubiquitination has an intrinsic preference for the K11 linkage of ubiquitin that is essential for substrate degradation. K11 specificity is determined by an E2 enzyme, UBE2S/E2-EPF, that elongates ubiquitin chains after the substrates are pre-ubiquitinated by UbcH10 or UbcH5. UBE2S copurifies with APC; dominant-negative Ube2S slows down APC substrate degradation in functional cell-cycle extracts. We propose that Ube2S is a critical, unique component of the APC ubiquitination pathway.

Assembly of k11-linked ubiquitin chains by the anaphase-promoting complex

Ubiquitin chains are assembled, when a ubiquitin is connected to one of the seven Lys residues or the amino-terminus of a ubiquitin molecule already attached to a substrate. K48-linked ubiquitin chains target proteins for degradation by the 26S proteasome, while those chains connected through K63 regulate intracellular signaling cascades independently of protein degradation. Although all other linkages are detected in cells, their function is not well understood. Here, we review recent progress in delineating substrates, enzymes and functions of K11-linked ubiquitin chains. In particular, we discuss the mechanism of assembly for K11-linked chains by the human anaphase-promoting complex and its physiological E2s UbcH10 and Ube2S and we speculate on the particularities of these noncanonical chains in cells.

HIF-1 regulation: not so easy come, easy go

The hypoxia-inducible factor-1 (HIF-1) is the master regulator of the cellular response to hypoxia and its expression levels are tightly controlled through synthesis and degradation. It is widely accepted that HIF-1alpha protein accumulation during hypoxia results from inhibition of its oxygen-dependent degradation by the von Hippel Lindau protein (pVHL) pathway. However, recent data describe new pVHL- or oxygen-independent mechanisms for HIF-1alpha degradation. Furthermore, the hypoxia-induced increase in HIF-1alpha levels is facilitated by the continued translation of HIF-1alpha during hypoxia despite the global inhibition of protein translation. Recent work has contributed to an increased understanding of the mechanisms that control the translation and degradation of HIF-1alpha under both normoxic and hypoxic conditions.

G2E3 is a dual function ubiquitin ligase required for early embryonic development

G2E3 is a putative ubiquitin ligase (E3) identified in a microarray screen for mitotic regulatory proteins. It shuttles between the cytoplasm and nucleus, concentrating in nucleoli and relocalizing to the nucleoplasm in response to DNA damage. In this study, we demonstrate that G2E3 is an unusual ubiquitin ligase that is essential in early embryonic development to prevent apoptotic death. This protein has a catalytically inactive HECT domain and two distinct RING-like ubiquitin ligase domains that catalyze lysine 48-linked polyubiquitination. To address in vivo function, we generated a knock-out mouse model of G2E3 deficiency that incorporates a beta-galactosidase reporter gene under control of the endogenous promoter. Animals heterozygous for G2E3 inactivation are phenotypically normal with no overt change in development, growth, longevity, or fertility, whereas G2E3 null embryos die prior to implantation. Although normal numbers of G2E3(-/-) blastocysts are present at embryonic day 3.5, these blastocysts involute in culture as a result of massive apoptosis. Using beta-galactosidase staining as a marker for protein expression, we demonstrate that G2E3 is predominantly expressed within the central nervous system and the early stages of limb bud formation of the developing embryo. In adult animals, the most intense staining is found in Purkinje cell bodies and cells lining the ductus deferens. In summary, G2E3 is a dual function ubiquitin ligase essential for prevention of apoptosis in early embryogenesis.

The ubiquitin-conjugating enzyme E2-EPF is overexpressed in primary breast cancer and modulates sensitivity to topoisomerase II inhibition

We identified the ubiquitin-conjugating enzyme E2-EPF mRNA as differentially expressed in breast tumors relative to normal tissues and performed studies to elucidate its putative role in cancer. We demonstrated that overexpression of E2-EPF protein correlated with estrogen receptor (ER) negativity in breast cancer specimens and that its expression is cell cycle-regulated, suggesting a potential function for E2-EPF in cell cycle progression. However, reduction of E2-EPF protein levels by > 80% using RNAi had no significant effects on the proliferation of HeLa cervical cancer cells or ER(-) MDA-MB-231 or MDA-MB-453 breast cancer cells. Because E2-EPF protein levels were elevated during the G(2)/M phase of the cell cycle and because E2-EPF mRNA in tumor specimens was frequently coexpressed with genes involved in cell cycle control, spindle assembly, and mitotic surveillance, the possibility that E2-EPF might have a function in the cellular response to agents that induce a G(2) checkpoint or an M checkpoint was investigated. E2-EPF knockdown sensitized HeLa cells to the topoisomerase (topo) II inhibitors etoposide and doxorubicin and also increased topo IIalpha protein levels. These data suggest that combined administration of topo II-directed drugs and E2-EPF inhibitors may enhance their clinical effectiveness.

Mapping the human proteome for non-redundant peptide islands

We describe immune-proteome structures using libraries of protein fragments that define a structural immunological alphabet. We propose and validate such an alphabet as i) composed of letters of five consecutive amino acids, pentapeptide units being sufficient minimal antigenic determinants in a protein, and ii) characterized by low-similarity to human proteins, so representing structures unknown to the host and potentially able to evoke an immune response. In this context, we have thoroughly sifted through the entire human proteome searching for non-redundant protein motifs. Here, for the first time, a complete sequence redundancy dissection of the human proteome has been conducted. The non-redundant peptide islands in the human proteome have been quantified and catalogued according to the amino acid length. The library of uniquely occurring n-peptide sequences that was obtained is characterized by a logarithmic decrease of the number of non-redundant peptides as a function of the peptide length. This library represents a highly specific catalogue of molecular protein signatures, the possible use of which in cancer/autoimmunity research is discussed, with a major focus on non-redundant dodecamer sequences.

Increased Expression of Insulin-like Growth Factor-II Messenger RNA–Binding Protein 1 Is Associated with Tumor Progression in Patients with Lung Cancer

PURPOSE

To identify novel biomarkers and therapeutic targets for lung cancers, we screened for genes that were highly transactivated in a large proportion of non-small cell lung cancers (NSCLC) using a cDNA microarray representing 27,648 genes.

EXPERIMENTAL DESIGN

A gene encoding insulin-like growth factor-II mRNA-binding protein 1 (IMP-1) was selected as a candidate (> or =3-fold expression than in normal lung tissue in about 70% of NSCLCs). Tumor tissue microarray was applied to examine expression of IMP-1 protein in archival lung cancer samples from 267 patients and investigated its clinicopathologic significance. A role of IMP-1 in cancer cell growth and/or survival was examined by small interfering RNA experiments. Cellular invasive activity of IMP-1 on mammalian cells was examined using Matrigel assays. mRNAs associated with IMP-1 in cancer cells were also isolated by RNA immunoprecipitation followed by cDNA microarray analysis.

RESULTS

Positive immunostaining of IMP-1 was correlated with male (P = 0.0001), tumor size (P = 0.0003), non-adenocarcinoma histology (P < 0.0001), smoking history (P = 0.0005), non-well-differentiated tumor grade (P = 0.0001), and poor prognosis (P = 0.0053). Suppression of IMP-1 expression with small interfering RNA effectively suppressed growth of NSCLC cells. In addition, we identified that exogenous expression of IMP-1 increased the migratory activity of mammalian cells. IMP-1 was able to bind to mRNAs encoding a variety of proteins involved in signal transduction, cell cycle progression, cell adhesion and cytoskeleton, and various types of enzymatic activities.

CONCLUSIONS

These results suggest that IMP-1 expression is likely to play important roles in lung cancer development and progression, and that IMP-1 is a prognostic biomarker and a promising therapeutic target for lung cancer.

Role of different pathways of the complement cascade in experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease associated with autoantibodies directed against the hemidesmosomal proteins BP180 and BP230 and inflammation. Passive transfer of antibodies to the murine BP180 (mBP180) induces a skin disease that closely resembles human BP. In the present study, we defined the roles of the different complement activation pathways in this model system. Mice deficient in the alternative pathway component factor B (Fb) and injected with pathogenic anti-mBP180 IgG developed delayed and less intense subepidermal blisters. Mice deficient in the classical pathway component complement component 4 (C4) and WT mice pretreated with neutralizing antibody against the first component of the classical pathway, C1q, were resistant to experimental BP. These mice exhibited a significantly reduced level of mast cell degranulation and polymorphonuclear neutrophil (PMN) infiltration in the skin. Intradermal administration of compound 48/80, a mast cell degranulating agent, restored BP disease in C4(-/-) mice. Furthermore, C4(-/-) mice became susceptible to experimental BP after local injection of PMN chemoattractant IL-8 or local reconstitution with PMNs. These findings provide the first direct evidence to our knowledge that complement activation via the classical and alternative pathways is crucial in subepidermal blister formation in experimental BP.

E2-EPF UCP targets pVHL for degradation and associates with tumor growth and metastasis

The von Hippel-Lindau tumor suppressor, pVHL, forms part of an E3 ubiquitin ligase complex that targets specific substrates for degradation, including hypoxia-inducible factor-1alpha (HIF-1alpha), which is involved in tumor progression and angiogenesis. It remains unclear, however, how pVHL is destabilized. Here we show that E2-EPF ubiquitin carrier protein (UCP) associates with and targets pVHL for ubiquitin-mediated proteolysis in cells, thereby stabilizing HIF-1alpha. UCP is detected coincidently with HIF-1alpha in human primary liver, colon and breast tumors, and metastatic cholangiocarcinoma and colon cancer cells. UCP level correlates inversely with pVHL level in most tumor cell lines. In vitro and in vivo, forced expression of UCP boosts tumor-cell proliferation, invasion and metastasis through effects on the pVHL-HIF pathway. Our results suggest that UCP helps stabilize HIF-1alpha and may be a new molecular target for therapeutic intervention in human cancers.

Genome analysis and functional characterization of the E2 and RING-type E3 ligase ubiquitination enzymes of Arabidopsis

Attachment of ubiquitin to substrate proteins is catalyzed by the three enzymes E1, E2 (ubiquitin conjugating [UBC]), and E3 (ubiquitin ligase). Forty-one functional proteins with a UBC domain and active-site cysteine are predicted in the Arabidopsis (Arabidopsis thaliana) genome, which includes four that are predicted or shown to function with ubiquitin-like proteins. Only nine were previously characterized biochemically as ubiquitin E2s. We obtained soluble protein for 22 of the 28 uncharacterized UBCs after expression in Escherichia coli and demonstrated that 16 function as ubiquitin E2s. Twelve, plus three previously characterized ubiquitin E2s, were also tested for the ability to catalyze ubiquitination in vitro in the presence of one of 65 really interesting new gene (RING) E3 ligases. UBC22, UBC19-20, and UBC1-6 had variable levels of E3-independent activity. Six UBCs were inactive with all RINGs tested. Closely related UBC8, 10, 11, and 28 were active with the largest number of RING E3s and with all RING types. Expression analysis was performed to determine whether E2s or E3s were expressed in specific organs or under specific environmental conditions. Closely related E2s show unique patterns of expression and most express ubiquitously. Some RING E3s are also ubiquitously expressed; however, others show organ-specific expression. Of all the organs tested, RING mRNAs are most abundant in floral organs. This study demonstrates that E2 diversity includes examples with broad and narrow specificity toward RINGs, and that most ubiquitin E2s are broadly expressed with each having a unique spatial and developmental pattern of expression.

Differential roles for beta2 integrins in experimental autoimmune bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune disease associated with autoantibodies directed against the hemidesmosomal antigens anti-BP230 and anti-B180. Neonatal mice injected with rabbit anti-mouse BP180 (mBP10) IgG develop a BP-like disease. Complement, immune complexes, mast cells, and neutrophils play a key role in subepidermal blistering in this animal model. In this study we investigated the role of beta2 integrins in experimental BP. Wild-type (WT) mice pretreated with neutralizing antibody against CD11a (LFA-1), CD11b (Mac-1), CD11a plus CD11b, or CD18 alone failed to develop BP when injected with pathogenic anti-mBP180 IgG. This was associated with a significant reduction in neutrophil accumulation in neutralizing antibody-treated mice. Mac-1-deficient (Mac-1 knockout [KO]) mice were resistant to experimental BP despite normal complement deposition and mast cell and neutrophil degranulation. Neutrophil infiltration in Mac-1 KO mice was severely impaired at 24 hours. However, more neutrophils accumulated in the skin of Mac-1 KO mice compared with WT mice at early time points (2-4 hours), which was associated with an increase in their survival as determined by apoptosis markers. These data suggest that beta2 integrins play differential roles in experimental BP: LFA-1 is required for neutrophil recruitment, while Mac-1 mediates late neutrophil accumulation and apoptosis of infiltrating neutrophils.

Profiling of Alopecia Areata Autoantigens Based on Protein Microarray Technology

Protein biochips have a great potential in future parallel processing of complex samples as a research tool and in diagnostics. For the generation of protein biochips, highly automated technologies have been developed for cDNA expression library production, high throughput protein expression, large scale analysis of proteins, and protein microarray generation. Using this technology, we present here a strategy to identify potential autoantigens involved in the pathogenesis of alopecia areata, an often chronic disease leading to the rapid loss of scalp hair. Only little is known about the putative autoantigen(s) involved in this process. By combining protein microarray technology with the use of large cDNA expression libraries, we profiled the autoantibody repertoire of sera from alopecia areata patients against a human protein array consisting of 37,200 redundant, recombinant human proteins. The data sets obtained from incubations with patient sera were compared with control sera from clinically healthy persons and to background incubations with anti-human IgG antibodies. From these results, a smaller protein subset was generated and subjected to qualitative and quantitative validation on highly sensitive protein microarrays to identify novel alopecia areata-associated autoantigens. Eight autoantigens were identified by protein chip technology and were successfully confirmed by Western blot analysis. These autoantigens were arrayed on protein microarrays to generate a disease-associated protein chip. To confirm the specificity of the results obtained, sera from patients with psoriasis or hand and foot eczema as well as skin allergy were additionally examined on the disease-associated protein chip. By using alopecia areata as a model for an autoimmune disease, our investigations show that the protein microarray technology has potential for the identification and evaluation of autoantigens as well as in diagnosis such as to differentiate alopecia areata from other skin diseases.

Glutathione S-transferase pull-down assays using dehydrated immobilized glutathione resin

We have developed an affinity-precipitation technique to facilitate conducting glutathione S-transferase (GST) pull-down assays. The dehydrated immobilized glutathione resin format, when combined with microcentrifuge spin columns, is a powerful tool that enables the simultaneous performance of resin hydration, the binding of the GST fusion protein, and the pull-down step with the appropriate protein partner in a semihigh-throughput fashion (multiple samples processed at the same time). The entire assay process is shortened and recovery is enhanced when coupled with a spin-column format, providing a convenient way to study protein-protein interactions. We successfully tested the resin format/technique in three common pull-down applications utilizing radiolabeled, overexpressed, and activated endogenous interacting protein partners.

A sensitive and restricted enzyme-linked immunosorbent assay for detecting a heterogeneous antibody population in serum from people suffering from a new variant of endemic pemphigus

We recently described a new variant of endemic pemphigus foliaceus (EPF) in El Bagre, Colombia, that resembles Senear-Usher syndrome and identified autoantibodies to desmoglein 1 (Dsg1), as well as to multiple known and unknown antigens including plectins, in the serum of these patients. Here, we developed a cost-effective ELISA assay capable of detecting the heterogeneous antibody population observed in these EPF patients, and useful for serum epidemiological studies. A protein extract obtained from trypsin-digested fresh bovine skin and further purified on a concanavalin A matrix was used as antigen. This extract contains an important conformational epitope (a 45 kDa tryptic fragment of the Dsg1 ectodomain), which is recognized by antibodies in serum from patients with all varieties of pemphigus foliaceus (PF), and from half of those with pemphigus vulgaris with active clinical disease. The cut-off and threshold values were normalized using human serum obtained from both endemic and non-endemic areas for PF. The efficiency of this ELISA was tested using 600 serum samples from controls and patients diagnosed with EPF, non-endemic PF and other bullous diseases. The overall sensitivity and specificity of the assay were determined to be 95% and 72%, respectively, with reproducibilities of 98% (intraassay) and 95% (interassay). Comparing the ELISA with other tests to detect EPF autoantibodies, this ELISA was the most sensitive, followed by direct immunofluorescence (DIF), indirect immunofluorescence using anti-IgG4 monoclonal antibodies and immunoprecipitation (IP), respectively. The most specific assay was IP, followed by DIF. Immunoblotting to Dsg1 exhibited both poor sensitivity and poor specificity, although plectins were well visualized. We conclude that this ELISA is an excellent tool for field serological studies, allowing testing of multiple serum samples simultaneously and for detecting, with appropriate restriction and sensitivity, the heterogeneous antibody population seen in patients with this variant of EPF. Finally, autoantibody serum levels obtained with this ELISA correlated well with the clinical activity and extent of disease in patients with El Bagre EPF.

Protein interactions within the N-end rule ubiquitin ligation pathway

Rate studies have been employed as a reporter function to probe protein-protein interactions within a biochemically defined reconstituted N-end rule ubiquitin ligation pathway. The concentration dependence for E1-catalyzed HsUbc2b/E2(14kb) transthiolation is hyperbolic and yields K(m) values of 102 +/- 13 nm and 123 +/- 19 nm for high affinity binding to rabbit and human E1/Uba1 orthologs. Competitive inhibition by the inactive substrate and product analogs HsUbc2bC88A (K(i) = 104 +/- 15 nm) and HsUbc2bC88S-ubiquitin oxyester (K(i) = 169 +/- 17 nm), respectively, indicates that the ubiquitin moiety contributes little to E1 binding. Under conditions of rate-limiting E3alpha-catalyzed conjugation to human alpha-lactalbumin, HsUbc2b-ubiquitin thiolester exhibits a K(i) of 54 +/- 18 nm and is competitively inhibited by the substrate analog HsUbc2bC88S-ubiquitin oxyester (K(i) = 66 +/- 29 nm). In contrast, the ligase product analog HsUbc2bC88A exhibits a K(i) of 440 +/- 55 nm with respect to the wild type HsUbc2b-ubiquitin thiolester, demonstrating that ubiquitin binding contributes to the ability of E3alpha to discriminate between substrate and product E2. A survey of E1 and E2 isoform distribution in selected cell lines demonstrates that Ubc2 isoforms are the predominant intracellular ubiquitin carrier protein. Intracellular levels of E1 and Ubc2 are micromolar and approximately equal based on in vitro quantitation by stoichiometric (125)I-ubiquitin thiolester formation. Comparison of intracellular E1 and Ubc2 pools with the corresponding ubiquitin pools reveals that most of the free ubiquitin in cells is present as thiolesters to the components of the conjugation pathways. The present data represent the first comprehensive analysis of protein interactions within a ubiquitin ligation pathway.

Ubiquitylation in plants: a post-genomic look at a post-translational modification

In this article, we summarize Arabidopsis genes encoding ubiquitin, ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzymes (E2s) and an additional selected set of proteins related to ubiquitylation. We emphasize comparisons to components from Saccharomyces cerevisiae, with occasional reference to animals. Among the E1 and E2s, Arabidopsis usually has two to four probable orthologs to one yeast gene. Also, Arabidopsis has genes with no likely ortholog in yeast, although they often have potential orthologs in animals. The large number of components with known function in ubiquitylation indicates that this process plays a complex role in cellular physiology.

Significance analysis of microarrays applied to the ionizing radiation response

Microarrays can measure the expression of thousands of genes to identify changes in expression between different biological states. Methods are needed to determine the significance of these changes while accounting for the enormous number of genes. We describe a method, Significance Analysis of Microarrays (SAM), that assigns a score to each gene on the basis of change in gene expression relative to the standard deviation of repeated measurements. For genes with scores greater than an adjustable threshold, SAM uses permutations of the repeated measurements to estimate the percentage of genes identified by chance, the false discovery rate (FDR). When the transcriptional response of human cells to ionizing radiation was measured by microarrays, SAM identified 34 genes that changed at least 1.5-fold with an estimated FDR of 12%, compared with FDRs of 60 and 84% by using conventional methods of analysis. Of the 34 genes, 19 were involved in cell cycle regulation and 3 in apoptosis. Surprisingly, four nucleotide excision repair genes were induced, suggesting that this repair pathway for UV-damaged DNA might play a previously unrecognized role in repairing DNA damaged by ionizing radiation.

The severity of cutaneous and oral pemphigus is related to desmoglein 1 and 3 antibody levels

BACKGROUND

Pemphigus vulgaris (PV) and foliaceus (PF) are characterized by antibodies to the desmosomal proteins desmoglein 3 (Dsg3) and desmoglein 1 (Dsg1), respectively. Past studies using indirect immunofluorescence (IIF) as a measure of pemphigus antibody levels have failed to demonstrate consistently a relationship between disease severity and IIF titres. However, IIF is not able to measure separately Dsg1 and 3 antibodies, unlike enzyme-linked immunosorbent assays (ELISA), which utilize recombinant proteins.

OBJECTIVES

To compare independently Dsg1 and 3 antibody levels with the severity of both cutaneous and oral involvement in PV and PF. Patients and methods Four hundred and twenty-four serum samples were analysed from 80 subjects with PV and 24 with PF. IgG antibodies to Dsg1 and 3 were measured by ELISA. For every sample analysed, the associated severity of skin and oral disease were graded from 0 to 3; quiescent, mild, moderate and severe.

RESULTS

A relationship between Dsg1 antibodies and skin severity was demonstrated such that a 10-unit increase in Dsg1 ELISA value was associated with a 34% chance of having a higher severity score [95% confidence interval (CI), 25-45%, P < 0.0005]. This was observed in both PV and PF. Oral severity was associated with Dsg3 antibody levels and a 10-unit increase in the Dsg3 ELISA value was associated with a 25% chance of a higher oral severity score (CI 17-33%, P < 0.0005). We were unable to demonstrate a relationship between Dsg1 antibodies and oral severity, even after adjusting for the effect of Dsg3 antibodies. Similarly, there was no relationship between Dsg3 antibodies and skin severity.

CONCLUSIONS

This study suggests that the clinical phenotype of pemphigus, in particular the balance of skin and oral disease, is determined principally by the quantities of Dsg1 and 3 autoantibodies, respectively.

Epitopes targeted by bullous pemphigoid T lymphocytes and autoantibodies map to the same sites on the bullous pemphigoid 180 ectodomain

Bullous pemphigoid is a blistering skin disease characterized by autoantibodies directed against the NC16A domain of bullous pemphigoid 180 (collagen XVII), a transmembrane protein of epidermal basal cells. Passive transfer studies in mice have shown that antibodies that bind to this immunodominant region of bullous pemphigoid 180 are capable of inducing a skin disease that closely mimics bullous pemphigoid, supporting the hypothesis that epitopes within NC16A are involved in the pathogenesis of bullous pemphigoid. In this study, we examined the autoimmune T cell response in bullous pemphigoid patients. T cells from eight of 12 bullous pemphigoid patients, all of whom had circulating anti-bullous pemphigoid 180 autoantibodies, showed a specific proliferative response to recombinant forms of NC16A. T cell lines and clones developed from four of these patients recognize the same NC16A peptides as those targeted by autoantibodies from the corresponding individuals. These NC16A-responding T lymphocytes express alpha/beta T cell receptors and CD4 memory T cell surface markers and exhibited a Th1/Th2 mixed cytokine profile that may support the production of antibodies. This new information will aid in defining the key steps involved in the development of the autoimmune response in bullous pemphigoid.

A critical role for neutrophil elastase in experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin disease characterized by subepidermal blisters and autoantibodies against 2 hemidesmosome-associated proteins, BP180 and BP230. The immunopathologic features of BP can be reproduced in mice by passive transfer of anti-BP180 antibodies. Lesion formation in this animal model depends upon complement activation and neutrophil recruitment. In the present study, we investigated the role of neutrophil elastase (NE) in antibody-induced blister formation in experimental BP. Abnormally high levels of caseinolytic activity, consistent with NE, were detected in extracts of lesional skin and blister fluid of mice injected with anti-BP180 IgG. The pathogenic anti-BP180 IgG failed to induce subepidermal blistering in NE-null (NE(-/-)) mutant mice. NE(-/-) mice reconstituted with neutrophils from wild-type mice became susceptible to experimental BP. Wild-type mice given NE inhibitors (alpha1-proteinase inhibitor and Me-O-Suc-Ala-Ala-Pro-Val-CH(2)Cl), but not mice given cathepsin G/chymase inhibitors (alpha1-antichymotrypsin or Z-Gly-Leu-Phe-CH(2)Cl), were resistant to the pathogenic activity of anti-BP180 antibodies. Incubation of murine skin with NE induced BP-like epidermal-dermal detachment. Finally, NE cleaved BP180 in vitro and in vivo. These results implicate NE directly in the dermal-epidermal cleavage induced by anti-BP180 antibodies in the experimental BP model.

Desmoglein-1-specific T lymphocytes from patients with endemic pemphigus foliaceus (fogo selvagem)

Fogo selvagem (FS), the endemic form of pemphigus foliaceus, is a cutaneous autoimmune disease characterized by subcorneal blistering of the epidermis and the production of autoantibodies against the desmosomal antigen desmoglein-1 (Dsg1). Previously, we showed that mice injected with autoantibodies from FS patients develop a skin disease that reproduces the clinical, histological, and immunological features of FS, indicating that autoantibodies play an essential role in the development of this disease. The purpose of this study was to characterize the autoimmune T-cell response associated with FS. We provide here the first evidence, to our knowledge, that the great majority of FS patients have circulating T lymphocytes that specifically proliferate in response to the extracellular domain of Dsg1. Long-term T cells developed from these patients also responded to Dsg1, and this antigen-specific response was shown to be restricted to HLA-DR molecules. These Dsg1-reactive FS T cells exhibited a CD4-positive memory T-cell phenotype and produced a T helper 2-like cytokine profile. These findings represent the initial steps in defining the role of T cells in FS autoimmunity.

Identification and Characterization of Epitopes Recognized by T Lymphocytes and Autoantibodies from Patients with Herpes Gestationis

Autoantibodies associated with herpes gestationis (HG), a pregnancy-associated autoimmune skin disease, target the hemidesmosomal protein BP180. It was shown that the major noncollagenous stretch of the BP180 ectodomain (NC16A) harbors epitopes recognized by HG sera. Furthermore, Abs reactive with the homologous domain of murine BP180 are known to trigger a cutaneous blistering disease in mice by passive transfer experiments. The present study was aimed at characterizing the T cell responses and specificities of autoantibodies from two HG patients. Using immunoblotting and T cell proliferation assays, we have identified a 14-amino-acid stretch of the BP180 ectodomain (MCW-1; aa 507–520) that is recognized by both T cells and autoantibodies produced by the HG patients. The neonate born to one of these HG patients showed no signs of skin disease and had no detectable T cell response to the BP180 Ag, but did have a low titer of circulating anti-BP180 autoantibodies, presumably of maternal origin. BP180-specific T cell lines and clones developed from an HG patient specifically reacted with the MCW-1 epitope. The proliferative responses of these clones were restricted to HLA-DR, but not -DQ or -DP. These Ag-specific T cells expressed α/β TCRs and a CD4 memory T cell phenotype and secreted IFN-γ and IL-2, but not IL-4 or IL-6, suggesting that they are Th1-type lymphocytes. Further characterization of these Ag-specific T cells and autoantibodies will aid in elucidating the autoimmune mechanism(s) leading to the development of HG.

Effect of nonenzymatic glycosylation on the titers of circulating autoantibodies in pemphigus and pemphigoid

Hyperglycemia is observed in some patients with autoimmune bullous diseases complicated by diabetes mellitus or treated with systemic corticosteroids. High concentrations of glucose can react with various proteins and change their structural and functional properties. We previously reported that nonenzymatic glycosylation of antibody can impair antigen-antibody binding. We ascertained whether glycosylation of autoantibody decreases the autoantibody titer by examining 30 sera from patients with pemphigus and pemphigoid. Nonenzymatic glycosylation in the physiological range was induced by incubation of sera with 1650 mM D-glucose at 4 degrees C for 7 days. The titers of sera were determined by indirect immunofluorescence (IIF). In all cases, the immunofluorescence intensity of glycosylated sera was weaker than that of nonglycosylated sera. Glycosylated sera showed a lower antibody titer by 1 doubling dilution in 18 out of 30 cases, compared with nonglycosylated sera. The ten BP patients' sera were also analyzed by immunoblotting for reactivity with the BP180-GST fusion proteins, S delta 1 and 4575. All BP sera reacted with S delta 1, and 5 out of 10 BP sera reacted with both S delta 1 and 4575. In all the sera that reacted only with S delta 1, the glycosylated sera showed a 1 doubling dilution decrease in autoantibody titer. Interestingly, in 4 out of 5 sera that reacted with both S delta 1 and 4575, there were no differences in the antibody titer between glycosylated and nonglycosylated sera. These results indicate the possibility of a false decrease in autoantibody titers of sera from patients with autoimmune bullous diseases complicated with hyperglycemia. Although the false decrease in titers of autoantibodies induced by nonenzymatic glycosylation is not dramatic, it must be considered in order not to underestimate the disease activity of pemphigus in such cases.

Gelatinase B-deficient mice are resistant to experimental bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease characterized by deposition of autoantibodies at the basement membrane zone. In an experimental BP model in mice, the subepidermal blistering is mediated by antibodies directed against the hemidesmosomal protein BP180 (collagen XVII, BPAG2), and depends on complement activation and neutrophil infiltration. Gelatinase B is present in BP blister fluid and can cleave BP180. In this study we investigated the role of gelatinase B in the immunopathogenesis of experimental BP using mice containing targeted disruption of the gelatinase B (MMP-9, 92 kD gelatinase) gene. Gelatinase B-deficient mice were resistant to the blistering effect of intracutaneous anti-mBP180 antibodies, although these mice showed deposition of autoantibodies at the basement membrane zone and neutrophil recruitment to the skin comparable to that observed in the control mice. Interleukin 8 given intradermally concomitantly with pathogenic anti-mBP180 elicited a significant neutrophil recruitment into the skin in gelatinase B-deficient mice, but blistering did not occur. However, gelatinase B-deficient mice reconstituted with neutrophils from normal mice developed blistering in response to anti-mBP180 antibodies. These results implicate neutrophil-derived gelatinase B in the pathogenesis of experimental BP and might lead to novel therapeutic strategies for BP.

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.

T lymphocytes from a subset of patients with pemphigus vulgaris respond to both desmoglein-3 and desmoglein-1

Pemphigus vulgaris and pemphigus foliaceus are cutaneous autoimmune diseases characterized by intraepithelial blisters and autoantibodies to desmosomal glycoproteins. The antigens recognized by pemphigus vulgaris and pemphigus foliaceus autoantibodies are desmoglein-3 (Dsg3) and desmoglein-1 (Dsg1), respectively. Dsg3 and Dsg1 are members of the desmoglein subfamily of the cadherin supergene family of cell adhesion molecules. It has been well documented that a subset of pemphigus vulgaris sera have IgG reactivity to both Dsg1 and Dsg3, suggesting that Dsg1 may also participate in the autoimmune response of these patients. The cellular mechanisms of T cell autoimmunity in these patients, however, are completely unknown. In this study, we tested the proliferative responses of T lymphocytes from eight pemphigus vulgaris patients after incubation with Dsg3 and Dsg1 fusion proteins. The sera of four of these PV patients showed reactivity with both Dsg1 and Dsg3, whereas the remaining four reacted only with Dsg3. We found that T cells obtained from those patients that exhibited the combined Dsg1/Dsg3 autoantibody reactivity showed a proliferative response after exposure to either Dsg1 or Dsg3 fusion proteins. The cellular responses to both of these recombinant proteins were highly specific and restricted to the CD4-positive T cell population. T cells from pemphigus vulgaris patients with no anti-Dsg1 serum reactivity showed a proliferative response to Dsg3, but not to Dsg1. The Dsg1 fusion protein used in this study has minimal sequence homology with Dsg3. Thus, this study provides the first evidence that T cells from a subset of pemphigus vulgaris patients respond to both Dsg1 and Dsg3.

Preferential interaction of sentrin with a ubiquitin-conjugating enzyme, Ubc9

Sentrin is a ubiquitin-like molecule that has been shown to interact with the death domains of Fas and tumor necrosis factor receptor 1 (TNFR1), PML, Rad51, Rad52, and RanGAP1. We have reported previously that sentrin can be conjugated to other proteins in a manner analogous to protein ubiquitination (Kamitani, T., Nguyen, H. P., and Yeh, E. T. H. (1997) J. Biol. Chem. 272, 14001-14004). Furthermore, the conserved C-terminal Gly-Gly residues are required for sentrinization to occur. To identify enzymes which play a role in sentrinization, the yeast two-hybrid system was used to screen a human placenta cDNA library using sentrin as bait. A strong positive interacting clone was found to contain a cDNA insert encoding the ubiquitin-conjugating enzyme, Ubc9. The interaction between sentrin and Ubc9 required the ubiquitin domain and the C-terminal Gly-Gly residues of sentrin. This interaction appears to be specific because sentrin could only interact weakly with UbcH5B, but could not interact with HHR6B, UbcH6 nor E2-EPF. In vitro translated sentrin could be precipitated by a GST-Ubc9 fusion protein, but not by glutathione S-transferase. A beta-mercaptoethanol-sensitive Ubc9-sentrin conjugate could also be identified in the in vitro binding assay. Substitution of the conserved cysteine residue of Ubc9 by serine abolished the formation of the Ubc9-sentrin conjugate. Taken together, Ubc9 is a strong candidate to be the key conjugating enzyme in the sentrinization pathway.

Beta2-microglobulin-deficient mice are resistant to bullous pemphigoid

Recent understanding of the mechanism of immunoglobulin G (IgG) catabolism has yielded new insight into antibody-mediated diseases. We proposed that beta2-microglobulin (beta2m)-deficient mice have been protected from systemic lupus erythematosis (SLE)-like syndromes because they lack the beta2m-associated IgG protection receptor (FcRn) and therefore catabolize IgG, including pathogenic IgG autoantibodies, considerably more rapidly than normal mice. Such an hypothesis would predict that beta2m-deficient mice would also be resistant to experimental bullous pemphigoid, a disease with a pathogenesis thought to be much simpler than SLE, being the result of antibody directed toward a pathogenic epitope on the epidermal hemidesmosome that anchors basal keratinocytes to the basement membrane. To test this hypothesis, we administered pathogenic rabbit antibody directed toward the hemidesmosome to beta2m-deficient mice and to normal control mice, both intraperitoneally and intradermally, and assessed the mice clinically, histologically, and immunologically for manifestations of skin disease. We found that the beta2m-deficient mice were protected when the antibody was given intraperitoneally whereas intradermal administration resulted in blisters only slightly less severe than those seen in normal mice. These data would indicate that autoantibody-mediated inflammation might be prevented or controlled by appropriate modulation of FcRn function.

Bullous pemphigoid and cicatricial pemphigoid autoantibodies react with ultrastructurally separable epitopes on the BP180 ectodomain: evidence that BP180 spans the lamina lucida

The BP180 antigen is a hemidesmosomal glycoprotein that is recognized by autoantibodies associated with three autoimmune disorders, bullous pemphigoid (BP), herpes gestationis (HG), and cicatricial pemphigoid (CP). BP and HG sera have been shown to recognize a common extracellular site located near the membrane-spanning domain of this protein, whereas CP sera react predominantly with a distinct site near the C terminus. In the current study, the main immunogenic sites on the BP180 ectodomain were ultrastructurally localized using six BP sera, four CP sera, and two rabbit antisera. The immunolocalization pattern of BP sera was largely restricted to the upper lamina lucida region immediately subjacent to the epidermal hemidesmosome and closely resembled that of a rabbit antiserum directed against the NC16A (membrane-proximal) domain of BP180. CP sera, on the other hand, exhibited a lower lamina lucida/lamina densa labeling pattern that was strikingly similar to that of rabbit antibodies to the BP180 C-terminal region. Finally, antibodies to the BP180 C-terminal region co-localized with an anti-laminin-5 antibody in the anchoring filament zone. These findings strongly suggest that the BP180 extracellular domain exists in an extended conformation, with the C terminus of this protein projecting into the lamina densa. These data support the hypothesis that BP180 contributes to the structure and function of the anchoring filaments. Differences in the ultrastructural mapping of BP and CP autoantibodies appear to correlate with epitope mapping data, which, together, may help to explain the clinical heterogeneity observed in this group of bullous disorders.

Novel multiubiquitin chain linkages catalyzed by the conjugating enzymes E2EPF and RAD6 are recognized by 26 S proteasome subunit 5

Targeting of substrates for degradation by the ATP, ubiquitin-dependent pathway requires formation of multiubiquitin chains in which the 8.6-kDa polypeptide is linked by isopeptide bonds between carboxyl termini and Lys-48 residues of successive monomers. Binding of Lys-48-linked chains by subunit 5 of the 26 S proteasome regulatory complex commits the attached target protein to degradation with concomitant release of free ubiquitin monomers following disassembly of the chains. Point mutants of ubiquitin (Lys-->Arg) were used to map the linkage specificity for ubiquitin-conjugating enzymes previously demonstrated to form novel multiubiquitin chains not attached through Lys-48. Recombinant human E2EPF catalyzed multiubiquitin chain formation exclusively through Lys-11 of ubiquitin while recombinant yeast RAD6 formed chains linked only through Lys-6. Multiubiquitin chains linked through Lys-6, Lys-11, or Lys-48 each bound to subunit 5 of partially purified human 26 S proteasome with comparable affinities. Since chains bearing different linkages are expected to pack into distinct structures, competition between Lys-11 and Lys-48 chains for binding to subunit 5 demonstrates that the latter possesses determinants for recognizing alternatively linked chains and precludes the existence of subunit 5 isoforms recognizing distinct structures. In addition, competition studies provided an estimate of Kd < or = 18 nM for the intrinsic binding of Lys-48-linked chains of linkage number n > 4. This result suggests that the principal mechanistic advantage of multiubiquitin chain formation is to enhance the affinity of the associated substrate for the 26 S complex relative to that of unconjugated target protein. Complementation studies with E1/E2-depleted rabbit reticulocyte extract demonstrated RAD6 supported isopeptide ligase-dependent degradation only through Lys-48-linked chains, while E2EPF retained the ability to target a model radiolabeled substrate through Lys-11-linked chains. Therefore, the linkage specificity exhibited by these E2 isozymes depends on their catalytic context with respect to isopeptide ligase.

Cloning of human ubiquitin-conjugating enzymes UbcH6 and UbcH7 (E2-F1) and characterization of their interaction with E6-AP and RSP5

E6-AP, a 100-kDa cellular protein, was originally identified through its interaction with the E6 protein of the oncogenic human papillomavirus types 16 and 18. The complex of E6-AP and E6 specifically interacts with p53 and mediates ubiquitination of p53 in concert with the E1 ubiquitin-activating enzyme and the E2 ubiquitin-conjugating enzyme UbcH5. Recent results suggest that E6-AP is representative of a family of putative ubiquitin-protein ligases. Members of this family are characterized by a conserved C-terminal region, termed hect domain. In this paper, we describe the isolation of two human E2s, designated as UbcH6 and UbcH7, that in addition to UbcH5 can interact with E6-AP. UbcH6 is a novel member of an evolutionally conserved subfamily of E2s that includes UbcH5 and Saccharomyces cerevisiae UBC4. Although UbcH7 does not appear to be a member of this subfamily, UbcH7 efficiently substitutes for UbcH5 in E6-AP-dependent ubiquitination. Surprisingly, UbcH6 was only weakly active in this particular assay. In addition, UbcH5 but not UbcH6 or UbcH7 efficiently interacts with the heet protein RSP5. These results indicate that E6-AP can interact with at least two species of E2 and that different hect proteins may interact with different E2s.

Characterization of a novel keratinocyte ubiquitin carrier protein

A novel member of the ubiquitin carrier protein family, designated E2EPF, has been cloned by our laboratory and expressed in a bacterial system in an active form. Ubiquitin carrier proteins, or E2s, catalyze one step in a multistep process that leads to the covalent conjugation of ubiquitin to substrate proteins. In this paper, we show that recombinant E2EPF catalyzes auto/multiubiquitination, the conjugation of multiple ubiquitin molecules to itself. Multiubiquitination has been shown previously to be required for targeting of a substrate protein for rapid degradation. Using a rabbit reticulocyte lysate system, E2EPF was shown to support the degradation of a model substrate in an ATP- and ubiquitin-dependent fashion. In contrast to a previous study which showed that selective protein degradation in one system is dependent upon multiubiquitination via the lysine 48 residue of ubiquitin, multiubiquitination, and proteolytic targeting by E2EPF was shown here to be independent of the lysine 48 multiubiquitin linkage. This functional characterization of E2EPF revealed a combination of features that distinguishes this enzyme from all previously characterized members of the ubiquitin carrier protein family. These results also suggest several possible autoregulatory models for E2EPF involving auto- and multiubiquitination.

Identification of a family of closely related human ubiquitin conjugating enzymes

Two very closely related human E2 ubiquitin conjugating enzymes, UbfH5B and UbcH5C, have been identified. These enzymes are products of distinct genes and are 88-89% identical in amino acid sequence to the recently described human E2, UbcH5 (now designated UbcH5A), UbcH5A-C are homologous to a family of five ubiquitin conjugating enzymes from Arabidopsis thaliana, AtUBC8-12. They are also closely related to Saccharomyces cerevisiae ScUBC4 and ScUBC5, which are involved in the stress response, and play a central role in the targeting of short-lived regulatory proteins for degradation. mRNAs encoding UbcH5A-C were co-expressed in all cell lines and tissues evaluated, with UbcH5C transcripts generally expressed at the highest levels. Analysis of Southern blots suggests that there are likely to be other related members of this family. Both UbcH5B and UbcH5C form thiol ester adducts with ubiquitin, and have activities similar to UbcH5A and AtUBC8 in the conjugation of ubiquitin to target proteins in the presence of the human ubiquitin protein ligase E6-AP. These results establish the existence of a highly conserved, and widely expressed, family of human ubiquitin conjugating enzymes.

Role of the ubiquitin-proteasome pathway in regulating abundance of the cyclin-dependent kinase inhibitor p27

The p27 mammalian cell cycle protein is an inhibitor of cyclin-dependent kinases. Both in vivo and in vitro, p27 was found to be degraded by the ubiquitin-proteasome pathway. The human ubiquitin-conjugating enzymes Ubc2 and Ubc3 were specifically involved in the ubiquitination of p27. Compared with proliferating cells, quiescent cells exhibited a smaller amount of p27 ubiquitinating activity, which accounted for the marked increase of p27 half-life measured in these cells. Thus, the abundance of p27 in cells is regulated by degradation. The specific proteolysis of p27 may represent a mechanism for regulating the activity of cyclin-dependent kinases.

Immunopathology of oral mucosal ulcerative, desquamative, and bullous diseases. Selective review of the literature

Cell/cell and cell/matrix adhesion proteins are responsible for maintaining the integrity of the mucosal lining of the oral cavity. Disease processes that destroy keratinocytes or adversely affect their adhesion to one another or to the subjacent basement membrane will result in erosions, ulcerations, and desquamations. Immunologic processes that have a deleterious effect on the integrity of the epithelial/basement membrane/submucosa complex are reviewed, and current research findings with respect to pathogenesis are discussed. In particular, T-cell-mediated hypersensitivity is involved in recurrent aphthous stomatitis and lichen planus; humoral-mediated immunity to cadherin intercellular adhesion molecules is important in the process of acantholysis in pemphigus vulgaris, and genetic defects and antibody-mediated processes give rise to junctional separation in epidermolysis bullosa and mucous membrane pemphigoid, respectively. An immune complex mechanism appears to underlie the pathogenesis of erythema multiforme.

Development of an ELISA to detect anti-BP180 autoantibodies in bullous pemphigoid and herpes gestationis

Autoantibodies associated with the subepidermal blistering disorders bullous pemphigoid and herpes gestationis react with a 180-kD transmembrane hemidesmosomal protein, designated BP180. The BP180 ectodomain is composed of a series of interrupted collagen triple helical domains. Located on one of the noncollagenous extracellular segments of this protein is an immunodominant epitope, designated MCW-1, recognized by patient autoantibodies. In this investigation we have developed an enzyme-linked immunosorbent assay system to detect antibody reactivity against the MCW-1 epitope with the use of a bacterial fusion protein containing the BP180 autoantibody-reactive site. The following sera were assayed for reactivity with this recombinant protein: bullous pemphigoid (n = 62), herpes gestationis (n = 28), endemic pemphigus foliaceus (n = 17), lupus erythematosus (n = 15), and normal human sera (n = 22). This enzyme-linked immunosorbent assay-based protocol was shown to be highly specific (98.3%) in detecting autoantibody activity in bullous pemphigoid and herpes gestationis patients. Fifty-three percent of bullous pemphigoid sera and 71% of herpes gestations sera, but none of the control sera, yielded positive results in this assay. Of the patient sera that were known to react with full-length BP180, almost all showed reactivity with the MCW-1 antigenic site of this protein. Autoantibodies detected in this assay were predominantly of the immunoglobulin G class. The results presented here lend support to the hypothesis that this well-defined antigen/antibody system may be relevant in pathogenesis.

Functional expression and molecular characterization of AtUBC2-1, a novel ubiquitin-conjugating enzyme (E2) from Arabidopsis thaliana

The first member of a novel subfamily of ubiquitin-conjugating E2-proteins was cloned from a cDNA library of Arabidopsis thaliana. Genomic blots indicate that this gene family (AtUBC2) consists of two members and is distinct from AtUBC1, the only other E2 enzyme known from this species to date (M.L. Sullivan and R.D. Vierstra, Proc. Natl. Acad. Sci. USA 86 (1989) 9861-9865). The cDNA sequence of AtUBC2-1 extends over 794 bp which would encode a protein of 161 amino acids and a calculated molecular mass of 18.25 kDa. The protein encoded by AtUBC2-1 is shown to accept 125I-ubiquitin from wheat E1 enzymes, when expressed from Escherichia coli hosts as fusion protein carrying N-terminal extensions. It is deubiquitinated in the presence of lysine and, by these criteria, is considered a functional E2 enzyme.