Ubiquitin-specific proteases are differentially expressed throughout the Schistosoma mansoni life cycle

Variety in the USP deubiquitinase catalytic mechanism

The ubiquitin-specific protease (USP) family of deubiquitinases (DUBs) controls cellular ubiquitin-dependent signaling events. This generates therapeutic potential, with active-site inhibitors in preclinical and clinical studies. Understanding of the USP active site is primarily guided by USP7 data, where the catalytic triad consists of cysteine, histidine, and a third residue (third critical residue), which polarizes the histidine through a hydrogen bond. A conserved aspartate (fourth critical residue) is directly adjacent to this third critical residue. Although both critical residues accommodate catalysis in USP2, these residues have not been comprehensively investigated in other USPs. Here, we quantitatively investigate their roles in five USPs. Although USP7 relies on the third critical residue for catalysis, this residue is dispensable in USP1, USP15, USP40, and USP48, where the fourth critical residue is vital instead. Furthermore, these residues vary in importance for nucleophilic attack. The diverging catalytic mechanisms of USP1 and USP7 are independent of substrate and retained in cells for USP1. This unexpected variety of catalytic mechanisms in this well-conserved protein family may generate opportunities for selective targeting of individual USPs.

microRNA silencing in a whole worm cestode model provides insight into miR-71 function

Parasites belonging to the class Cestoda include zoonotic species such as Echinococcus spp. and Taenia spp. that cause morbidity and mortality in endemic areas, mainly affecting pastoral and rural communities in low income countries but also upper middle income countries. Cestodes show remarkable developmental plasticity, implying tight regulation of gene expression throughout their complex life cycles. Despite the recent availability of genomic data for cestodes, little progress was made on postgenomic functional studies. MicroRNAs (miRNAs) are key components of gene regulatory systems that guide diverse developmental processes in multicellular organisms. miR-71 is a highly expressed miRNA in cestodes, which is absent in vertebrates and targets essential parasite genes, representing a potential key player in understanding the role of miRNAs in cestodes biology. Here we used transfection with antisense oligonucleotides to perform whole worm miRNA knockdown in tetrathyridia of Mesocestoides vogae (syn. Mesocestoides corti), a laboratory model of cestodes. We believe this is the first report of miRNA knockdown at the organism level in these parasites. Our results showed that M. vogae miR-71 is involved in the control of strobilation in vitro and in the establishment of murine infection. In addition, we identified miR-71 targets in M. vogae, several of them being de-repressed upon miR-71 knockdown. This study provides new knowledge on gene expression regulation in cestodes and suggests that miRNAs could be evaluated as new selective therapeutic targets for treating Neglected Tropical Diseases prioritised by the World Health Organization.

The ubiquitin-proteasome system in breast cancer

Ubiquitin-proteasome system (UPS) is a selective proteolytic system that is associated with the expression or function of target proteins and participates in various physiological and pathological processes of breast cancer. Inhibitors targeting the 26S proteasome in combination with other drugs have shown promising therapeutic effects in the clinical treatment of breast cancer. Moreover, several inhibitors/stimulators targeting other UPS components are also effective in preclinical studies, but have not yet been applied in the clinical treatment of breast cancer. Therefore, it is vital to comprehensively understand the functions of ubiquitination in breast cancer and to identify potential tumor promoters or tumor suppressors among UPS family members, with the aim of developing more effective and specific inhibitors/stimulators targeting specific components of this system.

A Plasmodium falciparum ubiquitin-specific protease (PfUSP) is essential for parasite survival and its disruption enhances artemisinin efficacy

Proteins associated with ubiquitin-proteasome system (UPS) are potential drug targets in the malaria parasite. The ubiquitination and deubiquitination are key regulatory processes for the functioning of UPS. In this study, we have characterized the biochemical and functional role of a novel ubiquitin-specific protease (USP) domain-containing protein of the human malaria parasite Plasmodium falciparum (PfUSP). We have shown that the PfUSP is an active deubiquitinase associated with parasite endoplasmic reticulum (ER). Selection linked integration (SLI) method for C-terminal tagging and GlmS-ribozyme mediated inducible knock-down (iKD) of PfUSP was utilized to assess its functional role. Inducible knockdown of PfUSP resulted in a remarkable reduction in parasite growth and multiplication; specifically, PfUSP-iKD disrupted ER morphology and development, blocked the development of healthy schizonts, and hindered proper merozoite development. PfUSP-iKD caused increased ubiquitylation of specific proteins, disrupted organelle homeostasis and reduced parasite survival. Since the mode of action of artemisinin and the artemisinin-resistance are shown to be associated with the proteasome machinery, we analyzed the effect of dihydroartemisinin (DHA) on PfUSP-iKD parasites. Importantly, the PfUSP-knocked-down parasite showed increased sensitivity to dihydroartemisinin (DHA), whereas no change in chloroquine sensitivity was observed, suggesting a role of PfUSP in combating artemisinin-induced cellular stress. Together, the results show that Plasmodium PfUSP is an essential protease for parasite survival, and its inhibition increases the efficacy of artemisinin-based drugs. Therefore, PfUSP can be targeted to develop novel scaffolds for developing new antimalarials to combat artemisinin resistance.

The emerging role of Deubiquitinases (DUBs) in parasites: A foresight review

Before the discovery of the proteasome complex, the lysosomes with acidic proteases and caspases in apoptotic pathways were thought to be the only pathways for the degradation of damaged, unfolded, and aged proteins. However, the discovery of 26S and 20S proteasome complexes in eukaryotes and microbes, respectively, established that the degradation of most proteins is a highly regulated ATP-dependent pathway that is significantly conserved across each domain of life. The proteasome is part of the ubiquitin-proteasome system (UPS), where the covalent tagging of a small molecule called ubiquitin (Ub) on the proteins marks its proteasomal degradation. The type and chain length of ubiquitination further determine whether a protein is designated for further roles in multi-cellular processes like DNA repair, trafficking, signal transduction, etc., or whether it will be degraded by the proteasome to recycle the peptides and amino acids. Deubiquitination, on the contrary, is the removal of ubiquitin from its substrate molecule or the conversion of polyubiquitin chains into monoubiquitin as a precursor to ubiquitin. Therefore, deubiquitylating enzymes (DUBs) can maintain the dynamic state of cellular ubiquitination by releasing conjugated ubiquitin from proteins and controlling many cellular pathways that are essential for their survival. Many DUBs are well characterized in the human system with potential drug targets in different cancers. Although, proteasome complex and UPS of parasites, like plasmodium and leishmania, were recently coined as multi-stage drug targets the role of DUBs is completely unexplored even though structural domains and functions of many of these parasite DUBs are conserved having high similarity even with its eukaryotic counterpart. This review summarizes the identification & characterization of different parasite DUBs based on in silico and a few functional studies among different phylogenetic classes of parasites including Metazoan (Schistosoma, Trichinella), Apicomplexan protozoans (Plasmodium, Toxoplasma, Eimeria, Cryptosporidium), Kinetoplastidie (Leishmania, Trypanosoma) and Microsporidia (Nosema). The identification of different homologs of parasite DUBs with structurally similar domains with eukaryotes, and the role of these DUBs alone or in combination with the 20S proteosome complex in regulating the parasite survival/death is further elaborated. We propose that small molecules/inhibitors of human DUBs can be potential antiparasitic agents due to their significant structural conservation.

USP41 promotes breast cancer via regulating RACK1

Background

Breast cancer (BC) is the most common cancer diagnosed among women and is the second leading cause of cancer death. It is of great significance to explore potential candidate targets for BC.

Methods

The expression of ubiquitin-specific protease 41 (USP41) and its prognosis prediction function was firstly evaluated by TCGA database analysis. Using BC cell lines and specimens from 10 patients with primary BC, the upregulation of USP41 in BC was ensured. By USP41 overexpression or knockdown, its function was studied by cell function assays, small interfering RNA (siRNA), western blot, mass spectrometry, and flow cytometry. The potential mechanism of USP41 was explored via Co-Immunoprecipitation mass spectrometry, and western blot.

Results

TCGA database analysis revealed that in metastatic BC, USP41 expression was upregulated and negatively correlated with BC prognosis. In BC cancer cells and cancer specimens, USP41 was also upregulated. Overexpression of USP41 greatly enhanced BC colony-forming ability, proliferation, and migration. In contrast, USP41 knockdown significantly inhibited BC colony-forming ability, proliferation, and migration. Moreover, Co-Immunoprecipitation mass spectrometry results indicated that USP41 could interact with RACK1. USP41 promoted the protein expression of RACK1. The expression of RACK1 in BC tissues was upregulated. Knockdown of RACK1 inhibited cell growth and migration, and reversed the oncogenic function of USP41 in BC cells.

Conclusions

USP41 can be a potential therapeutic target against BC via RACK1.

Assessment of reference genes at six different developmental stages of Schistosoma mansoni for quantitative RT-PCR

Reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) is the most used, fast, and reproducible method to confirm large-scale gene expression data. The use of stable reference genes for the normalization of RT-qPCR assays is recognized worldwide. No systematic study for selecting appropriate reference genes for usage in RT-qPCR experiments comparing gene expression levels at different Schistosoma mansoni life-cycle stages has been performed. Most studies rely on genes commonly used in other organisms, such as actin, tubulin, and GAPDH. Therefore, the present study focused on identifying reference genes suitable for RT-qPCR assays across six S. mansoni developmental stages. The expression levels of 25 novel candidates that we selected based on the analysis of public RNA-Seq datasets, along with eight commonly used reference genes, were systematically tested by RT-qPCR across six developmental stages of S. mansoni (eggs, miracidia, cercariae, schistosomula, adult males and adult females). The stability of genes was evaluated with geNorm, NormFinder and RefFinder algorithms. The least stable candidate reference genes tested were actin, tubulin and GAPDH. The two most stable reference genes suitable for RT-qPCR normalization were Smp_101310 (Histone H4 transcription factor) and Smp_196510 (Ubiquitin recognition factor in ER-associated degradation protein 1). Performance of these two genes as normalizers was successfully evaluated with females maintained unpaired or paired to males in culture for 8 days, or with worm pairs exposed for 16 days to double-stranded RNAs to silence a protein-coding gene. This study provides reliable reference genes for RT-qPCR analysis using samples from six different S. mansoni life-cycle stages.

Schistosoma mansoni Heterochromatin Protein 1 (HP1) nuclear interactome in cercariae

Schistosoma mansoni causes schistosomiasis, which affects 240 million people, and 700 million people are living at risk of infection. Epigenetic mechanisms are important for transcriptional control and are well-known conserved transcriptional co-regulators in evolution, already described in mammal, yeast, protozoa and S. mansoni, responsible for heterochromatization and gene silence mechanisms through the formation of complexes of transcriptional repression in chromatin. Previous results from another group have shown that HP1 (SmCBX) proteins form chromatin complexes with SmMDB2/3 proteins and regulate stem cells and oviposition in parasite adult worms. In addition, results from other groups have shown that cercariae are transcriptionally silent and epigenetic mechanisms are involved in the regulation of gene expression in this stage. In this work, our aim was to give insights into SmHP1 and proteins involved in transcriptional regulation in the cercariae stage. Using monoclonal anti-HP1 antibody for Western blotting, immunoprecipitation, and mass spectrometry, we preliminarily determined nuclear proteins that putatively interact with HP1 to form complexes to regulate gene expression, heterochromatin formation, and translational complexes in the cercariae stage. So far, our data is to give some insights into nuclear interactors in S. mansoni cercariae. SIGNIFICANCE: The significance of this original paper is the evidence for Heterochromatin Protein (HP1), interaction with nuclear proteins in the cercariae stage. Schistosoma mansoni cercariae are the infective stage of the human beings in endemic areas of schistosomiasis, a neglected disease, most prevalent in Brazil and Africa. While cercariae are waiting for a host, it does not feed, gene expression is silent and protein synthesis is stopped. These biochemical mechanisms are recovered when cercariae find a human host, but all proteins and mechanisms are not still elucidated. Until now, literature shows that these phenomena are regulated by epigenetics mechanisms, dependent of histone posttranslational modifications. But we have few pieces of evidence about the other proteins that participates in these processes and which are the co-regulators of expression.

Clinicopathological and prognostic significance of ubiquitin-specific peptidase 15 and its relationship with transforming growth factor-β receptors in patients with pancreatic ductal adenocarcinoma

BACKGROUND AND AIM

Ubiquitin-specific peptidase 15 (USP15) has been correlated to aggressive oncogenic behavior in several types of carcinomas, but its function in pancreatic ductal adenocarcinoma (PDAC) has not been clarified. This study aimed to evaluate the clinicopathological and prognostic value of USP15 and its relationship with transforming growth factor-β (TGF-β) receptors (TβRs) in PDAC.

METHODS

By immunohistochemical staining of tissue microarrays, the expression patterns of USP15 and TβRs were retrospectively analyzed in 287 PDAC patients who underwent radical surgical resection without neoadjuvant therapy. Cancer-specific survival was compared based on USP15 expression, and the correlations between USP15 and TβRs were analyzed.

RESULTS

Ubiquitin-specific peptidase 15 expression in tumor tissues was significantly higher than that in para-tumor tissues (P < 0.0001), and high USP15 expression was associated with the pathological N (pN) stage (P = 0.033). In addition, high USP15 expression was significantly associated with shorter cancer-specific survival (P = 0.019). Univariate analyses showed that high USP15 expression (P = 0.024), a poor histopathological grade (P = 0.003), and the pN1 stage (P = 0.009) were significantly correlated with shorter survival. Although the independent prognostic value of USP15 alone was not established, the combination of USP15 and the histological grade was identified as an independent prognostic factor in multivariate analyses (P = 0.015). USP15 expression was correlated with TβR-I, TβR-II, or TβR-III expression in PDAC.

CONCLUSIONS

High USP15 expression is a potential prognostic indicator in patients with PDAC, and it might affect the TGF-β signaling pathway in PDAC.

Proteomic analysis of Ascocotyle longa (Trematoda: Heterophyidae) metacercariae

Ascocotyle longa is parasitic trematode with wide distribution throughout America, Europe, Africa, and Middle East. Despite the fact that this fish-borne pathogen has been considered an agent of human heterophyiasis in Brazil, the molecules involved in the host-parasite interaction remain unknown. The present study reports the proteome profile of A. longa metacercariae collected from the fish Mugil liza from Brazil. This infective stage for humans, mammals and birds was analyzed using nLC-MS/MS approach. We identified a large repertoire of proteins, which are mainly involved in energy metabolism and cell structure. Peptidases and immunogenic proteins were also identified, which might play roles in host-parasite interface. Our data provided unprecedented insights into the biology of A. longa and represent a first step to understand the natural host-parasite interaction. Moreover, as the first proteome characterized in this trematode, it will provide an important resource for future studies.

Epigenetic and parasitological parameters are modulated in EBi3-/- mice infected with Schistosoma mansoni

Schistosoma mansoni adaptive success is related to regulation of replication, transcription and translation inside and outside the intermediate and definitive host. We hypothesize that S. mansoni alters its epigenetic state in response to the mammalian host immune system, reprogramming gene expression and altering the number of eggs. In response, a change in the DNA methylation profile of hepatocytes could occurs, modulating the extent of hepatic granuloma. To investigate this hypothesis, we used the EBi3-/- murine (Mus musculus) model of S. mansoni infection and evaluated changes in new and maintenance DNA methylation profiles in the liver after 55 days of infection. We evaluated expression of epigenetic genes and genes linked to histone deubiquitination in male and female S. mansoni worms. Comparing TET expression with DNMT expression indicated that DNA demethylation exceeds methylation in knockout infected and uninfected mice and in wild-type infected and uninfected mice. S. mansoni infection provokes activation of demethylation in EBi3-/-I mice (knockout infected). EBi3-/-C (knockout uninfected) mice present intrinsically higher DNA methylation than WTC (control uninfected) mice. EBi3-/-I mice show decreased hepatic damage considering volume and reduced number of granulomas compared to WTI mice; the absence of IL27 and IL35 pathways decreases the Th1 response resulting in minor liver damage. S. mansoni males and females recovered from EBi3-/-I mice have reduced expression of a deubiquitinating enzyme gene, orthologs of which target histones and affect chromatin state. SmMBD and SmHDAC1 expression levels are downregulated in male and female parasites recovered from EBi3-/-, leading to epigenetic gene downregulation in S. mansoni. Changes to the immunological background thus induce epigenetic changes in hepatic tissues and alterations in S. mansoni gene expression, which attenuate liver symptoms in the acute phase of schistosomiasis.

In vitro activity of aryl-thiazole derivatives against Schistosoma mansoni schistosomula and adult worms

Schistosomiasis is caused by a trematode of the genus Schistosoma and affects over 200 million people worldwide. The only drug recommended by the World Health Organization for treatment and control of schistosomiasis is praziquantel. Development of new drugs is therefore of great importance. Thiazoles are regarded as privileged structures with a broad spectrum of activities and are potential sources of new drug prototypes, since they can act through interactions with DNA and inhibition of DNA synthesis. In this context, we report the synthesis of a series of thiazole derivatives and their in vitro schistosomicidal activity by testing eight molecules (NJ03-08; NJ11-12) containing thiazole structures. Parameters such as motility and mortality, egg laying, pairing and parasite viability by ATP quantification, which were influenced by these compounds, were evaluated during the assays. Scanning electron microscopy (SEM) was utilized for evaluation of morphological changes in the tegument. Schistosomula and adult worms were treated in vitro with different concentrations (6.25 to 50 μM) of the thiazoles for up to 5 and 3 days, respectively. After in vitro treatment for five days with 6.25 μM NJ05 or NJ07 separately, we observed a decrease of 30% in schistosomula viability, whilst treatment with NJ05+NJ07 lead to a reduction of 75% in viability measured by ATP quantitation and propidium iodide labeling. Adult worms’ treatment with 50 μM NJ05, NJ07 or NJ05 + NJ07 showed decreased motility to 30–50% compared with controls. Compound NJ05 was more effective than NJ07, and adult worm viability after three days was reduced to 25% in parasites treated with 50 μM NJ05, compared with a viability reduction to 40% with 50 μM NJ07. SEM analysis showed severe alterations in adult worms with formation of bulges and blisters throughout the dorsal region of parasites treated with NJ05 or NJ07. Oviposition was extremely affected by treatment with the NJ series compounds; at concentrations of 25 μM and 50 μM, oviposition reached almost zero with NJ05, NJ07 or NJ05 + NJ07 already at day one. Tested genes involved in egg biosynthesis were all confirmed by qPCR as downregulated in females treated with 25 μM NJ05 for 2 days, with a significant reduction in expression of p14, Tyrosinase 2, p48 and fs800. NJ05, NJ07 or NJ05+NJ07 treatment of HEK293 (human embryonic cell line) and HES (human epithelial cell line) showed EC50 in the range of 18.42 to 145.20 μM. Overall, our results demonstrate that those molecules are suitable targets for further development into new drugs for schistosomiasis treatment, although progress is needed to lessen the cytotoxic effects on human cells. According to the present study, thiazole derivatives have schistosomicidal activities and may be part of a possible new arsenal of compounds against schistosomiasis.

FLA, which encodes a homolog of UBP, is required for chlorophyll accumulation and development of lemma and palea in rice

FLA, the homolog of ubiquitin-specific protease does not have deubiquitination activity, but it is essential for flower and chloroplast development in rice. Ubiquitin-specific proteases (UBPs) are widely distributed and highly conserved proteins and are also members of the most important family of deubiquitination enzymes. Although the functions and phylogenies of UBPs from yeast, mammals and Arabidopsis have been widely reported, the functions and evolutionary relationships of UBPs in rice remain unclear. In this study, we characterized the rice flower and leaf color aberrant mutant (fla), which exhibited a variety of developmental defects, including abnormal floral organs and pollen development, and leaf bleaching. We isolated FLA by positional cloning and found that it encodes a homolog of ubiquitin-specific protease. FLA is a ubiquitously expressed gene with the highest expression in floral organs. Subcellular localization analysis indicated that FLA is a cell membrane protein. Through searches of the rice genome database ( http://rice.plantbiology.msu.edu ), we identified 35 UBP family members in the rice genome. These proteins were grouped into 16 subfamilies based on phylogenetic analysis, and FLA was found to belong to the G8 subfamily. In vitro activity assays revealed that FLA does not have deubiquitination activity. Our data suggest that FLA plays an important role in the development of floral organs and chloroplast in rice, but that this role probably does not involve deubiquitination activity, because FLA does not have an active site and deubiquitination activity.

Effects of proteasome inhibitor MG-132 on the parasite Schistosoma mansoni

Proteasome is a proteolytic complex responsible for intracellular protein turnover in eukaryotes, archaea and in some actinobacteria species. Previous work has demonstrated that in Schistosoma mansoni parasites, the proteasome inhibitor MG-132 affects parasite development. However, the molecular targets affected by MG-132 in S. mansoni are not entirely known. Here, we used expression microarrays to measure the genome-wide changes in gene expression of S. mansoni adult worms exposed in vitro to MG-132, followed by in silico functional analyses of the affected genes using Ingenuity Pathway Analysis (IPA). Scanning electron microscopy was used to document changes in the parasites’ tegument. We identified 1,919 genes with a statistically significant (q-value ≤ 0.025) differential expression in parasites treated for 24 h with MG-132, when compared with control. Of these, a total of 1,130 genes were up-regulated and 790 genes were down-regulated. A functional gene interaction network comprised of MG-132 and its target genes, known from the literature to be affected by the compound in humans, was identified here as affected by MG-132. While MG-132 activated the expression of the 26S proteasome genes, it also decreased the expression of 19S chaperones assembly, 20S proteasome maturation, ubiquitin-like NEDD8 and its partner cullin-3 ubiquitin ligase genes. Interestingly, genes that encode proteins related to potassium ion binding, integral membrane component, ATPase and potassium channel activities were significantly down-regulated, whereas genes encoding proteins related to actin binding and microtubule motor activity were significantly up-regulated. MG-132 caused important changes in the worm tegument; peeling, outbreaks and swelling in the tegument tubercles could be observed, which is consistent with interference on the ionic homeostasis in S. mansoni. Finally, we showed the down-regulation of Bax pro-apoptotic gene, as well as up-regulation of two apoptosis inhibitor genes, IAP1 and BRE1, and in contrast, down-regulation of Apaf-1 apoptotic activator, thus suggesting that apoptosis is deregulated in S. mansoni exposed to MG-132. A considerable insight has been gained concerning the potential of MG-132 as a gene expression modulator, and overall the data suggest that the proteasome might be an important molecular target for the design of new drugs against schistosomiasis.

Ubiquitin-Dependent Modification of Skeletal Muscle by the Parasitic Nematode, Trichinella spiralis

Trichinella spiralis is a muscle-specific parasitic worm that is uniquely intracellular. T. spiralis reprograms terminally differentiated skeletal muscle cells causing them to de-differentiate and re-enter the cell cycle, a process that cannot occur naturally in mammalian skeletal muscle cells, but one that holds great therapeutic potential. Although the host ubiquitin pathway is a common target for viruses and bacteria during infection, its role in parasite pathogenesis has been largely overlooked. Here we demonstrate that the secreted proteins of T. spiralis contain E2 Ub-conjugating and E3 Ub-ligase activity. The E2 activity is attributed to TsUBE2L3, a novel and conserved T. spiralis enzyme located in the secretory organ of the parasite during the muscle stages of infection. TsUBE2L3 cannot function with any T.spiralis secreted E3, but specifically binds to a panel of human RING E3 ligases, including the RBR E3 ARIH2 with which it interacts with a higher affinity than the mammalian ortholog UbcH7/UBE2L3. Expression of TsUBE2L3 in skeletal muscle cells causes a global downregulation in protein ubiquitination, most predominantly affecting motor, sarcomeric and extracellular matrix proteins, thus mediating their stabilization with regards to proteasomal degradation. This effect is not observed in the presence of the mammalian ortholog, suggesting functional divergence in the evolution of the parasite protein. These findings demonstrate the first example of host-parasite interactions via a parasite-derived Ub conjugating enzyme; an E2 that demonstrates a novel muscle protein stabilization function.

Parasitic worms often establish long-lasting infections in their hosts; tightly regulating their surroundings to strike a delicate balance between host cell modulation and protection that will ensure their replication. This is accomplished via the active secretion of parasite glycolipids and glycoproteins into the host. Trichinella spiralis, a parasitic nematode that infects skeletal muscle of mammals, birds and reptiles, is the only parasitic worm with a true intracellular stage. T. spiralis invade terminally differentiated myotubes, reprogramming them to de-differentiate and re-enter the cell cycle, a process that cannot occur naturally in mammalian skeletal muscle cells, but one that holds great therapeutic potential. We have identified and characterized a novel T. spiralis secreted protein that, despite a high level of sequence identity, appears to have evolved a different function to its host ortholog. This protein is an active Ub conjugating enzyme that binds to a panel of human E3 Ub ligases with higher affinity than the host ortholog. Furthermore, when expressed in skeletal muscle cells in culture, its presence uniquely leads to the stabilization of muscle-specific proteins via the downregulation of their ubiquitination.