Tandem UIMs confer Lys48 ubiquitin chain substrate preference to deubiquitinase USP25

Smurf2 Suppresses Proliferation and Cell Cycle of Triple-Negative Breast Cancer Cells by Promoting the Polyubiquitination and Degradation of RPL35A

Human L35a ribosomal protein (RPL35A) has been reported to confer higher drug resistance and viability to triple-negative breast cancer (TNBC) cells, but the mechanism related to its promotion of TNBC malignant progression is still unclear. Here, we found that silencing of RPL35A could inhibit the proliferation of TNBC cells by suppressing the G1/S phase transition. Furthermore, SMAD-specific E3 ubiquitin protein ligase 2 (Smurf2) was found to be a potential upstream ubiquitin ligase of RPL35A. Smurf2 could interact with RPL35A and promote its degradation and K63-linked polyubiquitination, thereby suppressing the G1/S phase transition and proliferation of TNBC cells. In addition, the roles of Smurf2 were confirmed in a xenograft mouse model. Finally, we found a negative correlation between the protein levels of RPL35A and Smurf2 in human TNBC tissues. In summary, Smurf2 inhibits the proliferation of TNBC cells by blocking the cell cycle process, which is associated with regulating RPL35A.

Specificity profiling of deubiquitylases against endogenously generated ubiquitin-protein conjugates

Deubiquitylating enzymes (DUBs) remove ubiquitin from proteins thereby regulating their stability or activity. Our understanding of DUB-substrate specificity is limited because DUBs are typically not compared to each other against many physiological substrates. By broadly inhibiting DUBs in Xenopus egg extract, we generated hundreds of ubiquitylated proteins and compared the ability of 30 DUBs to deubiquitylate them using quantitative proteomics. We identified five high-impact DUBs (USP7, USP9X, USP36, USP15, and USP24) that each reduced ubiquitylation of over 10% of the isolated proteins. Candidate substrates of high-impact DUBs showed substantial overlap and were enriched for disordered regions, suggesting this feature may promote substrate recognition. Other DUBs showed lower impact and non-overlapping specificity, targeting distinct non-disordered proteins including complexes such as the ribosome or the proteasome. Altogether our study identifies candidate DUB substrates and defines patterns of functional redundancy and specificity, revealing substrate characteristics that may influence DUB-substrate recognition.

Substrate identification and specificity profiling of deubiquitylases against endogenously-generated ubiquitin-protein conjugates

Deubiquitylating enzymes (DUBs) remove ubiquitin from proteins thereby regulating their stability or activity. Our understanding of DUB-substrate specificity is limited because DUBs are typically not compared to each other against many physiological substrates. By broadly inhibiting DUBs in Xenopus egg extract, we generated hundreds of ubiquitylated proteins and compared the ability of 30 DUBs to deubiquitylate them using quantitative proteomics. We identified five high impact DUBs (USP7, USP9X, USP36, USP15 and USP24) that each reduced ubiquitylation of over ten percent of the isolated proteins. Candidate substrates of high impact DUBs showed substantial overlap and were enriched for disordered regions, suggesting this feature may promote substrate recognition. Other DUBs showed lower impact and non-overlapping specificity, targeting distinct non-disordered proteins including complexes such as the ribosome or the proteasome. Altogether our study identifies candidate DUB substrates and defines patterns of functional redundancy and specificity, revealing substrate characteristics that may influence DUB-substrate recognition.

USP25 Ameliorates Pathological Cardiac Hypertrophy by Stabilizing SERCA2a in Cardiomyocytes

BACKGROUND

Pathological cardiac hypertrophy can lead to heart failure and is one of the leading causes of death globally. Understanding the molecular mechanism of pathological cardiac hypertrophy will contribute to the treatment of heart failure. DUBs (deubiquitinating enzymes) are essential to cardiac pathophysiology by precisely controlling protein function, localization, and degradation. This study set out to investigate the role and molecular mechanism of a DUB, USP25 (ubiquitin-specific peptidase 25), in pathological cardiac hypertrophy.

METHODS

The role of USP25 in myocardial hypertrophy was evaluated in murine cardiomyocytes in response to Ang II (angiotensin II) and transverse aortic constriction stimulation and in hypertrophic myocardium tissues of heart failure patients. Liquid chromotography with mass spectrometry/mass spectrometry analysis combined with Co-IP was used to identify SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2A), an antihypertrophy protein, as an interacting protein of USP25. To clarify the molecular mechanism of USP25 in the regulation of SERCA2a, we constructed a series of mutant plasmids of USP25. In addition, we overexpressed USP25 and SERCA2a in the heart with adenoassociated virus serotype 9 vectors to validate the biological function of USP25 and SERCA2a interaction.

RESULTS

We revealed increased protein level of USP25 in murine cardiomyocytes subject to Ang II and transverse aortic constriction stimulation and in hypertrophic myocardium tissues of patients with heart failure. USP25 deficiency aggravated cardiac hypertrophy and cardiac dysfunction under Ang II and transverse aortic constriction treatment. Mechanistically, USP25 bound to SERCA2a directly via its USP (ubiquitin-specific protease) domain and cysteine at position 178 of USP25 exerts deubiquitination to maintain the stability of the SERCA2a protein by removing the K48 ubiquitin chain and preventing proteasomal pathway degradation, thereby maintaining calcium handling in cardiomyocytes. Moreover, restoration of USP25 expression via adenoassociated virus serotype 9 vectors in USP25^-/- mice attenuated Ang II-induced cardiac hypertrophy and cardiac dysfunction, whereas myocardial overexpression of SERCA2a could mimic the effect of USP25.

CONCLUSIONS

We confirmed that USP25 inhibited cardiac hypertrophy by deubiquitinating and stabilizing SERCA2a.

Probing protein ubiquitination in live cells

The reversible attachment of ubiquitin governs the interaction, activity and degradation of proteins whereby the type and target of this conjugation determine the biological response. The investigation of this complex and multi-faceted protein ubiquitination mostly relies on painstaking biochemical analyses. Here, we employ recombinant binding domains to probe the ubiquitination of proteins in living cells. We immobilize GFP-fused proteins of interest at a distinct cellular structure and detect their ubiquitination state with red fluorescent ubiquitin binders. With this ubiquitin fluorescent three-hybrid (ubiF3H) assay we identified HP1β as a novel ubiquitination target of UHRF1. The use of linkage specific ubiquitin binding domains enabled the discrimination of K48 and K63 linked protein ubiquitination. To enhance signal-to-noise ratio, we implemented fluorescence complementation (ubiF3Hc) with split YFP. Using in addition a cell cycle marker we could show that HP1β is mostly ubiquitinated by UHRF1 during S phase and deubiquitinated by the protease USP7. With this complementation assay we could also directly detect the ubiquitination of the tumor suppressor p53 and monitor its inhibition by the anti-cancer drug Nutlin-3. Altogether, we demonstrate the utility of the ubiF3H assay to probe the ubiquitination of specific proteins and to screen for ligases, proteases and small molecules controlling this posttranslational modification.

Panel of Engineered Ubiquitin Variants Targeting the Family of Human Ubiquitin Interacting Motifs

Ubiquitin (Ub)-binding domains embedded in intracellular proteins act as readers of the complex Ub code and contribute to regulation of numerous eukaryotic processes. Ub-interacting motifs (UIMs) are short α-helical modular recognition elements whose role in controlling proteostasis and signal transduction has been poorly investigated. Moreover, impaired or aberrant activity of UIM-containing proteins has been implicated in numerous diseases, but targeting modular recognition elements in proteins remains a major challenge. To overcome this limitation, we developed Ub variants (UbVs) that bind to 42 UIMs in the human proteome with high affinity and specificity. Structural analysis of a UbV:UIM complex revealed the molecular determinants of enhanced affinity and specificity. Furthermore, we showed that a UbV targeting a UIM in the cancer-associated Ub-specific protease 28 potently inhibited catalytic activity. Our work demonstrates the versatility of UbVs to target short α-helical Ub receptors with high affinity and specificity. Moreover, the UbVs provide a toolkit to investigate the role of UIMs in regulating and transducing Ub signals and establish a general strategy for the systematic development of probes for Ub-binding domains.

Emerging Roles of Ubiquitin-Specific Protease 25 in Diseases

The balance of ubiquitination and deubiquitination plays diverse roles in regulating protein stability and cellular homeostasis. Deubiquitinating enzymes catalyze the hydrolysis and removal of ubiquitin chains from target proteins and play critical roles in various disease processes, including cancer, immune responses to viral infections and neurodegeneration. This article aims to summarize roles of the deubiquitinating enzyme ubiquitin-specific protease 25 (USP25) in disease onset and progression. Previous studies have focused on the role of USP25 in antiviral immunity and neurodegenerative diseases. Recently, however, as the structural similarities and differences between USP25 and its homolog USP28 have become clear, mechanisms of action of USP25 in cancer and other diseases have been gradually revealed.

Deubiquitylase USP25 prevents degradation of BCR-ABL protein and ensures proliferation of Ph-positive leukemia cells

Fusion genes resulting from chromosomal rearrangements are frequently found in a variety of cancer cells. Some of these are known to be driver oncogenes, such as BCR-ABL in chronic myelogenous leukemia (CML). The products of such fusion genes are abnormal proteins that are ordinarily degraded in cells by a mechanism known as protein quality control. This suggests that the degradation of BCR-ABL protein is suppressed in CML cells to ensure their proliferative activity. Here, we show that ubiquitin-specific protease 25 (USP25) suppresses the degradation of BCR-ABL protein in cells harboring Philadelphia chromosome (Ph). USP25 was found proximal to BCR-ABL protein in cells. Depletion of USP25 using shRNA-mediated gene silencing increased the ubiquitylated BCR-ABL, and reduced the level of BCR-ABL protein. Accordingly, BCR-ABL-mediated signaling and cell proliferation were suppressed in BCR-ABL-positive leukemia cells by the depletion of USP25. We further found that pharmacological inhibition of USP25 induced rapid degradation of BCR-ABL protein in Ph-positive leukemia cells, regardless of their sensitivity to tyrosine kinase inhibitors. These results indicate that USP25 is a novel target for inducing the degradation of oncogenic BCR-ABL protein in Ph-positive leukemia cells. This could be an effective approach to overcome resistance to kinase inhibitors.

The 'dark matter' of ubiquitin-mediated processes: opportunities and challenges in the identification of ubiquitin-binding domains

Ubiquitin modifications of target proteins act to localise, direct and specify a diverse range of cellular processes, many of which are biomedically relevant. To allow this diversity, ubiquitin modifications exhibit remarkable complexity, determined by a combination of polyubiquitin chain length, linkage type, numbers of ubiquitin chains per target, and decoration of ubiquitin with other small modifiers. However, many questions remain about how different ubiquitin signals are specifically recognised and transduced by the decoding ubiquitin-binding domains (UBDs) within ubiquitin-binding proteins. This review briefly outlines our current knowledge surrounding the diversity of UBDs, identifies key challenges in their discovery and considers recent structural studies with implications for the increasing complexity of UBD function and identification. Given the comparatively low numbers of functionally characterised polyubiquitin-selective UBDs relative to the ever-expanding variety of polyubiquitin modifications, it is possible that many UBDs have been overlooked, in part due to limitations of current approaches used to predict their presence within the proteome. Potential experimental approaches for UBD discovery are considered; web-based informatic analyses, Next-Generation Phage Display, deubiquitinase-resistant diubiquitin, proximity-dependent biotinylation and Ubiquitin-Phototrap, including possible advantages and limitations. The concepts discussed here work towards identifying new UBDs which may represent the 'dark matter' of the ubiquitin system.

Differential Oligomerization of the Deubiquitinases USP25 and USP28 Regulates Their Activities

Deubiquitinases have emerged as promising drug targets for cancer therapy. The two DUBs USP25 and USP28 share high similarity but vary in their cellular functions. USP28 is known for its tumor-promoting role, whereas USP25 is a regulator of the innate immune system and, recently, a role in tumorigenesis was proposed. We solved the structures of the catalytic domains of both proteins and established substantial differences in their activities. While USP28 is a constitutively active dimer, USP25 presents an auto-inhibited tetramer. Our data indicate that the activation of USP25 is not achieved through substrate or ubiquitin binding. USP25 cancer-associated mutations lead to activation in vitro and in vivo, thereby providing a functional link between auto-inhibition and the cancer-promoting role of the enzyme. Our work led to the identification of significant differences between USP25 and USP28 and provided the molecular basis for the development of new and highly specific anti-cancer drugs.

The ubiquitin interacting motifs of USP37 act on the proximal Ub of a di-Ub chain to enhance catalytic efficiency

USP37 is a deubiquitinase (DUB) with roles in the regulation of DNA damage repair and the cohesion of sister chromatids during mitosis. USP37 contains a unique insert of three ubiquitin interacting motifs (UIMs) within its catalytic DUB domain. We investigated the role of the three UIMs in the ability of USP37 to cleave di-ubiquitin chains. We found that the third UIM of USP37 recognizes the proximal ubiquitin moiety of K48 di-Ub to potentiate cleavage activity and posit that this mechanism of action may be generalizable to other chain types. In the case of K48-linked ubiquitin chains this potentiation stemmed largely from a dramatic increase in catalytic rate (k_cat). We also developed and characterized three ubiquitin variant (UbV) inhibitors that selectively engage distinct binding sites in USP37. In addition to validating the deduced functional roles of the three UIMs in catalysis, the UbVs highlight a novel and effective means to selectively inhibit members of the difficult to drug DUB family.

Usp25m protease regulates ubiquitin-like processing of TUG proteins to control GLUT4 glucose transporter translocation in adipocytes

Insulin stimulates the exocytic translocation of specialized vesicles in adipocytes, which inserts GLUT4 glucose transporters into the plasma membrane to enhance glucose uptake. Previous results support a model in which TUG (Tether containing a UBX domain for GLUT4) proteins trap these GLUT4 storage vesicles at the Golgi matrix and in which insulin triggers endoproteolytic cleavage of TUG to translocate GLUT4. Here, we identify the muscle splice form of Usp25 (Usp25m) as a protease required for insulin-stimulated TUG cleavage and GLUT4 translocation in adipocytes. Usp25m is expressed in adipocytes, binds TUG and GLUT4, dissociates from TUG-bound vesicles after insulin addition, and colocalizes with TUG and insulin-responsive cargoes in unstimulated cells. Previous results show that TUG proteolysis generates the ubiquitin-like protein, TUGUL (for TUGubiquitin-like). We now show that TUGUL modifies the kinesin motor protein, KIF5B, and that TUG proteolysis is required to load GLUT4 onto these motors. Insulin stimulates TUG proteolytic processing independently of phosphatidylinositol 3-kinase. In nonadipocytes, TUG cleavage can be reconstituted by transfection of Usp25m, but not the related Usp25a isoform, together with other proteins present on GLUT4 vesicles. In rodents with diet-induced insulin resistance, TUG proteolysis and Usp25m protein abundance are reduced in adipose tissue. These effects occur soon after dietary manipulation, prior to the attenuation of insulin signaling to Akt. Together with previous data, these results support a model whereby insulin acts through Usp25m to mediate TUG cleavage, which liberates GLUT4 storage vesicles from the Golgi matrix and activates their microtubule-based movement to the plasma membrane. This TUG proteolytic pathway for insulin action is independent of Akt and is impaired by nutritional excess.

Ubiquitin-specific protease 8 deubiquitinates Sec31A and decreases large COPII carriers and collagen IV secretion

Nascent cargo proteins in the endoplasmic reticulum are transported to the Golgi by COPII carriers. Typical COPII vesicles are 60-70 nm in diameter, and much larger macromolecules, such as procollagen, are transported by atypical large COPII carriers in mammalian cells. The formation of large COPII carriers is enhanced by Cul3 ubiquitin ligase, which mono-ubiquitinates Sec31A, a COPII coat protein. However, the deubiquitinating enzyme for Sec31A was unclear. Here, we show that the deubiquitinating enzyme USP8 interacts with and deubiquitinates Sec31A. The interaction was mediated by the adaptor protein STAM1. USP8 overexpression inhibited the formation of large COPII carriers. By contrast, USP8 knockdown caused the accumulation of COPII coat proteins around the cis-Golgi, promoted the intracellular trafficking of procollagen IV from the endoplasmic reticulum to the Golgi, and increased collagen IV secretion. We concluded that USP8 deubiquitinates Sec31A and inhibits the formation of large COPII carriers, thereby suppressing collagen IV secretion.