Modulating cellular balance of Rps3 mono-ubiquitination by both Hel2 E3 ligase and Ubp3 deubiquitinase regulates protein quality control

Remodeling of the ribosomal quality control and integrated stress response by viral ubiquitin deconjugases

The strategies adopted by viruses to reprogram the translation and protein quality control machinery and promote infection are poorly understood. Here, we report that the viral ubiquitin deconjugase (vDUB)-encoded in the large tegument protein of Epstein-Barr virus (EBV BPLF1)-regulates the ribosomal quality control (RQC) and integrated stress responses (ISR). The vDUB participates in protein complexes that include the RQC ubiquitin ligases ZNF598 and LTN1. Upon ribosomal stalling, the vDUB counteracts the ubiquitination of the 40 S particle and inhibits the degradation of translation-stalled polypeptides by the proteasome. Impairment of the RQC correlates with the readthrough of stall-inducing mRNAs and with activation of a GCN2-dependent ISR that redirects translation towards upstream open reading frames (uORFs)- and internal ribosome entry sites (IRES)-containing transcripts. Physiological levels of active BPLF1 promote the translation of the EBV Nuclear Antigen (EBNA)1 mRNA in productively infected cells and enhance the release of progeny virus, pointing to a pivotal role of the vDUB in the translation reprogramming that enables efficient virus production.

Molecular Highway Patrol for Ribosome Collisions

During translation, messenger RNAs (mRNAs) are decoded by ribosomes which can stall for various reasons. These include chemical damage, codon composition, starvation, or translation inhibition. Trailing ribosomes can collide with stalled ribosomes, potentially leading to dysfunctional or toxic proteins. Such aberrant proteins can form aggregates and favor diseases, especially neurodegeneration. To prevent this, both eukaryotes and bacteria have evolved different pathways to remove faulty nascent peptides, mRNAs and defective ribosomes from the collided complex. In eukaryotes, ubiquitin ligases play central roles in triggering downstream responses and several complexes have been characterized that split affected ribosomes and facilitate degradation of the various components. As collided ribosomes signal translation stress to affected cells, in eukaryotes additional stress response pathways are triggered when collisions are sensed. These pathways inhibit translation and modulate cell survival and immune responses. Here, we summarize the current state of knowledge about rescue and stress response pathways triggered by ribosome collisions.

Loss of N-terminal acetyltransferase A activity induces thermally unstable ribosomal proteins and increases their turnover in Saccharomyces cerevisiae

Protein N-terminal (Nt) acetylation is one of the most abundant modifications in eukaryotes, covering ~50-80 % of the proteome, depending on species. Cells with defective Nt-acetylation display a wide array of phenotypes such as impaired growth, mating defects and increased stress sensitivity. However, the pleiotropic nature of these effects has hampered our understanding of the functional impact of protein Nt-acetylation. The main enzyme responsible for Nt-acetylation throughout the eukaryotic kingdom is the N-terminal acetyltransferase NatA. Here we employ a multi-dimensional proteomics approach to analyze Saccharomyces cerevisiae lacking NatA activity, which causes global proteome remodeling. Pulsed-SILAC experiments reveals that NatA-deficient strains consistently increase degradation of ribosomal proteins compared to wild type. Explaining this phenomenon, thermal proteome profiling uncovers decreased thermostability of ribosomes in NatA-knockouts. Our data are in agreement with a role for Nt-acetylation in promoting stability for parts of the proteome by enhancing the avidity of protein-protein interactions and folding.

Phosphorylation of rpS3 by Lyn increases translation of Multi-Drug Resistance (MDR1) gene

Lyn, a tyrosine kinase that is activated by double-stranded DNAdamaging agents, is involved in various signaling pathways, such as proliferation, apoptosis, and DNA repair. Ribosomal protein S3 (RpS3) is involved in protein biosynthesis as a component of the ribosome complex and possesses endonuclease activity to repair damaged DNA. Herein, we demonstrated that rpS3 and Lyn interact with each other, and the phosphorylation of rpS3 by Lyn, causing ribosome heterogeneity, upregulates the translation of p-glycoprotein, which is a gene product of multidrug resistance gene 1. In addition, we found that two different regions of the rpS3 protein are associated with the SH1 and SH3 domains of Lyn. An in vitro immunocomplex kinase assay indicated that the rpS3 protein acts as a substrate for Lyn, which phosphorylates the Y167 residue of rpS3. Furthermore, by adding various kinase inhibitors, we confirmed that the phosphorylation status of rpS3 was regulated by both Lyn and doxorubicin, and the phosphorylation of rpS3 by Lyn increased drug resistance in cells by upregulating p-glycoprotein translation. [BMB Reports 2023; 56(5): 302-307].

UBE2J1 inhibits colorectal cancer progression by promoting ubiquitination and degradation of RPS3

Ubiquitin-conjugating enzyme E2 J1 (UBE2J1) has been proven to participate in the ubiquitination of multiple substrate proteins. However, the underlying mechanisms of UBE2J1 as a ubiquitin-conjugating enzyme participating in cancer development and progression remain largely unknown. Here, we identified that UBE2J1 is downregulated in colorectal cancer (CRC) tissues and cell lines which are mediated by DNA hypermethylation of its promoter, and decreased UBE2J1 is associated with poor prognosis. Functionally, UBE2J1 serving as a suppressor gene inhibits the proliferation and metastasis of CRC cells. Mechanistically, UBE2J1-TRIM25, forming an E2-E3 complex, physically interacts with and targets RPS3 for ubiquitination and degradation at the K214 residue. The downregulated RPS3 caused by UBE2J1 overexpression restrains NF-κB translocation into the nucleus and therefore inactivates the NF-κB signaling pathway. Our study revealed a novel role of UBE2J1-mediated RPS3 poly-ubiquitination and degradation in disrupting the NF-κB signaling pathway, which may serve as a novel and promising biomarker and therapeutic target for CRC.

The deubiquitinase USP10 protects pancreatic cancer cells from endoplasmic reticulum stress

The ubiquitin-specific peptidase 10 (USP10) plays a context-specific, pro or anti-tumorigenic role in different malignancies. However, the role of USP10 in pancreatic cancer remains unclear. Our protein and RNA level analysis from archived specimens and public databases show that USP10 is overexpressed in pancreatic ductal adenocarcinoma (PDAC) and expression correlates with poor overall patient survival. Phenotypically, silencing USP10 decreased viability, clonal growth and invasive properties of pancreatic cancer cells. Mechanistically, silencing USP10 upregulated BiP and induced endoplasmic reticulum (ER) stress that led to an unfolded protein response (UPR) and upregulation of PERK, IRE1α. Decreased cell viability of USP10 silenced cells could be rescued by a chemical chaperone that promotes protein folding. Our studies suggest that USP10 by protecting pancreatic cancer cells from ER stress may support tumor progression.

USP10 as a Potential Therapeutic Target in Human Cancers

Deubiquitination is a major form of post-translational protein modification involved in the regulation of protein homeostasis and various cellular processes. Deubiquitinating enzymes (DUBs), comprising about five subfamily members, are key players in deubiquitination. USP10 is a USP-family DUB featuring the classic USP domain, which performs deubiquitination. Emerging evidence has demonstrated that USP10 is a double-edged sword in human cancers. However, the precise molecular mechanisms underlying its different effects in tumorigenesis remain elusive. A possible reason is dependence on the cell context. In this review, we summarize the downstream substrates and upstream regulators of USP10 as well as its dual role as an oncogene and tumor suppressor in various human cancers. Furthermore, we summarize multiple pharmacological USP10 inhibitors, including small-molecule inhibitors, such as spautin-1, and traditional Chinese medicines. Taken together, the development of specific and efficient USP10 inhibitors based on USP10’s oncogenic role and for different cancer types could be a promising therapeutic strategy.

Ubiquitination-mediated molecular pathway alterations in human lung squamous cell carcinomas identified by quantitative ubiquitinomics

Abnormal ubiquitination is extensively associated with cancers. To investigate human lung cancer ubiquitination and its potential functions, quantitative ubiquitinomics was carried out between human lung squamous cell carcinoma (LSCC) and control tissues, which characterized a total of 627 ubiquitin-modified proteins (UPs) and 1209 ubiquitinated lysine sites. Those UPs were mainly involved in cell adhesion, signal transduction, and regulations of ribosome complex and proteasome complex. Thirty three UPs whose genes were also found in TCGA database were significantly related to overall survival of LSCC. Six significant networks and 234 hub molecules were obtained from the protein-protein interaction (PPI) analysis of those 627 UPs. KEGG pathway analysis of those UPs revealed 47 statistically significant pathways, and most of which were tumor-associated pathways such as mTOR, HIF-1, PI3K-Akt, and Ras signaling pathways, and intracellular protein turnover-related pathways such as ribosome complex, ubiquitin-mediated proteolysis, ER protein processing, and proteasome complex pathways. Further, the relationship analysis of ubiquitination and differentially expressed proteins shows that ubiquitination regulates two aspects of protein turnover - synthesis and degradation. This study provided the first profile of UPs and molecular networks in LSCC tissue, which is the important resource to insight into new mechanisms, and to identify new biomarkers and therapeutic targets/drugs to treat LSCC.

Loss of Ras GTPase-activating protein SH3 domain-binding protein 1 (G3BP1) inhibits the progression of ovarian cancer in coordination with ubiquitin-specific protease 10 (USP10)

Ovarian cancer (OC) is one of the most lethal gynecological malignancies. However, the molecular mechanisms underlying the development of OC remain unclear. Here, we report that loss of Ras GTPase-activating protein SH3 domain-binding protein 1 (G3BP1) inhibits the progression of OC cells. Analysis of databases and clinical specimens showed that G3BP1 is upregulated in OC. The Kaplan-Meier plot results showed that G3BP1 is highly expressed in OC with a poor clinical outcome. Moreover, loss-of-G3BP1 suppresses the proliferation, migration, and invasion of OC cells. Protein-protein interaction network analysis and immunoprecipitation assay showed that ubiquitin-specific protease 10 (USP10) interacts with G3BP1. We next found that USP10 coordinately promotes tumor progression with G3BP1. Moreover, loss of USP10could restore the G3BP1-induced proliferation, migration, and invasion of OC cells. These data indicate that G3BP1 coordinated with USP10 to facilitate the progression of OC cells, and that G3BP1 may become a treatment target for OC.

Proteomic and metabolomic analysis of Nicotiana benthamiana under dark stress

Exposure to extended periods of darkness is a common source of abiotic stress that significantly affects plant growth and development. To understand how Nicotiana benthamiana responds to dark stress, the proteomes and metabolomes of leaves treated with darkness were studied. In total, 5763 proteins and 165 primary metabolites were identified following dark treatment. Additionally, the expression of autophagy-related gene (ATG) proteins was transiently upregulated. Weighted gene coexpression network analysis (WGCNA) was utilized to find the protein modules associated with the response to dark stress. A total of four coexpression modules were obtained. The results indicated that heat-shock protein (HSP70), SnRK1-interacting protein 1, 2A phosphatase-associated protein of 46 kDa (Tap46), and glutamate dehydrogenase (GDH) might play crucial roles in N. benthamiana's response to dark stress. Furthermore, a protein-protein interaction (PPI) network was constructed and top-degreed proteins were predicted to identify potential key factors in the response to dark stress. These proteins include isopropylmalate isomerase (IPMI), eukaryotic elongation factor 5A (ELF5A), and ribosomal protein 5A (RPS5A). Finally, metabolic analysis suggested that some amino acids and sugars were involved in the dark-responsive pathways. Thus, these results provide a new avenue for understanding the defensive mechanism against dark stress at the protein and metabolic levels in N. benthamiana.

Canary in a coal mine: collided ribosomes as sensors of cellular conditions

The recent discovery that collision of ribosomes triggers quality control and stress responses in eukaryotes has shifted the perspective of the field. Collided eukaryotic ribosomes adopt a unique structure, acting as a ubiquitin signaling platform for various response factors. While several of the signals that determine which downstream pathways are activated have been uncovered, we are only beginning to learn how the specificity for the activation of each process is achieved during collisions. This review will summarize those findings and how ribosome-associated factors act as molecular sentinels, linking aberrations in translation to the overall cellular state. Insights into how cells respond to ribosome collision events will provide greater understanding of the role of the ribosome in the maintenance of cellular homeostasis.

A Multi-Perspective Proximity View on the Dynamic Head Region of the Ribosomal 40S Subunit

A comparison of overlapping proximity captures at the head region of the ribosomal 40S subunit (hr40S) in Saccharomyces cerevisiae from four adjacent perspectives, namely Asc1/RACK1, Rps2/uS5, Rps3/uS3, and Rps20/uS10, corroborates dynamic co-localization of proteins that control activity and fate of both ribosomes and mRNA. Co-locating factors that associate with the hr40S are involved in (i) (de)ubiquitination of ribosomal proteins (Hel2, Bre5-Ubp3), (ii) clamping of inactive ribosomal subunits (Stm1), (iii) mRNA surveillance and vesicular transport (Smy2, Syh1), (iv) degradation of mRNA (endo- and exonucleases Ypl199c and Xrn1, respectively), (v) autophagy (Psp2, Vps30, Ykt6), and (vi) kinase signaling (Ste20). Additionally, they must be harmonized with translation initiation factors (eIF3, cap-binding protein Cdc33, eIF2A) and mRNA-binding/ribosome-charging proteins (Scp160, Sro9). The Rps/uS-BioID perspectives revealed substantial Asc1/RACK1-dependent hr40S configuration indicating a function of the β-propeller in context-specific spatial organization of this microenvironment. Toward resolving context-specific constellations, a Split-TurboID analysis emphasized the ubiquitin-associated factors Def1 and Lsm12 as neighbors of Bre5 at hr40S. These shuttling proteins indicate a common regulatory axis for the fate of polymerizing machineries for the biosynthesis of proteins in the cytoplasm and RNA/DNA in the nucleus.

iRQC, a surveillance pathway for 40S ribosomal quality control during mRNA translation initiation

Post-translational modification of ribosomal proteins enables rapid and dynamic regulation of protein biogenesis. Site-specific ubiquitylation of 40S ribosomal proteins uS10 and eS10 plays a key role during ribosome-associated quality control (RQC). Distinct, and previously functionally ambiguous, ubiquitylation events on the 40S proteins uS3 and uS5 are induced by diverse proteostasis stressors that impact translation activity. Here, we identify the ubiquitin ligase RNF10 and the deubiquitylating enzyme USP10 as the key enzymes that regulate uS3 and uS5 ubiquitylation. Prolonged uS3 and uS5 ubiquitylation results in 40S, but not 60S, ribosomal protein degradation in a manner independent of canonical autophagy. We show that blocking progression of either scanning or elongating ribosomes past the start codon triggers site-specific ubiquitylation events on ribosomal proteins uS5 and uS3. This study identifies and characterizes a distinct arm in the RQC pathway, initiation RQC (iRQC), that acts on 40S ribosomes during translation initiation to modulate translation activity and capacity.

Development of Carbazole Derivatives Compounds against Candida albicans: Candidates to Prevent Hyphal Formation via the Ras1-MAPK Pathway

Morphogenesis contributes to the virulence of the opportunistic human fungal pathogen Candida albicans. Ras1-MAPK pathways play a critical role in the virulence of C. albicans by regulating cell growth, morphogenesis, and biofilm formation. Ume6 acts as a transcription factor, and Nrg1 is a transcriptional repressor for the expression of hyphal-specific genes in morphogenesis. Azoles or echinocandin drugs have been extensively prescribed for C. albicans infections, which has led to the development of drug-resistant strains. Therefore, it is necessary to develop new molecules to effectively treat fungal infections. Here, we showed that Molecule B and Molecule C, which contained a carbazole structure, attenuated the pathogenicity of C. albicans through inhibition of the Ras1/MAPK pathway. We found that Molecule B and Molecule C inhibit morphogenesis through repressing protein and RNA levels of Ras/MAPK-related genes, including UME6 and NRG1. Furthermore, we determined the antifungal effects of Molecule B and Molecule C in vivo using a candidiasis murine model. We anticipate our findings are that Molecule B and Molecule C, which inhibits the Ras1/MAPK pathway, are promising compounds for the development of new antifungal agents for the treatment of systemic candidiasis and possibly for other fungal diseases.

The E3 ubiquitin ligase RNF10 modifies 40S ribosomal subunits of ribosomes compromised in translation

Reversible monoubiquitination of small subunit ribosomal proteins RPS2/uS5 and RPS3/uS3 has been noted to occur on ribosomes involved in ZNF598-dependent mRNA surveillance. Subsequent deubiquitination of RPS2 and RPS3 by USP10 is critical for recycling of stalled ribosomes in a process known as ribosome-associated quality control. Here, we identify and characterize the RPS2- and RPS3-specific E3 ligase Really Interesting New Gene (RING) finger protein 10 (RNF10) and its role in translation. Overexpression of RNF10 increases 40S ribosomal subunit degradation similarly to the knockout of USP10. Although a substantial fraction of RNF10-mediated RPS2 and RPS3 monoubiquitination results from ZNF598-dependent sensing of collided ribosomes, ZNF598-independent impairment of translation initiation and elongation also contributes to RPS2 and RPS3 monoubiquitination. RNF10 photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation (PAR-CLIP) identifies crosslinked mRNAs, tRNAs, and 18S rRNAs, indicating recruitment of RNF10 to ribosomes stalled in translation. These impeded ribosomes are tagged by ubiquitin at their 40S subunit for subsequent programmed degradation unless rescued by USP10.

Processing of the ribosomal ubiquitin-like fusion protein FUBI-eS30/FAU is required for 40S maturation and depends on USP36

In humans and other holozoan organisms, the ribosomal protein eS30 is synthesized as a fusion protein with the ubiquitin-like protein FUBI. However, FUBI is not part of the mature 40S ribosomal subunit and cleaved off by an as-of-yet unidentified protease. How FUBI-eS30 processing is coordinated with 40S subunit maturation is unknown. To study the mechanism and importance of FUBI-eS30 processing, we expressed non-cleavable mutants in human cells, which affected late steps of cytoplasmic 40S maturation, including the maturation of 18S rRNA and recycling of late-acting ribosome biogenesis factors. Differential affinity purification of wild-type and non-cleavable FUBI-eS30 mutants identified the deubiquitinase USP36 as a candidate FUBI-eS30 processing enzyme. Depletion of USP36 by RNAi or CRISPRi indeed impaired FUBI-eS30 processing and moreover, purified USP36 cut FUBI-eS30 in vitro. Together, these data demonstrate the functional importance of FUBI-eS30 cleavage and identify USP36 as a novel protease involved in this process.

Ribosomal protein S3 associates with the TFIIH complex and positively regulates nucleotide excision repair

In mammalian cells, the bulky DNA adducts caused by ultraviolet radiation are mainly repaired via the nucleotide excision repair (NER) pathway; some defects in this pathway lead to a genetic disorder known as xeroderma pigmentosum (XP). Ribosomal protein S3 (rpS3), a constituent of the 40S ribosomal subunit, is a multi-functional protein with various extra-ribosomal functions, including a role in the cellular stress response and DNA repair-related activities. We report that rpS3 associates with transcription factor IIH (TFIIH) via an interaction with the xeroderma pigmentosum complementation group D (XPD) protein and complements its function in the NER pathway. For optimal repair of UV-induced duplex DNA lesions, the strong helicase activity of the TFIIH complex is required for unwinding damaged DNA around the lesion. Here, we show that XP-D cells overexpressing rpS3 showed markedly increased resistance to UV radiation through XPD and rpS3 interaction. Additionally, the knockdown of rpS3 caused reduced NER efficiency in HeLa cells and the overexpression of rpS3 partially restored helicase activity of the TFIIH complex of XP-D cells in vitro. We also present data suggesting that rpS3 is involved in post-excision processing in NER, assisting TFIIH in expediting the repair process by increasing its turnover rate when DNA is damaged. We propose that rpS3 is an accessory protein of the NER pathway and its recruitment to the repair machinery augments repair efficiency upon UV damage by enhancing XPD helicase function and increasing its turnover rate.

UBE1a Suppresses Herpes Simplex Virus-1 Replication

Herpes simplex virus-1 (HSV-1) is the causative agent of cold sores, keratitis, meningitis, and encephalitis. HSV-1-encoded ICP5, the major capsid protein, is essential for capsid assembly during viral replication. Ubiquitination is a post-translational modification that plays a critical role in the regulation of cellular events such as proteasomal degradation, protein trafficking, and the antiviral response and viral events such as the establishment of infection and viral replication. Ub-activating enzyme (E1, also named UBE1) is involved in the first step in the ubiquitination. However, it is still unknown whether UBE1 contributes to viral infection or the cellular antiviral response. Here, we found that UBE1a suppressed HSV-1 replication and contributed to the antiviral response. The UBE1a inhibitor PYR-41 increased HSV-1 production. Immunofluorescence analysis revealed that UBE1a highly expressing cells presented low ICP5 expression, and vice versa. UBE1a inhibition by PYR-41 and shRNA increased ICP5 expression in HSV-1-infected cells. UBE1a reduced and retarded ICP5 protein expression, without affecting transcription of ICP5 mRNA or degradation of ICP5 protein. Additionally, UBE1a interacted with ICP27, and both partially co-localized at the Hsc70 foci/virus-induced chaperone-enriched (VICE) domains. PYR-41 reduced the co-localization of UBE1a and ICP27. Thus, our findings provide insights into the mechanism of UBE1a in the cellular response to viral infection.

When the chains do not break: the role of USP10 in physiology and pathology

Deubiquitination is now understood to be as important as its partner ubiquitination for the maintenance of protein half-life, activity, and localization under both normal and pathological conditions. The enzymes that remove ubiquitin from target proteins are called deubiquitinases (DUBs) and they regulate a plethora of cellular processes. DUBs are essential enzymes that maintain intracellular protein homeostasis by recycling ubiquitin. Ubiquitination is a post-translational modification where ubiquitin molecules are added to proteins thus influencing activation, localization, and complex formation. Ubiquitin also acts as a tag for protein degradation, especially by proteasomal or lysosomal degradation systems. With ~100 members, DUBs are a large enzyme family; the ubiquitin-specific peptidases (USPs) being the largest group. USP10, an important member of this family, has enormous significance in diverse cellular processes and many human diseases. In this review, we discuss recent studies that define the roles of USP10 in maintaining cellular function, its involvement in human pathologies, and the molecular mechanisms underlying its association with cancer and neurodegenerative diseases. We also discuss efforts to modulate USPs as therapy in these diseases.

A cellular handbook for collided ribosomes: surveillance pathways and collision types

Translating ribosomes slow down or completely stall when they encounter obstacles on mRNAs. Such events can lead to ribosomes colliding with each other and forming complexes of two (disome), three (trisome) or more ribosomes. While these events can activate surveillance pathways, it has been unclear if collisions are common on endogenous mRNAs and whether they are usually detected by these cellular pathways. Recent genome-wide surveys of collisions revealed widespread distribution of disomes and trisomes across endogenous mRNAs in eukaryotic cells. Several studies further hinted that the recognition of collisions and response to them by multiple surveillance pathways depend on the context and duration of the ribosome stalling. This review considers recent efforts in the identification of endogenous ribosome collisions and cellular pathways dedicated to sense their severity. We further discuss the potential role of collided ribosomes in modulating co-translational events and contributing to cellular homeostasis.

Disome and Trisome Profiling Reveal Genome-wide Targets of Ribosome Quality Control

The ribosome-associated protein quality control (RQC) system that resolves stalled translation events is activated when ribosomes collide and form disome, trisome, or higher-order complexes. However, it is unclear whether this system distinguishes collision complexes formed on defective mRNAs from those with functional roles on endogenous transcripts. Here, we performed disome and trisome footprint profiling in yeast and found collisions were enriched on diverse sequence motifs known to slow translation. When 60S recycling was inhibited, disomes accumulated at stop codons and could move into the 3' UTR to reinitiate translation. The ubiquitin ligase and RQC factor Hel2/ZNF598 generally recognized collisions but did not induce degradation of endogenous transcripts. However, loss of Hel2 triggered the integrated stress response, via phosphorylation of eIF2α, thus linking these pathways. Our results suggest that Hel2 has a role in sensing ribosome collisions on endogenous mRNAs, and such events may be important for cellular homeostasis.

Detection and Degradation of Stalled Nascent Chains via Ribosome-Associated Quality Control

Stalled protein synthesis produces defective nascent chains that can harm cells. In response, cells degrade these nascent chains via a process called ribosome-associated quality control (RQC). Here, we review the irregularities in the translation process that cause ribosomes to stall as well as how cells use RQC to detect stalled ribosomes, ubiquitylate their tethered nascent chains, and deliver the ubiquitylated nascent chains to the proteasome. We additionally summarize how cells respond to RQC failure.

DUBbing Down Translation: The Functional Interaction of Deubiquitinases with the Translational Machinery

Cancer cells revamp the regulatory processes that control translation to induce tumor-specific translational programs that can adapt to a hostile microenvironment as well as withstand anticancer therapeutics. Translational initiation has been established as a common downstream effector of numerous deregulated signaling pathways that together culminate in prooncogenic expression. Other mechanisms, including ribosomal stalling and stress granule assembly, also appear to be rewired in the malignant phenotype. Therefore, better understanding of the underlying perturbations driving oncogenic translation in the transformed state will provide innovative therapeutic opportunities. This review highlights deubiquitinating enzymes that are activated/dysregulated in hematologic malignancies, thereby altering the translational output and contributing to tumorigenesis.

Sequential Ubiquitination of Ribosomal Protein uS3 Triggers the Degradation of Non-functional 18S rRNA

18S non-functional rRNA decay (NRD) eliminates non-functional 18S rRNA with deleterious mutations in the decoding center. Dissociation of the non-functional 80S ribosome into 40S and 60S subunits is a prerequisite step for degradation of the non-functional 18S rRNA. However, the mechanisms by which the non-functional ribosome is recognized and dissociated into subunits remain elusive. Here, we report that the sequential ubiquitination of non-functional ribosomes is crucial for subunit dissociation. 18S NRD requires Mag2-mediated monoubiquitination followed by Hel2- and Rsp5-mediated K63-linked polyubiquitination of uS3 at the 212^th lysine residue. Determination of the aberrant 18S rRNA levels in sucrose gradient fractions revealed that the subunit dissociation of stalled ribosomes requires sequential ubiquitination of uS3 by E3 ligases and ATPase activity of Slh1 (Rqt2), as well as Asc1 and Dom34. We propose that sequential uS3 ubiquitination of the non-functional 80S ribosome induces subunit dissociation by Slh1, leading to degradation of the non-functional 18S rRNA.

Cracking the Monoubiquitin Code of Genetic Diseases

Ubiquitination is a versatile and dynamic post-translational modification in which single ubiquitin molecules or polyubiquitin chains are attached to target proteins, giving rise to mono- or poly-ubiquitination, respectively. The majority of research in the ubiquitin field focused on degradative polyubiquitination, whereas more recent studies uncovered the role of single ubiquitin modification in important physiological processes. Monoubiquitination can modulate the stability, subcellular localization, binding properties, and activity of the target proteins. Understanding the function of monoubiquitination in normal physiology and pathology has important therapeutic implications, as alterations in the monoubiquitin pathway are found in a broad range of genetic diseases. This review highlights a link between monoubiquitin signaling and the pathogenesis of genetic disorders.

The G3BP1-Family-USP10 Deubiquitinase Complex Rescues Ubiquitinated 40S Subunits of Ribosomes Stalled in Translation from Lysosomal Degradation

Ribosome-associated quality control (RQC) purges aberrant mRNAs and nascent polypeptides in a multi-step molecular process initiated by the E3 ligase ZNF598 through sensing of ribosomes collided at aberrant mRNAs and monoubiquitination of distinct small ribosomal subunit proteins. We show that G3BP1-family-USP10 complexes are required for deubiquitination of RPS2, RPS3, and RPS10 to rescue modified 40S subunits from programmed degradation. Knockout of USP10 or G3BP1 family proteins increased lysosomal ribosomal degradation and perturbed ribosomal subunit stoichiometry, both of which were rescued by a single K214R substitution of RPS3. While the majority of RPS2 and RPS3 monoubiquitination resulted from ZNF598-dependent sensing of ribosome collisions initiating RQC, another minor pathway contributed to their monoubiquitination. G3BP1 family proteins have long been considered RNA-binding proteins, however, our results identified 40S subunits and associated mRNAs as their predominant targets, a feature shared by stress granules to which G3BP1 family proteins localize under stress.

Different construction strategies affected on the physiology of Pichia pastoris strains highly expressed lipase by transcriptional analysis of key genes

We demonstrated previously that expression of Rhizomucor miehei lipase (RML) in Pichia pastoris could be significantly increased by addition of gene propeptide, optimized signal peptide codons and manipulation of gene dosage. In this study, effects of various strategies on the protein synthesis and secretion pathways were analyzed. Using nine strains previously constructed, we evaluated cell culture properties, enzymatic activities, and analyzed transcriptional levels of nine genes involved in protein synthesis and secretion pathways by qPCR. We observed that (i) Addition of propeptide decreased lipase folding stress by down-regulated four UPR-related genes. (ii) Signal peptide codons optimization had no effect on host with no change in the nine detected genes. (iii) Folding stress and limited transport capacity produced when rml gene dosage exceed 2. Different limiting factors on lipase expression in strains with different construction strategies were identified. This study provides a theoretical basis for further improving RML by transforming host.

yRACK1/Asc1 proxiOMICs-Towards Illuminating Ships Passing in the Night

Diverse signals and stress factors regulate the activity and homeostasis of ribosomes in all cells. The Saccharomyces cerevisiae protein Asc1/yRACK1 occupies an exposed site at the head region of the 40S ribosomal subunit (hr40S) and represents a central hub for signaling pathways. Asc1 strongly affects protein phosphorylation and is involved in quality control pathways induced by translation elongation arrest. Therefore, it is important to understand the dynamics of protein formations in the Asc1 microenvironment at the hr40S. We made use of the in vivo protein-proximity labeling technique Biotin IDentification (BioID). Unbiased proxiOMICs from two adjacent perspectives identified nucleocytoplasmic shuttling mRNA-binding proteins, the deubiquitinase complex Ubp3-Bre5, as well as the ubiquitin E3 ligase Hel2 as neighbors of Asc1. We observed Asc1-dependency of hr40S localization of mRNA-binding proteins and the Ubp3 co-factor Bre5. Hel2 and Ubp3-Bre5 are described to balance the mono-ubiquitination of Rps3 (uS3) during ribosome quality control. Here, we show that the absence of Asc1 resulted in massive exposure and accessibility of the C-terminal tail of its ribosomal neighbor Rps3 (uS3). Asc1 and some of its direct neighbors together might form a ribosomal decision tree that is tightly connected to close-by signaling modules.

SseL Deubiquitinates RPS3 to Inhibit Its Nuclear Translocation

Many Gram-negative bacterial pathogens use type III secretion systems to deliver virulence proteins (effectors) into host cells to counteract innate immunity. The ribosomal protein S3 (RPS3) guides NF-κB subunits to specific κB sites and plays an important role in the innate response to bacterial infection. Two E. coli effectors inhibit RPS3 nuclear translocation. NleH1 inhibits RPS3 phosphorylation by IKK-β, an essential aspect of the RPS3 nuclear translocation process. NleC proteolysis of p65 generates an N-terminal p65 fragment that competes for full-length p65 binding to RPS3, thus also inhibiting RPS3 nuclear translocation. Thus, E. coli has multiple mechanisms by which to block RPS3-mediated transcriptional activation. With this in mind, we considered whether other enteric pathogens also encode T3SS effectors that impact this important host regulatory pathway. Here we report that the Salmonella Secreted Effector L (SseL), which was previously shown to function as a deubiquitinase and inhibit NF-κB signaling, also inhibits RPS3 nuclear translocation by deubiquitinating this important host transcriptional co-factor. RPS3 deubiquitination by SseL was restricted to K63-linkages and mutating the active-site cysteine of SseL abolished its ability to deubiquitinate and subsequently inhibit RPS3 nuclear translocation. Thus, Salmonella also encodes at least one T3SS effector that alters RPS3 activities in the host nucleus.

Chaperone-E3 Ligase Complex HSP70-CHIP Mediates Ubiquitination of Ribosomal Protein S3

In addition to its role in ribosome biogenesis, ribosomal protein S3 (RPS3), a component of the 40S ribosomal subunit, has been suggested to possess several extraribosomal functions, including an apoptotic function. In this study, we demonstrated that in the mouse brain, the protein levels of RPS3 were altered by the degree of nutritional starvation and correlated with neuronal apoptosis. After endurable short-term starvation, the apoptotic function of RPS3 was suppressed by Akt activation and Akt-mediated T70 phosphorylation, whereas after prolonged starvation, the protein levels of RPS3 notably increased, and abundant neuronal death occurred. These events coincided with ubiquitination and subsequent degradation of RPS3, controlled by HSP70 and the cochaperone E3 ligase: carboxy terminus of heat shock protein 70-interacting protein (CHIP). Thus, our study points to an extraribosomal role of RPS3 in balancing neuronal survival or death depending on the degree of starvation through CHIP-mediated polyubiquitination and degradation.

Ssb2 is a novel factor in regulating synthesis and degradation of Gcn4 in Saccharomyces cerevisiae

Yeast cells respond to environmental stress by inducing the master regulator Gcn4 to control genes involved in biosynthesis of amino acids and purine pathways. Gcn4 is a member of the basic leucine Zipper family and binds directly as a homodimer to a conserved regulatory region of target genes. Ssb2 was discovered to rescue the mutant Gcn4 which has a point mutation that decreases DNA-binding affinity. Ssb2 is part of the Hsp70 protein family responsible for protein quality control and it is thought that Ssb2 assists the passage of nascent polypeptide chains from the ribosomes. To characterize the mechanism behind the rescue of the mutant gcn4 phenotype, transcriptional activity and protein levels of Gcn4 were analyzed. We found that Ssb2 improved the expression of Gcn4 target genes by increasing the DNA-binding affinity of gcn4 mutants to target gene promoters under conditions of amino acid starvation. Gcn4 levels increased at both translational and post-translational levels without regulating GCN4 steady-state mRNA levels. We also found that the nuclear export signal of Ssb2 is required for interaction with Gcn4 and rescue of the gcn4 mutant phenotype. These findings suggest that Ssb2 is a critical factor that modulates Gcn4 functions in the nucleus and cytosol.