Identification of Mycobacterium abscessus using the peaks of ribosomal protein L29, L30 and hemophore-related protein by MALDI-MS proteotyping.

Mycobacteroides (Mycobacterium) abscessus, which causes a variety of infectious diseases in humans, is becoming detected more frequently in clinical specimens as cases are spreading worldwide. Taxonomically, M. abscessus is composed of three subspecies of M. abscessus subsp. abscessus, M. abscessus subsp. bolletii, and M. abscessus subsp. massiliense, with different susceptibilities to macrolides. In order to identify rapidly these three subspecies, we determined useful biomarker proteins, including ribosomal protein L29, L30, and hemophore-related protein, for distinguishing the subspecies of M. abscessus using the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) profiles. Thirty-three clinical strains of M. abscessus were correctly identified at the subspecies-level by the three biomarker protein peaks. This study ultimately demonstrates the potential of routine MALDI-MS-based laboratory methods for early identification and treatment for M. abscessus infections.

Spatial mapping of hepatic ER and mitochondria architecture reveals zonated remodeling in fasting and obesity.

The hepatocytes within the liver present an immense capacity to adapt to changes in nutrient availability. Here, by using high resolution volume electron microscopy, we map how hepatic subcellular spatial organization is regulated during nutritional fluctuations and as a function of liver zonation. We identify that fasting leads to remodeling of endoplasmic reticulum (ER) architecture in hepatocytes, characterized by the induction of single rough ER sheet around the mitochondria, which becomes larger and flatter. These alterations are enriched in periportal and mid-lobular hepatocytes but not in pericentral hepatocytes. Gain- and loss-of-function in vivo models demonstrate that the Ribosome receptor binding protein1 (RRBP1) is required to enable fasting-induced ER sheet-mitochondria interactions and to regulate hepatic fatty acid oxidation. Endogenous RRBP1 is enriched around periportal and mid-lobular regions of the liver. In obesity, ER-mitochondria interactions are distinct and fasting fails to induce rough ER sheet-mitochondrion interactions. These findings illustrate the importance of a regulated molecular architecture for hepatocyte metabolic flexibility.

Mitoribosome structure with cofactors and modifications reveals mechanism of ligand binding and interactions with L1 stalk.

The mitoribosome translates mitochondrial mRNAs and regulates energy conversion that is a signature of aerobic life forms. We present a 2.2 Å resolution structure of human mitoribosome together with validated mitoribosomal RNA (rRNA) modifications, including aminoacylated CP-tRNAVal. The structure shows how mitoribosomal proteins stabilise binding of mRNA and tRNA helping to align it in the decoding center, whereas the GDP-bound mS29 stabilizes intersubunit communication. Comparison between different states, with respect to tRNA position, allowed us to characterize a non-canonical L1 stalk, and molecular dynamics simulations revealed how it facilitates tRNA transitions in a way that does not require interactions with rRNA. We also report functionally important polyamines that are depleted when cells are subjected to an antibiotic treatment. The structural, biochemical, and computational data illuminate the principal functional components of the translation mechanism in mitochondria and provide a description of the structure and function of the human mitoribosome.

Lentivirus-mediated gene therapy corrects ribosomal biogenesis and shows promise for Diamond Blackfan anemia.

This study lays the groundwork for future lentivirus-mediated gene therapy in patients with Diamond Blackfan anemia (DBA) caused by mutations in ribosomal protein S19 (RPS19), showing evidence of a new safe and effective therapy. The data show that, unlike patients with Fanconi anemia (FA), the hematopoietic stem cell (HSC) reservoir of patients with DBA was not significantly reduced, suggesting that collection of these cells should not constitute a remarkable restriction for DBA gene therapy. Subsequently, 2 clinically applicable lentiviral vectors were developed. In the former lentiviral vector, PGK.CoRPS19 LV, a codon-optimized version of RPS19 was driven by the phosphoglycerate kinase promoter (PGK) already used in different gene therapy trials, including FA gene therapy. In the latter one, EF1α.CoRPS19 LV, RPS19 expression was driven by the elongation factor alpha short promoter, EF1α(s). Preclinical experiments showed that transduction of DBA patient CD34+ cells with the PGK.CoRPS19 LV restored erythroid differentiation, and demonstrated the long-term repopulating properties of corrected DBA CD34+ cells, providing evidence of improved erythroid maturation. Concomitantly, long-term restoration of ribosomal biogenesis was verified using a potentially novel method applicable to patients' blood cells, based on ribosomal RNA methylation analyses. Finally, in vivo safety studies and proviral insertion site analyses showed that lentivirus-mediated gene therapy was nontoxic.

Nascent chains derived from a foldable protein sequence interact with specific ribosomal surface sites near the exit tunnel.

In order to become bioactive, proteins must be translated and protected from aggregation during biosynthesis. The ribosome and molecular chaperones play a key role in this process. Ribosome-bound nascent chains (RNCs) of intrinsically disordered proteins and RNCs bearing a signal/arrest sequence are known to interact with ribosomal proteins. However, in the case of RNCs bearing foldable protein sequences, not much information is available on these interactions. Here, via a combination of chemical crosslinking and time-resolved fluorescence-anisotropy, we find that nascent chains of the foldable globin apoHmp1-140 interact with ribosomal protein L23 and have a freely-tumbling non-interacting N-terminal compact region comprising 63-94 residues. Longer RNCs (apoHmp1-189) also interact with an additional yet unidentified ribosomal protein, as well as with chaperones. Surprisingly, the apparent strength of RNC/r-protein interactions does not depend on nascent-chain sequence. Overall, foldable nascent chains establish and expand interactions with selected ribosomal proteins and chaperones, as they get longer. These data are significant because they reveal the interplay between independent conformational sampling and nascent-protein interactions with the ribosomal surface.

Genetic compensation between ribosomal protein paralogs mediated by a cognate circular RNA.

Inter-regulation between related genes, such as ribosomal protein (RP) paralogs, has been observed to be important for genetic compensation and paralog-specific functions. However, how paralogs communicate to modulate their expression levels is unknown. Here, we report a circular RNA involved in the inter-regulation between RP paralogs RpL22 and RpL22-like during Drosophila spermatogenesis. Both paralogs are mutually regulated by the circular stable intronic sequence RNA (sisRNA) circRpL22(NE,3S) produced from the RpL22 locus. RpL22 represses itself and RpL22-like. Interestingly, circRpL22 binds to RpL22 to repress RpL22-like, but not RpL22, suggesting that circRpL22 modulates RpL22's function. circRpL22 is in turn controlled by RpL22-like, which regulates RpL22 binding to circRpL22 to indirectly modulate RpL22. This circRpL22-centric inter-regulatory circuit enables the loss of RpL22-like to be genetically compensated by RpL22 upregulation to ensure robust male germline development. Thus, our study identifies sisRNA as a possible mechanism of genetic crosstalk between paralogous genes.

MRTO4 Enhances Glycolysis to Facilitate HCC Progression by Inhibiting ALDOB.

BACKGROUND MRT4 Homolog, Ribosome Maturation Factor (MRTO4) is often upregulated in cancer cells. However, its impact in hepatocellular carcinoma (HCC) is less well understood. Herein, we explored the prognostic and energy metabolism reprogramming role of MRTO4 in HCC. MATERIAL AND METHODS Clinical data were obtained from The Cancer Genome Atlas (TCGA), and the expression of MRTO4 in clinical samples was analyzed. The association between different variables and overall survival (OS) was studied, as well as their potential as independent prognostic factors, using Cox regression analysis. We constructed a nomogram including clinical pathological variables and MRTO4 expression to provide a predictive model for prognosis. Heatmaps, Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed the relationship between energy metabolism pathways and MRTO4. We used classic molecular biology research methods, including RT-qPCR, Western blotting, CCK8, TUNEL, Clone formation, Transwell assay, ELISA, and immunohistochemistry, to study the role of MRTO4 in promoting the progression of HCC through glycolysis regulation. RESULTS Our study showed that MRTO4 is an independent prognostic risk factor for HCC and that MRTO4 accelerates glycolysis of HCC cells, promotes proliferation and invasion, and suppresses apoptosis of HCC cells. The underlying mechanism involves MRTO4 promoting glycolysis and accelerating HCC by inhibiting ALDOB. CONCLUSIONS Our study revealed a novel mechanism by which MRTO4 promotes glycolysis and accelerates HCC progression, and suggests that inhibiting MRTO4 could be a potential therapeutic strategy for HCC.

RNA Polymerase Subunits and Ribosomal Proteins: An Overview and Their Genetic Impact on Complex Human Traits.

Accurate gene expression is fundamental for sustaining life, enabling adaptive responses to routine tasks and management of urgent cellular environments. RNA polymerases (RNAP I, RNAP II, and RNAP III) and ribosomal proteins (RPs) play pivotal roles in the precise synthesis of proteins from DNA sequences. In this review, we briefly examined the structure and function of their constituent proteins and explored to characterize these proteins and the genes encoding them, particularly in terms of their expression quantitative trait loci (eQTL) associated with complex human traits. We gathered a comprehensive set of 4007 genome-wide association study (GWAS) signal-eQTL pairs, aligning GWAS Catalog signals with eQTLs across various tissues for the genes involved. These pairs spanned 16 experimental factor ontology (EFO) parent terms defined in European Bioinformatics Institute (EBI). A substantial majority (83.4%) of the pairs were attributed to the genes encoding RPs, especially RPS26 (32.9%). This large proportion was consistent across all tissues (15.5~81.9%), underscoring its extensive impact on complex human traits. Notably, these proportions of EFO terms differed significantly (p < 0.0031) from those for RNAPs. Brain-specific pairs for POLR3H, a component of RNAP III, were implicated in neurological disorders. The largest number of pairs in RNAP I was found for POLR1H, encoding RPA12, a built-in transcription factor essential for high transcriptional efficiency of RNAP I. RNAP II-related pairs were less abundant, with unique structural organization featuring minimal subunits for flexible transcription of a diverse range of genes with customized dissociable subunits. For instance, RPB4 encoded by POLR2D, the RNAP II gene with the most pairs, forms its dissociable stalk module with RPB7. This study provides insightful genetic characteristics of RPs and RNAPs, with a priority emphasis on RPS26, POLR1H, POLR2D, and POLR3H, for future studies on the impact of individual genetic variation on complex human traits.

Leishmania Ribosomal Protein (RP) paralogous genes compensate each other's expression maintaining protein native levels.

In the protozoan parasite Leishmania, most genes encoding for ribosomal proteins (RPs) are present as two or more copies in the genome. However, their untranslated regions (UTRs) are predominantly divergent and might be associated with a distinct regulation of the expression of paralogous genes. Herein, we investigated the expression profiles of two RPs (S16 and L13a) encoded by duplicated genes in Leishmania major. The genes encoding for the S16 protein possess identical coding sequences (CDSs) and divergent UTRs, whereas the CDSs of L13a diverge by two amino acids and by their UTRs. Using CRISPR/Cas9 genome editing, we generated knockout (Δ) and endogenously tagged transfectants for each paralog of L13a and S16 genes. Combining tagged and Δ cell lines we found evidence of differential expression of both RPS16 and RPL13a isoforms throughout parasite development, with one isoform consistently more abundant than its respective copy. In addition, compensatory expression was observed for each paralog upon deletion of the corresponding isoform, suggesting functional conservation between these proteins. This differential expression pattern relates to post-translational processes, given compensation occurs at the level of the protein, with no alterations detected at transcript level. Ribosomal profiles for RPL13a indicate a standard behavior for these paralogues suggestive of interaction with heavy RNA-protein complexes, as already reported for other RPs in trypanosomatids. We identified paralog-specific bound to their 3'UTRs which may be influential in regulating paralog expression. In support, we identified conserved cis-elements within the 3'UTRs of RPS16 and RPL13a; cis-elements exclusive to the UTR of the more abundant paralog or to the less abundant ones were identified.

Dynamic protein composition of Saccharomyces cerevisiae ribosomes in response to multiple stress conditions reflects alterations in translation activity.

Ribosomes, intercellular macromolecules responsible for translation in the cell, are composed of RNAs and proteins. While rRNA makes the scaffold of the ribosome and directs the catalytic steps of protein synthesis, ribosomal proteins play a role in the assembly of the subunits and are essential for the proper structure and function of the ribosome. To date researchers identified heterogeneous ribosomes in different developmental and growth stages. We hypothesized that under stress conditions the heterogeneity of the ribosomes may provide means to prepare the cells for quick recovery. Therefore the aim of the study was the identification of heterogeneity of ribosomal proteins within the ribosomes in response to eleven stress conditions in Saccharomyces cerevisiae, by means of a liquid chromatography/high resolution mass spectrometry (LC-HRMS) and translation activity tests. Out of the total of 74 distinct ribosomal proteins identified in the study 14 small ribosomal subunit (RPS) and 8 large ribosomal subunit (RPL) proteins displayed statistically significant differential abundances within the ribosomes under stress. Additionally, significant alterations in the ratios of 7 ribosomal paralog proteins were observed. Accordingly, the translational activity of yeast ribosomes was altered after UV exposure, during sugar starvation, cold shock, high salt, anaerobic conditions, and amino acid starvation.

PSPC1 Binds to HCV IRES and Prevents Ribosomal Protein S5 Binding, Inhibiting Viral RNA Translation.

Hepatitis C virus (HCV) infects the human liver, and its chronic infection is one of the major causes of Hepatocellular carcinoma. Translation of HCV RNA is mediated by an Internal Ribosome Entry Site (IRES) element located in the 5'UTR of viral RNA. Several RNA Binding proteins of the host interact with the HCV IRES and modulate its function. Here, we demonstrate that PSPC1 (Paraspeckle Component 1), an essential paraspeckle component, upon HCV infection is relocalized and interacts with HCV IRES to prevent viral RNA translation. Competition UV-crosslinking experiments showed that PSPC1 interacts explicitly with the SLIV region of the HCV IRES, which is known to play a vital role in ribosomal loading to the HCV IRES via interaction with Ribosomal protein S5 (RPS5). Partial silencing of PSPC1 increased viral RNA translation and, consequently, HCV replication, suggesting a negative regulation by PSPC1. Interestingly, the silencing of PSPC1 protein leads to an increased interaction of RPS5 at the SLIV region, leading to an overall increase in the viral RNA in polysomes. Overall, our results showed how the host counters viral infection by relocalizing nuclear protein to the cytoplasm as a survival strategy.

Free ribosomal proteins as culprits for nucleolar stress.

Nucleolar stress has been consistently linked to age-related diseases. In this issue, Sirozh et al.1 find that the common molecular signature of nucleolar stress is the accumulation of free ribosomal proteins, which leads to premature aging in mice; however, it can be reversed by mTOR inhibition.

Dysregulated Ribosome Biogenesis Is a Targetable Vulnerability in Triple-Negative Breast Cancer: MRPS27 as a Key Mediator of the Stemness-inhibitory Effect of Lovastatin.

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer with limited effective therapeutic options readily available. We have previously demonstrated that lovastatin, an FDA-approved lipid-lowering drug, selectively inhibits the stemness properties of TNBC. However, the intracellular targets of lovastatin in TNBC remain largely unknown. Here, we unexpectedly uncovered ribosome biogenesis as the predominant pathway targeted by lovastatin in TNBC. Lovastatin induced the translocation of ribosome biogenesis-related proteins including nucleophosmin (NPM), nucleolar and coiled-body phosphoprotein 1 (NOLC1), and the ribosomal protein RPL3. Lovastatin also suppressed the transcript levels of rRNAs and increased the nuclear protein level and transcriptional activity of p53, a master mediator of nucleolar stress. A prognostic model generated from 10 ribosome biogenesis-related genes showed outstanding performance in predicting the survival of TNBC patients. Mitochondrial ribosomal protein S27 (MRPS27), the top-ranked risky model gene, was highly expressed and correlated with tumor stage and lymph node involvement in TNBC. Mechanistically, MRPS27 knockdown inhibited the stemness properties and the malignant phenotypes of TNBC. Overexpression of MRPS27 attenuated the stemness-inhibitory effect of lovastatin in TNBC cells. Our findings reveal that dysregulated ribosome biogenesis is a targetable vulnerability and targeting MRPS27 could be a novel therapeutic strategy for TNBC patients.

[Clinical and genetic analysis of a patient with short stature due to variant of RPL13 gene].

OBJECTIVE: To analyze the clinical phenotype and genetic characteristics of a patient with Isidor-Toutain spinal epiphyseal dysplasia (SEMD) due to variant of RPL13 gene. METHODS: A pregnant woman at 18 weeks of gestation who had presented at Quzhou Maternal and Child Health Care Hospital on January 14, 2023 was selected as the study subject. Whole exome sequencing (WES) was carried out for the patient, and candidate variant was validated by Sanger sequencing and bioinformatic analysis. RESULTS: The woman was 37 years old with extremely short stature (135 cm) and "O" shaped legs. WES revealed that she has harbored a c.548G>C (p.Arg183Pro) missense variant of the RPL13 gene (NM_000977.4). The same variant was not found in her fetus. Based on the guidelines from the American College of Medical Genetics and Genomics (ACMG), the variant was predicted to be likely pathogenic (PS4+PM2_Supporting+PP3+PP4). CONCLUSION: Isidor-Toutain type SEMD due to variants of the RPL13 gene may have variable expressivity and diverse clinical phenotypes. Above finding has facilitated the differential diagnosis and genetic counseling for this family.

The DEAD-box ATPase Dbp10/DDX54 initiates peptidyl transferase center formation during 60S ribosome biogenesis.

DEAD-box ATPases play crucial roles in guiding rRNA restructuring events during the biogenesis of large (60S) ribosomal subunits, but their precise molecular functions are currently unknown. In this study, we present cryo-EM reconstructions of nucleolar pre-60S intermediates that reveal an unexpected, alternate secondary structure within the nascent peptidyl-transferase-center (PTC). Our analysis of three sequential nucleolar pre-60S intermediates reveals that the DEAD-box ATPase Dbp10/DDX54 remodels this alternate base pairing and enables the formation of the rRNA junction that anchors the mature form of the universally conserved PTC A-loop. Post-catalysis, Dbp10 captures rRNA helix H61, initiating the concerted exchange of biogenesis factors during late nucleolar 60S maturation. Our findings show that Dbp10 activity is essential for the formation of the ribosome active site and reveal how this function is integrated with subsequent assembly steps to drive the biogenesis of the large ribosomal subunit.

Identification of genes related to ribosomal proteins in colorectal cancer: exploring their potential as biomarkers, prognostic indicators, and therapeutic targets.

BACKGROUND: Colorectal cancer (CRC) ranks as the third most commonly diagnosed cancer in both females and males, underscoring the need for the identification of effective biomarkers. METHODS AND RESULTS: We assessed the expression levels of ribosomal proteins (RPs) at both mRNA and protein levels. Subsequently, leveraging the STRING database, we constructed a protein-protein interaction network and identified hub genes. The co-expression network of differentially expressed genes associated with CRC and their target hub RPs was constructed using the weighted gene co-expression network analysis algorithm. Gene ontology and molecular signatures database were conducted to gain insights into the biological roles of genes associated with the identified module. To confirm the results, the expression level of the candidate genes in the CRC samples compared to the adjacent healthy was evaluated by the RT-qPCR method. Our findings indicated that the genes related to RPs were predominantly enriched in biological processes associated with Myc Targets, Oxidative Phosphorylation, and cell proliferation. Also, results demonstrated that elevated levels of GRWD1, MCM5, IMP4, and RABEPK that related to RPs were associated with poor prognostic outcomes for CRC patients. Notably, IMP4 and RABEPK exhibited higher diagnostic value. Moreover, the expression of IMP4 and RABEPK showed a significant association with drug resistance using cancer cell line encyclopedia and genomics of drug sensitivity in cancer databases. Also, the results showed that the expression level of IMP4 and RABEPK in cancerous samples was significantly higher compared to the adjacent healthy ones. CONCLUSION: The general results of this study have shown that many genes related to RPs are increased in cancer and could be associated with the death rate of patients. We also highlighted the therapeutic and prognostic potentials of RPs genes in CRC.

RPL9 acts as an oncogene by shuttling miRNAs through exosomes in human hepatocellular carcinoma cells.

The exosomal pathway is an essential mechanism that regulates the abnormal content of microRNAs (miRNAs) in hepatocellular carcinoma (HCC). The directional transport of miRNAs requires the assistance of RNA­binding proteins (RBPs). The present study found that RBPs participate in the regulation of miRNA content through the exosomal pathway in HCC cells. First, differential protein expression profiles in the serum exosomes of patients with HCC and benign liver disease were detected using mass spectrometry. The results revealed that ribosomal protein L9 (RPL9) was highly expressed in serum exosomes of patients with HCC. In addition, the downregulation of RPL9 markedly suppressed the proliferation, migration and invasion of HCC cells and reduced the biological activity of HCC­derived exosomes. In addition, using miRNA microarrays, the changes in exosomal miRNA profiles in HCC cells caused by RPL9 knockdown were examined. miR­24­3p and miR­185­5p were most differentially expressed, as verified by reverse transcription­quantitative PCR. Additionally, using RNA immunoprecipitation, it was found that RPL9 was directly bound to the two miRNAs and immunofluorescence assays confirmed that RPL9 was able to carry miRNAs into recipient cells via exosomes. Overexpression of miR­24­3p in cells increased the accumulation of miR­24­3p in exosomes and simultaneously upregulated RPL9. Excessive expression of miR­24­3p in exosomes also increased their bioactivity. Exosome­mediated miRNA regulation and transfer require the involvement of RBPs. RPL9 functions as an oncogene, can directly bind to specific miRNAs and can be co­transported to receptor cells through exosomes, thereby exerting its biological functions. These findings provide a novel approach for modulating miRNA profiles in HCC.

Efficacy of maturase K and rpL20 protein extracted from C. procera leaves on Anophelesstephensi.

The present work is carried out to protein isolation, purification, and characterization from leaves, stem, and seed of C. procera and to evaluate the larvicidal potential on Anopheles stephensi. The whole protein was isolated using protein extraction buffer and precipitated by ammonium sulphate and larvicidal active protein was purified by the column chromatography. The homogeneity of larvicidal protein was confirmed by the SDS-PAGE. The identification of protein was done by the HPLC and LC-MS/ESI-MS. The crude protein from leaves showed 100% mortality of 3rd instar larvae of An. stephensi at the concentration of 5.5 mg/ml after 24 h of exposure. The crude protein from stem showed 25% mortality and no mortality observed was observed in seed protein. The leaves crude protein was further purified by ion exchange chromatography and eluted fractions were tested for larvicidal potential. The purified single protein fractions L2 and L3 from C. procera leaves showed 100% mortality at concentration of 0.06 mg/ml. The homogeneity of purified protein was confirmed by SDS-PAGE and two bands of 26 kDa and 15 kDa protein were observed. The peptide sequence "R.SQMLENSFLIENVMKR.L" was identified in the trypsin digested homogenous protein fraction L2 and "R.DRGSQKR.N" peptide sequence in L3 fraction by LC-MS/ESI-MS. The CprL2 peptide showed the sequence similarity with the protein maturase K and CprL3 peptide showed the sequence similarity with ribosomal protein L20 of C. procera. The conserved functional domain was also identified in both the CprL2 and CprL3 peptide. The identified proteins showed strong larvicidal efficacy at very low concentration. The identified proteins are novel and natural larvicidal agents against An. stephensi and hence can be used to control the malaria.

Inability to rescue stalled ribosomes results in overactivation of the integrated stress response.

Endogenous and exogenous chemical agents are known to compromise the integrity of RNA and cause ribosome stalling and collisions. Recent studies have shown that collided ribosomes serve as sensors for multiple processes, including ribosome quality control (RQC) and the integrated stress response (ISR). Since RQC and the ISR have distinct downstream consequences, it is of great importance that organisms activate the appropriate process. We previously showed that RQC is robustly activated in response to collisions and suppresses the ISR activation. However, the molecular mechanics behind this apparent competition were not immediately clear. Here we show that Hel2 does not physically compete with factors of the ISR, but instead its ribosomal-protein ubiquitination activity, and downstream resolution of collided ribosomes, is responsible for suppressing the ISR. Introducing a mutation in the RING domain of Hel2-which inhibits its ubiquitination activity and downstream RQC but imparts higher affinity of the factor for collided ribosomes-resulted in increased activation of the ISR upon MMS-induced alkylation stress. Similarly, mutating Hel2's lysine targets in uS10, which is responsible for RQC activation, resulted in increased Gcn4 target induction. Remarkably, the entire process of RQC appears to be limited by the action of Hel2, as the overexpression of this one factor dramatically suppressed the activation of the ISR. Collectively, our data suggest that cells evolved Hel2 to bind collided ribosomes with a relatively high affinity but kept its concentration relatively low, ensuring that it gets exhausted under stress conditions that cannot be resolved by quality control processes.

Mitochondrial Ribosomal Protein MRPS15 Is a Component of Cytosolic Ribosomes and Regulates Translation in Stressed Cardiomyocytes.

Regulation of mRNA translation is a crucial step in controlling gene expression in stressed cells, impacting many pathologies, including heart ischemia. In recent years, ribosome heterogeneity has emerged as a key control mechanism driving the translation of subsets of mRNAs. In this study, we investigated variations in ribosome composition in human cardiomyocytes subjected to endoplasmic reticulum stress induced by tunicamycin treatment. Our findings demonstrate that this stress inhibits global translation in cardiomyocytes while activating internal ribosome entry site (IRES)-dependent translation. Analysis of translating ribosome composition in stressed and unstressed cardiomyocytes was conducted using mass spectrometry. We observed no significant changes in ribosomal protein composition, but several mitochondrial ribosomal proteins (MRPs) were identified in cytosolic polysomes, showing drastic variations between stressed and unstressed cells. The most notable increase in polysomes of stressed cells was observed in MRPS15. Its interaction with ribosomal proteins was confirmed by proximity ligation assay (PLA) and immunoprecipitation, suggesting its intrinsic role as a ribosomal component during stress. Knock-down or overexpression experiments of MRPS15 revealed its role as an activator of IRES-dependent translation. Furthermore, polysome profiling after immunoprecipitation with anti-MRPS15 antibody revealed that the "MRPS15 ribosome" is specialized in translating mRNAs involved in the unfolded protein response.