The Saccharomyces cerevisiae TIF6 gene encoding translation initiation factor 6 is required for 60S ribosomal subunit biogenesis

p38α-eIF6-Nsun2 axis promotes ILC3's rapid response to protect host from intestinal inflammation

Group 3 innate lymphoid cells (ILC3s) are important for maintaining gut homeostasis. Upon stimulation, ILC3s can rapidly produce cytokines to protect against infections and colitis. However, the regulation of ILC3 quick response is still unclear. Here, we find that eIF6 aggregates with Nsun2 and cytokine mRNA in ILC3s at steady state, which inhibits the methyltransferase activity of Nsun2 and the nuclear export of cytokine mRNA, resulting in the nuclear reservation of cytokine mRNA. Upon stimulation, phosphorylated p38α phosphorylates eIF6, which in turn releases Nsun2 activity, and promotes the nuclear export of cytokine mRNA and rapid cytokine production. Genetic disruption of p38α, Nsun2, or eIF6 in ILC3s influences the mRNA nuclear export and protein expression of the protective cytokines, thus leading to increased susceptibility to colitis. Together, our data identify a crucial role of the p38α-eIF6-Nsun2 axis in regulating rapid ILC3 immune response at the posttranscriptional level, which is critical for gut homeostasis maintenance and protection against gut inflammation.

The CDK8 kinase module: A novel player in the transcription of translation initiation and ribosomal genes

Survival following stress is dependent upon reprogramming transcription and translation. Communication between these programs following stress is critical for adaptation but is not clearly understood. The Cdk8 kinase module (CKM) of the Mediator complex modulates the transcriptional response to various stresses. Its involvement in regulating translational machinery has yet to be elucidated, highlighting an existing gap in knowledge. Here, we report that the CKM positively regulates a subset of ribosomal protein (RP) and translation initiation factor (TIF)-encoding genes under physiological conditions in Saccharomyces cerevisiae. In mouse embryonic fibroblasts and HCT116 cells, the CKM regulates unique sets of RP and TIF genes, demonstrating some conservation of function across species. In yeast, this is mediated by Cdk8 phosphorylation of one or more transcription factors which control RP and TIF expression. Conversely, the CKM is disassembled following nutrition stress, permitting repression of RP and TIF genes. The CKM also plays a transcriptional role important for promoting cell survival, particularly during translational machinery stress triggered by ribosome-targeting antibiotics. Furthermore, in mammalian cells, the activity of CDK8 and its paralogue, CDK19, promotes cell survival following ribosome inhibition. These results provide mechanistic insights into the CKM's role in regulating expression of a subset of genes associated with translation.

Pre-ribosomal particles from nucleoli to cytoplasm

The analysis of nucleocytoplasmic transport of proteins and messenger RNA has been the focus of advanced microscopic approaches. Recently, it has been possible to identify and visualize individual pre-ribosomal particles on their way through the nuclear pore complex using both electron and light microscopy. In this review, we focused on the transport of pre-ribosomal particles in the nucleus on their way to and through the pores.

An ultraconserved snoRNA-like element in long noncoding RNA CRNDE promotes ribosome biogenesis and cell proliferation

Cancer cells frequently upregulate ribosome production to support tumorigenesis. While small nucleolar RNAs (snoRNAs) are critical for ribosome biogenesis, the roles of other classes of noncoding RNAs in this process remain largely unknown. Here we performed CRISPRi screens to identify essential long noncoding RNAs (lncRNAs) in renal cell carcinoma (RCC) cells. This revealed that an alternatively-spliced isoform of lncRNA Colorectal Neoplasia Differentially Expressed containing an ultraconserved element (UCE), referred to as CRNDE UCE, is required for RCC cell proliferation. CRNDE UCE localizes to the nucleolus and promotes 60S ribosomal subunit biogenesis. The UCE of CRNDE functions as an unprocessed C/D box snoRNA that directly interacts with ribosomal RNA precursors. This facilitates delivery of eIF6, a key 60S biogenesis factor, which binds to CRNDE UCE through a sequence element adjacent to the UCE. These findings highlight the functional versatility of snoRNA sequences and expand the known mechanisms through which noncoding RNAs orchestrate ribosome biogenesis.

Modulation of ribosomal subunit associations by eIF6 is critical for mitotic exit and cancer progression

Moderating the pool of active ribosomal subunits is critical for maintaining global translation rates. A factor crucial for modulating the 60S ribosomal subunits is eukaryotic translation initiation factor 6. Release of eIF6 from 60S is essential to permit 60S interactions with 40S. Here, using the N106S mutant of eIF6, we show that disrupting eIF6 interaction with 60S leads to an increase in vacant 80S. It further highlights a dichotomy in the anti-association activity of eIF6 that is distinct from its role in 60S biogenesis and shows that the nucleolar localization of eIF6 is not dependent on uL14-BCCIP interactions. Limiting active ribosomal pools markedly deregulates translation especially in mitosis and leads to chromosome segregation defects, mitotic exit delays and mitotic catastrophe. Ribo-Seq analysis of the eIF6-N106S mutant shows a significant downregulation in the translation efficiencies of mitotic factors and specifically transcripts with long 3'UTRs. eIF6-N106S mutation also limits cancer invasion, and this role is correlated with the overexpression of eIF6 only in high-grade invasive cancers suggesting that deregulation of eIF6 is probably not an early event in cancers. Thus, this study highlights the segregation of eIF6 functions and its role in moderating 80S availability for mitotic translation and cancer progression.

Rice OseIF6.1 encodes a eukaryotic translation initiation factor and is essential for the development of grain and anther

The eIF6 proteins are distributed extensively in eukaryotes and play diverse and essential roles. The bona fide eIF6 protein in Arabidopsis, At-eIF6;1, is essential for embryogenesis. However, the role of eIF6 proteins in rice growth and development remains elusive and requires further investigation. Here, we characterized the functions of OseIF6.1, which is homologous to At-eIF6;1. OseIF6.1 encodes an eukaryotic translation initiation factor with a conserved eIF6 domain. The knockdown of OseIF6.1 resulted in a decrease in grain length and pollen sterility, whereas the overexpression of OseIF6.1 displayed opposite phenotypes. Further studies revealed that OseIF6.1 regulates grain shape by influencing cell expansion and proliferation. In addition, OseIF6.1 interacts with OsNMD3, which is a nuclear export adaptor for the 60S ribosomal subunit. The knockdown of OsNMD3 in plants exhibited reduced fertility and seed setting. Therefore, our findings have significantly enriched the current understanding of the role of OseIF6.1 in rice growth and development.

On the origin of the nucleus: a hypothesis

SUMMARYIn this hypothesis article, we explore the origin of the eukaryotic nucleus. In doing so, we first look afresh at the nature of this defining feature of the eukaryotic cell and its core functions-emphasizing the utility of seeing the eukaryotic nucleoplasm and cytoplasm as distinct regions of a common compartment. We then discuss recent progress in understanding the evolution of the eukaryotic cell from archaeal and bacterial ancestors, focusing on phylogenetic and experimental data which have revealed that many eukaryotic machines with nuclear activities have archaeal counterparts. In addition, we review the literature describing the cell biology of representatives of the TACK and Asgardarchaeaota - the closest known living archaeal relatives of eukaryotes. Finally, bringing these strands together, we propose a model for the archaeal origin of the nucleus that explains much of the current data, including predictions that can be used to put the model to the test.

Dynamic states of eIF6 and SDS variants modulate interactions with uL14 of the 60S ribosomal subunit

Assembly of ribosomal subunits into active ribosomal complexes is integral to protein synthesis. Release of eIF6 from the 60S ribosomal subunit primes 60S to associate with the 40S subunit and engage in translation. The dynamics of eIF6 interaction with the uL14 (RPL23) interface of 60S and its perturbation by somatic mutations acquired in Shwachman-Diamond Syndrome (SDS) is yet to be clearly understood. Here, by using a modified strategy to obtain high yields of recombinant human eIF6 we have uncovered the critical interface entailing eight key residues in the C-tail of uL14 that is essential for physical interactions between 60S and eIF6. Disruption of the complementary binding interface by conformational changes in eIF6 disease variants provide a mechanism for weakened interactions of variants with the 60S. Hydrogen-deuterium exchange mass spectrometry (HDX-MS) analyses uncovered dynamic configurational rearrangements in eIF6 induced by binding to uL14 and exposed an allosteric interface regulated by the C-tail of eIF6. Disrupting key residues in the eIF6-60S binding interface markedly limits proliferation of cancer cells, which highlights the significance of therapeutically targeting this interface. Establishing these key interfaces thus provide a therapeutic framework for targeting eIF6 in cancers and SDS.

Ribosome biogenesis factors-from names to functions

The assembly of ribosomal subunits is a highly orchestrated process that involves a huge cohort of accessory factors. Most eukaryotic ribosome biogenesis factors were first identified by genetic screens and proteomic approaches of pre-ribosomal particles in Saccharomyces cerevisiae. Later, research on human ribosome synthesis not only demonstrated that the requirement for many of these factors is conserved in evolution, but also revealed the involvement of additional players, reflecting a more complex assembly pathway in mammalian cells. Yet, it remained a challenge for the field to assign a function to many of the identified factors and to reveal their molecular mode of action. Over the past decade, structural, biochemical, and cellular studies have largely filled this gap in knowledge and led to a detailed understanding of the molecular role that many of the players have during the stepwise process of ribosome maturation. Such detailed knowledge of the function of ribosome biogenesis factors will be key to further understand and better treat diseases linked to disturbed ribosome assembly, including ribosomopathies, as well as different types of cancer.

Critical Roles of METTL3 in Translation Regulation of Cancer

Aberrant translation, a characteristic feature of cancer, is regulated by the complex and sophisticated RNA binding proteins (RBPs) in the canonical translation machinery. N6-methyladenosine (m⁶A) modifications are the most abundant internal modifications in mRNAs mediated by methyltransferase-like 3 (METTL3). METTL3 is commonly aberrantly expressed in different tumors and affects the mRNA translation of many oncogenes or dysregulated tumor suppressor genes in a variety of ways. In this review, we discuss the critical roles of METTL3 in translation regulation and how METTL3 and m⁶A reader proteins in collaboration with RBPs within the canonical translation machinery promote aberrant translation in tumorigenesis, providing an overview of recent efforts aiming to 'translate' these results to the clinic.

Diagnostic and prognostic utility of eIF6 in glioblastoma: a study based on TCGA and CGGA databases

BACKGROUND

Among various glioma types, glioblastoma multiforme (GBM) is one of those with the highest malignancy. Although overexpression of eukaryotic translation initiation factor 6 (eIF6), a factor that regulates protein translation initiation, is believed to promote tumor development, its function and potential molecular mechanisms in glioma progression remain uncharacterized. Consequently, we evaluated its diagnostic and prognostic utility in GBM patients.

METHODS

Sample data from two databases, The Cancer Genome Atlas (TCGA) and the Chinese Glioma Genome Atlas (CGGA), were utilized to investigate the role of eIF6 as well as its mechanism of action in gliomas. We analyzed eIF6 expression in normal tissues as well as cancerous samples of different stages of glioma. The diagnostic and prognostic value of eIF6 were analyzed using the Receiver Operating Characteristic Curve (ROC) and Kaplan-Meier analysis, respectively. Furthermore, its underlying molecular mechanism in GBM was further revealed by gene set enrichment analysis (GSEA).

RESULTS

Transcriptome data analyses of the two databases showed that eIF6 was upregulated in glioma tissues compared with normal counterparts. eIF6 was at high levels in WHO grade IV gliomas versus grade II and III gliomas (P<0.05). In addition, eIF6 was highly expressed in elderly and Asian glioma patients. Furthermore, eIF6 expression was found to be lower in isocitrate dehydrogenase (IDH)-mutated tumors. Patients with high eIF6 level presented shorter overall survival than cases with low eIF6 level (P<0.05), and eIF6 had favorable accuracy in predicting the prognosis of glioma patients. GSEA revealed that high eIF6 expression was mainly concentrated in cell cycle and DNA repair related pathways.

CONCLUSIONS

eIF6 is highly expressed in gliomas and positively associated with the degree of malignancy. Patients with high eIF6 expression present poor survival. Therefore, eIF6 has the potential to be a diagnostic biomarker and a potential therapeutic target for glioma development and GBM.

The RNA binding protein OsLa influences grain and anther development in rice

La proteins are found widely in eukaryotes and play a variety of vital roles. AtLa1 has been identified as an La protein that is necessary for embryogenesis in Arabidopsis; however, the existence and biological functions of La proteins in rice (Oryza sativa L.) remain unclear. In this study, we identified and characterized two La proteins in rice that are homologous to AtLa1 and named them OsLa1 and OsLa2. Both the OsLa1 and OsLa2 genes encode RNA-binding proteins with an La domain and two RNA-binding domains. Mutant OsLa1 reduced grain length and pollen fertility, whereas OsLa1 overexpression caused the opposite phenotypes. Further experiments indicated that OsLa1 modulates grain size by influencing cell expansion. Interestingly, mutant OsLa2 resulted in thin grains with decreased weight and a low seed-setting rate. We also found that OsLa1 interacted with OsLa2 and that both OsLa1 and OsLa2 interacted with OseIF6.1, a eukaryotic translation initiation factor involved in ribosome biogenesis. In addition, OsLa1 was able to bind to OseIF6.1 mRNA to modulate its expression. Complete OseIF6.1 knockout caused lethality and OseIF6.1/oseif6.1 heterozygous plants displayed low fertility and low seed setting. Together, our results enrich our knowledge of the role of La proteins in rice growth and development, as well as the relationship between La and eIF6 in rice.

eIF6 rebinding dynamically couples ribosome maturation and translation

Protein synthesis is a cyclical process consisting of translation initiation, elongation, termination and ribosome recycling. The release factors SBDS and EFL1—both mutated in the leukemia predisposition disorder Shwachman-Diamond syndrome — license entry of nascent 60S ribosomal subunits into active translation by evicting the anti-association factor eIF6 from the 60S intersubunit face. We find that in mammalian cells, eIF6 holds all free cytoplasmic 60S subunits in a translationally inactive state and that SBDS and EFL1 are the minimal components required to recycle these 60S subunits back into additional rounds of translation by evicting eIF6. Increasing the dose of eIF6 in mice in vivo impairs terminal erythropoiesis by sequestering post-termination 60S subunits in the cytoplasm, disrupting subunit joining and attenuating global protein synthesis. These data reveal that ribosome maturation and recycling are dynamically coupled by a mechanism that is disrupted in an inherited leukemia predisposition disorder.

Jaako et al. discover a conserved tier of translational control that dynamically couples ribosome assembly and recycling. This mechanism is corrupted in an inherited bone marrow failure disorder associated with an increased risk of blood cancer.

Eukaryotic initiation factor 6 regulates mechanical responses in endothelial cells

The repertoire of extratranslational functions of components of the protein synthesis apparatus is expanding to include control of key cell signaling networks. However, very little is known about noncanonical functions of members of the protein synthesis machinery in regulating cellular mechanics. We demonstrate that the eukaryotic initiation factor 6 (eIF6) modulates cellular mechanobiology. eIF6-depleted endothelial cells, under basal conditions, exhibit unchanged nascent protein synthesis, polysome profiles, and cytoskeleton protein expression, with minimal effects on ribosomal biogenesis. In contrast, using traction force and atomic force microscopy, we show that loss of eIF6 leads to reduced stiffness and force generation accompanied by cytoskeletal and focal adhesion defects. Mechanistically, we show that eIF6 is required for the correct spatial mechanoactivation of ERK1/2 via stabilization of an eIF6-RACK1-ERK1/2-FAK mechanocomplex, which is necessary for force-induced remodeling. These results reveal an extratranslational function for eIF6 and a novel paradigm for how mechanotransduction, the cellular cytoskeleton, and protein translation constituents are linked.

Nuclear export of the pre-60S ribosomal subunit through single nuclear pores observed in real time

Ribosomal biogenesis has been studied by biochemical, genetic and electron microscopic approaches, but live cell data on the in vivo kinetics are still missing. Here we analyse the export kinetics of the large ribosomal subunit (pre-60S particle) through single NPCs in human cells. We established a stable cell line co-expressing Halo-tagged eIF6 and GFP-fused NTF2 to simultaneously label pre-60S particles and NPCs, respectively. By combining single molecule tracking and super resolution confocal microscopy we visualize the dynamics of single pre-60S particles during export through single NPCs. For export events, maximum particle accumulation is found in the centre of the pore, while unsuccessful export terminates within the nuclear basket. The export has a single rate limiting step and a duration of ∼24 milliseconds. Only about 1/3 of attempted export events are successful. Our results show that the mass flux through a single NPC can reach up to ~125 MDa·s^-1 in vivo.

eIF6 promotes the malignant progression of human hepatocellular carcinoma via the mTOR signaling pathway

BACKGROUND

Eukaryotic translation initiation factor 6 (eIF6) has a crucial function in the maturation of 60S ribosomal subunits, and it controls the initiation of protein translation. Although emerging studies indicate that eIF6 is aberrantly expressed in various types of cancers, the functions and underlying molecular mechanisms of eIF6 in the pathological progression of hepatocellular carcinoma (HCC) remain unclear. This study aimed to evaluate the potential diagnostic and prognostic value of eIF6 in patients with HCC.

METHODS

HCC samples enrolled from The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO) and our cohort were used to explore the role and mechanism of eIF6 in HCC. The diagnostic power of eIF6 was verified by receiver operating characteristic curve (ROC) analysis and its prognostic value was assessed by Kaplan-Meier analysis, and then related biological functions of eIF6 were determined in vitro and in vivo cancer models. In addition, potential molecular mechanism of eIF6 in HCC was unveiled by the gene set enrichment analysis and western blot assay.

RESULTS

We demonstrated that eIF6 expression was markedly increased in HCC, and elevated eIF6 expression correlated with pathological progression of HCC. Besides, eIF6 served as not only a new diagnostic biomarker but also an independent risk factor for OS in HCC patients. Functional studies indicated that the deletion of eIF6 displayed tumor-suppressor activity in HCC cells. Furthermore, we found that eIF6 could activate the mTOR-related signaling pathway and regulate the expression level of its target genes, such as CCND1, CDK4, CDK6, MYC, CASP3 and CTNNBL1, and these activities promoted proliferation and invasion of HCC cells.

CONCLUSIONS

The findings of this study provided a novel basis for understanding the potential role of eIF6 in promoting tumor growth and invasion, and exploited a promising strategy for improving diagnosis and prognosis of HCC.

The Archaeal Elongation Factor EF-2 Induces the Release of aIF6 From 50S Ribosomal Subunit

The translation factor IF6 is a protein of about 25 kDa shared by the Archaea and the Eukarya but absent in Bacteria. It acts as a ribosome anti-association factor that binds to the large subunit preventing the joining to the small subunit. It must be released from the large ribosomal subunit to permit its entry to the translation cycle. In Eukarya, this process occurs by the coordinated action of the GTPase Efl1 and the docking protein SBDS. Archaea do not possess a homolog of the former factor while they have a homolog of SBDS. In the past, we have determined the function and ribosomal localization of the archaeal (Sulfolobus solfataricus) IF6 homolog (aIF6) highlighting its similarity to the eukaryotic counterpart. Here, we analyzed the mechanism of aIF6 release from the large ribosomal subunit. We found that, similarly to the Eukarya, the detachment of aIF6 from the 50S subunit requires a GTPase activity which involves the archaeal elongation factor 2 (aEF-2). However, the release of aIF6 from the 50S subunits does not require the archaeal homolog of SBDS, being on the contrary inhibited by its presence. Molecular modeling, using published structural data of closely related homologous proteins, elucidated the mechanistic interplay between the aIF6, aSBDS, and aEF2 on the ribosome surface. The results suggest that a conformational rearrangement of aEF2, upon GTP hydrolysis, promotes aIF6 ejection. On the other hand, aSBDS and aEF2 share the same binding site, whose occupation by SBDS prevents aEF2 binding, thereby inhibiting aIF6 release.

Arabidopsis REI-LIKE proteins activate ribosome biogenesis during cold acclimation

Arabidopsis REIL proteins are cytosolic ribosomal 60S-biogenesis factors. After shift to 10 °C, reil mutants deplete and slowly replenish non-translating eukaryotic ribosome complexes of root tissue, while controlling the balance of non-translating 40S- and 60S-subunits. Reil mutations respond by hyper-accumulation of non-translating subunits at steady-state temperature; after cold-shift, a KCl-sensitive 80S sub-fraction remains depleted. We infer that Arabidopsis may buffer fluctuating translation by pre-existing non-translating ribosomes before de novo synthesis meets temperature-induced demands. Reil1 reil2 double mutants accumulate 43S-preinitiation and pre-60S-maturation complexes and alter paralog composition of ribosomal proteins in non-translating complexes. With few exceptions, e.g. RPL3B and RPL24C, these changes are not under transcriptional control. Our study suggests requirement of de novo synthesis of eukaryotic ribosomes for long-term cold acclimation, feedback control of NUC2 and eIF3C2 transcription and links new proteins, AT1G03250, AT5G60530, to plant ribosome biogenesis. We propose that Arabidopsis requires biosynthesis of specialized ribosomes for cold acclimation.

The composition and turnover of the Arabidopsis thaliana 80S cytosolic ribosome

Cytosolic 80S ribosomes contain proteins of the mature cytosolic ribosome (r-proteins) as well as proteins with roles in ribosome biogenesis, protein folding or modification. Here, we refined the core r-protein composition in Arabidopsis thaliana by determining the abundance of different proteins during enrichment of ribosomes from cell cultures using peptide mass spectrometry. The turnover rates of 26 40S subunit r-proteins and 29 60S subunit r-proteins were also determined, showing that half of the ribosome population is replaced every 3–4 days. Three enriched proteins showed significantly shorter half-lives; a protein annotated as a ribosomal protein uL10 (RPP0D, At1g25260) with a half-life of 0.5 days and RACK1b and c with half-lives of 1–1.4 days. The At1g25260 protein is a homologue of the human Mrt4 protein, a trans-acting factor in the assembly of the pre-60S particle, while RACK1 has known regulatory roles in cell function beyond its role in the 40S subunit. Our experiments also identified 58 proteins that are not from r-protein families but co-purify with ribosomes and co-express with r-proteins; 26 were enriched more than 10-fold during ribosome enrichment. Some of these enriched proteins have known roles in translation, while others are newly proposed ribosome-associated factors in plants. This analysis provides an improved understanding of A. thaliana ribosome protein content, shows that most r-proteins turnover in unison in vivo, identifies a novel set of potential plant translatome components, and how protein turnover can help identify r-proteins involved in ribosome biogenesis or regulation in plants.

BCCIP is required for nucleolar recruitment of eIF6 and 12S pre-rRNA production during 60S ribosome biogenesis

Ribosome biogenesis is a fundamental process required for cell proliferation. Although evolutionally conserved, the mammalian ribosome assembly system is more complex than in yeasts. BCCIP was originally identified as a BRCA2 and p21 interacting protein. A partial loss of BCCIP function was sufficient to trigger genomic instability and tumorigenesis. However, a complete deletion of BCCIP arrested cell growth and was lethal in mice. Here, we report that a fraction of mammalian BCCIP localizes in the nucleolus and regulates 60S ribosome biogenesis. Both abrogation of BCCIP nucleolar localization and impaired BCCIP-eIF6 interaction can compromise eIF6 recruitment to the nucleolus and 60S ribosome biogenesis. BCCIP is vital for a pre-rRNA processing step that produces 12S pre-rRNA, a precursor to the 5.8S rRNA. However, a heterozygous Bccip loss was insufficient to impair 60S biogenesis in mouse embryo fibroblasts, but a profound reduction of BCCIP was required to abrogate its function in 60S biogenesis. These results suggest that BCCIP is a critical factor for mammalian pre-rRNA processing and 60S generation and offer an explanation as to why a subtle dysfunction of BCCIP can be tumorigenic but a complete depletion of BCCIP is lethal.

Regulation of eukaryotic translation initiation factor 6 dynamics through multisite phosphorylation by GSK3

Eukaryotic translation initiation factor 6 (eIF6) is essential for the synthesis of 60S ribosomal subunits and for regulating the association of 60S and 40S subunits. A mechanistic understanding of how eIF6 modulates translation in response to stress, specifically starvation-induced stress, is lacking. We here show a novel mode of eIF6 regulation by glycogen synthase kinase 3 (GSK3) that is predominantly active in response to serum starvation. Both GSK3α and GSK3β phosphorylate human eIF6. Multiple residues in the C terminus of eIF6 are phosphorylated by GSK3 in a sequential manner. In response to serum starvation, eIF6 accumulates in the cytoplasm, and this altered localization depends on phosphorylation by GSK3. Disruption of eIF6 phosphorylation exacerbates the translation inhibitory response to serum starvation and stalls cell growth. These results suggest that eIF6 regulation by GSK3 contributes to the attenuation of global protein synthesis that is critical for adaptation to starvation-induced stress.

Systematic Review of Plant Ribosome Heterogeneity and Specialization

Plants dedicate a high amount of energy and resources to the production of ribosomes. Historically, these multi-protein ribosome complexes have been considered static protein synthesis machines that are not subject to extensive regulation but only read mRNA and produce polypeptides accordingly. New and increasing evidence across various model organisms demonstrated the heterogeneous nature of ribosomes. This heterogeneity can constitute specialized ribosomes that regulate mRNA translation and control protein synthesis. A prominent example of ribosome heterogeneity is seen in the model plant, Arabidopsis thaliana, which, due to genome duplications, has multiple paralogs of each ribosomal protein (RP) gene. We support the notion of plant evolution directing high RP paralog divergence toward functional heterogeneity, underpinned in part by a vast resource of ribosome mutants that suggest specialization extends beyond the pleiotropic effects of single structural RPs or RP paralogs. Thus, Arabidopsis is a highly suitable model to study this phenomenon. Arabidopsis enables reverse genetics approaches that could provide evidence of ribosome specialization. In this review, we critically assess evidence of plant ribosome specialization and highlight steps along ribosome biogenesis in which heterogeneity may arise, filling the knowledge gaps in plant science by providing advanced insights from the human or yeast fields. We propose a data analysis pipeline that infers the heterogeneity of ribosome complexes and deviations from canonical structural compositions linked to stress events. This analysis pipeline can be extrapolated and enhanced by combination with other high-throughput methodologies, such as proteomics. Technologies, such as kinetic mass spectrometry and ribosome profiling, will be necessary to resolve the temporal and spatial aspects of translational regulation while the functional features of ribosomal subpopulations will become clear with the combination of reverse genetics and systems biology approaches.

Mechanism of ribosome shutdown by RsfS in Staphylococcus aureus revealed by integrative structural biology approach

For the sake of energy preservation, bacteria, upon transition to stationary phase, tone down their protein synthesis. This process is favored by the reversible binding of small stress-induced proteins to the ribosome to prevent unnecessary translation. One example is the conserved bacterial ribosome silencing factor (RsfS) that binds to uL14 protein onto the large ribosomal subunit and prevents its association with the small subunit. Here we describe the binding mode of Staphylococcus aureus RsfS to the large ribosomal subunit and present a 3.2 Å resolution cryo-EM reconstruction of the 50S-RsfS complex together with the crystal structure of uL14-RsfS complex solved at 2.3 Å resolution. The understanding of the detailed landscape of RsfS-uL14 interactions within the ribosome shed light on the mechanism of ribosome shutdown in the human pathogen S. aureus and might deliver a novel target for pharmacological drug development and treatment of bacterial infections.

Eukaryotic translation initiation factor 6 overexpression plays a major role in the translational control of gallbladder cancer

BACKGROUND

Gallbladder cancer (GBC) is a rare neoplasia of the biliary tract with high mortality rates and poor prognosis. Signs and symptoms of GBC are not specific and often arise at late stage of disease. For this reason, diagnosis is typically made when the cancer is already in advanced stages, and prognosis for survival is less than 5 years in 90% of cases. Biomarkers to monitor disease progression and novel therapeutic alternative targets for these tumors are strongly required. Commonly, dysregulated protein synthesis contributes to carcinogenesis and cancer progression. In this case, protein synthesis directs translation of specific mRNAs, and, in turn, promotes cell survival, invasion, angiogenesis, and metastasis of tumors. In eukaryotes, protein synthesis is regulated at its initiation, which is a rate-limiting step involving eukaryotic translation initiation factors (eIFs). We hypothesize that eIFs represent crossroads in the development of GBC, and might serve as potential biomarkers. The study focus was the role of eIF6 (an anti-association factor for the ribosomal subunits) in GBC.

METHODS

In human GBC samples, the expression of eIF6 was analyzed biochemically at the protein (immunohistochemistry, immunoblot analyses) and mRNA levels (qRT-PCR).

RESULTS

High levels of eIF6 correlated with shorter overall survival in biliary tract cancer (BTC) patients (n = 28). Immunohistochemical data from tissue microarrays (n = 114) demonstrated significantly higher expression levels of eIF6 in GBC compared to non-neoplastic tissue. Higher eIF6 expression on protein (immunoblot) and mRNA (qRT-PCR) level was confirmed by analyzing fresh frozen GBC patient samples (n = 14). Depletion of eIF6 (using specific siRNA-mediated knockdown) in Mz-ChA-2 and TFK-1 cell lines inhibited cell proliferation and induced apoptosis.

CONCLUSION

Our data indicates that eIF6 overexpression plays a major role in the translational control of GBC, and indicates its potential as a new biomarker and therapeutic target in GBC.

Extensions, Extra Factors, and Extreme Complexity: Ribosomal Structures Provide Insights into Eukaryotic Translation

Although the basic aspects of protein synthesis are preserved in all kingdoms of life, there are many important structural and functional differences between bacterial and the more complex eukaryotic ribosomes. High-resolution cryo-electron microscopy (cryo-EM) and X-ray crystallography structures of eukaryotic ribosomes have revealed the complex architectures of eukaryotic ribosomes and species-specific variations in protein and ribosomal RNA (rRNA) extensions. They also enabled structural studies of a range of eukaryotic ribosomal complexes involved in translation initiation, elongation, and termination, revealing unique mechanistic features of the eukaryotic translation process, especially with respect to the identification and recognition of translation start and stop codons on messenger RNAs (mRNAs). Most recently, structural biology has provided insights into the eukaryotic ribosomal biogenesis pathway by visualizing several of its complex intermediates. This review highlights the past decade's structural work on eukaryotic ribosomes and its implications on our understanding of eukaryotic translation.

Ribosomal protein L14 contributes to the early assembly of 60S ribosomal subunits in Saccharomyces cerevisiae

The contribution of most ribosomal proteins to ribosome synthesis has been quite well analysed in Saccharomyces cerevisiae. However, few yeast ribosomal proteins still await characterization. Herein, we show that L14, an essential 60S ribosomal protein, assembles in the nucleolus at an early stage into pre-60S particles. Depletion of L14 results in a deficit in 60S subunits and defective processing of 27SA2 and 27SA3 to 27SB pre-rRNAs. As a result, 27S pre-rRNAs are subjected to turnover and export of pre-60S particles is blocked. These phenotypes likely appear as the direct consequence of the reduced pre-60S particle association not only of L14 upon its depletion but also of a set of neighboring ribosomal proteins located at the solvent interface of 60S subunits and the adjacent region surrounding the polypeptide exit tunnel. These pre-60S intermediates also lack some essential trans-acting factors required for 27SB pre-rRNA processing but accumulate practically all factors required for processing of 27SA3 pre-rRNA. We have also analysed the functional interaction between the eukaryote-specific carboxy-terminal extensions of the neighboring L14 and L16 proteins. Our results indicate that removal of the most distal parts of these extensions cause slight translation alterations in mature 60S subunits.

Lucky, times ten: A career in Texas science

On January 21, 2017, I received an E-mail from Herb Tabor that I had been simultaneously hoping for and dreading for several years: an invitation to write a "Reflections" article for the Journal of Biological Chemistry On the one hand, I was honored to receive an invitation from Herb, a man I have admired for over 40 years, known for 24 years, and worked with as a member of the Editorial Board and Associate Editor of the Journal of Biological Chemistry for 17 years. On the other hand, the invitation marked the waning of my career as an academic scientist. With these conflicting emotions, I wrote this article with the goals of recording my career history and recognizing the many mentors, trainees, and colleagues who have contributed to it and, perhaps with pretension, with the desire that students who are beginning a career in research will find inspiration in the path I have taken and appreciate the importance of luck.

Functional Interactions Between rsks-1/S6K, glp-1/Notch, and Regulators of Caenorhabditis elegans Fertility and Germline Stem Cell Maintenance

The proper accumulation and maintenance of stem cells is critical for organ development and homeostasis. The Notch signaling pathway maintains stem cells in diverse organisms and organ systems. In Caenorhabditis elegans, GLP-1/Notch activity prevents germline stem cell (GSC) differentiation. Other signaling mechanisms also influence the maintenance of GSCs, including the highly-conserved TOR substrate ribosomal protein S6 kinase (S6K). Although C. elegans bearing either a null mutation in rsks-1/S6K or a reduction-of-function (rf) mutation in glp-1/Notch produce half the normal number of adult germline progenitors, virtually all these single mutant animals are fertile. However, glp-1(rf) rsks-1(null) double mutant animals are all sterile, and in about half of their gonads, all GSCs differentiate, a distinctive phenotype associated with a significant reduction or loss of GLP-1 signaling. How rsks-1/S6K promotes GSC fate is unknown. Here, we determine that rsks-1/S6K acts germline-autonomously to maintain GSCs, and that it does not act through Cyclin-E or MAP kinase in this role. We found that interfering with translation also enhances glp-1(rf), but that regulation through rsks-1 cannot fully account for this effect. In a genome-scale RNAi screen for genes that act similarly to rsks-1/S6K, we identified 56 RNAi enhancers of glp-1(rf) sterility, many of which were previously not known to interact functionally with Notch. Further investigation revealed at least six candidates that, by genetic criteria, act linearly with rsks-1/S6K. These include genes encoding translation-related proteins, cacn-1/Cactin, an RNA exosome component, and a Hedgehog-related ligand. We found that additional Hedgehog-related ligands may share functional relationships with glp-1/Notch and rsks-1/S6K in maintaining germline progenitors.

Plant Temperature Acclimation and Growth Rely on Cytosolic Ribosome Biogenesis Factor Homologs

Arabidopsis (Arabidopsis thaliana) REI1-LIKE (REIL) proteins, REIL1 and REIL2, are homologs of a yeast ribosome biogenesis factor that participates in late cytoplasmic 60S ribosomal subunit maturation. Here, we report that the inhibited growth of the reil1-1 reil2-1 mutant at 10°C can be rescued by the expression of amino-terminal FLUORESCENT PROTEIN (FP)-REIL fusions driven by the UBIQUITIN10 promoter, allowing the analysis of REIL function in planta. Arabidopsis REIL1 appears to be functionally conserved, based on the cytosolic localization of FP-REIL1 and the interaction of native REIL1 with the 60S subunit in wild-type plants. In contrast to its yeast homologs, REIL1 also was present in translating ribosome fractions. Systems analysis revealed that wild-type Arabidopsis remodels the cytosolic translation machinery when grown at 10°C by accumulating cytosolic ribosome subunits and inducing the expression of cytosolic ribosomal RNA, ribosomal genes, ribosome biogenesis factors, and translation initiation or elongation factors. In the reil1-1 reil2-1 mutant, all processes associated with inhibited growth were delayed, although the plants maintained cellular integrity or acquired freezing tolerance. REIL proteins also were implicated in plant-specific processes: nonacclimated reil1-1 reil2-1 exhibited cold-acclimation responses, including activation of the DREB/CBF regulon. In addition, acclimated reil1-1 reil2-1 plants failed to activate FLOWERING LOCUS T expression in mature leaves. Therefore, in the wild type, REIL function may contribute to temperature perception by suppressing premature cold responses during growth at nonstressful temperatures. In conclusion, we suggest that Arabidopsis REIL proteins influence cold-induced plant ribosome remodeling and enhance the accumulation of cytosolic ribosome subunits after cold shift either by de novo synthesis or by recycling them from the translating ribosome fraction.

The assembly factor Erb1 functions in multiple remodeling events during 60S ribosomal subunit assembly in S. cerevisiae

A major gap in our understanding of ribosome assembly is knowing the precise function of each of the ∼200 assembly factors. The steps in subunit assembly in which these factors participate have been examined for the most part by depleting each protein from cells. Depletion of the assembly factor Erb1 prevents stable assembly of seven other interdependent assembly factors with pre-60S subunits, resulting in turnover of early preribosomes, before the ITS1 spacer can be removed from 27SA3 pre-rRNA. To investigate more specific functions of Erb1, we constructed eight internal deletions of 40-60 amino acid residues each, spanning the amino-terminal half of Erb1. The erb1Δ161-200 and erb1Δ201-245 deletion mutations block a later step than depletion of Erb1, namely cleavage of the C2 site that initiates removal of the ITS2 spacer. Two other remodeling events fail to occur in these erb1 mutants: association of twelve different assembly factors with domain V of 25S rRNA, including the neighborhood surrounding the peptidyl transferase center, and stable association of ribosomal proteins with rRNA surrounding the polypeptide exit tunnel. This suggests that successful initiation of construction of these functional centers is a checkpoint for committing to spacer removal.

Bcp1 Is the Nuclear Chaperone of Rpl23 in Saccharomyces cerevisiae

Eukaryotic ribosomes are composed of rRNAs and ribosomal proteins. Ribosomal proteins are translated in the cytoplasm and imported into the nucleus for assembly with the rRNAs. It has been shown that chaperones or karyopherins responsible for import can maintain the stability of ribosomal proteins by neutralizing unfavorable positive charges and thus facilitate their transports. Among 79 ribosomal proteins in yeast, only a few are identified with specific chaperones. Besides the classic role in maintaining protein stability, chaperones have additional roles in transport, chaperoning the assembly site, and dissociation of ribosomal proteins from karyopherins. Bcp1 has been shown to be necessary for the export of Mss4, a phosphatidylinositol 4-phosphate 5-kinase, and required for ribosome biogenesis. However, its specific function in ribosome biogenesis has not been described. Here, we show that Bcp1 dissociates Rpl23 from the karyopherins and associates with Rpl23 afterward. Loss of Bcp1 causes instability of Rpl23 and deficiency of 60S subunits. In summary, Bcp1 is a novel 60S biogenesis factor via chaperoning Rpl23 in the nucleus.

Mechanistic insight into eukaryotic 60S ribosomal subunit biogenesis by cryo-electron microscopy

Eukaryotic ribosomes, the protein-producing factories of the cell, are composed of four ribosomal RNA molecules and roughly 80 proteins. Their biogenesis is a complex process that involves more than 200 biogenesis factors that facilitate the production, modification, and assembly of ribosomal components and the structural transitions along the maturation pathways of the pre-ribosomal particles. Here, I review recent structural and mechanistic insights into the biogenesis of the large ribosomal subunit that were furthered by cryo-electron microscopy of natively purified pre-60S particles and in vitro reconstituted ribosome assembly factor complexes. Combined with biochemical, genetic, and previous structural data, these structures have provided detailed insights into the assembly and maturation of the central protuberance of the 60S subunit, the network of biogenesis factors near the ribosomal tunnel exit, and the functional activation of the large ribosomal subunit during cytoplasmic maturation.

The Rice Eukaryotic Translation Initiation Factor 3 Subunit e (OseIF3e) Influences Organ Size and Pollen Maturation

Eukaryotic translation initiation factor 3 (eIF3) is a large protein complex that participates in most translation initiation processes. While eIF3 has been well characterized, less is known about the roles of individual eIF3 subunits, particularly in plants. Here, we identified and characterized OseIF3e in rice (Oryza sativa L.). OseIF3e was constitutively expressed in various tissues, but most strongly in vigorously growing organs. Transgenic OseIF3e-silenced rice plants showed inhibited growth in seedling and vegetative stages. Repression of OseIF3e led to defects in pollen maturation but did not affect pollen mitosis. In rice, eIF3e interacted with eIF3 subunits b, d, e, f, h, and k, and with eIF6, forming homo- and heterodimers to initiate translation. Furthermore, OseIF3e was shown by yeast two-hybrid assay to specifically bind to inhibitors of cyclin-dependent kinases 1, 5, and 6. This interaction was mediated by the sequence of amino acid residues at positions 118-138, which included a conserved motif (IGPEQIETLYQFAKF). These results suggested although OseIF3e is not a "functional core" subunit of eIF3, it still plays crucial roles in rice growth and development, in combination with other factors. We proposed a pathway by which OseIF3e influence organ size and pollen maturation in rice, providing an opportunity to optimize plant architecture for crop breeding.

The Roles of Puf6 and Loc1 in 60S Biogenesis Are Interdependent, and Both Are Required for Efficient Accommodation of Rpl43

Puf6 and Loc1 have two important functional roles in the cells, asymmetric mRNA distribution and ribosome biogenesis. Puf6 and Loc1 are localized predominantly in the nucleolus. They bind ASH1 mRNA, repress its translation, and facilitate the transport to the daughter cells. Asymmetric mRNA distribution is important for cell differentiation. Besides their roles in mRNA localization, Puf6 and Loc1 have been shown to be involved in 60S biogenesis. In puf6Δ or loc1Δ cells, pre-rRNA processing and 60S export are impaired and 60S subunits are underaccumulated. The functional studies of Puf6 and Loc1 have been focused on ASH1 mRNA pathway, but their roles in 60S biogenesis are still not clear. In this study, we found that Puf6 and Loc1 interact directly with each other and both proteins interact with the ribosomal protein Rpl43 (L43e). Notably, the roles of Puf6 and Loc1 in 60S biogenesis are interdependent, and both are required for efficient accommodation of Rpl43. Loc1 is further required to maintain the protein level of Rpl43. Additionally, the recruitment of Rpl43 is required for the release of Puf6 and Loc1. We propose that Puf6 and Loc1 facilitate Rpl43 loading and are sequentially released from 60S after incorporation of Rpl43 into ribosomes in yeast.

Disruption of ribosome assembly in yeast blocks cotranscriptional pre-rRNA processing and affects the global hierarchy of ribosome biogenesis

In higher eukaryotes, pre-rRNA processing occurs almost exclusively post-transcriptionally. This is not the case in rapidly dividing yeast, as the majority of nascent pre-rRNAs are processed cotranscriptionally, with cleavage at the A2 site first releasing a pre-40S ribosomal subunit followed by release of a pre-60S ribosomal subunit upon transcription termination. Ribosome assembly is driven in part by hierarchical association of assembly factors and r-proteins. Groups of proteins are thought to associate with pre-ribosomes cotranscriptionally during early assembly steps, whereas others associate later, after transcription is completed. Here we describe a previously uncharacterized phenotype observed upon disruption of ribosome assembly, in which normally late-binding proteins associate earlier, with pre-ribosomes containing 35S pre-rRNA. As previously observed by many other groups, we show that disruption of 60S subunit biogenesis results in increased amounts of 35S pre-rRNA, suggesting that a greater fraction of pre-rRNAs are processed post-transcriptionally. Surprisingly, we found that early pre-ribosomes containing 35S pre-rRNA also contain proteins previously thought to only associate with pre-ribosomes after early pre-rRNA processing steps have separated maturation of the two subunits. We believe the shift to post-transcriptional processing is ultimately due to decreased cellular division upon disruption of ribosome assembly. When cells are grown under stress or to high density, a greater fraction of pre-rRNAs are processed post-transcriptionally and follow an alternative processing pathway. Together, these results affirm the principle that ribosome assembly occurs through different, parallel assembly pathways and suggest that there is a kinetic foot-race between the formation of protein binding sites and pre-rRNA processing events.

Functional Dissection of the Nascent Polypeptide-Associated Complex in Saccharomyces cerevisiae

Both the yeast nascent polypeptide-associated complex (NAC) and the Hsp40/70-based chaperone system RAC-Ssb are systems tethered to the ribosome to assist cotranslational processes such as folding of nascent polypeptides. While loss of NAC does not cause phenotypic changes in yeast, the simultaneous deletion of genes coding for NAC and the chaperone Ssb (nacΔssbΔ) leads to strongly aggravated defects compared to cells lacking only Ssb, including impaired growth on plates containing L-canavanine or hygromycin B, aggregation of newly synthesized proteins and a reduced translational activity due to ribosome biogenesis defects. In this study, we dissected the functional properties of the individual NAC-subunits (α-NAC, β-NAC and β’-NAC) and of different NAC heterodimers found in yeast (αβ-NAC and αβ’-NAC) by analyzing their capability to complement the pleiotropic phenotype of nacΔssbΔ cells. We show that the abundant heterodimer αβ-NAC but not its paralogue αβ’-NAC is able to suppress all phenotypic defects of nacΔssbΔ cells including global protein aggregation as well as translation and growth deficiencies. This suggests that αβ-NAC and αβ’-NAC are functionally distinct from each other. The function of αβ-NAC strictly depends on its ribosome association and on its high level of expression. Expression of individual β-NAC, β’-NAC or α-NAC subunits as well as αβ’-NAC ameliorated protein aggregation in nacΔssbΔ cells to different extents while only β-NAC was able to restore growth defects suggesting chaperoning activities for β-NAC sufficient to decrease the sensitivity of nacΔssbΔ cells against L-canavanine or hygromycin B. Interestingly, deletion of the ubiquitin-associated (UBA)-domain of the α-NAC subunit strongly enhanced the aggregation preventing activity of αβ-NAC pointing to a negative regulatory role of this domain for the NAC chaperone activity in vivo.

Protective Effects of Scutellarin on Human Cardiac Microvascular Endothelial Cells against Hypoxia-Reoxygenation Injury and Its Possible Target-Related Proteins

Scutellarin (SCU) is one of the main components of traditional Chinese medicine plant Erigeron breviscapus (Vant.) Hand.-Mazz. In this paper, we studied the protective effects of SCU on human cardiac microvascular endothelial cells (HCMECs) against hypoxia-reoxygenation (HR) injury and its possible target-related proteins. Results of MTT assay showed that pretreatment of SCU at doses of 1, 5, and 10 μM for 2 h could significantly inhibit the decrease in cell viability of HCMECs induced by HR injury. Subcellular fractions of cells treated with vehicle control, 1 μM SCU, HR injury, or 1 μM SCU + HR injury were separated by ultracentrifugation. The protein expression profiles of cytoplasm and membrane/nuclei fractions were checked using protein two-dimensional electrophoresis (2-DE). Proteins differentially expressed between control and SCU-treated group, control and HR group, or HR and SCU + HR group were identified using mass spectrometry (MS/MS). Possible interaction network of these target-related proteins was predicted using bioinformatic analysis. The influence of SCU on the expression levels of these proteins was confirmed using Western blotting assay. The results indicated that proteins such as p27BBP protein (EIF6), heat shock 60 kDa protein 1 (HSPD1), and chaperonin containing TCP1 subunit 6A isoform (CCT6A) might play important roles in the effects of SCU.

Eukaryotic translation initiation factor 6 is a novel regulator of reactive oxygen species-dependent megakaryocyte maturation

BACKGROUND

Ribosomopathies constitute a class of inherited disorders characterized by defects in ribosome biogenesis and function. Classically, bone marrow (BM) failure is a clinical symptom shared between these syndromes, including Shwachman-Bodian-Diamond syndrome (SBDS). Eukaryotic translation initiation factor 6 (eIF6) is a critical translation factor that rescues the quasilethal effect of the loss of the SBDS protein.

OBJECTIVES

To determine whether eIF6 activity is necessary for BM development.

METHODS

We used eIF6(+/-) mice and primary BM megakaryocytes to investigate the involvement of eIF6 in the regulation of hematopoiesis.

RESULTS

We provide evidence that reduced eIF6 expression negatively impacts on megakaryopoiesis. We show that inhibition of eIF6 leads to a reduction in cell size and mean ploidy level of megakaryocytes and a delay in megakaryocyte maturation by blocking the G1 /S transition. Consistent with this phenotype, only few megakaryocyte-forming proplatelets were found in eIF6(+/-) cells. We also discovered that, in eIF6(+/-) cells, the steady-state abundance of mitochondrial respiratory chain complex I-encoding mRNAs is decreased, resulting in decreased reactive oxygen species (ROS) production. Intriguingly, connectivity map analysis showed that eIF6-mediated changes overlap with specific translational inhibitors. eIF6 is a translation factor acting downstream of insulin/phorbol 12-myristate 13-acetate (PMA) stimulation. PMA treatment significantly restored eIF6(+/-) megakaryocyte maturation, indicating that activation of eIF6 is essential for the rescue of the phenotype.

CONCLUSIONS

Taken together, our results show a role for eIF6-driven translation in megakaryocyte development, and unveil the novel connection between translational control and ROS production in this cell subset.

Mechanism of eIF6 release from the nascent 60S ribosomal subunit

SBDS protein (deficient in the inherited leukemia-predisposition disorder Shwachman-Diamond syndrome) and the GTPase EFL1 (an EF-G homolog) activate nascent 60S ribosomal subunits for translation by catalyzing eviction of the antiassociation factor eIF6 from nascent 60S ribosomal subunits. However, the mechanism is completely unknown. Here, we present cryo-EM structures of human SBDS and SBDS-EFL1 bound to Dictyostelium discoideum 60S ribosomal subunits with and without endogenous eIF6. SBDS assesses the integrity of the peptidyl (P) site, bridging uL16 (mutated in T-cell acute lymphoblastic leukemia) with uL11 at the P-stalk base and the sarcin-ricin loop. Upon EFL1 binding, SBDS is repositioned around helix 69, thus facilitating a conformational switch in EFL1 that displaces eIF6 by competing for an overlapping binding site on the 60S ribosomal subunit. Our data reveal the conserved mechanism of eIF6 release, which is corrupted in both inherited and sporadic leukemias.

eIF6 anti-association activity is required for ribosome biogenesis, translational control and tumor progression

Here we discuss the function of eukaryotic initiation factor 6 (eIF6; Tif6 in yeast). eIF6 binds 60S ribosomal subunits and blocks their joining to 40S. In this context, we propose that eIF6 impedes unproductive 80S formation, namely, the formation of 80S subunits without mRNA. Genetic evidence shows that eIF6 has a dual function: in yeast and mammals, nucleolar eIF6 is necessary for the biogenesis of 60S subunits. In mammals, cytoplasmic eIF6 is required for insulin and growth factor-stimulated translation. In contrast to other translation factors, eIF6 activity is not under mTOR control. The physiological significance of eIF6 impacts on cancer and on inherited Shwachman-Bodian-Diamond syndrome. eIF6 is overexpressed in specific human tumors. In a murine model of lymphomagenesis, eIF6 depletion leads to a striking increase of survival, without adverse effects. Shwachman-Bodian-Diamond syndrome is caused by loss of function of SBDS protein. In yeast, point mutations of Tif6, the yeast homolog of eIF6, rescue the quasi-lethal effect due to the loss of the SBDS homolog, Sdo1. We propose that eIF6 is a node regulator of ribosomal function and predict that prioritizing its pharmacological targeting will be of benefit in cancer and Shwachman-Bodian-Diamond syndrome. This article is part of a Special Issue entitled: Translation and Cancer.

EIF6 over-expression increases the motility and invasiveness of cancer cells by modulating the expression of a critical subset of membrane-bound proteins

BACKGROUND

Eukaryotic Initiation factor 6 (eIF6) is a peculiar translation initiation factor that binds to the large 60S ribosomal subunits, controlling translation initiation and participating in ribosome biogenesis. In the past, knowledge about the mechanisms adopted by the cells for controlling protein synthesis by extracellular stimuli has focused on two translation initiation factors (eIF4E and eIF2), however, recent data suggest eIF6 as a newcomer in the control of downstream of signal transduction pathways. eIF6 is over-expressed in tumors and its decreased expression renders cells less prone to tumor growth. A previous work from our laboratory has disclosed that over-expression of eIF6 in transformed cell lines markedly increased cell migration and invasion.

METHODS

Here, we performed a quantitative proteomic analysis of membrane-associated proteins in A2780 ovarian cancer cells over-expressing eIF6. Differentially expressed proteins upon eIF6 overproduction were further investigated in silico by Ingenuity Pathway Analysis (IPA). RT-qPCR and Western blot were performed in order to validate the proteomic data. Furthermore, the effects of a potent and selective inhibitor ML-141 in A2780 cells were evaluated using transwell migration assay. Finally, we explored the effects of eIF6 over-expression on WM793 primary melanoma cell lines.

RESULTS

We demonstrated that: (i) the genes up-regulated upon eIF6 overproduction mapped to a functional network corresponding to cellular movements in a highly significant way; (ii) cdc42 plays a pivotal role as an effector of enhanced migratory phenotype induced upon eIF6 over-expression; (iii) the variations in abundance observed for cdc42 protein occur at a post-transcriptional level; (iv) the increased cell migration/invasion upon eIF6 over-expression was generalizable to other cell line models.

CONCLUSIONS

Collectively, our data confirm and further extend the role of eIF6 in enhancing cell migration/invasion. We show that a number of membrane-associated proteins indeed vary in abundance upon eIF6 over-expression, and that the up-regulated proteins can be located within a functional network controlling cell motility and tumor metastasis. Full understanding of the role eIF6 plays in the metastatic process is important, also in view of the fact that this factor is a potentially druggable target to be exploited for new anti-cancer therapies.

Marrow failure: a window into ribosome biology

Diamond-Blackfan anemia, Shwachman-Diamond syndrome, and dyskeratosis congenita are inherited syndromes characterized by marrow failure, congenital anomalies, and cancer predisposition. Genetic and molecular studies have uncovered distinct abnormalities in ribosome biogenesis underlying each of these 3 disorders. How defects in ribosomes, the essential organelles required for protein biosynthesis in all cells, cause tissue-specific abnormalities in human disease remains a question of fundamental scientific and medical importance. Here we review the overlapping and distinct clinical features of these 3 syndromes and discuss current knowledge regarding the ribosomal pathways disrupted in each of these disorders. We also explore the increasing complexity of ribosome biology and how this informs our understanding of developmental biology and human disease.

Identification of essential proteins based on ranking edge-weights in protein-protein interaction networks

Essential proteins are those that are indispensable to cellular survival and development. Existing methods for essential protein identification generally rely on knock-out experiments and/or the relative density of their interactions (edges) with other proteins in a Protein-Protein Interaction (PPI) network. Here, we present a computational method, called EW, to first rank protein-protein interactions in terms of their Edge Weights, and then identify sub-PPI-networks consisting of only the highly-ranked edges and predict their proteins as essential proteins. We have applied this method to publicly-available PPI data on Saccharomyces cerevisiae (Yeast) and Escherichia coli (E. coli) for essential protein identification, and demonstrated that EW achieves better performance than the state-of-the-art methods in terms of the precision-recall and Jackknife measures. The highly-ranked protein-protein interactions by our prediction tend to be biologically significant in both the Yeast and E. coli PPI networks. Further analyses on systematically perturbed Yeast and E. coli PPI networks through randomly deleting edges demonstrate that the proposed method is robust and the top-ranked edges tend to be more associated with known essential proteins than the lowly-ranked edges.

Cytoskeletal proteins associate with components of the ribosomal maturation and translation apparatus in Xenopus stage I oocytes

Actin-based cytoskeleton (CSK) and microtubules may bind to RNAs and related molecules implicated in translation. However, many questions remain to be answered regarding the role of cytoskeletal components in supporting the proteins involved in steps in the maturation and translation processes. Here, we performed co-immunoprecipitation and immunofluorescence to examine the association between spectrins, keratins and tubulin and proteins involved in 60S ribosomal maturation and translation in Xenopus stage I oocytes, including ribosomal rpl10, eukaryotic initiation factor 6 (Eif6), thesaurins A/B, homologs of the eEF1α elongation factor, and P0, the ribosomal stalk protein. We found that rpl10 and eif6 cross-reacted with the actin-based CSK and with tubulin. rpl10 co-localizes with spectrin, particularly in the perinuclear region. eif6 is similarly localized. Given that upon ribosomal maturation, the insertion of rpl10 into the 60S subunit occurs simultaneously with the release of eif6, one can hypothesise that actin-based CSK and microtubules provide the necessary scaffold for the insertion/release of these two molecules and, subsequently, for eif6 transport and binding to the mature 60S subunit. P0 and thesaurins cross-reacted with only spectrin and cytokeratins. Thesaurins aggregated at the oocyte periphery, rendering this a territory favourable site for protein synthesis; the CSK may support the interaction between thesaurins and sites of the translating ribosome. Moreover, given that the assembly of the ribosome stalk, where P0 is located, to the 60S subunit is essential for the release of eif6, it can be hypothesised that the CSK can facilitate the binding of the stalk to the 60S.

Ribosome assembly factors Pwp1 and Nop12 are important for folding of 5.8S rRNA during ribosome biogenesis in Saccharomyces cerevisiae

Previous work from our lab suggests that a group of interdependent assembly factors (A(3) factors) is necessary to create early, stable preribosomes. Many of these proteins bind at or near internal transcribed spacer 2 (ITS2), but in their absence, ITS1 is not removed from rRNA, suggesting long-range communication between these two spacers. By comparing the nonessential assembly factors Nop12 and Pwp1, we show that misfolding of rRNA is sufficient to perturb early steps of biogenesis, but it is the lack of A(3) factors that results in turnover of early preribosomes. Deletion of NOP12 significantly inhibits 27SA(3) pre-rRNA processing, even though the A(3) factors are present in preribosomes. Furthermore, pre-rRNAs are stable, indicating that the block in processing is not sufficient to trigger turnover. This is in contrast to the absence of Pwp1, in which the A(3) factors are not present and pre-rRNAs are unstable. In vivo RNA structure probing revealed that the pre-rRNA processing defects are due to misfolding of 5.8S rRNA. In the absence of Nop12 and Pwp1, rRNA helix 5 is not stably formed. Interestingly, the absence of Nop12 results in the formation of an alternative yet unproductive helix 5 when cells are grown at low temperatures.

CK1δ and CK1ε are components of human 40S subunit precursors required for cytoplasmic 40S maturation

Biogenesis of 40S pre-ribosomal subunits requires many trans-acting factors, among them several protein kinases. In this study, we show that the human casein kinase 1 (CK1) isoforms δ and ε are required for cytoplasmic maturation steps of 40S subunit precursors. We show that both CK1δ and CK1ε isoforms are components of pre-40S subunits, on which they phosphorylate the ribosome biogenesis factors ENP1/BYSL and LTV1. Inhibition or co-depletion of CK1δ and CK1ε results in failure to recycle a series of trans-acting factors including ENP1/BYSL, LTV1, RRP12, DIM2/PNO1, RIO2 and NOB1 from pre-40S particles after nuclear export. Furthermore, co-depletion of CK1δ and CK1ε leads to defects in 18S-E pre-rRNA processing. Together, these data demonstrate that CK1δ and CK1ε play a decisive role in triggering late steps of pre-40S maturation that are required for acquisition of functionality of 40S ribosomal subunits in protein translation.

The REIL1 and REIL2 proteins of Arabidopsis thaliana are required for leaf growth in the cold

The evolutionarily conserved proteins REI1-LIKE (REIL1) and REIL2 have four conserved zinc finger domains and are Arabidopsis thaliana homologs of the cytosolic 60S ribosomal maturation factor Rei1p (for Required for isotropic bud growth1 protein) from yeast (Saccharomyces cerevisiae) and its paralog Reh1p (for REI1 homologue1 protein). The yeast and A. thaliana paralogs result from independent gene duplications. The A. thaliana REIL paralogs are required specifically in the cold (10°C) but not for growth at optimal temperature (20°C). A reil1-1 reil2-1 double mutant is arrested at 10°C prior to the emergence of the first rosette leaf. Two allelic reil2 mutants, reil2-1 and reil2-2, form small spoon-shaped leaves at 10°C. This phenomenon reverts after emergence of the inflorescence in the cold or upon shift to 20°C. Except for a slightly delayed germination, a reil1-1 mutant shows no further growth phenotype under the currently investigated conditions. A comparative analysis demonstrates conserved coexpression of orthologous genes from yeast and A. thaliana that are coregulated with yeast rei1 or with A. thaliana REIL2, respectively. The conserved correlations point to a role of A. thaliana REIL proteins in the maturation of the eukaryotic ribosomal 60S subunit. We support this conclusion by heterologous complementation of the cold-induced growth defect of the yeast Δrei1 deletion.