mTORC1 regulates cell survival under glucose starvation through 4EBP1/2-mediated translational reprogramming of fatty acid metabolism.

Energetic stress compels cells to evolve adaptive mechanisms to adjust their metabolism. Inhibition of mTOR kinase complex 1 (mTORC1) is essential for cell survival during glucose starvation. How mTORC1 controls cell viability during glucose starvation is not well understood. Here we show that the mTORC1 effectors eukaryotic initiation factor 4E binding proteins 1/2 (4EBP1/2) confer protection to mammalian cells and budding yeast under glucose starvation. Mechanistically, 4EBP1/2 promote NADPH homeostasis by preventing NADPH-consuming fatty acid synthesis via translational repression of Acetyl-CoA Carboxylase 1 (ACC1), thereby mitigating oxidative stress. This has important relevance for cancer, as oncogene-transformed cells and glioma cells exploit the 4EBP1/2 regulation of ACC1 expression and redox balance to combat energetic stress, thereby supporting transformation and tumorigenicity in vitro and in vivo. Clinically, high EIF4EBP1 expression is associated with poor outcomes in several cancer types. Our data reveal that the mTORC1-4EBP1/2 axis provokes a metabolic switch essential for survival during glucose starvation which is exploited by transformed and tumor cells.

Poly(A)-binding protein promotes VPg-dependent translation of potyvirus through enhanced binding of phosphorylated eIFiso4F and eIFiso4F∙eIF4B.

The phosphorylation of eukaryotic translational initiation factors has been shown to play a significant role in controlling the synthesis of protein. Viral infection, environmental stress, and growth circumstances cause phosphorylation or dephosphorylation of plant initiation factors. Our findings indicate that casein kinase 2 can phosphorylate recombinant wheat eIFiso4E and eIFiso4G generated from E. coli in vitro. For wheat eIFiso4E, Ser-207 was found to be the in vitro phosphorylation site. eIFiso4E lacks an amino acid that can be phosphorylated at the position corresponding to Ser-209, the phosphorylation site in mammalian eIF4E, yet phosphorylation of eIFiso4E has effects on VPg binding affinity that are similar to those of phosphorylation of mammalian eIF4E. The addition of VPg and phosphorylated eIFiso4F to depleted wheat germ extract (WGE) leads to enhancement of translation of both uncapped and capped viral mRNA. The addition of PABP together with eIFiso4Fp and eIF4B to depleted WGE increases both uncapped and capped mRNA translation. However, it exhibits a translational advantage specifically for uncapped mRNA, implying that the phosphorylation of eIFiso4F hinders cap binding while promoting VPg binding, thereby facilitating uncapped translation. These findings indicate TEV virus mediates VPg-dependent translation by engaging a mechanism entailing phosphorylated eIFiso4Fp and PABP. To elucidate the molecular mechanisms underlying these observed effects, we studied the impact of PABP and/or eIF4B on the binding of VPg with eIFiso4Fp. The inclusion of PABP and eIF4B with eIFiso4Fp resulted in about 2-fold increase in affinity for VPg (Kd = 24 ± 1.7 nM), as compared to the affinity of eIFiso4Fp alone (Kd = 41.0 ± 3.1 nM). The interactions between VPg and eIFiso4Fp were determined to be both enthalpically and entropically favorable, with the enthalpic contribution accounting for 76-97% of the ΔG at 25°C, indicating a substantial role of hydrogen bonding in enhancing the stability of the complex. The binding of PABP to eIFiso4Fp·4B resulted in a conformational alteration, leading to a significant enhancement in the binding affinity to VPg. These observations suggest PABP enhances the affinity between eIFiso4Fp and VPg, leading to an overall conformational change that provides a stable platform for efficient viral translation.

AGO1 controls protein folding in mouse embryonic stem cell fate decisions.

Argonaute (AGO) proteins are evolutionarily conserved RNA-binding proteins that control gene expression through the small RNAs they interact with. Whether AGOs have regulatory roles independent of RNAs, however, is unknown. Here, we show that AGO1 controls cell fate decisions through facilitating protein folding. We found that in mouse embryonic stem cells (mESCs), while AGO2 facilitates differentiation via the microRNA (miRNA) pathway, AGO1 controls stemness independently of its binding to small RNAs. We determined that AGO1 specifically interacts with HOP, a co-chaperone for the HSP70 and HSP90 chaperones, and enhances the folding of a set of HOP client proteins with intrinsically disordered regions. This AGO1-mediated facilitation of protein folding is important for maintaining stemness in mESCs. Our results demonstrate divergent functions between AGO1 and AGO2 in controlling cellular states and identify an RNA-independent function of AGO1 in controlling gene expression and cell fate decisions.

A Translational Regulatory Mechanism Mediated by Hypusinated Eukaryotic Initiation Factor 5A Facilitates ß-Cell Identity and Function.

As professional secretory cells, ß-cells require adaptable mRNA translation to facilitate a rapid synthesis of proteins, including insulin, in response to changing metabolic cues. Specialized mRNA translation programs are essential drivers of cellular development and differentiation. However, in the pancreatic ß-cell, the majority of factors identified to promote growth and development function primarily at the level of transcription. Therefore, despite its importance, the regulatory role of mRNA translation in the formation and maintenance of functional ß-cells is not well defined. In this study, we have identified a translational regulatory mechanism mediated by the specialized mRNA translation factor eukaryotic initiation factor 5A (eIF5A), which facilitates the maintenance of ß-cell identity and function. The mRNA translation function of eIF5A is only active when it is posttranslationally modified ("hypusinated") by the enzyme deoxyhypusine synthase (DHPS). We have discovered that the absence of ß-cell DHPS in mice reduces the synthesis of proteins critical to ß-cell identity and function at the stage of ß-cell maturation, leading to a rapid and reproducible onset of diabetes. Therefore, our work has revealed a gatekeeper of specialized mRNA translation that permits the ß-cell, a metabolically responsive secretory cell, to maintain the integrity of protein synthesis necessary during times of induced or increased demand.

The intrinsically disordered region of eIF5B stimulates IRES usage and nucleates biological granule formation.

Cells activate stress response pathways to survive adverse conditions. Such responses involve the inhibition of global cap-dependent translation. This inhibition is a block that essential transcripts must escape via alternative methods of translation initiation, e.g., an internal ribosome entry site (IRES). IRESs have distinct structures and generally require a limited repertoire of translation factors. Cellular IRESs have been identified in many critical cellular stress response transcripts. We previously identified cellular IRESs in the murine insulin receptor (Insr) and insulin-like growth factor 1 receptor (Igf1r) transcripts and demonstrated their resistance to eukaryotic initiation factor 4F (eIF4F) inhibition. Here, we find that eIF5B preferentially promotes Insr, Igf1r, and hepatitis C virus IRES activity through a non-canonical mechanism that requires its highly charged and disordered N terminus. We find that the N-terminal region of eIF5B can drive cytoplasmic granule formation. This eIF5B granule is triggered by cellular stress and is sufficient to specifically promote IRES activity.

Kinetics of Translating Ribosomes Determine the Efficiency of Programmed Stop Codon Readthrough.

During translation, a stop codon on the mRNA signals the ribosomes to terminate the process. In certain mRNAs, the termination fails due to the recoding of the canonical stop codon, and ribosomes continue translation to generate C-terminally extended protein. This process, termed stop codon readthrough (SCR), regulates several cellular functions. SCR is driven by elements/factors that act immediately downstream of the stop codon. Here, we have analysed the process of SCR using a simple mathematical model to investigate how the kinetics of translating ribosomes influences the efficiency of SCR. Surprisingly, the analysis revealed that the rate of translation inversely regulates the efficiency of SCR. We tested this prediction experimentally in mammalian AGO1 and MTCH2 mRNAs. Reduction in translation either globally by harringtonine or locally by rare codons caused an increase in the efficiency of SCR. Thus, our study has revealed a hitherto unknown mode of regulation of SCR.

Backbone resonance assignments of the C-terminal region of human translation initiation factor eIF4B.

Translation initiation in eukaryotes is an early step in protein synthesis, requiring multiple factors to recruit the ribosomal small subunit to the mRNA 5' untranslated region. One such protein factor is the eukaryotic translation initiation factor 4B (eIF4B), which increases the activity of the eIF4A RNA helicase, and is linked to cell survival and proliferation. We report here the protein backbone chemical shift assignments corresponding to the C-terminal 279 residues of human eIF4B. Analysis of the chemical shift values identifies one main helical region in the area previously linked to RNA binding, and confirms that the overall C-terminal region is intrinsically disordered.

Dihydropyrimidinase-Related Protein 2 Is a New Partner in the Binding between 4E-BP2 and eIF4E Related to Neuronal Death after Cerebral Ischemia.

Transient cerebral ischemia induces neuronal degeneration, followed in time by secondary delayed neuronal death that is strongly correlated with a permanent inhibition of protein synthesis in vulnerable brain regions, while protein translational rates are recovered in resistant areas. In the translation-regulation initiation step, the eukaryotic initiation factor (eIF) 4E is a key player regulated by its association with eIF4E-binding proteins (4E-BPs), mostly 4E-BP2 in brain tissue. In a previous work, we identified dihydropyrimidinase-related protein 2 (DRP2) as a 4E-BP2-interacting protein. Here, using a proteomic approach in a model of transient cerebral ischemia, a detailed study of DRP2 was performed in order to address the challenge of translation restoration in vulnerable regions. In this report, several DRP2 isoforms that have a specific interaction with both 4E-BP2 and eIF4E were identified, showing significant and opposite differences in this association, and being differentially detected in resistant and vulnerable regions in response to ischemia reperfusion. Our results provide the first evidence of DRP2 isoforms as potential regulators of the 4E-BP2-eIF4E association that would have consequences in the delayed neuronal death under ischemic-reperfusion stress. The new knowledge reported here identifies DRP2 as a new target to promote neuronal survival after cerebral ischemia.

Comparison of Human Eukaryotic Translation Initiation Factors 5A1 and 5AL1: Identification of Amino Acid Residues Important for EIF5A1 Lysine 50 Hypusination and Its Protein Stability.

The human eukaryotic translation initiation factor 5A (EIF5A) family consists of three members, namely EIF5A1, EIF5A2, and EIF5AL1. Recent studies have shown that the expression of EIF5As is related to many human diseases, such as diabetes, viral infection, central nervous system injury, and cancer. Among them, EIF5A1 plays different functions in various cancers, possibly as a tumor-suppressor or oncogene, while EIF5A2 promotes the occurrence and development of cancer. Yet, the biological function of EIF5AL1 is not being studied so far. Interestingly, although there are only three amino acid (at residues 36, 45, and 109) differences between EIF5A1 and EIF5AL1, we demonstrate that only EIF5A1 can be hypusinated while EIF5AL1 cannot, and EIF5AL1 has a tumor-suppressor-like function by inhibiting cell proliferation and migration. We also show that EIF5AL1 protein turnover is mediated through the proteasomal pathway, and EIF5AL1 protein turnover is much faster than that of EIF5A1, which may explain their differential protein expression level in cells. By engineering single and double mutations on these three amino acids, we pinpoint which of these amino acids are critical for hypusination and protein stability. The data of this work should fill in the gaps in EIF5As research and pave the way for future studies on EIF5AL1.

Neuronal RNA-Binding Protein HuD Interacts with Translation Initiation Factor eIF3.

Translation initiation is the rate-limiting step of protein synthesis and is the main target of translation regulation. RNA-binding proteins (RBPs) are key mediators of the spatiotemporal control of translation and are critical for cell proliferation, development, and differentiation. We have previously shown that HuD, one of the neuronal RBPs, enhances cap-dependent translation through the direct interaction with eukaryotic initiation factor 4A (eIF4A) and poly(A) tail using a HeLa-derived in vitro translation system. We have also found that translation stimulation of HuD is essential for HuD-induced neurite outgrowth in PC12 cells. However, it remains unclear how HuD is involved in the regulation of translation initiation. Here, we report that HuD binds to eukaryotic initiation factor 3 (eIF3) via the eIF3b subunit, which belongs to the functional core of mammalian eIF3. eIF3 plays an essential role in recruiting the 40S ribosomal subunit onto mRNA in translation initiation. We hypothesize that the interaction between HuD and eIF3 stabilizes the translation initiation complex and increases translation efficiency. We also showed that the linker region of HuD is required for the interaction with eIF3b. Moreover, we found that eIF3b-binding region of HuD is conserved in all Hu proteins (HuB, HuC, HuD, and HuR). These data might also help to explain how Hu proteins stimulate translation in a cap- and poly(A)-dependent way.

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.

A noncanonical function of EIF4E limits ALDH1B1 activity and increases susceptibility to ferroptosis.

Ferroptosis is a type of lipid peroxidation-dependent cell death that is emerging as a therapeutic target for cancer. However, the mechanisms of ferroptosis during the generation and detoxification of lipid peroxidation products remain rather poorly defined. Here, we report an unexpected role for the eukaryotic translation initiation factor EIF4E as a determinant of ferroptotic sensitivity by controlling lipid peroxidation. A drug screening identified 4EGI-1 and 4E1RCat (previously known as EIF4E-EIF4G1 interaction inhibitors) as powerful inhibitors of ferroptosis. Genetic and functional studies showed that EIF4E (but not EIF4G1) promotes ferroptosis in a translation-independent manner. Using mass spectrometry and subsequent protein-protein interaction analysis, we identified EIF4E as an endogenous repressor of ALDH1B1 in mitochondria. ALDH1B1 belongs to the family of aldehyde dehydrogenases and may metabolize the aldehyde substrate 4-hydroxynonenal (4HNE) at high concentrations. Supraphysiological levels of 4HNE triggered ferroptosis, while low concentrations of 4HNE increased the cell susceptibility to classical ferroptosis inducers by activating the NOX1 pathway. Accordingly, EIF4E-dependent ALDH1B1 inhibition enhanced the anticancer activity of ferroptosis inducers in vitro and in vivo. Our results support a key function of EIF4E in orchestrating lipid peroxidation to ignite ferroptosis.

[Effect of eIF4B knockout on apoptosis of mouse fetal liver cells].

Eukaryotic translation initiation factor 4B (eIF4B) plays an important role in mRNA translation initiation, cell survival and proliferation in vitro, but the in vivo function is poorly understood. In this study, via various experimental techniques such as hematoxylin-eosin (HE) staining, flow cytometry, Western blotting, and immunohistochemistry, we investigated the role of eIF4B in mouse embryo development using an eIF4B knockout (KO) mouse model and explored the mechanism. We found that the livers, but not lungs, brain, stomach, or pancreas, derived from eIF4B KO mouse embryos displayed severe pathological changes characterized by enhanced apoptosis and necrosis. Accordingly, high expression of cleaved-caspase 3, and excessive activation of mTOR signaling as evidenced by increased expression and phosphorylation of p70S6K and enhanced phosphorylation of 4EBP1, were observed in mouse embryonic fibroblasts and fetal livers from eIF4B KO mice. These results uncover a critical role of eIF4B in mouse embryo development and provide important insights into the biological functions of eIF4B in vivo.

High expression of serine and arginine-rich splicing factor 9 (SRSF9) is associated with hepatocellular carcinoma progression and a poor prognosis.

BACKGROUND: Serine and arginine-rich splicing factor 9 (SRSF9) has been linked to the occurrence and progression of various cancers; however, its effects and mechanism of action hepatocellular carcinoma (HCC) have not been reported. In this study, we used a bioinformatics approach and in vitro assays to evaluate the expression of SRSF9 in HCC, its prognostic value, and its underlying regulatory mechanisms, including analyses of related pathways and the role of methylation. METHODS: Transcriptomic and DNA methylation data for 357 HCC cases and 50 paratumor tissues in The Cancer Genome Atlas database were obtained. Additionally, protein expression data for cell lines and tissue samples were obtained from the Human Protein Atlas. The CMap databased was used to predict candidate drugs targeting SRSF9. Various cell lines were used for in vitro validation. RESULTS: SRSF9 expression was significantly elevated in HCC and was negatively regulated by its methylation site cg06116271. The low expression of SRSF9 and hypermethylation of cg06116271 were both associated with a longer overall survival time. A correlation analysis revealed ten genes that were co-expressed with SRSF9; levels of CDK4, RAN, DENR, RNF34, and ANAPC5 were positively correlated and levels of RBP4, APOC1, MASP2, HP, and HPX were negatively correlated with SRSF9 expression. The knockdown of SRSF9 in vitro inhibited the proliferation and migration of HCC cells and significantly reduced the expression of proteins in the Wnt signaling pathway (DVL2 and ß-catenin) and cell cycle pathway (Cyclin D and Cyclin E). A CMap analysis identified two drugs, camptothecin and apigenin, able to target and inhibit the expression of SRSF9. CONCLUSIONS: This study expands our understanding of the molecular biological functions of SRSF9 and cg06116271 and provides candidate diagnostic and therapeutic targets for HCC.

Inhibitors of eIF4G1-eIF1 uncover its regulatory role of ER/UPR stress-response genes independent of eIF2α-phosphorylation.

During translation initiation, eIF4G1 dynamically interacts with eIF4E and eIF1. While the role of eIF4E-eIF4G1 is well established, the regulatory functions of eIF4G1-eIF1 are poorly understood. Here, we report the identification of the eIF4G1-eIF1 inhibitors i14G1-10 and i14G1-12. i14G1s directly bind eIF4G1 and inhibit translation in vitro and in the cell, and their effects on translation are dependent on eIF4G1 levels. Translatome analyses revealed that i14G1s mimic eIF1 and eIF4G1 perturbations on the stringency of start codon selection and the opposing roles of eIF1-eIF4G1 in scanning-dependent and scanning-independent short 5' untranslated region (UTR) translation. Remarkably, i14G1s activate ER/unfolded protein response (UPR) stress-response genes via enhanced ribosome loading, elevated 5'UTR translation at near-cognate AUGs, and unexpected concomitant up-regulation of coding-region translation. These effects are, at least in part, independent of eIF2α-phosphorylation. Interestingly, eIF4G1-eIF1 interaction itself is negatively regulated by ER stress and mTOR inhibition. Thus, i14G1s uncover an unknown mechanism of ER/UPR translational stress response and are valuable research tools and potential drugs against diseases exhibiting dysregulated translation.

Multisite phosphorylation and binding alter conformational dynamics of the 4E-BP2 protein.

Intrinsically disordered proteins (IDPs) play critical roles in regulatory protein interactions, but detailed structural/dynamic characterization of their ensembles remain challenging, both in isolation and when they form dynamic "fuzzy" complexes. Such is the case for mRNA cap-dependent translation initiation, which is regulated by the interaction of the predominantly folded eukaryotic initiation factor 4E (eIF4E) with the intrinsically disordered eIF4E binding proteins (4E-BPs) in a phosphorylation-dependent manner. Single-molecule Förster resonance energy transfer showed that the conformational changes of 4E-BP2 induced by binding to eIF4E are non-uniform along the sequence; while a central region containing both motifs that bind to eIF4E expands and becomes stiffer, the C-terminal region is less affected. Fluorescence anisotropy decay revealed a non-uniform segmental flexibility around six different labeling sites along the chain. Dynamic quenching of these fluorescent probes by intrinsic aromatic residues measured via fluorescence correlation spectroscopy report on transient intra- and inter-molecular contacts on nanosecond-to-microsecond timescales. Upon hyperphosphorylation, which induces folding of ∼40 residues in 4E-BP2, the quenching rates decreased at most labeling sites. The chain dynamics around sites in the C-terminal region far away from the two binding motifs significantly increased upon binding to eIF4E, suggesting that this region is also involved in the highly dynamic 4E-BP2:eIF4E complex. Our time-resolved fluorescence data paint a sequence-level rigidity map of three states of 4E-BP2 differing in phosphorylation or binding status and distinguish regions that form contacts with eIF4E. This study adds complementary structural and dynamics information to recent studies of 4E-BP2, and it constitutes an important step toward a mechanistic understanding of this important IDP via integrative modeling.

PABP/purine-rich motif as an initiation module for cap-independent translation in pattern-triggered immunity.

Upon stress, eukaryotes typically reprogram their translatome through GCN2-mediated phosphorylation of the eukaryotic translation initiation factor, eIF2α, to inhibit general translation initiation while selectively translating essential stress regulators. Unexpectedly, in plants, pattern-triggered immunity (PTI) and response to other environmental stresses occur independently of the GCN2/eIF2α pathway. Here, we show that while PTI induces mRNA decapping to inhibit general translation, defense mRNAs with a purine-rich element ("R-motif") are selectively translated using R-motif as an internal ribosome entry site (IRES). R-motif-dependent translation is executed by poly(A)-binding proteins (PABPs) through preferential association with the PTI-activating eIFiso4G over the repressive eIF4G. Phosphorylation by PTI regulators mitogen-activated protein kinase 3 and 6 (MPK3/6) inhibits eIF4G's activity while enhancing PABP binding to the R-motif and promoting eIFiso4G-mediated defense mRNA translation, establishing a link between PTI signaling and protein synthesis. Given its prevalence in both plants and animals, the PABP/R-motif translation initiation module may have a broader role in reprogramming the stress translatome.

Inhibition of eIF6 Activity Reduces Hepatocellular Carcinoma Growth: An In Vivo and In Vitro Study.

Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of lipids in the liver. Given the high prevalence of NAFLD, its evolution to nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC) is of global concern. Therapies for managing NASH-driven HCC can benefit from targeting factors that play a continuous role in NAFLD evolution to HCC. Recent work has shown that postprandial liver translation exacerbates lipid accumulation through the activity of a translation factor, eukaryotic initiation factor 6 (eIF6). Here, we test the effect of eIF6 inhibition on the progression of HCC. Mice heterozygous for eIF6 express half the level of eIF6 compared to wt mice and are resistant to the formation of HCC nodules upon exposure to a high fat/high sugar diet combined with liver damage. Histology showed that nodules in eIF6 het mice were smaller with reduced proliferation compared to wt nodules. By using an in vitro model of human HCC, we confirm that eIF6 depletion reduces the growth of HCC spheroids. We also tested three pharmacological inhibitors of eIF6 activity-eIFsixty-1, eIFsixty-4, and eIFsixty-6-and all three reduced eIF6 binding to 60S ribosomes and limited the growth of HCC spheroids. Thus, inhibition of eIF6 activity is feasible and limits HCC formation.

Role of aIF5B in archaeal translation initiation.

In eukaryotes and in archaea late steps of translation initiation involve the two initiation factors e/aIF5B and e/aIF1A. In eukaryotes, the role of eIF5B in ribosomal subunit joining is established and structural data showing eIF5B bound to the full ribosome were obtained. To achieve its function, eIF5B collaborates with eIF1A. However, structural data illustrating how these two factors interact on the small ribosomal subunit have long been awaited. The role of the archaeal counterparts, aIF5B and aIF1A, remains to be extensively addressed. Here, we study the late steps of Pyrococcus abyssi translation initiation. Using in vitro reconstituted initiation complexes and light scattering, we show that aIF5B bound to GTP accelerates subunit joining without the need for GTP hydrolysis. We report the crystallographic structures of aIF5B bound to GDP and GTP and analyze domain movements associated to these two nucleotide states. Finally, we present the cryo-EM structure of an initiation complex containing 30S bound to mRNA, Met-tRNAiMet, aIF5B and aIF1A at 2.7 Å resolution. Structural data shows how archaeal 5B and 1A factors cooperate to induce a conformation of the initiator tRNA favorable to subunit joining. Archaeal and eukaryotic features of late steps of translation initiation are discussed.

Mechanism of mitoribosomal small subunit biogenesis and preinitiation.

Mitoribosomes are essential for the synthesis and maintenance of bioenergetic proteins. Here we use cryo-electron microscopy to determine a series of the small mitoribosomal subunit (SSU) intermediates in complex with auxiliary factors, revealing a sequential assembly mechanism. The methyltransferase TFB1M binds to partially unfolded rRNA h45 that is promoted by RBFA, while the mRNA channel is blocked. This enables binding of METTL15 that promotes further rRNA maturation and a large conformational change of RBFA. The new conformation allows initiation factor mtIF3 to already occupy the subunit interface during the assembly. Finally, the mitochondria-specific ribosomal protein mS37 (ref. 1) outcompetes RBFA to complete the assembly with the SSU-mS37-mtIF3 complex2 that proceeds towards mtIF2 binding and translation initiation. Our results explain how the action of step-specific factors modulate the dynamic assembly of the SSU, and adaptation of a unique protein, mS37, links the assembly to initiation to establish the catalytic human mitoribosome.