Alain Scaiola

Last Name

Scaiola

First Name

Alain

ORCID

0000-0003-3233-3910

Organisational unit

03556 - Ban, Nenad / Ban, Nenad

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Publications 1 - 10 of 14

Mechanism of cotranslational protein N-myristoylation in human cells
Gamerdinger, Martin; Echeverria, Blanca; Lentzsch, Alfred M.; et al. (2025)
Molecular Cell
N-myristoyltransferases (NMTs) cotranslationally transfer the fatty acid myristic acid to the N terminus of newly synthesized proteins, regulating their function and cellular localization. These enzymes are important drug targets for the treatment of cancer and viral infections. N-myristoylation of nascent proteins occurs specifically on N-terminal glycine residues after the excision of the initiator methionine by methionine aminopeptidases (METAPs). How NMTs interact with ribosomes and gain timely and specific access to their substrates remains unknown. Here, we show that human NMT1 exchanges with METAP1 at the ribosomal tunnel exit to form an active cotranslational complex together with the nascent polypeptide-associated complex (NAC). NMT1 binding is sequence selective and specifically triggered by methionine excision, which exposes the N-myristoylation motif in the nascent chain. The revealed mode of interaction of NMT1 with NAC and the methionine-cleaved nascent protein elucidates how a specific subset of proteins can be efficiently N-myristoylated in human cells.
Mitoribosomal small subunit biogenesis in trypanosomes involves an extensive assembly machinery
Saurer, Martin; Ramrath, David J.F.; Niemann, Moritz; et al. (2019)
Science
Mitochondrial ribosomes (mitoribosomes) are large ribonucleoprotein complexes that synthesize proteins encoded by the mitochondrial genome. An extensive cellular machinery responsible for ribosome assembly has been described only for eukaryotic cytosolic ribosomes. Here we report that the assembly of the small mitoribosomal subunit in Trypanosoma brucei involves a large number of factors and proceeds through the formation of assembly intermediates, which we analyzed by using cryo–electron microscopy. One of them is a 4-megadalton complex, referred to as the small subunit assemblosome, in which we identified 34 factors that interact with immature ribosomal RNA (rRNA) and recognize its functionally important regions. The assembly proceeds through large-scale conformational changes in rRNA coupled with successive incorporation of mitoribosomal proteins, providing an example for the complexity of the ribosomal assembly process in mitochondria.
Molecular basis of translation termination at noncanonical stop codons in human mitochondria
Saurer, Martin; Leibundgut, Marc; Nadimpalli, Hima Priyanka; et al. (2023)
Science
The genetic code that specifies the identity of amino acids incorporated into proteins during protein synthesis is almost universally conserved. Mitochondrial genomes feature deviations from the standard genetic code, including the reassignment of two arginine codons to stop codons. The protein required for translation termination at these noncanonical stop codons to release the newly synthesized polypeptides is not currently known. In this study, we used gene editing and ribosomal profiling in combination with cryo-electron microscopy to establish that mitochondrial release factor 1 (mtRF1) detects noncanonical stop codons in human mitochondria by a previously unknown mechanism of codon recognition. We discovered that binding of mtRF1 to the decoding center of the ribosome stabilizes a highly unusual conformation in the messenger RNA in which the ribosomal RNA participates in specific recognition of the noncanonical stop codons.
Structural basis of ribosomal frameshifting during translation of the SARS-CoV-2 RNA genome
Bhatt, Pramod R.; Scaiola, Alain; Loughran, Gary; et al. (2021)
Science
Programmed ribosomal frameshifting is a key event during translation of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA genome that allows synthesis of the viral RNA-dependent RNA polymerase and downstream proteins. Here, we present the cryo–electron microscopy structure of a translating mammalian ribosome primed for frameshifting on the viral RNA. The viral RNA adopts a pseudoknot structure that lodges at the entry to the ribosomal messenger RNA (mRNA) channel to generate tension in the mRNA and promote frameshifting, whereas the nascent viral polyprotein forms distinct interactions with the ribosomal tunnel. Biochemical experiments validate the structural observations and reveal mechanistic and regulatory features that influence frameshifting efficiency. Finally, we compare compounds previously shown to reduce frameshifting with respect to their ability to inhibit SARS-CoV-2 replication, establishing coronavirus frameshifting as a target for antiviral intervention.
Structural basis of translation inhibition by cadazolid, a novel quinoxolidinone antibiotic
Scaiola, Alain; Leibundgut, Marc; Boehringer, Daniel; et al. (2019)
Scientific Reports
Oxazolidinones are synthetic antibiotics used for treatment of infections caused by Gram-positive bacteria. They target the bacterial protein synthesis machinery by binding to the peptidyl transferase centre (PTC) of the ribosome and interfering with the peptidyl transferase reaction. Cadazolid is the first member of quinoxolidinone antibiotics, which are characterized by combining the pharmacophores of oxazolidinones and fluoroquinolones, and it is evaluated for treatment of Clostridium difficile gastrointestinal infections that frequently occur in hospitalized patients. In vitro protein synthesis inhibition by cadazolid was shown in Escherichia coli and Staphylococcus aureus, including an isolate resistant against linezolid, the prototypical oxazolidinone antibiotic. To better understand the mechanism of inhibition, we determined a 3.0 Å cryo-electron microscopy structure of cadazolid bound to the E. coli ribosome in complex with mRNA and initiator tRNA. Here we show that cadazolid binds with its oxazolidinone moiety in a binding pocket in close vicinity of the PTC as observed previously for linezolid, and that it extends its unique fluoroquinolone moiety towards the A-site of the PTC. In this position, the drug inhibits protein synthesis by interfering with the binding of tRNA to the A-site, suggesting that its chemical features also can enable the inhibition of linezolid-resistant strains.
NAC guides a ribosomal multienzyme complex for nascent protein processing
Lentzsch, Alfred M.; Yudin, Denis; Gamerdinger, Martin; et al. (2024)
Nature
Approximately 40% of the mammalian proteome undergoes N-terminal methionine excision and acetylation, mediated sequentially by methionine aminopeptidase (MetAP) and N-acetyltransferase A (NatA), respectively(1). Both modifications are strictly cotranslational and essential in higher eukaryotic organisms(1). The interaction, activity and regulation of these enzymes on translating ribosomes are poorly understood. Here we perform biochemical, structural and in vivo studies to demonstrate that the nascent polypeptide-associated complex(2,3) (NAC) orchestrates the action of these enzymes. NAC assembles a multienzyme complex with MetAP1 and NatA early during translation and pre-positions the active sites of both enzymes for timely sequential processing of the nascent protein. NAC further releases the inhibitory interactions from the NatA regulatory protein huntingtin yeast two-hybrid protein K-4,K-5 (HYPK) to activate NatA on the ribosome, enforcing cotranslational N-terminal acetylation. Our results provide a mechanistic model for the cotranslational processing of proteins in eukaryotic cells.
Mechanism of signal sequence handover from NAC to SRP on ribosomes during ER-protein targeting
Jomaa, Ahmad; Gamerdinger, Martin; Hsieh, Hao-Hsuan; et al. (2022)
Science
The nascent polypeptide-associated complex (NAC) interacts with newly synthesized proteins at the ribosomal tunnel exit and competes with the signal recognition particle (SRP) to prevent mistargeting of cytosolic and mitochondrial polypeptides to the endoplasmic reticulum (ER). How NAC antagonizes SRP and how this is overcome by ER targeting signals are unknown. Here, we found that NAC uses two domains with opposing effects to control SRP access. The core globular domain prevented SRP from binding to signal-less ribosomes, whereas a flexibly attached domain transiently captured SRP to permit scanning of nascent chains. The emergence of an ER-targeting signal destabilized NAC's globular domain and facilitated SRP access to the nascent chain. These findings elucidate how NAC hands over the signal sequence to SRP and imparts specificity of protein localization.
Stepwise maturation of the peptidyl transferase region of human mitoribosomes
Lenarcic, Tea; Jaskolowski, Mateusz; Leibundgut, Marc; et al. (2021)
Nature Communications
Mitochondrial ribosomes are specialized for the synthesis of membrane proteins responsible for oxidative phosphorylation. Mammalian mitoribosomes have diverged considerably from the ancestral bacterial ribosomes and feature dramatically reduced ribosomal RNAs. The structural basis of the mammalian mitochondrial ribosome assembly is currently not well understood. Here we present eight distinct assembly intermediates of the human large mitoribosomal subunit involving seven assembly factors. We discover that the NSUN4-MTERF4 dimer plays a critical role in the process by stabilizing the 16S rRNA in a conformation that exposes the functionally important regions of rRNA for modification by the MRM2 methyltransferase and quality control interactions with the conserved mitochondrial GTPase MTG2 that contacts the sarcin-ricin loop and the immature active site. The successive action of these factors leads to the formation of the peptidyl transferase active site of the mitoribosome and the folding of the surrounding rRNA regions responsible for interactions with tRNAs and the small ribosomal subunit.
NAC controls cotranslational N-terminal methionine excision in eukaryotes
Gamerdinger, Martin; Jia, Min; Schloemer, Renate; et al. (2023)
Science
N-terminal methionine excision from newly synthesized proteins, catalyzed cotranslationally by methionine aminopeptidases (METAPs), is an essential and universally conserved process that plays a key role in cell homeostasis and protein biogenesis. However, how METAPs interact with ribosomes and how their cleavage specificity is ensured is unknown. We discovered that in eukaryotes the nascent polypeptide-associated complex (NAC) controls ribosome binding of METAP1. NAC recruits METAP1 using a long, flexible tail and provides a platform for the formation of an active methionine excision complex at the ribosomal tunnel exit. This mode of interaction ensures the efficient excision of methionine from cytosolic proteins, whereas proteins targeted to the endoplasmic reticulum are spared. Our results suggest a broader mechanism for how access of protein biogenesis factors to translating ribosomes is controlled.
The 3.2-Å resolution structure of human mTORC2
Scaiola, Alain; Mangia, Francesca; Imseng, Stefan; et al. (2020)
Science Advances
The protein kinase mammalian target of rapamycin (mTOR) is the central regulator of cell growth. Aberrant mTOR signaling is linked to cancer, diabetes, and neurological disorders. mTOR exerts its functions in two distinct multiprotein complexes, mTORC1 and mTORC2. Here, we report a 3.2-Å resolution cryo-EM reconstruction of mTORC2. It reveals entangled folds of the defining Rictor and the substrate-binding SIN1 subunits, identifies the carboxyl-terminal domain of Rictor as the source of the rapamycin insensitivity of mTORC2, and resolves mechanisms for mTORC2 regulation by complex destabilization. Two previously uncharacterized small-molecule binding sites are visualized, an inositol hexakisphosphate (InsP6) pocket in mTOR and an mTORC2-specific nucleotide binding site in Rictor, which also forms a zinc finger. Structural and biochemical analyses suggest that InsP6 and nucleotide binding do not control mTORC2 activity directly but rather have roles in folding or ternary interactions. These insights provide a firm basis for studying mTORC2 signaling and for developing mTORC2-specific inhibitors.

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Alain Scaiola

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