NAC controls cotranslational N-terminal methionine excision in eukaryotes

Protein targeting to the ER membrane: multiple pathways and shared machinery

The endoplasmic reticulum (ER) serves as a central hub for protein production and sorting in eukaryotic cells, processing approximately one-third of the cellular proteome. Protein targeting to the ER occurs through multiple pathways that operate both during and independent of translation. The classical translation-dependent pathway, mediated by cytosolic factors like signal recognition particle, recognizes signal peptides or transmembrane helices in nascent proteins, while translation-independent mechanisms utilize RNA-based targeting through specific sequence elements and RNA-binding proteins. At the core of these processes lies the Sec61 complex, which undergoes dynamic conformational changes and coordinates with numerous accessory factors to facilitate protein translocation and membrane insertion across and into the endoplasmic reticulum membrane. This review focuses on the molecular mechanisms of protein targeting to the ER, from the initial recognition of targeting signals to the dynamics of the translocation machinery, highlighting recent discoveries that have revealed unprecedented complexity in these cellular trafficking pathways.

The nascent polypeptide-associated complex (NAC) as regulatory hub on ribosomes

The correct synthesis of new proteins is essential for maintaining a functional proteome and cell viability. This process is tightly regulated, with ribosomes and associated protein biogenesis factors ensuring proper protein production, modification, and targeting. In eukaryotes, the conserved nascent polypeptide-associated complex (NAC) plays a central role in coordinating early protein processing by regulating the ribosome access of multiple protein biogenesis factors. NAC recruits modifying enzymes to the ribosomal exit site to process the N-terminus of nascent proteins and directs secretory proteins into the SRP-mediated targeting pathway. In this review we will focus on these pathways, which are critical for proper protein production, and summarize recent advances in understanding the cotranslational functions and mechanisms of NAC in higher eukaryotes.

Protein N-terminal modifications: molecular machineries and biological implications

The majority of eukaryotic proteins undergo N-terminal (Nt) modifications facilitated by various enzymes. These enzymes, which target the initial amino acid of a polypeptide in a sequence-dependent manner, encompass peptidases, transferases, cysteine oxygenases, and ligases. Nt modifications - such as acetylation, fatty acylations, methylation, arginylation, and oxidation - enhance proteome complexity and regulate protein targeting, stability, and complex formation. Modifications at protein N termini are thereby core components of a large number of biological processes, including cell signaling and motility, autophagy regulation, and plant and animal oxygen sensing. Dysregulation of Nt-modifying enzymes is implicated in several human diseases. In this feature review we provide an overview of the various protein Nt modifications occurring either co- or post-translationally, the enzymes involved, and the biological impact.

Mechanistic Insights into Protein Biogenesis and Maturation on the Ribosome

The ribosome is a major cellular machine that converts genetic information into biological function. Emerging data show that the ribosome is not only a protein synthesis machine, but also participates in the maturation of the nascent protein into properly folded and active molecules. The ribosome surface near the opening of the polypeptide exit tunnel can interact directly with the newly synthesized proteins and, more importantly, provides a platform where numerous protein biogenesis factors assemble, gain access to the nascent chain, and direct them into diverse biogenesis pathways. In this article, we review the current understanding of cotranslational protein maturation pathways, with an emphasis on systems in which biochemical studies provided a high-resolution molecular understanding and yielded generalizable mechanistic principles.

A ribosome-associating chaperone mediates GTP-driven vectorial folding of nascent eEF1A

Eukaryotic translation elongation factor 1A (eEF1A) is a highly abundant, multi-domain GTPase. Post-translational steps essential for eEF1A biogenesis are carried out by bespoke chaperones but co-translational mechanisms tailored to eEF1A folding remain unexplored. Here, we use AlphaPulldown to identify Ypl225w (also known as Chp1, Chaperone 1 for eEF1A) as a conserved yeast protein predicted to stabilize the N-terminal, GTP-binding (G) domain of eEF1A against its misfolding propensity, as predicted by computational simulations and validated by microscopy analysis of ypl225wΔ cells. Proteomics and biochemical reconstitution reveal that Ypl225w functions as a co-translational chaperone by forming dual interactions with the eEF1A G domain nascent chain and the UBA domain of ribosome-bound nascent polypeptide-associated complex (NAC). Lastly, we show that Ypl225w primes eEF1A nascent chains for binding to GTP as part of a folding mechanism tightly coupled to chaperone recycling. Our work shows that an ATP-independent chaperone can drive vectorial folding of nascent chains by co-opting G protein nucleotide binding.

Bacterial Metallostasis: Metal Sensing, Metalloproteome Remodeling, and Metal Trafficking

Transition metals function as structural and catalytic cofactors for a large diversity of proteins and enzymes that collectively comprise the metalloproteome. Metallostasis considers all cellular processes, notably metal sensing, metalloproteome remodeling, and trafficking (or allocation) of metals that collectively ensure the functional integrity and adaptability of the metalloproteome. Bacteria employ both protein and RNA-based mechanisms that sense intracellular transition metal bioavailability and orchestrate systems-level outputs that maintain metallostasis. In this review, we contextualize metallostasis by briefly discussing the metalloproteome and specialized roles that metals play in biology. We then offer a comprehensive perspective on the diversity of metalloregulatory proteins and metal-sensing riboswitches, defining general principles within each sensor superfamily that capture how specificity is encoded in the sequence, and how selectivity can be leveraged in downstream synthetic biology and biotechnology applications. This is followed by a discussion of recent work that highlights selected metalloregulatory outputs, including metalloproteome remodeling and metal allocation by metallochaperones to both client proteins and compartments. We close by briefly discussing places where more work is needed to fill in gaps in our understanding of metallostasis.

Functional similarities and differences among subunits of the nascent polypeptide-associated complex (NAC) of Saccharomyces cerevisiae

Protein factors bind ribosomes near the tunnel exit, facilitating protein trafficking and folding. In eukaryotes, the heterodimeric nascent polypeptide-associated complex (NAC) is the most abundant-equimolar to ribosomes. Saccharomyces cerevisiae has a minor β-type subunit (Nacβ2) in addition to abundant Nacβ1, and therefore two NAC heterodimers, α/β1 and α/β12. The additional beta NAC gene arose at the time of the whole genome duplication that occurred in the S. cerevisiae lineage. Nacβ2 has been implicated in regulating the fate of messenger RNA encoding ribosomal protein Rpl4 during translation via its interaction with the Caf130 subunit of the regulatory CCR4-Not complex. We found that Nacβ2 residues just C-terminal to the globular domain are required for its interaction with Caf130 and its negative effect on the growth of cells lacking Acl4, the specialized chaperone for Rpl4. Substitution of these Nacβ2 residues at homologous positions in Nacβ1 results in a chimeric protein that interacts with Caf130 and slows the growth of ∆acl4 cells lacking Nacβ2. Furthermore, alteration of residues in the N-terminus of Nacβ2 or chimeric Nacβ1 previously shown to affect ribosome binding overcomes the growth defect of ∆acl4. Our results are consistent with a model in which Nacβ2's ribosome association per se or its precise positioning is necessary for productive recruitment of CCR4-Not via its interaction with the Caf130 subunit to drive Rpl4 messenger RNA degradation.

High-resolution reconstruction of a C. elegans ribosome sheds light on evolutionary dynamics and tissue specificity

Caenorhabditis elegans is an important model organism for human health and disease, with foundational contributions to the understanding of gene expression and tissue patterning in animals. An invaluable tool in modern gene expression research is the presence of a high-resolution ribosome structure, though no such structure exists for C. elegans Here, we present a high-resolution single-particle cryogenic electron microscopy (cryo-EM) reconstruction and molecular model of a C. elegans ribosome, revealing a significantly streamlined animal ribosome. Many facets of ribosome structure are conserved in C. elegans, including overall ribosomal architecture and the mechanism of cycloheximide, whereas other facets, such as expansion segments and eL28, are rapidly evolving. We identify uL5 and uL23 as two instances of tissue-specific ribosomal protein paralog expression conserved in Caenorhabditis, suggesting that C. elegans ribosomes vary across tissues. The C. elegans ribosome structure will provide a basis for future structural, biochemical, and genetic studies of translation in this important animal system.

N-degron pathways

An N-degron is a degradation signal whose main determinant is a "destabilizing" N-terminal residue of a protein. Specific N-degrons, discovered in 1986, were the first identified degradation signals in short-lived intracellular proteins. These N-degrons are recognized by a ubiquitin-dependent proteolytic system called the Arg/N-degron pathway. Although bacteria lack the ubiquitin system, they also have N-degron pathways. Studies after 1986 have shown that all 20 amino acids of the genetic code can act, in specific sequence contexts, as destabilizing N-terminal residues. Eukaryotic proteins are targeted for the conditional or constitutive degradation by at least five N-degron systems that differ both functionally and mechanistically: the Arg/N-degron pathway, the Ac/N-degron pathway, the Pro/N-degron pathway, the fMet/N-degron pathway, and the newly named, in this perspective, GASTC/N-degron pathway (GASTC = Gly, Ala, Ser, Thr, Cys). I discuss these systems and the expanded terminology that now encompasses the entire gamut of known N-degron pathways.

RAPIDASH: Tag-free enrichment of ribosome-associated proteins reveals composition dynamics in embryonic tissue, cancer cells, and macrophages

Ribosomes are emerging as direct regulators of gene expression, with ribosome-associated proteins (RAPs) allowing ribosomes to modulate translation. Nevertheless, a lack of technologies to enrich RAPs across sample types has prevented systematic analysis of RAP identities, dynamics, and functions. We have developed a label-free methodology called RAPIDASH to enrich ribosomes and RAPs from any sample. We applied RAPIDASH to mouse embryonic tissues and identified hundreds of potential RAPs, including Dhx30 and Llph, two forebrain RAPs important for neurodevelopment. We identified a critical role of LLPH in neural development linked to the translation of genes with long coding sequences. In addition, we showed that RAPIDASH can identify ribosome changes in cancer cells. Finally, we characterized ribosome composition remodeling during immune cell activation and observed extensive changes post-stimulation. RAPIDASH has therefore enabled the discovery of RAPs in multiple cell types, tissues, and stimuli and is adaptable to characterize ribosome remodeling in several contexts.

Multi-protein assemblies orchestrate co-translational enzymatic processing on the human ribosome

Nascent chains undergo co-translational enzymatic processing as soon as their N-terminus becomes accessible at the ribosomal polypeptide tunnel exit (PTE). In eukaryotes, N-terminal methionine excision (NME) by Methionine Aminopeptidases (MAP1 and MAP2), and N-terminal acetylation (NTA) by N-Acetyl-Transferase A (NatA), is the most common combination of subsequent modifications carried out on the 80S ribosome. How these enzymatic processes are coordinated in the context of a rapidly translating ribosome has remained elusive. Here, we report two cryo-EM structures of multi-enzyme complexes assembled on vacant human 80S ribosomes, indicating two routes for NME-NTA. Both assemblies form on the 80S independent of nascent chain substrates. Irrespective of the route, NatA occupies a non-intrusive 'distal' binding site on the ribosome which does not interfere with MAP1 or MAP2 binding nor with most other ribosome-associated factors (RAFs). NatA can partake in a coordinated, dynamic assembly with MAP1 through the hydra-like chaperoning function of the abundant Nascent Polypeptide-Associated Complex (NAC). In contrast to MAP1, MAP2 completely covers the PTE and is thus incompatible with NAC and MAP1 recruitment. Together, our data provide the structural framework for the coordinated orchestration of NME and NTA in protein biogenesis.

NAC guides a ribosomal multienzyme complex for nascent protein processing

Approximately 40% of the mammalian proteome undergoes N-terminal methionine excision and acetylation, mediated sequentially by methionine aminopeptidase (MetAP) and N-acetyltransferase A (NatA), respectively1. Both modifications are strictly cotranslational and essential in higher eukaryotic organisms1. 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 complex2,3 (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 K4,5 (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.

Cytoplasmic ribosomes on mitochondria alter the local membrane environment for protein import

Most of the mitochondria proteome is nuclear-encoded, synthesized by cytoplasmic ribosomes, and targeted to mitochondria post-translationally. However, a subset of mitochondrial-targeted proteins is imported co-translationally, although the molecular mechanisms governing this process remain unclear. We employ cellular cryo-electron tomography to visualize interactions between cytoplasmic ribosomes and mitochondria in Saccharomyces cerevisiae. We use surface morphometrics tools to identify a subset of ribosomes optimally oriented on mitochondrial membranes for protein import. This allows us to establish the first subtomogram average structure of a cytoplasmic ribosome on the surface of the mitochondria in the native cellular context, which showed three distinct connections with the outer mitochondrial membrane surrounding the peptide exit tunnel. Further, this analysis demonstrated that cytoplasmic ribosomes primed for mitochondrial protein import cluster on the outer mitochondrial membrane at sites of local constrictions of the outer and inner mitochondrial membrane. Overall, our study reveals the architecture and the spatial organization of cytoplasmic ribosomes at the mitochondrial surface, providing a native cellular context to define the mechanisms that mediate efficient mitochondrial co-translational protein import.

Improving dermal fibroblast-to-epidermis communications and aging wound repair through extracellular vesicle-mediated delivery of Gstm2 mRNA

Skin aging is characterized by the disruption of skin homeostasis and impaired skin injury repair. Treatment of aging skin has long been limited by the unclear intervention targets and delivery techniques. Engineering extracellular vesicles (EVs) as an upgraded version of natural EVs holds great potential in regenerative medicine. In this study, we found that the expression of the critical antioxidant and detoxification gene Gstm2 was significantly reduced in aging skin. Thus, we constructed the skin primary fibroblasts-derived EVs encapsulating Gstm2 mRNA (EVsGstm2), and found that EVsGstm2 could significantly improve skin homeostasis and accelerate wound healing in aged mice. Mechanistically, we found that EVsGstm2 alleviated oxidative stress damage of aging dermal fibroblasts by modulating mitochondrial oxidative phosphorylation, and promoted dermal fibroblasts to regulate skin epidermal cell function by paracrine secretion of Nascent Polypeptide-Associated Complex Alpha subunit (NACA). Furthermore, we confirmed that NACA is a novel skin epidermal cell protective molecule that regulates skin epidermal cell turnover through the ROS-ERK-ETS-Cyclin D pathway. Our findings demonstrate the feasibility and efficacy of EVs-mediated delivery of Gstm2 for aged skin treatment and unveil novel roles of GSTM2 and NACA for improving aging skin.

Navigating the complexities of multi-domain protein folding

Proteome complexity has expanded tremendously over evolutionary time, enabling biological diversification. Much of this complexity is achieved by combining a limited set of structural units into long polypeptides. This widely used evolutionary strategy poses challenges for folding of the resulting multi-domain proteins. As a consequence, their folding differs from that of small single-domain proteins, which generally fold quickly and reversibly. Co-translational processes and chaperone interactions are important aspects of multi-domain protein folding. In this review, we discuss some of the recent experimental progress toward understanding these processes.

NAC and Zuotin/Hsp70 chaperone systems coexist at the ribosome tunnel exit in vivo

The area surrounding the tunnel exit of the 60S ribosomal subunit is a hub for proteins involved in maturation and folding of emerging nascent polypeptide chains. How different factors vie for positioning at the tunnel exit in the complex cellular environment is not well understood. We used in vivo site-specific cross-linking to approach this question, focusing on two abundant factors-the nascent chain-associated complex (NAC) and the Hsp70 chaperone system that includes the J-domain protein co-chaperone Zuotin. We found that NAC and Zuotin can cross-link to each other at the ribosome, even when translation initiation is inhibited. Positions yielding NAC-Zuotin cross-links indicate that when both are present the central globular domain of NAC is modestly shifted from the mutually exclusive position observed in cryogenic electron microscopy analysis. Cross-linking results also suggest that, even in NAC's presence, Hsp70 can situate in a manner conducive for productive nascent chain interaction-with the peptide binding site at the tunnel exit and the J-domain of Zuotin appropriately positioned to drive stabilization of nascent chain binding. Overall, our results are consistent with the idea that, in vivo, the NAC and Hsp70 systems can productively position on the ribosome simultaneously.

Triaging of α-helical proteins to the mitochondrial outer membrane by distinct chaperone machinery based on substrate topology

Mitochondrial outer membrane ⍺-helical proteins play critical roles in mitochondrial-cytoplasmic communication, but the rules governing the targeting and insertion of these biophysically diverse proteins remain unknown. Here, we first defined the complement of required mammalian biogenesis machinery through genome-wide CRISPRi screens using topologically distinct membrane proteins. Systematic analysis of nine identified factors across 21 diverse ⍺-helical substrates reveals that these components are organized into distinct targeting pathways that act on substrates based on their topology. NAC is required for the efficient targeting of polytopic proteins, whereas signal-anchored proteins require TTC1, a cytosolic chaperone that physically engages substrates. Biochemical and mutational studies reveal that TTC1 employs a conserved TPR domain and a hydrophobic groove in its C-terminal domain to support substrate solubilization and insertion into mitochondria. Thus, the targeting of diverse mitochondrial membrane proteins is achieved through topological triaging in the cytosol using principles with similarities to ER membrane protein biogenesis systems.

Cotranslational sorting and processing of newly synthesized proteins in eukaryotes

Ribosomes interact with a variety of different protein biogenesis factors that guide newly synthesized proteins to their native 3D shapes and cellular localization. Depending on the type of translated substrate, a distinct set of cotranslational factors must interact with the ribosome in a timely and coordinated manner to ensure proper protein biogenesis. While cytonuclear proteins require cotranslational maturation and folding factors, secretory proteins must be maintained in an unfolded state and processed cotranslationally by transport and membrane translocation factors. Here we explore the specific cotranslational processing steps for cytonuclear, secretory, and membrane proteins in eukaryotes and then discuss how the nascent polypeptide-associated complex (NAC) cotranslationally sorts these proteins into the correct protein biogenesis pathway.

Methionine aminopeptidase 2 and its autoproteolysis product have different binding sites on the ribosome

Excision of the initiator methionine is among the first co-translational processes that occur at the ribosome. While this crucial step in protein maturation is executed by two types of methionine aminopeptidases in eukaryotes (MAP1 and MAP2), additional roles in disease and translational regulation have drawn more attention to MAP2. Here, we report several cryo-EM structures of human and fungal MAP2 at the 80S ribosome. Irrespective of nascent chains, MAP2 can occupy the tunnel exit. On nascent chain displaying ribosomes, the MAP2-80S interaction is highly dynamic and the MAP2-specific N-terminal extension engages in stabilizing interactions with the long rRNA expansion segment ES27L. Loss of this extension by autoproteolytic cleavage impedes interactions at the tunnel, while promoting MAP2 to enter the ribosomal A-site, where it engages with crucial functional centers of translation. These findings reveal that proteolytic remodeling of MAP2 severely affects ribosome binding, and set the stage for targeted functional studies.

Protein quality control: from molecular mechanisms to therapeutic intervention-EMBO workshop, May 21-26 2023, Srebreno, Croatia

Protein quality control pathways ensure a functional proteome and rely on a complex proteostasis network (PN) that is composed of molecular chaperones and proteases. Failures in the PN can lead to a broad spectrum of diseases, including neurodegenerative disorders like Alzheimer's, Parkinson's, and a range of motor neuron diseases. The EMBO workshop "Protein quality control: from molecular mechanisms to therapeutic intervention" covered all aspects of protein quality control from underlying molecular mechanisms of chaperones and proteases to stress signaling pathways and medical implications. This report summarizes the workshop and highlights selected presentations.

Triaging of α-helical proteins to the mitochondrial outer membrane by distinct chaperone machinery based on substrate topology

Mitochondrial outer membrane α-helical proteins play critical roles in mitochondrial-cytoplasmic communication, but the rules governing the targeting and insertion of these biophysically diverse substrates remain unknown. Here, we first defined the complement of required mammalian biogenesis machinery through genome-wide CRISPRi screens using topologically distinct membrane proteins. Systematic analysis of nine identified factors across 21 diverse α-helical substrates reveals that these components are organized into distinct targeting pathways which act on substrates based on their topology. NAC is required for efficient targeting of polytopic proteins whereas signal-anchored proteins require TTC1, a novel cytosolic chaperone which physically engages substrates. Biochemical and mutational studies reveal that TTC1 employs a conserved TPR domain and a hydrophobic groove in its C-terminal domain to support substrate solubilization and insertion into mitochondria. Thus, targeting of diverse mitochondrial membrane proteins is achieved through topological triaging in the cytosol using principles with similarities to ER membrane protein biogenesis systems.