Concerted action of the ribosome and the associated chaperone trigger factor confines nascent polypeptide folding

Thermodynamic profiles for cotranslational trigger factor substrate recognition

Molecular chaperones are central to the maintenance of proteostasis in living cells. A key member of this protein family is trigger factor (TF), which acts throughout the protein life cycle and has a ubiquitous role as the first chaperone encountered by proteins during synthesis. However, our understanding of how TF achieves favorable interactions with such a diverse substrate base remains limited. Here, we use microfluidics to reveal the thermodynamic determinants of this process. We find that TF binding to empty 70S ribosomes is enthalpy-driven, with micromolar affinity, while nanomolar affinity is achieved through a favorable entropic contribution for both intrinsically disordered and folding-competent nascent chains. These findings suggest a general mechanism for cotranslational TF function, which relies on occupation of the exposed TF-substrate binding groove rather than specific complementarity between chaperone and nascent chain. These insights add to our wider understanding of how proteins can achieve broad substrate specificity.

Mechanism of chaperone coordination during cotranslational protein folding in bacteria

Protein folding is assisted by molecular chaperones that bind nascent polypeptides during mRNA translation. Several structurally distinct classes of chaperones promote de novo folding, suggesting that their activities are coordinated at the ribosome. We used biochemical reconstitution and structural proteomics to explore the molecular basis for cotranslational chaperone action in bacteria. We found that chaperone binding is disfavored close to the ribosome, allowing folding to precede chaperone recruitment. Trigger factor recognizes compact folding intermediates that expose an extensive unfolded surface, and dictates DnaJ access to nascent chains. DnaJ uses a large surface to bind structurally diverse intermediates and recruits DnaK to sequence-diverse solvent-accessible sites. Neither Trigger factor, DnaJ, nor DnaK destabilize cotranslational folding intermediates. Instead, the chaperones collaborate to protect incipient structure in the nascent polypeptide well beyond the ribosome exit tunnel. Our findings show how the chaperone network selects and modulates cotranslational folding intermediates.

Resolving chaperone-assisted protein folding on the ribosome at the peptide level

Protein folding in vivo begins during synthesis on the ribosome and is modulated by molecular chaperones that engage the nascent polypeptide. How these features of protein biogenesis influence the maturation pathway of nascent proteins is incompletely understood. Here, we use hydrogen-deuterium exchange mass spectrometry to define, at peptide resolution, the cotranslational chaperone-assisted folding pathway of Escherichia coli dihydrofolate reductase. The nascent polypeptide folds along an unanticipated pathway through structured intermediates not populated during refolding from denaturant. Association with the ribosome allows these intermediates to form, as otherwise destabilizing carboxy-terminal sequences remain confined in the ribosome exit tunnel. Trigger factor binds partially folded states without disrupting their structure, and the nascent chain is poised to complete folding immediately upon emergence of the C terminus from the exit tunnel. By mapping interactions between the nascent chain and ribosomal proteins, we trace the path of the emerging polypeptide during synthesis. Our work reveals new mechanisms by which cellular factors shape the conformational search for the native state.

AP profiling resolves co-translational folding pathway and chaperone interactions in vivo

Natural proteins have evolved to fold robustly along specific pathways. Folding begins during synthesis, guided by interactions of the nascent protein with the ribosome and molecular chaperones. However, the timing and progression of co-translational folding remain largely elusive, in part because the process is difficult to measure in the natural environment of the cytosol. We developed a high-throughput method to quantify co-translational folding in live cells that we term Arrest Peptide profiling (AP profiling). We employed AP profiling to delineate co-translational folding for a set of GTPase domains with very similar structures, defining how topology shapes folding pathways. Genetic ablation of major nascent chain-binding chaperones resulted in localized folding changes that suggest how functional redundancies among chaperones are achieved by distinct interactions with the nascent protein. Collectively, our studies provide a window into cellular folding pathways of complex proteins and pave the way for systematic studies on nascent protein folding at unprecedented resolution and throughput.

Human Small Heat Shock Protein B8 Inhibits Protein Aggregation without Affecting the Native Folding Process

Small Heat Shock Proteins (sHSPs) are key components of our Protein Quality Control system and are thought to act as reservoirs that neutralize irreversible protein aggregation. Yet, sHSPs can also act as sequestrases, promoting protein sequestration into aggregates, thus challenging our understanding of their exact mechanisms of action. Here, we employ optical tweezers to explore the mechanisms of action of the human small heat shock protein HSPB8 and its pathogenic mutant K141E, which is associated with neuromuscular disease. Through single-molecule manipulation experiments, we studied how HSPB8 and its K141E mutant affect the refolding and aggregation processes of the maltose binding protein. Our data show that HSPB8 selectively suppresses protein aggregation without affecting the native folding process. This anti-aggregation mechanism is distinct from previous models that rely on the stabilization of unfolded polypeptide chains or partially folded structures, as has been reported for other chaperones. Rather, it appears that HSPB8 selectively recognizes and binds to aggregated species formed at the early stages of aggregation, preventing them from growing into larger aggregated structures. Consistently, the K141E mutation specifically targets the affinity for aggregated structures without impacting native folding, and hence impairs its anti-aggregation activity.

Selective ribosome profiling reveals a role for SecB in the co-translational inner membrane protein biogenesis

The chaperone SecB has been implicated in de novo protein folding and translocation across the membrane, but it remains unclear which nascent polypeptides SecB binds, when during translation SecB acts, how SecB function is coordinated with other chaperones and targeting factors, and how polypeptide engagement contributes to protein biogenesis. Using selective ribosome profiling, we show that SecB binds many nascent cytoplasmic and translocated proteins generally late during translation and controlled by the chaperone trigger factor. Revealing an uncharted role in co-translational translocation, inner membrane proteins (IMPs) are the most prominent nascent SecB interactors. Unlike other substrates, IMPs are bound early during translation, following the membrane targeting by the signal recognition particle. SecB remains bound until translation is terminated, and contributes to membrane insertion. Our study establishes a role of SecB in the co-translational maturation of proteins from all cellular compartments and functionally implicates cytosolic chaperones in membrane protein biogenesis.

The biogenesis of β-lactamase enzymes

The discovery of penicillin by Alexander Fleming marked a new era for modern medicine, allowing not only the treatment of infectious diseases, but also the safe performance of life-saving interventions, like surgery and chemotherapy. Unfortunately, resistance against penicillin, as well as more complex β-lactam antibiotics, has rapidly emerged since the introduction of these drugs in the clinic, and is largely driven by a single type of extra-cytoplasmic proteins, hydrolytic enzymes called β-lactamases. While the structures, biochemistry and epidemiology of these resistance determinants have been extensively characterized, their biogenesis, a complex process including multiple steps and involving several fundamental biochemical pathways, is rarely discussed. In this review, we provide a comprehensive overview of the journey of β-lactamases, from the moment they exit the ribosomal channel until they reach their final cellular destination as folded and active enzymes.

Trigger factor both holds and folds its client proteins

ATP-independent chaperones like trigger factor are generally assumed to play passive roles in protein folding by acting as holding chaperones. Here we show that trigger factor plays a more active role. Consistent with a role as an aggregation inhibiting chaperone, we find that trigger factor rapidly binds to partially folded glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and prevents it from non-productive self-association by shielding oligomeric interfaces. In the traditional view of holding chaperone action, trigger factor would then be expected to transfer its client to a chaperone foldase system for complete folding. Unexpectedly, we noticed that GAPDH folds into a monomeric but otherwise rather native-like intermediate state while trigger factor-bound. Upon release from trigger factor, the mostly folded monomeric GAPDH rapidly self-associates into its native tetramer and acquires enzymatic activity without needing additional folding factors. The mechanism we propose here for trigger factor bridges the holding and folding activities of chaperone function.

ATP-Independent Chaperones

The folding of proteins into their native structure is crucial for the functioning of all biological processes. Molecular chaperones are guardians of the proteome that assist in protein folding and prevent the accumulation of aberrant protein conformations that can lead to proteotoxicity. ATP-independent chaperones do not require ATP to regulate their functional cycle. Although these chaperones have been traditionally regarded as passive holdases that merely prevent aggregation, recent work has shown that they can directly affect the folding energy landscape by tuning their affinity to various folding states of the client. This review focuses on emerging paradigms in the mechanism of action of ATP-independent chaperones and on the various modes of regulating client binding and release.

Biosynthetic Protein Folding and Molecular Chaperons

The problem of linear polypeptide chain folding into a unique tertiary structure is one of the fundamental scientific challenges. The process of folding cannot be fully understood without its biological context, especially for big multidomain and multisubunit proteins. The principal features of biosynthetic folding are co-translational folding of growing nascent polypeptide chains and involvement of molecular chaperones in the process. The review summarizes available data on the early events of nascent chain folding, as well as on later advanced steps, including formation of elements of native structure. The relationship between the non-uniformity of translation rate and folding of the growing polypeptide is discussed. The results of studies on the effect of biosynthetic folding features on the parameters of folding as a physical process, its kinetics and mechanisms, are presented. Current understanding and hypotheses on the relationship of biosynthetic folding with the fundamental physical parameters and current views on polypeptide folding in the context of energy landscapes are discussed.

Protein folding in vitro and in the cell: From a solitary journey to a team effort

Correct protein folding is essential for the health and function of living organisms. Yet, it is not well understood how unfolded proteins reach their native state and avoid aggregation, especially within the cellular milieu. Some proteins, especially small, single-domain and apparent two-state folders, successfully attain their native state upon dilution from denaturant. Yet, many more proteins undergo misfolding and aggregation during this process, in a concentration-dependent fashion. Once formed, native and aggregated states are often kinetically trapped relative to each other. Hence, the early stages of protein life are absolutely critical for proper kinetic channeling to the folded state and for long-term solubility and function. This review summarizes current knowledge on protein folding/aggregation mechanisms in buffered solution and within the bacterial cell, highlighting early stages. Remarkably, teamwork between nascent chain, ribosome, trigger factor and Hsp70 molecular chaperones enables all proteins to overcome aggregation propensities and reach a long-lived bioactive state.

Common sequence motifs of nascent chains engage the ribosome surface and trigger factor

Proteins are produced by ribosomes in the cell, and during this process, can begin to adopt their biologically active forms assisted by molecular chaperones such as trigger factor. This fundamental cellular mechanism is crucial to maintaining a functional proteome and avoiding deleterious misfolding. Here, we study how disordered nascent chains emerge from the ribosome exit tunnel, and find that interactions with the ribosome surface dominate their dynamics in vitro and in vivo. Moreover, we show that the types of amino acids that mediate such interactions are also those that recruit trigger factor. This lays the foundation to describe how nascent chains are handed over from the ribosome surface to chaperones during biosynthesis within the crowded cytosol.

In the cell, the conformations of nascent polypeptide chains during translation are modulated by both the ribosome and its associated molecular chaperone, trigger factor. The specific interactions that underlie these modulations, however, are still not known in detail. Here, we combine protein engineering, in-cell and in vitro NMR spectroscopy, and molecular dynamics simulations to explore how proteins interact with the ribosome during their biosynthesis before folding occurs. Our observations of α-synuclein nascent chains in living Escherichia coli cells reveal that ribosome surface interactions dictate the dynamics of emerging disordered polypeptides in the crowded cytosol. We show that specific basic and aromatic motifs drive such interactions and directly compete with trigger factor binding while biasing the direction of the nascent chain during its exit out of the tunnel. These results reveal a structural basis for the functional role of the ribosome as a scaffold with holdase characteristics and explain how handover of the nascent chain to specific auxiliary proteins occurs among a host of other factors in the cytosol.

Proteomic analysis of the colistin-resistant E. coli clinical isolate: Explorations of the resistome

BACKGROUND

Antimicrobial resistance is a worldwide problem after the emergence of colistin resistance since it was the last option left to treat carbapenemase-resistant bacterial infections. The mcr gene and its variants are one of the causes for colistin resistance. Besides mcr genes, some other intrinsic genes are also involved in colistin resistance but still need to be explored.

OBJECTIVE

The aim of this study was to investigate differential proteins expression of colistin-resistant E. coli clinical isolate and to understand their interactive partners as future drug targets.

METHODS

In this study, we have employed the whole proteome analysis through LC-MS/MS. The advance proteomics tools were used to find differentially expressed proteins in the colistin-resistant Escherichia coli clinical isolate compared to susceptible isolate. Gene ontology and STRING were used for functional annotation and protein-protein interaction networks, respectively.

RESULTS

LC-MS/MS analysis showed overexpression of 47 proteins and underexpression of 74 proteins in colistin-resistant E. coli. These proteins belong to DNA replication, transcription and translational process; defense and stress related proteins; proteins of phosphoenol pyruvate phosphotransferase system (PTS) and sugar metabolism. Functional annotation and protein-protein interaction showed translational and cellular metabolic process, sugar metabolism and metabolite interconversion.

CONCLUSION

We conclude that these protein targets and their pathways might be used to develop novel therapeutics against colistin-resistant infections. These proteins could unveil the mechanism of colistin resistance.

Zinc-Dependent Oligomerization of Thermus thermophilus Trigger Factor Chaperone

Simple Summary

Metal ions often play important roles in biological processes. Thermus thermophilus trigger factor (TtTF) is a zinc-dependent molecular chaperone where Zn²⁺ has been shown to enhance its folding-arrest activity. However, the mechanisms of how Zn²⁺ binds to TtTF and how Zn²⁺ affects the activity of TtTF are yet to be elucidated. As a first step in understanding the mechanism, we performed in vitro biophysical experiments on TtTF to investigate the zinc-binding site on TtTF and unveil how Zn²⁺ alters the physical properties of TtTF, including secondary structure, thermal stability, and oligomeric state. Our results showed that TtTF binds Zn²⁺ in a 1:1 ratio, and all three domains of TtTF are involved in zinc-binding. We found that Zn²⁺ does not affect the thermal stability of TtTF, whereas it does induce partial structural change and promote the oligomerization of TtTF. Given that the folding-arrest activity of Escherichia coli TF (EcTF) is regulated by its oligomerization, our results imply that TtTF exploits Zn²⁺ to modulate its oligomeric state to regulate the activity.

Abstract

Thermus thermophilus trigger factor (TtTF) is a zinc-dependent molecular chaperone whose folding-arrest activity is regulated by Zn²⁺. However, little is known about the mechanism of zinc-dependent regulation of the TtTF activity. Here we exploit in vitro biophysical experiments to investigate zinc-binding, the oligomeric state, the secondary structure, and the thermal stability of TtTF in the absence and presence of Zn²⁺. The data show that full-length TtTF binds Zn²⁺, but the isolated domains and tandem domains of TtTF do not bind to Zn²⁺. Furthermore, circular dichroism (CD) and nuclear magnetic resonance (NMR) spectra suggested that Zn²⁺-binding induces the partial structural changes of TtTF, and size exclusion chromatography-multi-angle light scattering (SEC-MALS) showed that Zn²⁺ promotes TtTF oligomerization. Given the previous work showing that the activity regulation of E. coli trigger factor is accompanied by oligomerization, the data suggest that TtTF exploits zinc ions to induce the structural change coupled with the oligomerization to assemble the client-binding site, thereby effectively preventing proteins from misfolding in the thermal environment.

Structural polymorphism and substrate promiscuity of a ribosome-associated molecular chaperone

Trigger factor (TF) is a highly conserved multi-domain molecular chaperone that exerts its chaperone activity at the ribosomal tunnel exit from which newly synthesized nascent chains emerge. TF also displays promiscuous substrate binding for a large number of cytosolic proteins independent of ribosome binding. We asked how TF recognizes a variety of substrates while existing in a monomer-dimer equilibrium. Paramagnetic nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy were used to show that dimeric TF displays a high degree of structural polymorphism in solution. A series of peptides has been generated to quantify their TF binding affinities in relation with their sequence compositions. The results confirmed a previous predication that TF preferentially binds to peptide fragments that are rich in aromatic and positively charged amino acids. NMR paramagnetic relaxation enhancement analysis showed that TF utilizes multiple binding sites, located in the chaperone domain and part of the prolyl trans-cis isomerization domain, to interact with these peptides. Dimerization of TF effectively sequesters most of the substrate binding sites, which are expected to become accessible upon binding to the ribosome as a monomer. As TF lacks ATPase activity, which is commonly used to trigger conformational changes within molecular chaperones in action, the ribosome-binding-associated disassembly and conformational rearrangements may be the underlying regulatory mechanism of its chaperone activity.

Mechanisms of Cotranslational Protein Maturation in Bacteria

Growing cells invest a significant part of their biosynthetic capacity into the production of proteins. To become functional, newly-synthesized proteins must be N-terminally processed, folded and often translocated to other cellular compartments. A general strategy is to integrate these protein maturation processes with translation, by cotranslationally engaging processing enzymes, chaperones and targeting factors with the nascent polypeptide. Precise coordination of all factors involved is critical for the efficiency and accuracy of protein synthesis and cellular homeostasis. This review provides an overview of the current knowledge on cotranslational protein maturation, with a focus on the production of cytosolic proteins in bacteria. We describe the role of the ribosome and the chaperone network in protein folding and how the dynamic interplay of all cotranslationally acting factors guides the sequence of cotranslational events. Finally, we discuss recent data demonstrating the coupling of protein synthesis with the assembly of protein complexes and end with a brief discussion of outstanding questions and emerging concepts in the field of cotranslational protein maturation.

Co-translational folding of nascent polypeptides: Multi-layered mechanisms for the efficient biogenesis of functional proteins

The coupling of protein synthesis and folding is a crucial yet poorly understood aspect of cellular protein folding. Over the past few years, it has become possible to experimentally follow and define protein folding on the ribosome, revealing principles that shape co-translational folding and distinguish it from refolding in solution. Here, we highlight some of these recent findings from biochemical and biophysical studies and their potential significance for cellular protein biogenesis. In particular, we focus on nascent chain interactions with the ribosome, interactions within the nascent protein, modulation of translation elongation rates, and the role of mechanical force that accompanies nascent protein folding. The ability to obtain mechanistic insight in molecular detail has set the stage for exploring the intricate process of nascent protein folding. We believe that the aspects discussed here will be generally important for understanding how protein synthesis and folding are coupled and regulated.

Trigger factor is a bona fide secretory pathway chaperone that interacts with SecB and the translocase

Bacterial secretory preproteins are translocated across the inner membrane post-translationally by the SecYEG-SecA translocase. Mature domain features and signal peptides maintain preproteins in kinetically trapped, largely soluble, folding intermediates. Some aggregation-prone preproteins require chaperones, like trigger factor (TF) and SecB, for solubility and/or targeting. TF antagonizes the contribution of SecB to secretion by an unknown molecular mechanism. We reconstituted this interaction in vitro and studied targeting and secretion of the model preprotein pro-OmpA. TF and SecB display distinct, unsuspected roles in secretion. Tightly associating TF:pro-OmpA targets the translocase at SecA, but TF prevents pro-OmpA secretion. In solution, SecB binds TF:pro-OmpA with high affinity. At the membrane, when bound to the SecA C-tail, SecB increases TF and TF:pro-OmpA affinities for the translocase and allows pro-OmpA to resume translocation. Our data reveal that TF, a main cytoplasmic folding pathway chaperone, is also a bona fide post-translational secretory chaperone that directly interacts with both SecB and the translocase to mediate regulated protein secretion. Thus, TF links the cytoplasmic folding and secretion chaperone networks.

Codon Selection Affects Recruitment of Ribosome-Associating Factors during Translation

An intriguing aspect of protein synthesis is how cotranslational events are managed inside the cell. In this study, we developed an in vivo bimolecular fluorescence complementation assay coupled to SecM stalling (BiFC-SecM) to study how codon usage influences the interactions of ribosome-associating factors that occur cotranslationally. We profiled ribosomal associations of a number of proteins, and observed differential association of chaperone proteins TF, DnaK, GroEL, and translocation factor Ffh as a result of introducing synonymous codon substitutions that change the affinity of the translating sequence to the ribosomal anti-Shine-Dalgarno (aSD) sequence. The use of pausing sequences within proteins regulates their transit within the translating ribosome. Our results indicate that the dynamics between cellular factors and the new polypeptide chain are affected by how codon composition is designed. Furthermore, associating factors may play a role in processes including protein quality control (folding and degradation) and cellular respiration.

Protein Transport Across the Bacterial Plasma Membrane by the Sec Pathway

More than a third of all bacterial polypeptides, comprising the 'exportome', are transported to extracytoplasmic locations. Most of the exportome is targeted and inserts into ('membranome') or crosses ('secretome') the plasma membrane. The membranome and secretome use distinct targeting signals and factors, and driving forces, but both use the ubiquitous and essential Sec translocase and its SecYEG protein-conducting channel. Membranome export is co-translational and uses highly hydrophobic N-terminal signal anchor sequences recognized by the signal recognition particle on the ribosome, that also targets C-tail anchor sequences. Translating ribosomes drive movement of these polypeptides through the lateral gate of SecY into the inner membrane. On the other hand, secretome export is post-translational and carries two types of targeting signals: cleavable N-terminal signal peptides and multiple short hydrophobic targeting signals in their mature domains. Secretome proteins remain translocation competent due to occupying loosely folded to completely non-folded states during targeting. This is accomplished mainly by the intrinsic properties of mature domains and assisted by signal peptides and/or chaperones. Secretome proteins bind to the dimeric SecA subunit of the translocase. SecA converts from a dimeric preprotein receptor to a monomeric ATPase motor and drives vectorial crossing of chains through SecY aided by the proton motive force. Signal peptides are removed by signal peptidases and translocated chains fold or follow subsequent trafficking.

Energetic dependencies dictate folding mechanism in a complex protein

Large proteins composed of multiple domains are abundant in all proteomes, but their folding and structural dynamics remain poorly understood. Using single-molecule force spectroscopy, we have defined how stabilizing interfaces among the domains of elongation factor G (EF-G) shape its folding pathway. Contrary to the expectation that multidomain proteins fold sequentially as they emerge from the ribosome, we find that folding cannot be completed until the full protein has been synthesized. This posttranslational folding mechanism results in a propensity for misfolding. It is dictated by an energetic coupling among domains that enables conformational flexibility crucial for EF-G function. EF-G thus provides an example of how distinct biological ends—robust folding and functionally important flexibility—come into conflict during protein biogenesis.

Large proteins with multiple domains are thought to fold cotranslationally to minimize interdomain misfolding. Once folded, domains interact with each other through the formation of extensive interfaces that are important for protein stability and function. However, multidomain protein folding and the energetics of domain interactions remain poorly understood. In elongation factor G (EF-G), a highly conserved protein composed of 5 domains, the 2 N-terminal domains form a stably structured unit cotranslationally. Using single-molecule optical tweezers, we have defined the steps leading to fully folded EF-G. We find that the central domain III of EF-G is highly dynamic and does not fold upon emerging from the ribosome. Surprisingly, a large interface with the N-terminal domains does not contribute to the stability of domain III. Instead, it requires interactions with its folded C-terminal neighbors to be stably structured. Because of the directionality of protein synthesis, this energetic dependency of domain III on its C-terminal neighbors disrupts cotranslational folding and imposes a posttranslational mechanism on the folding of the C-terminal part of EF-G. As a consequence, unfolded domains accumulate during synthesis, leading to the extensive population of misfolded species that interfere with productive folding. Domain III flexibility enables large-scale conformational transitions that are part of the EF-G functional cycle during ribosome translocation. Our results suggest that energetic tuning of domain stabilities, which is likely crucial for EF-G function, complicates the folding of this large multidomain protein.

Chaperone Interactions at the Ribosome

The continuous refreshment of the proteome is critical to maintain protein homeostasis and to adapt cells to changing conditions. Thus, de novo protein biogenesis by ribosomes is vitally important to every cellular system. This process is delicate and error-prone and requires, besides cytosolic chaperones, the guidance by a specialized set of molecular chaperones that bind transiently to the translation machinery and the nascent protein to support early folding events and to regulate cotranslational protein transport. These chaperones include the bacterial trigger factor (TF), the archaeal and eukaryotic nascent polypeptide-associated complex (NAC), and the eukaryotic ribosome-associated complex (RAC). This review focuses on the structures, functions, and substrates of these ribosome-associated chaperones and highlights the most recent findings about their potential mechanisms of action.

Mapping a single-molecule folding process onto a topological space

Physics of protein folding has been dominated by conceptual frameworks including the nucleation-propagation mechanism and the diffusion-collision model, and none address the topological properties of a chain during a folding process. Single-molecule interrogation of folded biomolecules has enabled real-time monitoring of folding processes at an unprecedented resolution. Despite these advances, the topology landscape has not been fully mapped for any chain. Using a novel circuit topology approach, we map the topology landscape of a model polymeric chain. Inspired by single-molecule mechanical interrogation studies, we restrained the ends of a chain and followed fold nucleation dynamics. We find that, before the nucleation, transient local entropic loops dominate. Although the nucleation length of globules is dependent on the cohesive interaction, the ultimate topological states of the collapsed polymer are largely independent of the interaction but depend on the speed of the folding process. After the nucleation, transient topological rearrangements are observed that converge to a steady-state, where the fold grows in a self-similar manner.

Nature and Regulation of Protein Folding on the Ribosome

Co-translational protein folding is an essential process by which cells ensure the safe and efficient production and assembly of new proteins in their functional native states following biosynthesis on the ribosome. In this review, we describe recent progress in probing the changes during protein synthesis of the free energy landscapes that underlie co-translational folding and discuss the critical coupling between these landscapes and the rate of translation that ultimately determines the success or otherwise of the folding process. Recent developments have revealed a variety of mechanisms by which both folding and translation can be modulated or regulated, and we discuss how these effects are utilised by the cell to optimise the outcome of protein biosynthesis.

Mechanisms of Cotranslational Maturation of Newly Synthesized Proteins

The timely production of functional proteins is of critical importance for the biological activity of cells. To reach the functional state, newly synthesized polypeptides have to become enzymatically processed, folded, and assembled into oligomeric complexes and, for noncytosolic proteins, translocated across membranes. Key activities of these processes occur cotranslationally, assisted by a network of machineries that transiently engage nascent polypeptides at distinct phases of translation. The sequence of events is tuned by intrinsic features of the nascent polypeptides and timely association of factors with the translating ribosome. Considering the dynamics of translation, the heterogeneity of cellular proteins, and the diversity of interaction partners, it is a major cellular achievement that these processes are temporally and spatially so precisely coordinated, minimizing the generation of damaged proteins. This review summarizes the current progress we have made toward a comprehensive understanding of the cotranslational interactions of nascent chains, which pave the way to their functional state.

The Ribosome Cooperates with a Chaperone to Guide Multi-domain Protein Folding

Multi-domain proteins, containing several structural units within a single polypeptide, constitute a large fraction of all proteomes. Co-translational folding is assumed to simplify the conformational search problem for large proteins, but the events leading to correctly folded, functional structures remain poorly characterized. Similarly, how the ribosome and molecular chaperones promote efficient folding remains obscure. Using optical tweezers, we have dissected early folding events of nascent elongation factor G, a multi-domain protein that requires chaperones for folding. The ribosome and the chaperone trigger factor reduce inter-domain misfolding, permitting folding of the N-terminal G-domain. Successful completion of this step is a crucial prerequisite for folding of the next domain. Unexpectedly, co-translational folding does not proceed unidirectionally; emerging unfolded polypeptide can denature an already-folded domain. Trigger factor, but not the ribosome, protects against denaturation. The chaperone thus serves a previously unappreciated function, helping multi-domain proteins overcome inherent challenges during co-translational folding.

The Role of SurA PPIase Domains in Preventing Aggregation of the Outer-Membrane Proteins tOmpA and OmpT

SurA is a conserved ATP-independent periplasmic chaperone involved in the biogenesis of outer-membrane proteins (OMPs). Escherichia coli SurA has a core domain and two peptidylprolyl isomerase (PPIase) domains, the role(s) of which remain unresolved. Here we show that while SurA homologues in early proteobacteria typically contain one or no PPIase domains, the presence of two PPIase domains is common in SurA in later proteobacteria, implying an evolutionary advantage for this domain architecture. Bioinformatics analysis of >350,000 OMP sequences showed that their length, hydrophobicity and aggregation propensity are similar across the proteobacterial classes, ruling out a simple correlation between SurA domain architecture and these properties of OMP sequences. To investigate the role of the PPIase domains in SurA activity, we deleted one or both PPIase domains from E.coli SurA and investigated the ability of the resulting proteins to bind and prevent the aggregation of tOmpA (19 kDa) and OmpT (33 kDa). The results show that wild-type SurA inhibits the aggregation of both OMPs, as do the cytoplasmic OMP chaperones trigger factor and SecB. However, while the ability of SurA to bind and prevent tOmpA aggregation does not depend on its PPIase domains, deletion of even a single PPIase domain ablates the ability of SurA to prevent OmpT aggregation. The results demonstrate that the core domain of SurA endows its generic chaperone ability, while the presence of PPIase domains enhances its chaperone activity for specific OMPs, suggesting one reason for the conservation of multiple PPIase domains in SurA in proteobacteria.

The Hsp70-Hsp90 Chaperone Cascade in Protein Folding

Conserved families of molecular chaperones assist protein folding in the cell. Here we review the conceptual advances on three major folding routes: (i) spontaneous, chaperone-independent folding; (ii) folding assisted by repetitive Hsp70 cycles; and (iii) folding by the Hsp70-Hsp90 cascades. These chaperones prepare their protein clients for folding on their own, without altering their folding path. A particularly interesting role is reserved for Hsp90. The function of Hsp90 in folding is its ancient function downstream of Hsp70, free of cochaperone regulation and present in all kingdoms of life. Eukaryotic signalling networks, however, embrace Hsp90 by a plethora of cochaperones, transforming the profolding machinery to a folding-on-demand factor. We discuss implications for biology and molecular medicine.

Folding up and Moving on-Nascent Protein Folding on the Ribosome

All cellular proteins are synthesized by the ribosome, an intricate molecular machine that translates the information of protein coding genes into the amino acid alphabet. The linear polypeptides synthesized by the ribosome must generally fold into specific three-dimensional structures to become biologically active. Folding has long been recognized to begin before synthesis is complete. Recently, biochemical and biophysical studies have shed light onto how the ribosome shapes the folding pathways of nascent proteins. Here, we discuss recent progress that is beginning to define the role of the ribosome in the folding of newly synthesized polypeptides.

Exploring the roles of substrate-binding surface of the chaperone site in the chaperone activity of trigger factor

Trigger factor (TF) is a key component of the prokaryotic chaperone network, which is involved in many basic cellular processes, such as protein folding, protein trafficking, and ribosome assembly. The major chaperone site of TF has a cradle-like structure in which protein substrate may fold without interference from other proteins. Here, we investigated in vivo and in vitro the roles of hydrophobic and charged patches on the edge and interior of cradle during TF-assisted protein folding. Our results showed that most of the surface of the cradle was involved in TF-assisted protein folding, which was larger than found in early studies. Although the inner surface of cradle was mostly hydrophobic, both hydrophobic and electrostatic patches were indispensable for TF to facilitate correct protein folding. However, hydrophobic patches were more important for the antiaggregation activity of TF. Furthermore, it was found that the patches on the surface of cradle were involved in TF-assisted protein folding in a spatial and temporal order. These results suggest that the folding-favorable interface between the cradle and substrate was dynamic during TF-assisted protein folding, which enabled TF to be involved in the folding of substrate in an aggressive manner rather than acting as a classic holdase.-Fan, D., Cao, S., Zhou, Q., Zhang, Y., Yue, L., Han, C., Yang, B., Wang, Y., Ma, Z., Zhu, L., Liu, C. Exploring the roles of substrate-binding surface of chaperone site in the chaperone activity of trigger factor.

Molecular Mechanisms of Fast Neurotransmitter Release

This review summarizes current knowledge of synaptic proteins that are central to synaptic vesicle fusion in presynaptic active zones, including SNAREs (soluble N-ethylmaleimide sensitive factor attachment protein receptors), synaptotagmin, complexin, Munc18 (mammalian uncoordinated-18), and Munc13 (mammalian uncoordinated-13), and highlights recent insights in the cooperation of these proteins for neurotransmitter release. Structural and functional studies of the synaptic fusion machinery suggest new molecular models of synaptic vesicle priming and Ca2+-triggered fusion. These studies will be a stepping-stone toward answering the question of how the synaptic vesicle fusion machinery achieves such high speed and sensitivity.

Oligomerization of a molecular chaperone modulates its activity

Molecular chaperones alter the folding properties of cellular proteins via mechanisms that are not well understood. Here, we show that Trigger Factor (TF), an ATP-independent chaperone, exerts strikingly contrasting effects on the folding of non-native proteins as it transitions between a monomeric and a dimeric state. We used NMR spectroscopy to determine the atomic resolution structure of the 100 kDa dimeric TF. The structural data show that some of the substrate-binding sites are buried in the dimeric interface, explaining the lower affinity for protein substrates of the dimeric compared to the monomeric TF. Surprisingly, the dimeric TF associates faster with proteins and it exhibits stronger anti-aggregation and holdase activity than the monomeric TF. The structural data show that the dimer assembles in a way that substrate-binding sites in the two subunits form a large contiguous surface inside a cavity, thus accounting for the observed accelerated association with unfolded proteins. Our results demonstrate how the activity of a chaperone can be modulated to provide distinct functional outcomes in the cell.

Protein Folding Mediated by Trigger Factor and Hsp70: New Insights from Single-Molecule Approaches

Chaperones assist in protein folding, but what this common phrase means in concrete terms has remained surprisingly poorly understood. We can readily measure chaperone binding to unfolded proteins, but how they bind and affect proteins along folding trajectories has remained obscure. Here we review recent efforts by our labs and others that are beginning to pry into this issue, with a focus on the chaperones trigger factor and Hsp70. Single-molecule methods are central, as they allow the stepwise process of folding to be followed directly. First results have already revealed contrasts with long-standing paradigms: rather than acting only "early" by stabilizing unfolded chain segments, these chaperones can bind and stabilize partially folded structures as they grow to their native state. The findings suggest a fundamental redefinition of the protein folding problem and a more extensive functional repertoire of chaperones than previously assumed.

Profiling Ssb-Nascent Chain Interactions Reveals Principles of Hsp70-Assisted Folding

The yeast Hsp70 chaperone Ssb interacts with ribosomes and nascent polypeptides to assist protein folding. To reveal its working principle, we determined the nascent chain-binding pattern of Ssb at near-residue resolution by in vivo selective ribosome profiling. Ssb associates broadly with cytosolic, nuclear, and hitherto unknown substrate classes of mitochondrial and endoplasmic reticulum (ER) nascent proteins, supporting its general chaperone function. Ssb engages most substrates by multiple binding-release cycles to a degenerate sequence enriched in positively charged and aromatic amino acids. Timely association with this motif upon emergence at the ribosomal tunnel exit requires ribosome-associated complex (RAC) but not nascent polypeptide-associated complex (NAC). Ribosome footprint densities along orfs reveal faster translation at times of Ssb binding, mainly imposed by biases in mRNA secondary structure, codon usage, and Ssb action. Ssb thus employs substrate-tailored dynamic nascent chain associations to coordinate co-translational protein folding, facilitate accelerated translation, and support membrane targeting of organellar proteins.

An in vitro tag-and-modify protein sample generation method for single-molecule fluorescence resonance energy transfer

Biomolecular systems exhibit many dynamic and biologically relevant properties, such as conformational fluctuations, multistep catalysis, transient interactions, folding, and allosteric structural transitions. These properties are challenging to detect and engineer using standard ensemble-based techniques. To address this drawback, single-molecule methods offer a way to access conformational distributions, transient states, and asynchronous dynamics inaccessible to these standard techniques. Fluorescence-based single-molecule approaches are parallelizable and compatible with multiplexed detection; to date, however, they have remained limited to serial screens of small protein libraries. This stems from the current absence of methods for generating either individual dual-labeled protein samples at high throughputs or protein libraries compatible with multiplexed screening platforms. Here, we demonstrate that by combining purified and reconstituted in vitro translation, quantitative unnatural amino acid incorporation via AUG codon reassignment, and copper-catalyzed azide-alkyne cycloaddition, we can overcome these challenges for target proteins that are, or can be, methionine-depleted. We present an in vitro parallelizable approach that does not require laborious target-specific purification to generate dual-labeled proteins and ribosome-nascent chain libraries suitable for single-molecule FRET-based conformational phenotyping. We demonstrate the power of this approach by tracking the effects of mutations, C-terminal extensions, and ribosomal tethering on the structure and stability of three protein model systems: barnase, spectrin, and T4 lysozyme. Importantly, dual-labeled ribosome-nascent chain libraries enable single-molecule co-localization of genotypes with phenotypes, are well suited for multiplexed single-molecule screening of protein libraries, and should enable the in vitro directed evolution of proteins with designer single-molecule conformational phenotypes of interest.

Bottleneck in secretion of α-amylase in Bacillus subtilis

Amylase plays an important role in biotechnology industries, and Gram-positive bacterium Bacillus subtilis is a major host to produce heterogeneous α-amylases. However, the secretion stress limits the high yield of α-amylase in B. subtilis although huge efforts have been made to address this secretion bottleneck. In this question-oriented review, every effort is made to answer the following questions, which look simple but are long-standing, through reviewing of literature: (1) Does α-amylase need a specific and dedicated chaperone? (2) What signal sequence does CsaA recognize? (3) Does CsaA require ATP for its operation? (4) Does an unfolded α-amylase is less soluble than a folded one? (5) Does α-amylase aggregate before transporting through Sec secretion system? (6) Is α-amylase sufficient stable to prevent itself from misfolding? (7) Does α-amylase need more disulfide bonds to be stabilized? (8) Which secretion system does PrsA pass through? (9) Is PrsA ATP-dependent? (10) Is PrsA reused after folding of α-amylase? (11) What is the fate of PrsA? (12) Is trigger factor (TF) ATP-dependent? The literature review suggests that not only the most of those questions are still open to answers but also it is necessary to calculate ATP budget in order to better understand how B. subtilis uses its energy for production and secretion.

The ribosome destabilizes native and non-native structures in a nascent multidomain protein

Correct folding is a prerequisite for the biological activity of most proteins. Folding has largely been studied using in vitro refolding assays with isolated small, robustly folding proteins. A substantial fraction of all cellular proteomes is composed of multidomain proteins that are often not amenable to this approach, and their folding remains poorly understood. These large proteins likely begin to fold during their synthesis by the ribosome, a large molecular machine that translates the genetic code. The ribosome affects how folding proceeds, but the underlying mechanisms remain largely obscure. We have utilized optical tweezers to study the folding of elongation factor G, a multidomain protein composed of five domains. We find that interactions among unfolded domains interfere with productive folding in the full-length protein. The N-terminal G-domain constitutes an independently folding unit that, upon in vitro refolding, adopts two similar states that correspond to the natively folded and a non-native, possibly misfolded structure. The ribosome destabilizes both of these states, suggesting a mechanism by which terminal misfolding into highly stable, non-native structures is avoided. The ribosome may thus directly contribute to efficient folding by modulating the folding of nascent multidomain proteins.

Folding of proteins with a flavodoxin-like architecture

The flavodoxin-like fold is a protein architecture that can be traced back to the universal ancestor of the three kingdoms of life. Many proteins share this α-β parallel topology and hence it is highly relevant to illuminate how they fold. Here, we review experiments and simulations concerning the folding of flavodoxins and CheY-like proteins, which share the flavodoxin-like fold. These polypeptides tend to temporarily misfold during unassisted folding to their functionally active forms. This susceptibility to frustration is caused by the more rapid formation of an α-helix compared to a β-sheet, particularly when a parallel β-sheet is involved. As a result, flavodoxin-like proteins form intermediates that are off-pathway to native protein and several of these species are molten globules (MGs). Experiments suggest that the off-pathway species are of helical nature and that flavodoxin-like proteins have a nonconserved transition state that determines the rate of productive folding. Folding of flavodoxin from Azotobacter vinelandii has been investigated extensively, enabling a schematic construction of its folding energy landscape. It is the only flavodoxin-like protein of which cotranslational folding has been probed. New insights that emphasize differences between in vivo and in vitro folding energy landscapes are emerging: the ribosome modulates MG formation in nascent apoflavodoxin and forces this polypeptide toward the native state.

Conformational dynamics of bacterial trigger factor in apo and ribosome-bound states

The chaperone trigger factor (TF) binds to the ribosome exit tunnel and helps cotranslational folding of nascent chains (NC) in bacterial cells and chloroplasts. In this study, we aim to investigate the functional dynamics of fully-atomistic apo TF and its complex with 50S. As TF accomodates a high percentage of charged residues on its surface, the effect of ionic strength on TF dynamics is assessed here by performing five independent molecular dynamics (MD) simulations (total of 1.3 micro-second duration) at 29 mM and 150 mM ionic strengths. At both concentrations, TF exhibits high inter- and intra-domain flexibility related to its binding (BD), core (CD), and head (HD) domains. Even though large oscillations in gyration radius exist during each run, we do not detect the so-called ‘fully collapsed’ state with both HD and BD collapsed upon the core. In fact, the extended conformers with relatively open HD and BD are highly populated at the physiological concentration of 150 mM. More importantly, extended TF snapshots stand out in terms of favorable docking onto the 50S subunit. Elastic network modeling (ENM) indicates significant changes in TF’s functional dynamics and domain decomposition depending on its conformation and positioning on the 50S. The most dominant slow motions are the lateral sweeping and vertical opening/closing of HD relative to 50S. Finally, our ENM-based efficient technique -ClustENM- is used to sample atomistic conformers starting with an extended TF-50S complex. Specifically, BD and CD motions are restricted near the tunnel exit, while HD is highly mobile. The atomistic conformers generated without an NC are in agreement with the cryo-EM maps available for TF-ribosome-NC complex.

The chaperone toolbox at the single-molecule level: From clamping to confining

Protein folding is well known to be supervised by a dedicated class of proteins called chaperones. However, the core mode of action of these molecular machines has remained elusive due to several reasons including the promiscuous nature of the interactions between chaperones and their many clients, as well as the dynamics and heterogeneity of chaperone conformations and the folding process itself. While troublesome for traditional bulk techniques, these properties make an excellent case for the use of single-molecule approaches. In this review, we will discuss how force spectroscopy, fluorescence microscopy, FCS, and FRET methods are starting to zoom in on this intriguing and diverse molecular toolbox that is of direct importance for protein quality control in cells, as well as numerous degenerative conditions that depend on it.

Quantitative determination of ribosome nascent chain stability

Accurate protein folding is essential for proper cellular and organismal function. In the cell, protein folding is carefully regulated; changes in folding homeostasis (proteostasis) can disrupt many cellular processes and have been implicated in various neurodegenerative diseases and other pathologies. For many proteins, the initial folding process begins during translation while the protein is still tethered to the ribosome; however, most biophysical studies of a protein's energy landscape are carried out in isolation under idealized, dilute conditions and may not accurately report on the energy landscape in vivo. Thus, the energy landscape of ribosome nascent chains and the effect of the tethered ribosome on nascent chain folding remain unclear. Here we have developed a general assay for quantitatively measuring the folding stability of ribosome nascent chains, and find that the ribosome exerts a destabilizing effect on the polypeptide chain. This destabilization decreases as a function of the distance away from the peptidyl transferase center. Thus, the ribosome may add an additional layer of robustness to the protein-folding process by avoiding the formation of stable partially folded states before the protein has completely emerged from the ribosome.

The Ribosome Restrains Molten Globule Formation in Stalled Nascent Flavodoxin

Folding of proteins usually involves intermediates, of which an important type is the molten globule (MG). MGs are ensembles of interconverting conformers that contain (non-)native secondary structure and lack the tightly packed tertiary structure of natively folded globular proteins. Whereas MGs of various purified proteins have been probed to date, no data are available on their presence and/or effect during protein synthesis. To study whether MGs arise during translation, we use ribosome-nascent chain (RNC) complexes of the electron transfer protein flavodoxin. Full-length isolated flavodoxin, which contains a non-covalently bound flavin mononucleotide (FMN) as cofactor, acquires its native α/β parallel topology via a folding mechanism that contains an off-pathway intermediate with molten globular characteristics. Extensive population of this MG state occurs at physiological ionic strength for apoflavodoxin variant F44Y, in which a phenylalanine at position 44 is changed to a tyrosine. Here, we show for the first time that ascertaining the binding rate of FMN as a function of ionic strength can be used as a tool to determine the presence of the off-pathway MG on the ribosome. Application of this methodology to F44Y apoflavodoxin RNCs shows that at physiological ionic strength the ribosome influences formation of the off-pathway MG and forces the nascent chain toward the native state.

A strategy for co-translational folding studies of ribosome-bound nascent chain complexes using NMR spectroscopy

During biosynthesis on the ribosome, an elongating nascent polypeptide chain can begin to fold, in a process that is central to all living systems. Detailed structural studies of co-translational protein folding are now beginning to emerge; such studies were previously limited, at least in part, by the inherently dynamic nature of emerging nascent chains, which precluded most structural techniques. NMR spectroscopy is able to provide atomic-resolution information for ribosome-nascent chain complexes (RNCs), but it requires large quantities (≥10 mg) of homogeneous, isotopically labeled RNCs. Further challenges include limited sample working concentration and stability of the RNC sample (which contribute to weak NMR signals) and resonance broadening caused by attachment to the large (2.4-MDa) ribosomal complex. Here, we present a strategy to generate isotopically labeled RNCs in Escherichia coli that are suitable for NMR studies. Uniform translational arrest of the nascent chains is achieved using a stalling motif, and isotopically labeled RNCs are produced at high yield using high-cell-density E. coli growth conditions. Homogeneous RNCs are isolated by combining metal affinity chromatography (to isolate ribosome-bound species) with sucrose density centrifugation (to recover intact 70S monosomes). Sensitivity-optimized NMR spectroscopy is then applied to the RNCs, combined with a suite of parallel NMR and biochemical analyses to cross-validate their integrity, including RNC-optimized NMR diffusion measurements to report on ribosome attachment in situ. Comparative NMR studies of RNCs with the analogous isolated proteins permit a high-resolution description of the structure and dynamics of a nascent chain during its progressive biosynthesis on the ribosome.

In vivo aspects of protein folding and quality control

Most proteins must fold into unique three-dimensional structures to perform their biological functions. In the crowded cellular environment, newly synthesized proteins are at risk of misfolding and forming toxic aggregate species. To ensure efficient folding, different classes of molecular chaperones receive the nascent protein chain emerging from the ribosome and guide it along a productive folding pathway. Because proteins are structurally dynamic, constant surveillance of the proteome by an integrated network of chaperones and protein degradation machineries is required to maintain protein homeostasis (proteostasis). The capacity of this proteostasis network declines during aging, facilitating neurodegeneration and other chronic diseases associated with protein aggregation. Understanding the proteostasis network holds the promise of identifying targets for pharmacological intervention in these pathologies.

Structural characterization of the interaction of α-synuclein nascent chains with the ribosomal surface and trigger factor

The ribosome is increasingly becoming recognized as a key hub for integrating quality control processes associated with protein biosynthesis and cotranslational folding (CTF). The molecular mechanisms by which these processes take place, however, remain largely unknown, in particular in the case of intrinsically disordered proteins (IDPs). To address this question, we studied at a residue-specific level the structure and dynamics of ribosome-nascent chain complexes (RNCs) of α-synuclein (αSyn), an IDP associated with Parkinson's disease (PD). Using solution-state nuclear magnetic resonance (NMR) spectroscopy and coarse-grained molecular dynamics (MD) simulations, we find that, although the nascent chain (NC) has a highly disordered conformation, its N-terminal region shows resonance broadening consistent with interactions involving specific regions of the ribosome surface. We also investigated the effects of the ribosome-associated molecular chaperone trigger factor (TF) on αSyn structure and dynamics using resonance broadening to define a footprint of the TF-RNC interactions. We have used these data to construct structural models that suggest specific ways by which emerging NCs can interact with the biosynthesis and quality control machinery.