Measuring cotranslational folding of nascent polypeptide chains on ribosomes

Predictive biophysical modeling and understanding of the dynamics of mRNA translation and its evolution

mRNA translation is the fundamental process of decoding the information encoded in mRNA molecules by the ribosome for the synthesis of proteins. The centrality of this process in various biomedical disciplines such as cell biology, evolution and biotechnology, encouraged the development of dozens of mathematical and computational models of translation in recent years. These models aimed at capturing various biophysical aspects of the process. The objective of this review is to survey these models, focusing on those based and/or validated on real large-scale genomic data. We consider aspects such as the complexity of the models, the biophysical aspects they regard and the predictions they may provide. Furthermore, we survey the central systems biology discoveries reported on their basis. This review demonstrates the fundamental advantages of employing computational biophysical translation models in general, and discusses the relative advantages of the different approaches and the challenges in the field.

Cell-Free Synthesis of Macromolecular Complexes

Cell-free protein synthesis based on E. coli cell extracts has been described for the first time more than 50 years ago. To date, cell-free synthesis is widely used for the preparation of toxic proteins, for studies of the translation process and its regulation as well as for the incorporation of artificial or labeled amino acids into a polypeptide chain. Many efforts have been directed towards establishing cell-free expression as a standard method for gene expression, with limited success. In this chapter we will describe the state-of-the-art of cell-free expression, extract preparation methods and recent examples for successful applications of cell-free synthesis of macromolecular complexes.

Computational evidence that fast translation speed can increase the probability of cotranslational protein folding

Translation speed can affect the cotranslational folding of nascent peptide. Experimental observations have indicated that slowing down translation rates of codons can increase the probability of protein cotranslational folding. Recently, a kinetic modeling indicates that fast translation can also increase the probability of cotranslational protein folding by avoiding misfolded intermediates. We show that the villin headpiece subdomain HP35 is an ideal model to demonstrate this phenomenon. We studied cotranslational folding of HP35 with different fast translation speeds by all-atom molecular dynamics simulations and found that HP35 can fold along a well-defined pathway that passes the on-pathway intermediate but avoids the misfolded off-pathway intermediate in certain case. This greatly increases the probability of HP35 cotranslational folding and the approximate mean first passage time of folding into native state is about 1.67μs. Since we also considered the space-confined effect of the ribosomal exit tunnel on the cotranslational folding, our simulation results suggested alternative mechanism for the increasing of cotranslational folding probability by fast translation speed.

A comparison of the folding characteristics of free and ribosome-tethered polypeptide chains using limited proteolysis and mass spectrometry

The kinetics and thermodynamics of protein folding are commonly studied in vitro by denaturing/renaturing intact protein sequences. How these folding mechanisms relate to de novo folding that occurs as the nascent polypeptide emerges from the ribosome is much less well understood. Here, we have employed limited proteolysis followed by mass spectrometry analyses to compare directly free and ribosome-tethered polypeptide chains of the Src-homology 3 (SH3) domain and its unfolded variant, SH3-m10. The disordered variant was found to undergo faster proteolysis than SH3. Furthermore, the trypsin cleavage patterns observed show minor, but significant, differences for the free and ribosome-bound nascent chains, with significantly fewer tryptic peptides detected in the presence of ribosome. The results highlight the utility of limited proteolysis coupled with mass spectrometry for the structural analysis of these complex systems, and pave the way for detailed future analyses by combining this technique with chemical labeling methods (for example, hydrogen-deuterium exchange, photochemical oxidation) to analyze protein folding in real time, including in the presence of additional ribosome-associated factors.

Roles for Synonymous Codon Usage in Protein Biogenesis

Owing to the degeneracy of the genetic code, a protein sequence can be encoded by many different synonymous mRNA coding sequences. Synonymous codon usage was once thought to be functionally neutral, but evidence now indicates it is shaped by evolutionary selection and affects other aspects of protein biogenesis beyond specifying the amino acid sequence of the protein. Synonymous rare codons, once thought to have only negative impacts on the speed and accuracy of translation, are now known to play an important role in diverse functions, including regulation of cotranslational folding, covalent modifications, secretion, and expression level. Mutations altering synonymous codon usage are linked to human diseases. However, much remains unknown about the molecular mechanisms connecting synonymous codon usage to efficient protein biogenesis and proper cell physiology. Here we review recent literature on the functional effects of codon usage, including bioinformatics approaches aimed at identifying general roles for synonymous codon usage.

Shedding light on protein folding landscapes by single-molecule fluorescence

Single-molecule (SM) fluorescence methods have been increasingly instrumental in our current understanding of a number of key aspects of protein folding and aggregation landscapes over the past decade. With the advantage of a model free approach and the power of probing multiple subpopulations and stochastic dynamics directly in a heterogeneous structural ensemble, SM methods have emerged as a principle technique for studying complex systems such as intrinsically disordered proteins (IDPs), globular proteins in the unfolded basin and during folding, and early steps of protein aggregation in amyloidogenesis. This review highlights the application of these methods in investigating the free energy landscapes, folding properties and dynamics of individual protein molecules and their complexes, with an emphasis on inherently flexible systems such as IDPs.

Structural and energetic determinants of co-translational folding

We performed extensive lattice Monte Carlo simulations of ribosome-bound stalled nascent chains (RNCs) to explore the relative roles of native topology and non-native interactions in co-translational folding of small proteins. We found that the formation of a substantial part of the native structure generally occurs towards the end of protein synthesis. However, multi-domain structures, which are rich in local interactions, are able to develop gradually during chain elongation, while those with proximate chain termini require full protein synthesis to fold. A detailed assessment of the conformational ensembles populated by RNCs with different lengths reveals that the directionality of protein synthesis has a fine-tuning effect on the probability to populate low-energy conformations. In particular, if the participation of non-native interactions in folding energetics is mild, the formation of native-like conformations is majorly determined by the properties of the contact map around the tethering terminus. Likewise, a pair of RNCs differing by only 1-2 residues can populate structurally well-resolved low energy conformations with significantly different probabilities. An interesting structural feature of these low-energy conformations is that, irrespective of native structure, their non-native interactions are always long-ranged and marginally stabilizing. A comparison between the conformational spectra of RNCs and chain fragments folding freely in the bulk reveals drastic changes amongst the two set-ups depending on the native structure. Furthermore, they also show that the ribosome may enhance (up to 20%) the population of low energy conformations for chains folding to native structures dominated by local interactions. In contrast, a RNC folding to a non-local topology is forced to remain largely unstructured but can attain low energy conformations in bulk.

Atomic force microscopy captures folded ribosome bound nascent chains

Direct visualization of co-translational folding of nascent polypeptide chains is challenging. Here we present, for the first time, AFM images of large protein constructs based on the membrane binding domain of ankyrin-R, complexed with the ribosome. The characteristic "horse-shoe" shape of ankyrin-R emerging from the ribosome was captured.

Autotransporters: The Cellular Environment Reshapes a Folding Mechanism to Promote Protein Transport

We know very little about how the cellular environment affects protein folding mechanisms. Here, we focus on one unique aspect of that environment that is difficult to recapitulate in the test tube: the effect of a folding vector. When protein folding is initiated at one end of the polypeptide chain, folding starts from a much smaller ensemble of conformations than during refolding of a full-length polypeptide chain. But to what extent can vectorial folding affect protein folding kinetics and the conformations of folding intermediates? We focus on recent studies of autotransporter proteins, the largest class of virulence proteins from pathogenic Gram-negative bacteria. Autotransporter proteins are secreted across the bacterial inner membrane from N→C-terminus, which, like refolding in vitro, retards folding. But in contrast, upon C→N-terminal secretion across the outer membrane autotransporter folding proceeds orders of magnitude faster. The potential impact of vectorial folding on the folding mechanisms of other proteins is also discussed.