Cotranslational folding of proteins on the ribosome.

Many proteins in the cell fold cotranslationally within the restricted space of the polypeptide exit tunnel or at the surface of the ribosome. A growing body of evidence suggests that the ribosome can alter the folding trajectory in many different ways. In this review, we summarize the recent examples of how translation affects folding of single-domain, multiple-domain and oligomeric proteins. The vectorial nature of translation, the spatial constraints of the exit tunnel, and the electrostatic properties of the ribosome-nascent peptide complex define the onset of early folding events. The ribosome can facilitate protein compaction, induce the formation of intermediates that are not observed in solution, or delay the onset of folding. Examples of single-domain proteins suggest that early compaction events can define the folding pathway for some types of domain structures. Folding of multi-domain proteins proceeds in a domain-wise fashion, with each domain having its role in stabilizing or destabilizing neighboring domains. Finally, the assembly of protein complexes can also begin cotranslationally. In all these cases, the ribosome helps the nascent protein to attain a native fold and avoid the kinetic traps of misfolding

Translational control by ribosome pausing in bacteria: How a non-uniform pace of translation affects protein production and folding

Protein homeostasis of bacterial cells is maintained by coordinated processes of protein production, folding, and degradation. Translational efficiency of a given mRNA depends on how often the ribosomes initiate synthesis of a new polypeptide and how quickly they read the coding sequence to produce a full-length protein. The pace of ribosomes along the mRNA is not uniform: periods of rapid synthesis are separated by pauses. Here, we summarize recent evidence on how ribosome pausing affects translational efficiency and protein folding. We discuss the factors that slow down translation elongation and affect the quality of the newly synthesized protein. Ribosome pausing emerges as important factor contributing to the regulatory programs that ensure the quality of the proteome and integrate the cellular and environmental cues into regulatory circuits of the cell

A switch from α‐helical to β‐strand conformation during co‐translational protein folding

Cellular proteins begin to fold as they emerge from the ribosome.The folding landscape of nascent chains is not only shaped by theiramino acid sequence but also by the interactions with the ribo-some. Here, we combine biophysical methods with cryo-EM struc-ture determination to show that folding of aβ-barrel proteinbegins with formation of a dynamicα-helix inside the ribosome. Asthe growing peptide reaches the end of the tunnel, the N-terminalpart of the nascent chain refolds to aβ-hairpin structure thatremains dynamic until its release from the ribosome. Contactswith the ribosome and structure of the peptidyl transferase centerdepend on nascent chain conformation. These results indicate thatproteins may start out asα-helices inside the tunnel and switchinto their native folds only as they emerge from the ribosome.Moreover, the correlation of nascent chain conformations withreorientation of key residues of the ribosomal peptidyl-transferasecenter suggest that protein folding could modulate ribosome activity

Gradual compaction of nascent peptide during cotranslational folding on the ribosome

Nascent polypeptides begin to fold in the constrained space of the ribosomal peptide exit tunnel. Here we use force-profile analysis (FPA) and photo-induced energy-transfer fluorescence correlation spectroscopy (PET-FCS) to show how a small α-helical domain, the N-terminal domain of HemK, folds cotranslationally. Compaction starts vectorially as soon as the first α-helical segments are synthesized. As nascent chain grows, emerging helical segments dock onto each other and continue to rearrange at the vicinity of the ribosome. Inside or in the proximity of the ribosome, the nascent peptide undergoes structural fluctuations on the µs time scale. The fluctuations slow down as the domain moves away from the ribosome. Mutations that destabilize the packing of the domain’s hydrophobic core have little effect on folding within the exit tunnel, but abolish the final domain stabilization. The results show the power of FPA and PET-FCS in solving the trajectory of cotranslational protein folding and in characterizing the dynamic properties of folding intermediates