Chain Dynamics of Nascent Polypeptides Emerging from the Ribosome

Very little is known about the conformation of polypeptides emerging from the ribosome during protein biosynthesis. Here, we explore the dynamics of ribosome-bound nascent polypeptides and proteins in Escherichia coli by dynamic fluorescence depolarization and assess the population of cotranslationally active chaperones trigger factor (TF) and DnaK. E. coli cell-free technology and fluorophore-linked E. coli Met-tRNA_f^Met enable selective site-specific labeling of nascent proteins at the N-terminal methionine. For the first time, direct spectroscopic evidence captures the generation of independent nascent chain motions for a single-domain protein emerging from the ribosome (apparent rotational correlation time ∼5 ns), during the intermediate and late stages of polypeptide elongation. Such motions are detected only for a sequence encoding a globular protein and not for a natively unfolded control, suggesting that the independent nascent chain dynamics may be a signature of folding-competent sequences. In summary, we observe multicomponent, severely rotationally restricted, and strongly chain length/sequence-dependent nascent chain dynamics.