Confined dynamics of a ribosome‐bound nascent globin: Cone angle analysis of fluorescence depolarization decays in the presence of two local motions

We still know very little about how proteins achieve their native three‐dimensional structure in vitro and in the cell. Folding studies as proteins emerge from the mega Dalton‐sized ribosome pose special challenges due to the large size and complicated nature of the ribosome‐nascent chain complex. This work introduces a combination of three‐component analysis of fluorescence depolarization decays (including the presence of two local motions) and in‐cone analysis of diffusive local dynamics to investigate the spatial constraints experienced by a protein emerging from the ribosomal tunnel. We focus on E. coli ribosomes and an all‐α‐helical nascent globin in the presence and absence of the cotranslationally active chaperones DnaK and trigger factor. The data provide insights on the dynamic nature and structural plasticity of ribosome‐nascent chain complexes. We find that the sub‐ns motions of the N‐terminal fluorophore, reporting on the globin dynamics in the vicinity of the N terminus, are highly constrained both inside and outside the ribosomal tunnel, resulting in high‐order parameters (>0.85) and small cone semiangles (<30°). The shorter globin chains buried inside the tunnel are less spatially constrained than those of a reference sequence from a natively unfolded protein, suggesting either that the two nascent chain sequences have a different secondary structure and therefore sample different regions of the tunnel or that the tunnel undergoes local structural adjustments to accommodate the globin sequence. Longer globins emerging out of the ribosomal tunnel are also found to have highly spatially constrained slow (ns) motions. There are no observable spectroscopic changes in the absence of bound chaperones.