Cysteine oxidation and disulfide formation in the ribosomal exit tunnel

Understanding the conformational sampling of translation-arrested ribosome nascent chain complexes is key to understand co-translational folding. Up to now, coupling of cysteine oxidation, disulfide bond formation and structure formation in nascent chains has remained elusive. Here, we investigate the eye-lens protein gamma B-crystallin in the ribosomal exit tunnel. Using mass spectrometry, theoretical simulations, dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance and cryo-electron microscopy, we show that thiol groups of cysteine residues undergo S-glutathionylation and S-nitrosylation and form non-native disulfide bonds. Thus, covalent modification chemistry occurs already prior to nascent chain release as the ribosome exit tunnel provides sufficient space even for disulfide bond formation which can guide protein folding. As protein synthesis takes place, newly synthesized polypeptide chain passes through the ribosomal exit tunnel, which can accommodate up to 70 residues in the case of a helical peptide. Here the authors show that oxidation of cysteine residues in the nascent chain can occur within the ribosome exit tunnel, where sufficient space exists for the formation of disulfide bonds.