Characterization of a ribonuclease S refolding intermediate

D uring ribonuclease S (RN ase S) refolding, two peptide fragm ents recognize each other, and bind to g eth er to form a refolding in term ediate which slowly converts to th e nativ e state. We have characterized this refolding interm ediate using absorbance, circular dichroism (CD), and nuclear m agnetic resonance (NMR) spectroscopies. These techniques reveal significant am ounts of bo th secondary and te rtia ry stru ctu re ; the in term ediate differs from a m olten globule in being packed and native-like, b u t it resembles a m olten globule in having no near-ultraviolet (UV) CD spectrum . Final refolding is slow and accom panies proline isom erization. The results show th a t a t least two separate stages are observed in the form ation of the te rtia ry stru ctu re of RNaseS.