EFFECT OF PROLYL ISOMERASE ON THE FOLDING REACTIONS OF STAPHYLOCOCCALNUCLEASE

The low temperature fluorescence-detected refolding of staphylococcal nuclease (SNase) can be described by three slow kinetic phases, The sl owest phase is absent in the P117G mutant of SNase, Peptidyl prolyl ci s-trans isomerase (cyclophilin), which has been shown to catalyze the slow folding reactions of some proteins, was employed to determine whi ch of the refolding reactions of SNase and P117G SNase involve proline isomerization. We report here that all three folding phases of the wi ld type and the slower phase of P117G SNase are catalyzed by prolyl is omerase, indicating that proline isomerization is involved in these fl uorescence-detected phases in the refolding of SNase. Since the rates of these phases are denaturant-dependent, we conclude that the slow fo lding steps involve isomerization of non-native cis proline peptide bo nds and are tightly coupled to denaturant-sensitive structural changes .