AUTOCATALYZED PROTEIN-FOLDING

Proline isomerization, an intrinsically slow process, kinetically trap s intermediates in slow protein folding reactions. Thus, enzymes that catalyze proline isomerization (prolyl isomerases) often catalyze prot ein folding. We have investigated the folding kinetics of FKBP, a prol yl isomerase. The main conclusion is that FKBP catalyzes its own foldi ng. Altogether, the FKBP refolding kinetics are resolved into three ex ponential phases: a fast phase, tau(3); an intermediate phase, tau(2); and a slow phase, tau(1). Unfolding occurs in a single phase, the unf olding branch of phase tau(2). In the presence of native FKBP, both th e intermediate (tau(2)) and slow (tau(1)) phases are faster, suggestin g that folding phases tau(1) and tau(2) involve proline cis-trans isom erization. In the absence of added native FKBP, autocatalytic folding of FKBP is detected. For refolding starting with all the FKBP unfolded initially, the slowest folding phase (tau(1)) is almost 2-fold faster at a final concentration of 14 mu M FKBP than at 2 mu M FKBP, suggest ing that catalytically active FKBP formed in the fast (tau(3)) or inte rmediate (tau(2)) folding phases catalyzes the slow folding phase (tau (1)). Moreover, autocatalysis of folding is inhibited by FK506, an inh ibitor of the FKBP prolyl isomerase activity. The results show that th e slow phase in FKBP folding is an autocatalyzed formation of native F KBP from kinetically trapped species with non-native proline isomers. While the magnitude of the catalytic effects reported here are modest, FKBP folding may provide a prototype for autocatalysis of kinetically trapped macromolecular conformational changes in other systems.