THE CHAPERONIN CYCLE CANNOT SUBSTITUTE FOR PROLYL ISOMERASE ACTIVITY,BUT GROEL ALONE PROMOTES PRODUCTIVE FOLDING OF A CYCLOPHILIN-SENSITIVE SUBSTRATE TO A CYCLOPHILIN-RESISTANT FORM

The chaperonin GroEL and the peptidyl-prolyl cis-trans isomerase cyclo philin are major representatives of two distinct cellular systems that help proteins to adopt their native three-dimensional structure: mole cular chaperones and folding catalysts, Little is known about whether and how these proteins cooperate in protein folding, In this study, we have examined the action of GroEL and cyclophilin on a substrate prot ein in two distinct prolyl isomerization states, Our results indicate that: (i) GroEL binds the same substrate in different prolyl isomeriza tion states, (ii) GroEL-ES does not promote prolyl isomerizations, but even retards isomerizations. (iii) Cyclophilin cannot promote the cor rect isomerization of prolyl bonds of a GroEL-bound substrate, but act s sequentially after release of the substrate from GroEL, (iv) A denat ured substrate with all-native prolyl bonds is delayed in folding by c yclophilin due to isomerization to non-native prolyl bonds; a substrat e that has proceeded in folding beyond a stage where it can be bound b y GroEL is still sensitive to cyclophilin. (v) If a denatured cyclophi lin-sensitive substrate is first bound to GroEL, however, productive f olding to a cyclophilin-resistant form can be promoted, even without G roES, We conclude that GroEL and cyclophilin act sequentially and exer t complementary functions in protein folding.