Chaperonin-Mediated Protein Folding

Abstract

Chaperonins are remarkable ring assemblies that provide kinetic assistance to protein folding to the native state. They function through the action of the central cavities of their rings. The best studied such system is bacterial GroEL and its cooperating co-chaperonin GroES, which mediate productive folding of a large number of proteins in the presence of ATP. I will recount the early functional studies of chaperonins in mitochondria, carried out collaboratively with Ulrich Hartl, and describe our subsequent X-ray, EM, and biochemical studies of GroEL leading to an understanding of the structure and reaction cycle, studies carried out with Paul Sigler and Helen Saibil. The structure and mechanism studies revealed that an open ring of GroEL exposes hydrophobic surfaces that capture non-native proteins and prevent them from aggregation. In the presence of ATP, which binds cooperatively in the seven equatorial sites of an open ring, the cochaperonin ring GroES associates, producing large rigid body movements of the apical and intermediate domains of the bound, so-called cis ring, releasing substrate protein into a now-encapsulated central cavity that has hydrophilic walls. Folding occurs in this space in what is the longest-lived state of the reaction cycle. ATP hydrolysis gates rapid entry of ATP into the opposite ring, which both allosterically ejects the cis ligands and sets up a new folding active ring. I will present recent cryoEM studies of the allosteric movements driven by ATP binding, which both facilitate substrate protein binding and enable initial contact of GroES with GroEL, carried out with Helen Saibil, as well as discuss cis folding studies, carried out in part with Kurt Wüthrich.