Chaperonin complex with a newly folded protein encapsulated in the folding chamber

Abstract

In Escherichia coli the chaperonins GroEL and GroES form a double-ring complex that binds and folds nascent or unfolded proteins. The T4 bacteriophage has its own version of GroES, gp31, that forms a taller folding chamber to fold the major virus capsid protein, gp23. Clare et al. present the structure of gp23–chaperonin complexes showing gp23 encapsulated in the folding chamber. The folding chamber is distorted in order to enclose a large substrate. This is the first study to present an image of a newly folded protein just before its release from the GroEL folding chamber. This study presents the structure of gp23-chaperonin complexes showing gp23 encapsulated in the folding chamber. The folding chamber is distorted to enclose a large substrate, and this is the first study that visualizes of a newly folded physiological substrate trapped inside the folding chamber of GroEL. A subset of essential cellular proteins requires the assistance of chaperonins (in Escherichia coli, GroEL and GroES), double-ring complexes in which the two rings act alternately to bind, encapsulate and fold a wide range of nascent or stress-denatured proteins1,2,3,4,5. This process starts by the trapping of a substrate protein on hydrophobic surfaces in the central cavity of a GroEL ring6,7,8,9,10. Then, binding of ATP and co-chaperonin GroES to that ring ejects the non-native protein from its binding sites, through forced unfolding or other major conformational changes, and encloses it in a hydrophilic chamber for folding11,12,13,14,15. ATP hydrolysis and subsequent ATP binding to the opposite ring trigger dissociation of the chamber and release of the substrate protein3. The bacteriophage T4 requires its own version of GroES, gp31, which forms a taller folding chamber, to fold the major viral capsid protein gp23 (refs 16–20). Polypeptides are known to fold inside the chaperonin complex, but the conformation of an encapsulated protein has not previously been visualized. Here we present structures of gp23–chaperonin complexes, showing both the initial captured state and the final, close-to-native state with gp23 encapsulated in the folding chamber. Although the chamber is expanded, it is still barely large enough to contain the elongated gp23 monomer, explaining why the GroEL–GroES complex is not able to fold gp23 and showing how the chaperonin structure distorts to enclose a large, physiological substrate protein.