Structural basis of substrate progression through the chaperonin cycle

ABSTRACT The bacterial chaperonin GroEL-GroES promotes protein folding through ATP-regulated cycles of substrate protein binding, encapsulation, and release. Here, we have used cryoEM to determine structures of GroEL, GroEL-ADP·BeF 3 , and GroEL-ADP·AlF 3 -GroES all complexed with the model substrate Rubisco. Our structures provide a series of snapshots that show how the conformation and interactions of non-native Rubisco change as it proceeds through the GroEL-GroES reaction cycle. We observe specific charged and hydrophobic GroEL residues forming strong initial contacts with non-native Rubisco. Binding of ATP or ADP·BeF 3 to GroEL-Rubisco results in the formation of an intermediate GroEL complex displaying striking asymmetry in the ATP/ADP·BeF 3 -bound ring. In this ring, four GroEL subunits bind Rubisco and the other three are in the GroES-accepting conformation, explaining how GroEL can recruit GroES without releasing bound substrate. Our cryoEM structures of stalled GroEL-ADP·AlF 3 -Rubisco-GroES complexes show Rubisco folding intermediates interacting with GroEL-GroES via different sets of residues.