Eukaryotic chaperonin CCT stabilizes actin and tubulin folding intermediates in open quasi-native conformations

Three-dimensional reconstruction from cryoelectron micrographs of the eukar yotic cytosolic chaperonin CCT complexed to tubulin shows that CCT interact s with tubulin (both the a and beta isoforms) using five specific CCT subun its, The CCT-tubulin interaction has a different geometry to the CCT-actin interaction, and a mixture of shared and unique CCT subunits is used in bin ding the two substrates. Docking of the atomic structures of both actin and tubulin to their CCT-bound conformation suggests a common mode of chaperon in-substrate interaction. CCT stabilizes quasi-native structures in both pr oteins that are open through their domain-connecting hinge regions, suggest ing a novel mechanism and function of CCT in assisted protein folding.