Point Mutations in a Hinge Linking the Small and Large Domains of β-Actin Result in Trapped Folding Intermediates Bound to Cytosolic Chaperonin CCT

Abstract

The30-Å cryo-EM-derived structure of apo-CCT–α-actin shows actin opened up across its nucleotide-binding cleft and binding to either of two CCT subunit pairs, CCTβ–CCTδ or CCTϵ–CCTδ, in a similar 1:4 arrangement. The two main duplicated domains of native actin are linked twice, topologically, by the connecting residues, Q137–S145 and P333–S338, and are tightly held together by hydrogen bonding with bound adenine nucleotide. We carried out a mutational screen to find residues in actin that might be involved in the huge rotations observed in the CCT-bound folding intermediate. When two evolutionarily highly conserved glycine residues of β-actin, G146 and G150, were changed to proline, both mutant actin proteins were poorly processed by CCT in in vitro translation assays; they become arrested on CCT. A three-dimensional reconstruction of the substrate-bound ring of the apo-CCT–β-actin complex shows that β-actin G150P is not able to bind across the chaperonin cavity to interact with the CCTδ subunit. β-actin G150P seems tightly packed and apparently bound only to the CCTβ and CCTϵ subunits, which further indicates that these CCT subunits drive the interaction between CCT and actin. Hinge opening seems to be critical for actin folding, and we suggest that residues G146 and G150 are important components of the hinge around which the rigid subdomains, presumably already present in early actin folding intermediates, rotate during CCT-assisted folding.