Cytosolic chaperonin protects folding intermediates of Gß from aggregation by recognizing hydrophobic ß-strands

Cytosolic chaperonin containing t-complex polypepticle 1 (CCT)/TRiC is a group 11 chaperonin that assists in the folding of newly synthesized proteins. It is a eukaryotic homologue of the bacterial group I chaperonin GroEL. In contrast to the well studied functions of GroEL, the substrate recognition mechanism of CCT/TRiC is poorly understood. Here, we established a system for analyzing CCT/TRiC functions by using a reconstituted protein synthesis by using recombinant elements system and show that CCT/TRiC strongly recognizes WD40 proteins particularly at hydrophobic beta-strands. Using the G protein beta subunit (G beta), a WD40 protein that is very rich in P-sheets, as a model substrate, we found that CCT/TRiC prevents aggregation and assists in folding of G beta, whereas GroEL does not. G beta has a seven-bladed beta-propeller structure; each blade is formed from a WD40 repeat sequence encoding four beta-strands. Detailed mutational analysis of G beta indicated that CCT/TRiC, but not GroEL, preferentially recognizes hydrophobic residues aligned on surfaces of beta-strands in the second WD40 repeat of G beta. These findings indicate that one of the CCT/TRiC-specific targets is hydrophobic beta-strands, which are highly prone to aggregation.