Visualizing the chaperone-mediated folding trajectory of the G protein β5 β-propeller

The cytosolic Chaperonin Containing Tailless polypeptide 1 (CCT) complex is an essential protein folding machine with a diverse clientele of substrates, including many proteins with β-propeller domains. Here, we determined structures of CCT in complex with its accessory co-chaperone, phosducin-like protein 1 (PhLP1), in the process of folding Gβ₅, a component of Regulator of G protein Signaling (RGS) complexes. Cryo-EM and image processing revealed an ensemble of distinct snapshots that represent the folding trajectory of Gβ₅ from an unfolded molten globule to a fully folded β-propeller. These structures reveal the mechanism by which CCT directs Gβ₅ folding through initiating specific intermolecular contacts that facilitate the sequential folding of individual β-sheets until the propeller closes into its native structure. This work directly visualizes chaperone-mediated protein folding and establishes that CCT directs folding by stabilizing intermediates through interactions with surface residues that permit the hydrophobic core to coalesce into its folded state.