Structural analysis of G protein beta(5) subunit folding by the cytosolic chaperonin CCT/TRiC and its co-chaperone phosducin-like protein 1 reveal molecular mechanisms of chaperone-mediated beta-propeller protein folding

The cytosolic chaperonin CCT and its co-chaperone phosducin-like protein 1 (PhLP1) play important roles in G protein complex assembly by folding G protein β subunits (Gβ) into β-propeller structures. To understand this process at the molecular level, we have isolated the CCT-Gβ -PhLP1 folding intermediate in both the open and closed CCT conformations and determined its structure by high resolution cryo-electron microscopy (cryo-EM). In the open structures, Gβ interact with the N- and C-termini of the CCT subunits deep inside the folding chamber between the CCT rings in a closed β-propeller conformation. PhLP1 allosterically enhances Gβ binding to CCT by interacting with the CCT apical domains at the rim of the folding chamber without contacting Gβ directly. In the closed CCT structures, Gβ move from between the CCT rings into one of the folding chambers, suggesting a path for release of Gβ5 from CCT during its ATPase cycle. These findings provide a molecular explanation for CCT-dependent folding and release of Gβ to interact with RGS proteins and perform their essential functions in G protein signaling.