Disease-causing mutations in the G protein β5 β-propeller disrupt its chaperonin-mediated folding trajectory.

The Chaperonin Containing Tailless polypeptide 1 (CCT or TRiC) is an essential cytosolic chaperone that folds multiple protein substrates, including many with β-propeller folds. One β-propeller substrate is the G protein β(5) subunit (Gβ(5)) of Regulator of G protein Signaling (RGS) complexes that determine the duration of G protein signals in neurons. In recent work, we used cryo-electron microscopy (cryo-EM) to visualize the complete CCT-mediated folding trajectory for Gβ(5), from an initiating electrostatic interaction of a single β-strand in Gβ(5) with CCT5 to a completely folded β-propeller structure. Here, we used biochemistry and cryo-EM to determine how missense mutations in Gβ(5), including those that cause severe neurological diseases, alter the Gβ(5) folding trajectory and lead to incompletely folded, trapped intermediates. These findings highlight how defects in chaperonin-mediated folding contribute to disease and suggest potential strategies for stabilizing misfolded proteins to restore function.