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 β₅ subunit (Gβ₅) 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 Gβ₅, from an initiating electrostatic interaction of a single β-strand in Gβ₅ with CCT5 to a completely folded β-propeller structure. Here, we used biochemistry and cryo-EM to determine how missense mutations in Gβ₅, including those that cause severe neurological diseases, alter the Gβ₅ 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.