The group II chaperonin Mm‐Cpn binds and refolds mutant H[&gamma]D Crystallin to a native‐like structure

The aggregation of damaged or misfolded proteins is associated with a number of human diseases, including Alzheimer's disease, Huntington's disease, and cataract. In this study, we investigate the ability of the Group II chaperonin from Methanococcus marapaludis , Mm‐Cpn, a homolog of the eukaryotic chaperonin TRiC, to bind and refold human γD crystallins. Crystallins are a family of structural proteins found in the lens of the human eye, and aggregation of these proteins is thought to be the cause of cataract. Mm‐Cpn interactions with both wild type HγD‐Crys, and damage and disease model mutant HγD‐Crys were evaluated. Solution turbidity studies indicate that Mm‐Cpn suppresses aggregation of both wild type and disease model mutant HγD‐Crys, and exhibits a greater affinity for the destabilized mutant HγD‐Crys. In addition, size exclusion chromatography and fluorescence spectroscopy show that Mm‐Cpn can refold HγD to a native like state, as well as form a long‐lived Mm‐Cpn/HγD complex with both the wild type and mutant HγD crystallins. This long‐lived complex may be ideal for imaging of the chaperonin/substrate complex by cryo‐EM or x‐ray crystallography. These data suggest that the Mm‐Cpn/HγD interaction may serve as a model for general chaperonin/substrate interaction, as well as provide a better understanding of the mechanism of binding and refolding by the Group II chaperonins.