Protein folding in the cell is critically dependent on the assistance of molecular chaperones. The chaperonin of eukaryotic cells, TRiC, is a 1 MDa ring-shaped complex that folds many essential cellular proteins. TRiC is hetero-oligomeric and uses ATP binding and hydrolysis to open and close a built-in lid over the central cavity. Notably, TRiC has the ability to fold some eukaryotic proteins, such as actin, that cannot be folded by any other chaperone. Thus understanding TRiC mechanism is an

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... tegral first step to deciphering the role of this chaperonin in cellular folding and potentially ameliorating human misfolding disorders. Recent insights into the structural and mechanistic basis of the conformational cycle of TRiC will be discussed, as well as their implications for polypeptide-mediated folding.