A novel chaperonin-encapsulation system for NMR measurements has been designed. The single-ring variant SR398 with an ATPase deficient mutation of GroEL, also known as chaperonin, bound co-chaperonin GroES irreversibly, forming a stable cage to encapsulate a target protein. A small GroEL-binding tag made it possible to perform all steps of the encapsulation under near physiological conditions while retaining the native conformation of the target protein. About half of the SR398/GroES cages encapsulated target protein molecules. As binding only depends on the 12-residue tag sequence, this encapsulation method is applicable to a large number of proteins. Isolation of the target proteins in the molecular cage of chaperonin will allow the study of highly aggregation-prone proteins by solution NMR. (C) 2010 Elsevier B.V. All rights reserved.