Monomeric chaperonin-60 and its 50-kDa fragment possess the ability to interact with non-native proteins, to suppress aggregation, and to promote protein folding.

Chaperonin-60 is usually isolated as a tetradecameric form arranged as two stacked seven-member rings, and this structure has been considered to be required for promoting protein folding. However, monomeric chaperonin-60 (cpn60m), isolated from holo-chaperonin of Thermus thermophilus, and its proteolytic 50-kDa fragment, which lacks amino-terminal 78 amino acid residues, can interact with non-native rhodanese and lactate dehydrogenase, suppress formation of aggregates, and promote productive folding under the appropriate conditions. However, different from tetradecameric chaperonin-60, folding promoted by cpn60m and the 50-kDa fragment produces lower yields of active enzymes and does not require ATP or chaperonin-10. These effects are not due to transient reassembly of cpn60m into a tetradecamer during the reaction, since immobilized cpn60m and the 50-kDa fragment, both of which can not reassemble into a tetradecamer, can still promote protein folding. An excess amount of the 50-kDa fragment shows an inhibitory effect on MgATP-triggered holochaperonin-dependent folding, indicating the 50-kDa fragment and holo-chaperonin can interact with the same species of non-native proteins. Thus, cpn60m has an intrinsic activity as a molecular chaperone and amino-terminal region of cpn60 is dispensable for this activity.