ON THE ROLE OF GROES IN THE CHAPERONIN-ASSISTED FOLDING REACTION - 3 CASE-STUDIES

The mechanism by which correctly folded proteins are recovered from st able complexes with groEL is not well understood. Certain target prote ins require ATP and groES, while others seemingly dispense with the co -chaperonin. Here, we examine the chaperonin-assisted folding of ribul ose-1,5-bisphosphate carboxylase, malate dehydrogenase, and citrate sy nthase, three proteins that are believed to require both chaperonin co mponents for successful reactivation. Surprisingly, in all cases, the need for groES depended on the folding environment. Under ''non-permis sive'' conditions, where unassisted spontaneous folding could not occu r, reactivation to the native state required the complete chaperonin s ystem (e.g. groEL, groES, and MgATP). However, under ''permissive'' co nditions where spontaneous folding could occur groES was no longer man datory. Instead, upon the addition of ATP alone, all three target prot eins could be released from groEL, in a form that was capable of reach ing the native state. In the permissive setting, groES merely accelera ted the rate of the ATP dependent release process. The results suggest that the incompletely folded protein species that are released from g roEL, in the absence of groES, are not necessarily committed to the na tive state. Similar to the unassisted folding reaction, they still par tition between productive and unproductive folding pathways in an envi ronment-dependent manner. It follows that the mechanistic contribution of the co-chaperonin, groES, and its physiological significance in ce llular protein folding, could be entirely missed in a permissive in vi tro environment.