QUASI-NATIVE CHAPERONIN-BOUND INTERMEDIATES IN FACILITATED PROTEIN-FOLDING

Chaperonins are known to facilitate protein folding, but their mechani sm of action is not well understood. The fact that target proteins are released from and rebind to different chaperonin molecules (''cycling '') during a folding reaction suggests that chaperonins function by un folding aberrantly folded molecules, allowing them multiple opportunit ies to reach the native state in bulk solution. Here we show that the cycling of alpha-tubulin by cytosolic chaperonin (c-cpn) can be uncoup led from the action of cofactors required to complete the folding reac tion. This results in the accumulation of folding intermediates which are chaperonin-bound, stable, and quasi-native in that they bind GTP n onex-changeably. We present evidence that these intermediates can be g enerated without the target protein leaving c-cpn. These data show tha t, in contrast to prevailing models, target proteins can maintain, and possibly acquire, significant native-like structure while chaperonin- bound.