We have isolated a Cytoplasmic chaperonin based on its ability to catalyze the folding of denatured β-actin. The cytoplasmic chaperonin is organized as a multisubunit torold and requires Mg2+ and ATP for activity. The folding reaction proceeds via the rapid ATP-Independent formation of a binary complex, followed by a slower ATP-dependent release of the native product. Electron microscopic observations reveal a striking structural change that occurs upon addition of Mg2+ and ATP. The eukaryotic cytoplasm thus contains a chaperonin that Is functionally analagous to its prokaryotic, mitochondrial, and chloroplastic counterparts.
