FACILITATED FOLDING OF ACTINS AND TUBULINS OCCURS VIA A NUCLEOTIDE-DEPENDENT INTERACTION BETWEEN CYTOPLASMIC CHAPERONIN AND DISTINCTIVE FOLDING INTERMEDIATES

In the cytoplasm of eukaryotes, the folding of actins and tubulins is facilitated via interaction with a heteromeric toroidal complex (cytop lasmic chaperonin), The folding reaction consists of the formation of a binary complex between the unfolded target protein and the chaperoni n, followed by the ultimate release of the native polypeptide in an AT P-dependent reaction. Here we show that the mitochondrial chaperonin ( cpn60) and the cytoplasmic chaperonin both recognize a range of target proteins with different relative affinities; however, the cytoplasmic chaperonin shows the highest affinity for intermediates derived from unfolded tubulins and actins. These high-affinity actin and tubulin fo lding intermediates are distinct from the ''molten globule'' intermedi ates formed-by noncytoskeletal target proteins in that they form relat ively slowly. We show that the interaction between cytoplasmic chapero nin and unfolded target proteins depends on the chaperonin being in it s ADP-bound state and that the release of the target protein occurs af ter a transition of the chaperonin to the ATP-bound state. Our data su ggest a model in which ATP hydrolysis acts as a switch between conform ational forms of the cytoplasmic chaperonin that interact either stron gly or weakly,vith unfolded substrates.