COFACTOR-A IS A MOLECULAR CHAPERONE REQUIRED FOR BETA-TUBULIN FOLDING- FUNCTIONAL AND STRUCTURAL CHARACTERIZATION

Actin and tubulin polypeptide drains acquire their native conformation in the presence of the chaperonin containing TCP-1 (CCT) and, in the case of alpha- and beta-tubulin additional protein cofactors. We recen tly identified one of these cofactors, termed cofactor A, that is requ ired for the proper folding of the beta-tubulin chain [Gao et al. (199 4) J. Cell. Biol. 125, 989-996]. We show here that cofactor A, a monom eric protein that has no measurable affinity for nucleotides, is a hig hly conserved protein among vertebrates. Its NH2-terminal region is es sential for the structural integrity of the protein and consequently f or its activity. We demonstrate that cofactor A does not interact with CCT nor does it affect the intrinsic ATPase activity of CCT, alone or in the presence of different target proteins. Thus, unlike GroES, cof actor A does not modulate or coordinate ATP hydrolysis. It does not ac t as a nucleotide exchange factor or a catalyst in tubulin folding. Ra ther, we demonstrate that cofactor A participates in the tubulin foldi ng process by interacting with a folding intermediate of beta-tubulin that is released from CCT. Our data imply that cofactor A is a chapero ne involved in tubulin folding.