CYSTOSOLIC CHAPERONIN SUBUNITS HAVE A CONSERVED ATPASE DOMAIN BUT DIVERGED POLYPEPTIDE-BINDING DOMAINS

CCT (also called the TCP-1 complex or TriC) is a chaperonin found in t he eukaryotic cytosol, and has unique structural and functional featur es. Unlike homo-oligomeric chaperonins, CCT comprises at least eight d ifferent subunits, and appears to have a limited range of physiologica l substrates. We have analysed CCT sequences in light of the recent de termination of the crystal structure and mutational identification of the functional domains of the bacterial chaperonin GroEL. A high level of identity among all chaperonin subunits is observed in those region s that correspond to the ATP-binding site of GroEL. By contrast, no si gnificant identity is shared in the region corresponding to the polype ptide-binding region of GroEL, either between CCT subunits or between CCT subunits and GroEL. This suggests that the polypeptide-binding sit es of CCT subunits have diverged both from each other and from GroEL, which may explain the apparently different range of substrates recogni zed by CCT.