Analysis of the interaction between the eukaryotic chaperonin CCT and its substrates actin and tubulin

Two mechanisms have thus far been characterized for the assistance by chape ronins of the folding of other proteins. The first and best described is th at of the prokaryotic chaperonin GroEL, which interacts with a large spectr um of proteins. GroEL uses a nonspecific mechanism by which any conformatio n of practically any unfolded polypeptide interacts with it through exposed , hydrophobic residues. ATP binding liberates the substrate in the GroEL ca vity where it is given a chance to fold. A second mechanism has been descri bed for the eukaryotic chaperonin CCT, which interacts mainly with the cyto skeletal proteins actin and tubulin. Cryoelectron microscopy and biochemica l studies have revealed that both of these proteins interact with CCT in qu asi-native, defined conformations. Here we have performed a detailed study of the docking of the actin and tubulin molecules extracted from their corr esponding CCT:substrate complexes obtained from cryoelectron microscopy and image processing to localize certain regions in actin and tubulin that are involved in the interaction with CCT. These regions of actin and tubulin, which are not present in their prokaryotic counterparts FtsA and FtsZ, are involved in the polymerization of the two cytoskeletal proteins. These find ings suggest coevolution of CCT with actin and tubulin in order to countera ct the folding problems associated with the generation in these two cytoske letal protein families of new domains involved in their polymerization. (C) 2001 Academic Press.