Structure of the complex between the cytosolic chaperonin CCT and phosducin-like protein.

The three-dimensional structure of the complex formed between the cytosolic chaperonin CCT (chaperonin containing TCP-1) and phosducin (Pdc)-like protein (PhLP), a regulator of CCT activity, has been solved by cryoelectron microscopy. Binding of PhLP to CCT occurs through only one of the chaperonin rings, and the protein does not occupy the central folding cavity but rather sits above it through interactions with two regions on opposite sides of the ring. This causes the apical domains of the CCT subunits to close in, thus excluding access to the folding cavity. The atomic model of PhLP generated from several atomic structures of the homologous Pdc fits very well with the mass of the complex attributable to PhLP and predicts the involvement of several sequences of PhLP in CCT binding. Binding experiments performed with PhLP/Pdc chimeric proteins, taking advantage of the fact that Pdc does not interact with CCT, confirm that both the N- and C-terminal domains of PhLP are involved in CCT binding and that several regions suggested by the docking experiment are indeed critical in the interaction with the cytosolic chaperonin.