3D reconstruction of the ATP-bound form of CCT reveals the asymmetric folding conformation of a type II chaperonin.

Abstract

The type II chaperonin CCT (chaperonin containing Tcp-1) of eukaryotic cytosol is a heteromeric 16-mer particle composed of eight different subunits. Three-dimensional reconstructions of apo-CCT and ATP-CCT have been obtained at 28 A resolution by cryo-electron microscopy. Binding of ATP generates an asymmetric particle; one ring has a slightly different conformation from the apo-CCT ring, while the other has undergone substantial movements in the apical domains. Upon ATP binding the apical domains rotate and point towards the cylinder axis, so that the helical protrusions present at their tips could act as a lid closing the ring cavity.