Eukaryotic type II chaperonin CCT interacts with actin through specific subunits

Chaperonins assist the folding of other proteins. Type II chaperonins, such as chaperonin containing TCP-1 (CCT), are found in archaea and in the eukaryotic cytosol. They are hexadecameric or nonadecameric oligomers composed of one to eight different polypeptides. Whereas type I chaperonins like GroEL are promiscuous, assisting in the folding of many other proteins, only a small number of proteins, mainly actin and tubulin, have been described as natural substrates of CCT. This specificity may be related to the divergence of the eight CCT subunits. Here we have obtained a three-dimensional reconstruction of the complex between CCT and α-actin by cryo-electron microscopy and image processing. This shows that α-actin interacts with the apical domains of either of two CCT subunits. Immunolabelling of CCT-substrate complexes with antibodies against two specific CCT subunits showed that actin binds to CCT using two specific and distinct interactions: the small domain of actin binds to CCTδ and the large domain to CCTβ or CCTε (both in position 1,4 with respect to δ). These results indicate that the binding of actin to CCT is both subunit-specific and geometry-dependent. Thus, the substrate recognition mechanism of eukaryotic CCT may differ from that of prokaryotic GroEL.