Eukaryotic type II chaperonin CCT interacts with actin through specific subunits

Chaperonins assist the folding of other proteins(1). Type II chaperonins, s uch as chaperonin containing TCP-1(CCT), are found in archaea and in the eu karyotic cytosol(2). They are hexadecameric or nonadecameric oligomers comp osed of one to eight different polypeptides. Whereas type I chaperonins lik e GroEL are promiscuous, assisting in the folding of many other proteins(1) only a small number of proteins, mainly actin and tubulin, have been descr ibed as natural substrates of CCT, This specificity may be related to the d ivergence of the eight CCT subunits(3). Here we have obtained a three-dimen sional reconstruction of the complex between CCT and alpha-actin by cryo-el ectron microscopy and image processing. This shows that cr-actin interacts with the apical domains of either of two CCT subunits. Immunolabelling of C CT-substrate complexes with antibodies against two specific CCT subunits sh owed that actin binds to CCT using two specific and distinct interactions: the small domain of actin binds to CCT delta and the large domain to CCT be ta or CCT epsilon (both in position 1,4 with respect to delta). These resul ts indicate that the binding of actin to CCT is both subunit-specific and g eometry-dependent, Thus, the substrate recognition mechanism of eukaryotic CCT may differ from that of prokaryotic GroEL.