Studies on the eukaryotic chaperonin CCT

CCT (chaperonin containing TCP-1) is a barrel-shaped chaperone complex (16-mer) of eight different subunits, organised in two rings. CCT is essential in folding the cytoskeletal proteins actin and tubulin but also interacts with a wider range of proteins. A strong functional link was predicted by bioinformatics (Marcotte 05 al. Nature 402 p83 1999) with yeast Sup35p, a model prion-like protein in yeast, which is the eukaryotic translation release factor 3 (eRF3). Sup35p shares 43% amino acid similarity with its mammalian homologue named G to S-phase transition protein 1 (GSPT1). In mammalian systems there are two GSPT homologues GSPT1 and GSPT2. Unlike Sup35p, GSPT aggregation is unknown and it lacks the N-terminal repeats implicated in Sup35p prionogenesis. In this study a non-substrate like interaction has been demonstrated between Sup35p/GSPT and CCT. This interaction has been demonstrated both in vitro and in vivo by a number of methods. The nature of the interaction indicates that sub-16mer complexes of CCT are involved as opposed to the whole complex. The presence of these microcomplexes has been both implied and demonstrated in a study of the disassembly process of CCT. It has been demonstrated that the CCT complex disassembles in a single ring mediated manner and requires the presence of both ATP and K+ ions. The order in which the CCT subunits are able to leave the complex appears to be related to the size of a variable loop as reported by Roobol et al (1999) and not to their individual ATPase activities. The ATPase activities of four of the subunits are reported and vary enormously. It is proposed that the function of the CCT/eRF3 interaction is to couple translation termination and protein folding in the cytosol, a process not previously put forward involving these proteins.