Structure and function of a protein folding machine: the eukaryotic cytosolic chaperonin CCT

Chaperonins are large oligomers made up of two superimposed rings, each enclosing a cavity used for the folding of other proteins. Among the chaperonins, the eukaryotic cytosolic chaperonin CCT is the most complex, not only with regard to its subunit composition but also with respect to its function, still not well understood. Unlike the more well studied eubacterial chaperonin GroEL, which binds any protein that presents stretches of hydrophobic residues, CCT recognises in its substrates specific binding determinants and interacts with them through particular combinations of CCT subunits. Folding then occurs after the conformational changes induced in the chaperonin upon nucleotide binding have occurred, through a mechanism that, although still poorly defined, clearly differs from the one established for GroEL. Although CCT seems to be mainly involved in the folding of actin and tubulin, other substrates involved in various cellular roles are beginning to be characterised, including many WD40-repeat, 7-blade propeller proteins.