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.