Thermodynamic stability and folding of GroEL minichaperones.

The apical domain of GroEL (residues 191 to 376) and its C-terminally truncated fragment GroEL(191-345) are expressed with high yield in Escherichia coli to give functional monomeric minichaperones. Owing to the reversible folding behaviour of the minichaperones we can analyse the folding of the polypeptide binding domain of the multidomain GroEL protein, the folding of which is known to be irreversible. The apical domain shows two reversible temperature transitions with transition midpoints at 35 degrees C and at 67 degrees C that can be attributed to the unfolding of the C-terminal helices and the domain core, respectively. The native state of the domain core is stabilized by 5.5 kcal mol-1 relative to the unfolded state. The rate constant of folding of the apical domain core is independent of the minichaperone concentration and the presence of the C-terminal alpha-helices. A folding intermediate on the folding pathway is destabilized relative to the native state by 1.6 kcal mol-1, which is also detected by equilibrium and kinetic binding of the dye bis-ANS. Reversible folding of the polypeptide domain of GroEL guarantees highly efficient chaperonin activity within the GroEL toroid.