THE STRUCTURAL STABILITY OF THE CO-CHAPERONIN GROES

The structural stability of the co-chaperonin GroES has been studied b y high sensitivity differential scanning calorimetry and circular dich roism under different solvent conditions. The thermal folding/unfoldin g of GroES is a spontaneous reversible process involving a highly coop erative transition between folded heptamers and unfolded monomers. Dur ing the-denaturation process folded monomers are energetically unfavor able and consequently never become populated to an appreciable degree. Analysis of the high resolution structure indicates that isolated fol ded monomers of GroES bury a significantly smaller fraction of their t otal surface than typical globular proteins of similar molecular mass. For this reason the intramolecular interactions within each GroES mon omer appear not to be sufficient for thermodynamic stabilization. The stabilization of the heptameric structure is due primarily to intersub unit interactions rather than intrasubunit interactions. These interac tions favor oligomerization both enthalpically and entropically. Despi te the high density of charged residues, the stability of GroES shows no measurable dependence on salt concentration at PH 7. On the other h and, millimolar concentrations of magnesium stabilize GroES, presumabl y by specific binding. The stabilization elicited by Mg2+ is consisten t with a dissociation constant of the order of 0.5 mM and approximatel y three binding sites per heptamer, These results emphasize the role o f quaternary structure in the stabilization of small oligomeric protei ns. (C) 1997 Academic Press Limited.