Structure and Stability of Hsp60 and Groel in Solution

Molecular chaperones are a class of proteins able to prevent non-specific aggregation of mitochondrial proteins and to promote their proper folding. Among them, human Hsp60 is currently considered as a ubiquitous molecule with multiple roles both in maintaining health conditions and as a trigger of several diseases. Of particular interest is its role in neurodegenerative disorders since it is able to inhibit the formation of amyloid fibrils.Hsp60 structure was considered, until recent years, analogue to the one of its bacterial homolog GroEL, one of the most investigated chaperones, whose crystallographic structure is a homo-tetradecamer, made up of two seven member rings. On the contrary, it has been recently demonstrated by some of us that Hsp60 shows an equilibrium of tetradecamers and heptamers in solution.We present the results of combined investigations including complementary approaches such as molecular dynamics (MD), circular dichroism (CD) and small-angle X-ray scattering (SAXS) concerning structure, self-organization, and stability of naive Hsp60 in solution in comparison with bacterial GroEL. We analyzed the structural differences between the two proteins in solution, evidencing their different oligomeric equilibria, too. Experiments at increasing concentrations of guanidinium hydrochloride provided further information on the stability of the oligomeric species, on the dissociation of tetradecamers into heptamers and monomers, until the unfolding. The unfolding free energy was obtained, proving that GroEL is more stable in respect to Hsp60.