The 60-kDa heat shock protein (mHsp60) is a vital cellular complex that mediates the folding of many of the mitochondrial proteins. Its function is executed in cooperation with the cochaperonin, mHsp10, and requires ATP. Recently, the discovery of a new mHsp60-associated neurodegenerative disorder, Mit-CHAP-60 disease, has been reported. The disease is caused by a point mutation at position 3 (D3G) of the mature mitochondrial Hsp60 protein, which renders it unable to complement the deletion of the homologous bacterial protein in Escherichia coli (Magen, D., Georgopoulos, C., Bross, P., Ang, D., Segev, Y., Goldsher, D., Nemirovski, A., Shahar, E., Ravid, S., Luder, A., Heno, B., Gershoni-Baruch, R., Skorecki, K., and Mandel, H. (2008) Am. J. Hum. Genet. 83, 30 -42). The molecular basis of the MitCHAP-60 disease is still unknown. In this study, we present an in vitro structural and functional analysis of the purified wild-type human mHsp60 and the MitCHAP-60 mutant. We show that the D3G mutation leads to destabilization of the mHsp60 oligomer and causes its disassembly at low protein concentrations. We also show that the mutant protein has impaired protein folding and ATPase activities. An additional mutant that lacks the first three amino acids (N-del), including Asp-3, is similarly impaired in refolding activity. Surprisingly, however, this mutant exhibits profound stabilization of its oligomeric structure. These results suggest that the D3G mutation leads to entropic destabilization of the mHsp60 oligomer, which severely impairs its chaperone function, thereby causing the disease.Type I chaperonins are essential molecular chaperones of the Hsp60 family found in eubacteria, mitochondria, and chloroplasts (1). They are key players in mediating the correct folding of newly translated, translocated, and stress-denatured proteins. The folding function of chaperonins is executed by the coordinated action of two oligomeric proteins, Hsp60 (also named cpn60 and in bacteria GroEL) and its co-chaperonin Hsp10 (also named cpn10 and in bacteria GroES). The GroEL molecule is composed of 14 subunits that form a barrel-like structure that consists of two back-to-back stacked heptameric rings with a large cavity at each end termed the "Anfinsen cage." GroES is a heptameric ring formed by ϳ10-kDa subunits. The co-chaperonin binds to the chaperonin in the presence of ATP and Mg 2ϩ via a short, unstructured, but highly conserved region known as the mobile loop (2, 3). Due to the stability of the GroEL/ES oligomers and the ease with which they can be purified from bacteria, they have become the primary targets for study in the field of chaperonins. As a result, almost all of our knowledge concerning the structure and mechanism of chaperonins, both Hsp60 and Hsp10, is based largely on data from experiments on these E. coli proteins. The chaperonin reaction cycle starts when a non-folded protein substrate binds to the surface of the cavity of one of the GroEL rings. ATP-dependent GroES binding to that (cis) ring causes a dramatic co...