Purification of functional recombinant human mitochondrial Hsp60.

The mitochondrial 60 kDa heat shock protein (mHsp60) is an oligomeric, barrel-like structure that mediates protein folding in cooperation with its cochaperonin Hsp10, in an ATP-dependent manner. In contrast to the extremely stable oligomeric structure of the bacterial chaperonin, GroEL, the human mHsp60 exists in equilibrium between single and double heptameric units, which dissociate easily to inactive monomers under laboratory conditions. Consequently, purification and manipulation of active mHsp60 oligomers is not straightforward. In this manuscript, we present an improved protocol for the purification of functional mHsp60, following its expression in bacteria. This method is based upon a previously published strategy that exploits the notorious instability of mHsp60 to purify the monomeric form, which is subsequently reconstituted to functional oligomers under controlled conditions. In our protocol, we use affinity chromatography on a Ni NTA-agarose resin as the initial step, facilitating purification of substantial amounts of highly pure active protein. The resulting Hsp60 is suitable for both functional and structural analyses, including crystallography and electron cryo-microscopy (cryo-EM) studies, to obtain high resolution structures of the mHsp60 oligomers alone and in various complexes.