On the brotherhood of the mitochondrial chaperones mortalin and heat shock protein 60

Abstract

The heat shock chaperones mortalin/mitochondrial heat shock protein 70 (mtHsp70) and Hsp60 are found in multiple subcellular sites and function in the folding and intracellular trafficking of many proteins. The chaperoning activity of these 2 proteins involves different structural and functional mechanisms. In spite of providing an excellent model for an evolutionarily conserved molecular "brotherhood", their individual functions, although overlapping, are nonredundant. As they travel to various locations, both chaperones acquire different binding partners and exert a more divergent involvement in tumorigenesis, cellular senescence, and immunology. An understanding of their functional biology may lead to novel designing and development of therapeutic strategies for cancer and aging.

Fig 1.

(A) A “kettle pot” model for the structure of mortalin. Above, For simplicity, we can compare the structure of the mortalin with a kettle pot showing its handle (ATPase domain), which consists of 4 subdomains that fold into a pair of lobes to form a deep catalytic cleft and regulates the opening of the lid, and the pot (substrate-binding cleft), which contains the substrate. Upon binding to ATP, the Hsp70 ATPase confers the lid an altered conformation, opening it by bending the hinge (shown by an empty square), and allows the substrate to enter the cleft. Below, Ribbon diagrams of the peptide-binding domain (in standard stereo [left] and one rotated 90° counterclockwise [right]) (from the Protein Data Bank, 1dkx.pdb). Four loops emanate within this β-subdomain: an inner pair (L_1,2 and L_3,4) that establishes a hydrophobic substrate cleft, and a flanking outer pair (L_4,5 and L_5,6) that stabilizes the substrate-binding cleft. (B) A “donut and munchkin” model for Hsp60. Shown in side view is the structure of the GroEL–GroES–(ADP)₇ complex with the 4-banded appearance of the “doubly stacked donut” GroEL and its apical spherical-shaped GroES (munchkin) with a central substrate-binding cavity. Each “donut” is composed of a ring of 7 subunits. Each subunit (right) is composed of 3 domains: apical, intermediate, and equatorial. Upon binding of 7 molecules of ATP to the equatorial domains, sequential conformational changes occur in both the intermediate and apical domains, and allosteric interactions are transmitted from one ring to the other causing the expansion of the cavity size that gives an illusion that the complex “breathes.” If substrate is not folded properly, it can rebind to the same or different GroEL molecule, and the cycle of regurgitation is repeated until the protein substrate attains its native state (from the Protein Data Bank, 1a6d.pdb).

Fig 2.

Mortalin and Hsp60 cooperate during mitochondrial biogenesis and maintenance. After protein synthesis, preproteins enter the mitochondria through the outer membrane (TOM) followed by the inner membrane (TIM) translocation channels. Mortalin, bound to Tim44, brings the preprotein into the matrix by acting either as a molecular ratchet or motor (see box and see Schneider et al 1994; Neupert and Brunner 2002). During its transit, the preprotein is simultaneously refolded by mortalin, or is handed over to the Hsp60 complex for further cycles of protein refolding. Alternatively, some proteins are synthesized by resident mitochondrial ribosomes, and the nascent proteins are also assisted to fold by mortalin and Hsp60. Upon reaching the native state, mature mitochondrial proteins are further trafficked, assembled, and sorted to their proper places to start functioning in various mitochondrial processes. Because proteins may be damaged, either by wear-and-tear or by ROS attack during their lifetime, the repair of misfolded domains or degradation is achieved with the assistance of the 2 chaperones.