Unusual Cellular Disposition of the Mitochondrial Molecular Chaperones Hsp60, Hsp70 and Hsp10

A number of molecular chaperones in mammalian cells are localized in mitochondria and they are presumed to function mainly within this organelle. However, there is now compelling evidence that these chaperones are also localized at a variety of other sites/compartments in cells where they perform important functions. These proteins include: (i) the major chaperonin Hsp60 (or P1), which was identified in mammalian cells as a protein altered in mutants resistant to microtubule inhibitors and is involved in numerous functions at the cell surface and in other compartments; (ii) the Hspl0 or Cpn10 protein, which is a co-chaperone for Hsp60 in protein folding but also serves as an early pregnancy factor in maternal serum; and (iii) the mHsp70 protein, which plays a central role in mitochondrial protein import but is also important for cellular senescence (mortalin) and antigen presentation processes. The presence of these mitochondrial chaperones at specific extramitochondrial locations greatly broadens the range of functions that they can carry out in cells. However, these observations also raise important questions regarding the mechanisms by which these proteins reach these extramitochondrial locations. My paper will review some work in this area and discuss the significance of these results.