Hsp10 (10-kDa
heat shock protein, also known as
chaperonin 10 or Cpn10) is a co-chaperone for Hsp60 in the protein folding process. This
protein has also been shown to be identical to the
early pregnancy factor, which is an immunosuppressive
growth factor found in maternal serum. In this study we have used immunogold electron microscopy to study the subcellular localization of Hsp10 in rat tissues sections embedded in
LR Gold resin employing polyclonal
antibodies raised against different regions of human Hsp10. In all rat tissues examined including liver, heart, pancreas, kidney, anterior pituitary, salivary gland, thyroid, and adrenal gland,
antibodies to Hsp10 showed strong labeling of mitochondria. However, in a number of tissues, in addition to the mitochondrial labeling, strong and highly specific labeling with the Hsp10
antibodies was also observed in several extramitochondrial compartments. These sites included zymogen granules in pancreatic acinar cells,
growth hormone granules in anterior pituitary, and secretory granules in PP pancreatic islet cells. Additionally, the mature red blood cells which lack mitochondria, also showed strong reactivity with the Hsp10
antibodies. The observed labeling with the Hsp10
antibodies, both within mitochondria as well as in other compartments/cells, was abolished upon omission of the primary
antibodies or upon preadsorption of the primary
antibodies with the purified recombinant human Hsp10. These results provide evidence that similar to a number of other recently described
mitochondrial proteins (viz., Hsp60,
tumor necrosis factor receptor-associated
protein-1, P32 (gC1q-R)
protein, and
cytochrome c), Hsp10 is also found at a variety of specific extramitochondrial sites in normal rat tissue. These results raise important questions as to how these
mitochondrial proteins are translocated to other compartments and their possible function(s) at these sites. The presence of these
proteins at extramitochondrial sites in normal tissues has important implications concerning the role of mitochondria in apoptosis and
genetic diseases.