Localization of mitochondrial 60-kD heat shock chaperonin protein (Hsp60) in pituitary growth hormone secretory granules and pancreatic zymogen granules

Abstract

We used quantitative immunogold electron microscopy and biochemical analysis to evaluate the subcellular distribution of Hsp60 in rat tissues. Western blot analysis, employing both monoclonal and polyclonal antibodies raised against mammalian Hsp60, shows that only a single 60-kD protein is reactive with the antibodies in brain, heart, kidney, liver, pancreas, pituitary, spleen, skeletal muscle, and adrenal gland. Immunogold labeling of tissues embedded in the acrylic resin LR Gold shows strong labeling of mitochondria in all tissues. However, in the anterior pitutary and in pancreatic acinar cells, Hsp60 also localizes in secretory granules. The labeled granules in the pituitary and pancreas were determined to be growth hormone granules and zymogen granules, respectively, using antibodies to growth hormone and carboxypeptidase A. Immunogold labeling of Hsp60 in all compartments was prevented by preadsorption of the antibodies with recombinant Hsp60. Biochemically purified zymogen granules free of mitochondrial contamination are shown by Western blot analysis to contain Hsp60, confirming the morphological localization results in pancreatic acinar cells. In kidney distal tubule cells, low Hsp60 reactivity is associated with infoldings of the basal plasma membrane. In comparison, the plasma membrane in kidney proximal tubule cells and in other tissues examined showed only background labeling. These findings raise interesting questions concerning translocation mechanisms and the cellular roles of Hsp60.