Intramitochondrial Localization of the Main 70-kDa Heat-Shock Cognate Protein in Drosophila Cells

The main heat shock cognate protein (hsc) of 70 kDa in Drosophila, hsc 4, was localized in cultured cells using a specific affinity-purified antibody and colloidal gold immunoelectron microscopy. This constitutively expressed member of the heat shock protein (hsp) 70 family is found in the cytosol, in mitochondria, and in the nucleus of unstressed cells. The identity of hsc 4 in these three cellular compartments was confirmed by two-dimensional gel immunoblots and partial proteolytic digestion patterns. In mitochondria, the colloidal gold particles are observed in close proximity to or on the inner membranes. The intramitochondrial localization of this hsc was confirmed by density gradient purification and by resistance of hsc 4 to externally added trypsin. In the nucleus, the labeling is found on nucleoplasmic perichromatin RNP fibrils and is not detected in the nucleolus. Heat shock induces an intracellular redistribution of hsc 4 with an enrichment in the nucleus. The localization of this hsc in different cellular compartments is consistent with the previously suggested functions of some members of this family of proteins in basic cellular processes such as protein folding. Moreover, the present data suggests that the main constitutively expressed member of the hsp 70 family, hsc 4, functions both within the mitochondrial compartment and in the nucleus.