Localization of P32 protein (gC1q-R) in mitochondria and at specific extramitochondrial locations in normal tissues

P32 protein, also known as the gC1q receptor for complement component C1q, is a binding protein for nuclear pre-mRNA splicing factor SF2/ASF and numer ous other nuclear and cell surface proteins, yet is targeted to the mitocho ndrial matrix compartment where these proteins are not present. In the pres ent study, we use immunogold electron microscopy to evaluate the subcellula r distribution of P32 protein (gC1q-R) in cultured cell lines and in rat ti ssues embedded in the acrylic resin LR Gold. Immunogold labeling of Raji ly mphoma, CHO, human fibroblasts, HeLa and B-SC-1 cells shows reactivity prim arily within mitochondria. Highly specific labeling of mitochondria is also obtained in rat tissues, including adrenal gland, cerebellum, cerebral cor tex, heart, kidney, liver, pituitary, pancreas, skeletal muscle, spleen, te stes and thyroid. However, strong P32 (gC1q-R) reactivity is also present i n (i) zymogen granules, condensing vacuoles, endoplasmic reticulum, and on the cell surface of pancreatic acinar cells, (ii) on the cell surface of mi crovascular endothelial cells in pancreas and kidney, (iii) on the cell sur face and in nuclei of splenic lymphocytes, and (iv) in the acrosome of deve loping spermatids in testes. Western immunoblots show that the polyclonal a ntibody to P32 (gC1q-R) used in this study reacts specifically with a 32-kD a protein in both purified pancreatic zymogen granules and in mitochondria, and no other proteins are reactive. These results provide evidence that P3 2 (gC1q-R) is a mitochondrial protein that also localizes outside mitochond ria in certain cells and tissues under normal physiological conditions.