Identification of calpastatin and μ-calpain and studies of their association in pulmonary smooth muscle mitochondria

Using calpastatin antibody we have identified a 145 kDa major band along with two relatively minor bands at 120 kDa and 110 kDa calpastatin molecules in bovine pulmonary artery smooth muscle mitochondria. To the best of our knowledge this is first report regarding the identification of calpastatin in mitochondria. We also demonstrated the presence of μ-calpain in the mitochondria by immunoblot and casein zymogram studies. Immunoblot studies identified two major bands corresponding to the 80 kDa large and the 28 kDa small subunit of μ-calpain. Additionally 76 kDa, 40 kDa and 18 kDa immunoreactive bands have also been detected. Purification and N-terminal amino acid sequence analysis of the identified proteins confirmed their identity as μ-calpain and calpastatins. Immunoprecipitation study revealed molecular association between μ-calpain and calpastatin in the mitochondria indicating that calpastatin could play an important role in preventing uncontrolled activity of μ-calpain which otherwise may facilitate pulmonary hypertension, smooth muscle proliferation and apoptosis.