The binding of large calpastatin to biologic membranes is mediated in part by interaction of an amino terminal region with acidic phospholipids.

Animal cells contain a non-lysosomal proteolytic system which degrades various protein substrates in the presence of calcium ion. The calcium-dependent proteinases (calpains) co-exist in cells with a specific protein inhibitor called calpastatin. Distribution of this inhibitor to different subcellular sites could be important in overall regulation of the calpains. Previously, myocardial calpastatin was shown to be present in preparations of sarcoplasmic reticulum and sarcolemma. In the present work, we show that purified bovine myocardial calpastatin binds to the acidic phospholipids, phosphatidylinositol and phosphatidylserine, but not to the neutral phospholipids, phosphatidylcholine and phosphatidylethanolamine. Large forms of calpastatin from canine myocardium and rabbit liver also were bound to phosphatidylinositol. Smaller forms of calpastatin present in the preparations did not bind to acidic phospholipids. Bovine large calpastatin was subjected to CNBr digestion, and a phospholipid-binding fragment representing approximately one-sixth of the intact protein mass was purified. Amino acid sequence analysis indicated that the phospholipid-binding fragment was derived from the amino terminus of the large calpastatin.