MOLECULAR-CLONING OF HUMAN PLASMA-MEMBRANE PHOSPHOLIPID SCRAMBLASE - A PROTEIN MEDIATING TRANSBILAYER MOVEMENT OF PLASMA-MEMBRANE PHOSPHOLIPIDS

The rapid movement of phospholipids (PL) between plasma membrane leafl ets in response to increased intracellular Ca2+ is thought to play a k ey role in expression of platelet procoagulant activity and in clearan ce of injured or apoptotic cells. We recently reported isolation of a similar to 37-kDa protein in erythrocyte membrane that mediates Ca2+-d ependent movement of PL between membrane leaflets, similar to that obs erved upon elevation of Ca2+ in the cytosol (Basse, F., Stout, J. G., Sims, P. J., and Wiedmer, T. (1996) J. Biol. Chem. 271, 17205-17210). Based on internal peptide sequence obtained from this protein, a 1,445 -base pair cDNA was cloned from a K-562 cDNA Library, The deduced ''PL scramblase'' protein is a proline-rich, type II plasma membrane prote in with a single transmembrane segment near the C terminus. Antibody a gainst the deduced C-terminal peptide was found to precipitate the sim ilar to 37-kDa red blood cell protein and absorb PL scramblase activit y, confirming the identity of the cloned cDNA to erythrocyte PL scramb lase, Ca2+-dependent PL scramblase activity was also demonstrated in r ecombinant protein expressed from plasmid containing the cDNA. Quantit ative immunoblotting revealed an approximately 10-fold higher abundanc e of PL scramblase in platelet (similar to 10(4) molecules/cell) than in erythrocyte (similar to 10(3) molecules/cell), consistent with appa rent increased PL scramblase activity of the platelet plasma membrane, PL scramblase mRNA was found in a variety of hematologic and nonhemat ologic cells and tissues, suggesting that this protein functions in al l cells.