The C-terminal transmembrane domain of human phospholipid scramblase 1 is essential for the protein flip-flop activity and Ca2+-binding

Human phospholipid scramblase 1 (SCR) is a 318 amino acid protein that was originally described as catalyzing phospholipid transbilayer (flip-flop) motion in plasma membranes in a Ca2+-dependent, ATP-independent way. Further studies have suggested an intranuclear role for this protein in addition. A putative transmembrane domain located at the C terminus (aa 291-309) has been related to the flip-flop catalysis. In order to clarify the role of the C-terminal region of SCR, a mutant was produced (SCR Delta) in which the last 28 amino acid residues were lacking, including the alpha-helix. SCR Delta had lost the scramblase activity and its affinity for Ca2+ was decreased by one order of magnitude. Fluorescence and IR spectroscopic studies revealed that the C-terminal region of SCR was essential for the proper folding of the protein. Moreover, it was found that Ca2+ exerted an overall destabilizing effect on SCR, which might facilitate its binding to membranes.