The single C-terminal helix of human phospholipid scramblase1 is required for membrane insertion and scrambling activity

Human phospholipid scramblase1 (hPLSCR1) belongs to the ATP-independent class of phospholipid translocators which possess a single EF-hand-like Ca2+-binding motif and also a C-terminal helix (CTH). The CTH domain of hPLSCR1 was believed to be a putative single transmembrane helix at the C-terminus. Recent homology modeling studies by Bateman etal. predicted that the hydrophobic nature of this helix is due to its packing in the core of the protein domain and proposed that this is not a true transmembrane helix [Bateman A, Finn RD, Sims PJ, Wiedmer T, Biegert A & Johannes S. Bioinformatics 2008, 25, 159]. To determine the exact function of the CTH of hPLSCR1, we deleted the CTH domain and determined: (a) whether CTH plays any role beyond membrane anchorage, (b) the functional consequences of CTH deletion, and (c) any conformational changes associated with CTH in a lipid environment. Invitro reconstitution studies confirm that the predicted CTH is required for membrane insertion and scrambling activity. CTH deletion caused a 50% decrease in binding affinity of Ca2+ for CTH-hPLSCR1 (Ka=115m) compared with hPLSCR1 (Ka=249m). Far UV-CD studies revealed that the CTH peptide adopts -helicity only in the presence of SDS micelles and negatively charged vesicles, indicating that electrostatic interactions are required for insertion of the peptide. CTH peptide-quenching studies confirm that the predicted CTH inserts into the membrane and its ability to interact with the membrane depends on the presence of charge interactions. TOXCAT assay revealed that CTH of hPLSCR1 does not oligomerize in the membrane. We conclude that CTH is required for membrane insertion and Ca2+ coordination and also plays an important role in the functional conformation of hPLSCR1.