The role of the transmembrane alpha-helix of human Phospholipid Scramblase 1 in protein structure and function

Human Phospholipid Scramblase 1 (hPLSCR1) is an endofacial plasma membrane protein that is multipalmitoylated and is widely expressed in most human tissues. hPLSCR1 is involved in the rapid calcium dependent translocation of plasma membrane phospholipids, although neither the detailed calcium-induced conformational change nor the mechanism of phospholipid scrambling are known yet. In addition to this role in phospholipid scrambling, hPLSCR1 may also function to regulate processes including signaling, cell differentiation, apoptosis, injury, cell proliferation and transcription.We have constructed a truncated mutant lacking the transmembrane alpha helix at C-terminal end (hPLSCR1ΔC290) in order to make the protein more stable and soluble. Preliminary structural studies(fluorescence and infrared spectroscopies) in the presence of calcium suggest that the truncated protein binds the ion and undergoes conformational changes similarly to the wild type.Functional studies using the Langmuir balance show that the ion increases the mutant adsorption to lipid monolayers, and this increase is enhanced by the presence of PS. Nevertheless the protein reconstituted in liposomes does not have any scramblase activity. We conclude that the mutant lacking the transmembrane domain retains two important properties of the native protein, the calcium-dependent conformational change and the capacity of membrane interaction.