Unraveling the Mysteries of Phospholipid Scrambling
Peter J. Sims (1), (2) , Therese Wiedmer (1)
Departments of (1) Molecular and Experimental Medicine and (2) Vascular Biology, The Scripps Research Institute, La Jolla, CA, USA
Summary Plasma membrane phospholipid asymmetry is maintained by anaminophospholipid translocase that transports phosphatidylserine (PS)and phosphatidylethanolamine (PE) from outer to inner membraneleaflet. Cell activation or injury leads to redistribution of all major lipidclasses within the plasma membrane, resulting in surface exposure ofPS and PE. Cell surface-exposed PS can serve as receptor sites forcoagulation enzyme complexes, and contributes to cell clearance by thereticuloendothelial system. The mechanism(s) by which this PL"scrambling" occurs is poorly understood. A protein called phospholipidscramblase (PLSCR1) has been cloned that exhibits Ca2+-activatedPL scrambling activity in vitro. PLSCR1 belongs to a new family ofproteins with no apparent homology to other known proteins. PLSCR1is palmitoylated and contains a potential protein kinase C phosphorylationsite. It further contains multiple PxxP and PPxY motifs, representingpotential binding motifs for SH3 and WW domains implicated inmediating protein-protein interactions. Although at least two proteinshave been shown to associate with PLSCR1, the functional significanceof such interaction remains to be elucidated. Evidence that PLSCR1may serve functions other than its proposed activity as PL scramblase isalso presented.