Discovery and mechanism of lipid scramblases

About This Grant

The translocation of polar lipids from one side of a membrane bilayer to the other is critically important in physiology. This process, termed lipid flip-flop, is variously required for biogenesis of mitochondria, growth of the endoplasmic reticulum (ER), all forms of protein glycosylation, and synthesis of glycolipid blood group antigens. It is also needed to expose the signaling lipid phosphatidylserine (PS) at the cell surface in response to physiological triggers – PS promotes blood clotting by activated platelets, marks apoptotic cells for clearance by macrophages and induces cell-cell fusion needed to produce myotubes and osteclasts. Specific proteins – termed scramblases – catalyze lipid flip-flop by acting as lipid channels. These proteins have attracted considerable recent interest, resulting in new information and new concepts concerning intracellular lipid transport and homeostasis, yet many fundamental mechanistic and biological questions remain. The overall goals of our research are to investigate these questions broadly, by discovering the molecular identity of scramblases in key processes where their activity is implicated but not yet been assigned to a specific protein(s), understanding the molecular mechanisms by which these novel lipid channels work, evaluating their specific contributions to physiological processes, and understanding how their activity is regulated. To achieve these goals, we focus here on two understudied research areas which exemplify key knowledge gaps. We recently discovered phospholipid scramblases in the outer mitochondrial membrane. These proteins likely provide the means by which polar lipids enter mitochondria to support the growth and function of the organelle. We will use a multidisciplinary suite of methods to understand how these proteins work and to define their specific contributions to mitochondrial biogenesis. Second, we are interested in the molecular identity of glycolipid scramblases that are implicated in all forms of protein glycosylation in the ER. These scramblases have eluded discovery, and their identification is a major goal for the field. We propose a multi-pronged experimental approach to identify these proteins and validate their function using biochemistry and genetics. Mitochondrial dysfunction is a common source of inborn errors of metabolism, and is associated with cancer, cardiovascular and neurodegenerative diseases, as well as with lipid syndromes such as Barth. Protein glycosylation is essential for life, and specific defects in glycosylation pathways can lead to a number of diseases including muscular dystrophy. In addition to advancing the fields of mitochondrial cell biology and glycobiology and providing unifying biophysical insights into a fundamental membrane transport process, the research proposed here and the overall program that we have conceived will contribute to an understanding of the etiology and presentation of many diseases.