Abstract 2311 Two Bridge-like lipid transfer proteins (BLTPs) transport phosphatidylethanolamine at membrane contact sites to facilitate GPI-anchored biogenesis and maintain plasma membrane fluidity

Intracellular lipid trafficking is essential for lipid homeostasis and membrane biogenesis. Lipids are transported within cells by vesicular trafficking and at membrane contacts sites by lipid transfer proteins, which facilitate nonvesicular lipid exchange between cellular compartments. Recently, a family of bridge-like lipid transfer protein (BLTPs) has been described that are thought to have long hydrophobic grooves that allow lipids to flow between the two membranes at contact sites. Our group is studying the function of two BLTPs: Csf1p and Fmp27p in S. cerevisiae and their respective human homologs BLTP1 and BLTP2. We found that both proteins localize primarily to contact sites between the endoplasmic reticulum (ER) and plasma membrane (PM), where they probably facilitate phosphatidylethanolamine (PE) exchange between these compartments. Csf1p/BLTP1 facilitate biosynthesis of glycosylphosphatidylinositol (GPI), a lipidic protein anchor generated in the ER. The synthesis of each GPI anchor requires three molecules of PE and our findings suggest Csf1p/BLTP1 transfers PE from the PM to the ER and may channel PE to the ER enzyme that transfers ethanolamine phosphate from PE to nascent GPI anchors. Our results suggest Fmp27p/BLTP2 transfers PE from the ER to the PM to maintain PM fluidity. In yeast, Fmp27p is necessary for growth at low temperature, which requires adjustment of membrane lipid composition. Human cells lacking BLTP2 have reduced rates of proliferation and, interestingly, BLTP2 is highly expressed and near essential in several breast cancer cell lines. We found BLTP2 is necessary to sustain the aggressiveness of a triple negative breast cancer cell line in an vivo model of metastasis, suggesting maintenance of PM fluidity by BLTP2 is critical for cell proliferation in humans. Together, our results suggest that Csf1p/BLTP1 and Fmp27 play critical roles in regulating intracellular PE homeostasis to maintain metabolism and membrane function during stress. UTSW medical center starting grant, NIDDK NIH intramural funding