Lipopolysaccharide binding protein and soluble CD14 catalyze exchange of phospholipids.
Lipopolysaccharide binding protein (LBP) is a plasma protein known to facilitate the diffusion of bacterial LPS (endotoxin). LBP catalyzes movement of LPS monomers from LPS aggregates to HDL particles, to phospholipid bilayers, and to a binding site on a second plasma protein, soluble CD14 (sCD14). sCD14 can hasten transfer by receiving an LPS monomer from an LPS aggregate, and then surrendering it to an HDL particle, thus acting as a soluble "shuttle" for an insoluble lipid. Here we show that LBP and sCD14 shuttle not only LPS, but also phospholipids. Phosphatidylinositol (PI), phosphatidylcholine, and a fluorescently labeled derivative of phosphatidylethanolamine (R-PE) are each transferred by LBP from membranes to HDL particles. The transfer could be observed using recombinant LBP and sCD14 or whole human plasma, and the plasma-mediated transfer of PI could be blocked by anti-LBP and partially inhibited by anti-CD14. sCD14 appears to act as a soluble shuttle for phospholipids since direct binding of PI and R-PE to sCD14 was observed and because addition of sCD14 accelerated transfer of these lipids. These studies define a new function for LBP and sCD14 and describe a novel mechanism for the transfer of phospholipids in blood. In further studies, we show evidence suggesting that LBP transfers LPS and phospholipids by reciprocal exchange: LBP-catalyzed binding of R-PE to LPS x sCD14 complexes was accompanied by the exit of LPS from sCD14, and LBP-catalyzed binding of R-PE to sCD14 was accelerated by prior binding of LPS to sCD14. Binding of one lipid is thus functionally coupled with the release of a second. These results suggest that LBP acts as a lipid exchange protein.
Yu
B
B
Laboratory of Cellular Physiology and Immunology, The Rockefeller University, New York, New York 10021, USA.
Hailman
E
E
Wright
S D
SD
D000209:Acute-Phase Proteins; D001692:Biological Transport; D001798:Blood Proteins / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008051:Lipid Bilayers / Q000378:metabolism; D018950:Lipopolysaccharide Receptors / Q000235:genetics / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000378:metabolism*; D008075:Lipoproteins, HDL / Q000378:metabolism; D008562:Membrane Glycoproteins; D008956:Models, Chemical; D010713:Phosphatidylcholines / Q000378:metabolism; D010714:Phosphatidylethanolamines / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D012475:Salmonella / Q000737:chemistry; D012995:Solubility; D013050:Spectrometry, Fluorescence; D000099231:Lipopolysaccharide-Binding Protein
D000209:Acute-Phase Proteins; D001798:Blood Proteins; D002352:Carrier Proteins; D008051:Lipid Bilayers; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; D008075:Lipoproteins, HDL; D008562:Membrane Glycoproteins; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D010716:Phosphatidylinositols; D010743:Phospholipids; D011994:Recombinant Proteins; D000099231:Lipopolysaccharide-Binding Protein