Phospholipid Bilayer Assembly: Facilitated Transmembrane Movement of Phosphatidylcholine
The transmembrane movement of phospholipids is a fundamental step in membrane assembly and intracellular lipid sorting. Phospholipid synthesis occurs on the cytoplasmic surface of the endoplasmic reticulum. Translocation to the lumenal surface is required for assembly of the bilayer. Studies employing a spectrophotometric assay to measure the transmembrane movement of phosphatidylglycerol and diacylglycerol analogs in phospholipid vesicles revealed half-times of greater than 8 days and less than 15 seconds, respectively. The hypothesis that rat liver microsomes possess a “PC flippase” was tested by determining whether sn 1,2-dibutyroylphosphatidylcholine (diC4PC) was transported into the lumen. This compound was chosen for study because it retains the polar head group, the portion of PC unable to spontaneously move across the membrane. Since diC4PC is water soluble, standard transport methods could be applied. DiC4PC uptake into the microsomal lumen was saturable, time dependent and proportional to the amount of microsomes employed. An intact permeability barrier was required. DiC4PC transport was inhibited by structural analogs (but not sn-2,3-diC4PC) and was inhibited by proteases and protein modification reagents. Transport of diC4PC was not observed across PC vesicles or red cell membranes where PC movement is slow. The transport of diC4PC is a protein facilitated process. It is inferred that the same system functions in PC translocation in the assembly of the endoplasmic reticulum bilayer.
KeywordsCytoplasmic Surface Polar Headgroup Phospholipid Asymmetry Lipid Asymmetry Transmembrane Movement
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