Protein Involvement in Transmembrane Lipid Asymmetry

Abstract

Transmembrane asymmetry has been extensively studied in eukaryotic cells. It is as yet only clearly demonstrated in the plasma membrane of a few cells. Subcellular organelles have evidence of lipid asymmetry, but very little consistent quantitative data exist. Proteins involved in transmembrane passage of lipids comprise enzymes of lipid metabolism and also the so-called phospholipid flippases that are either passive or active putative lipid transporters. The aminophospholipid translocase that pumps amino-phospholipids from the outer to the inner monolayer of the plasma membrane of eukaryotes is a Mg(2+)-ATP dependent protein with a high lipid selectivity. Lipid asymmetry provides an asymmetrical environment for membrane enzymes. Thus, PS (and PE) reorientation could be a way of controlling or triggering specific enzymes. Also, the asymmetrical distribution of phospholipids most likely determines the fusion-competent membranes and/or which sides of membranes should fuse. Finally, the lipid pump as well as all enzymes responsible for the net transmembrane flux of phospholipids may provide the driving force for membrane bending, notably during the formation of endocytic vesicles. Clearly, real progress in this area will be made only if the proteins of the flippase family are purified and antibodies obtained that will permit the recognition and localization of these proteins in various cells. Also, specific inhibitors as well as mutants would allow one to infer more directly what are the real functions of these proteins. At a late stage, the protein purification will eventually permit speculation on the mechanism of action of a pump that must transport simultaneously hydrophilic and hydrophobic groups through a membrane.

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

“…More recently, the distribution of the phosphoinositides was established (Butikofer et al, 1990;Gascard et al, 1991): 100 % of the phosphatidylinositol 4-phosphate, and 80 % of the phosphatidylinositol and of its 4,5-bisphosphate derivative are located in the cytoplasmic leaflet, as is 800% of the phosphatidic acid. In fact, this distribution, with an outer surface composed principally of the choline-containing phospholipids phosphatidylcholine and sphingomyelin, seems to be a general trend for the plasma membrane of animal cells (see Table 1 and reviews by Devaux, 1992;Op den Kamp, 1979;Zachowski and Devaux, 1990). This outer monolayer is often poor in aminophospholipids, phosphatidylethanolamine and phosphatidylserine, but some plasma membranes can contain large amounts of them.…”

γ-Cyclodextrins Greatly Enhance Translocation of Hydrophobic Fluorescent Phospholipids from Vesicles to Cells in Culture

“…Labeling of these organelles is in agreement with previous data obtained for NBD 6 PS (44), NBD 12 PS (45), and DiPyr 4 PS (25) and can be explained as follows. The PS molecules are initially introduced to the outer leaflet of the plasma membrane, but they move rapidly to the inner leaflet with the assistance of the aminophospholipid translocase (46). From the inner leaflet, the fluorescent PS molecules move further, possibly via spontaneous diffusion (see below), to various organelles, including mitochondria, where they can be decarboxylated to PE (45,47).…”

Membrane Asymmetry Is Maintained during Influenza-induced Fusion

“…Moreover, the integral membrane protein hemagglutinin, which is present on the outer leaflet of the viral membrane, was quantitatively digested with protease after fusion, indicating that hemagglutinin remained on the outer leaflet of the fusion product. Therefore, there is no merger of the inner with outer leaflets of the viral or the liposomal membrane during fusion, and transverse membrane asymmetry is maintained.The constituents of biological membranes, such as the plasma membranes of cells, are distributed in an asymmetric manner between the two leaflets of the membranes (Devaux, 1991(Devaux, , 1992. Although these membranes continuously undergo membrane fusion and fission, their asymmetry is maintained.…”

“…After 60 min, all non-metabolized probe could be extracted from the outer leaflet of the membrane again. Thus, although cells possess amino phospholipid translocases and other "flippases" that could restore membrane asymmetry (Devaux, 1991(Devaux, , 1992, it seems more logical to assume that asymmetry is maintained during fusion. In line with this, in most models for the molecular mechanism of biological membrane fusion (Ohki et al, 1987;Wilschut and Hoekstra, 1990; Bentz, 1993) it is proposed that fusion involves merger of the outer leaflets of the participant membranes followed by that of the inner leaflets, maintaining the asymmetry of the bulk of the lipids.…”

“…The constituents of biological membranes, such as the plasma membranes of cells, are distributed in an asymmetric manner between the two leaflets of the membranes (Devaux, 1991(Devaux, , 1992. Although these membranes continuously undergo membrane fusion and fission, their asymmetry is maintained.…”