1.Transbilayer distribution of lipids at nano scale

Motohide Murate, Toshihide Kobayashi
Lipid Biology Laboratory, RIKEN

There is a limited number of methods to examine transbilayer lipid distribution in biomembranes. We employed freeze-fracture replica labelling immunoelectron microscopy in combination with lipid binding proteins and a peptide to examine both transbilayer distribution and lateral distribution of various phospholipids in mammalian cells. Our results indicate that phospholipids are exclusively distributed either in the outer or inner leaflet of human red blood cell (RBC) membranes. In contrast, in nucleated cells, such as human skin fibroblasts and neutrophils, sphingomyelin was distributed in both leaflets while exhibiting characteristic lipid domains in the inner leaflet. Similar to RBCs, lipid asymmetry was maintained both in resting and thrombin-activated platelets. However, the microparticles released from thrombin-activated platelets lost membrane asymmetry. Our results suggest that the microparticles were shed from platelet plasma membrane domains enriched with phosphatidylserine and/or phosphatidylinositol at the outer leaflet. These findings underscore the strict regulation and cell-type specificity of lipid asymmetry in the plasma membrane.

Fig.1. Distribution of sphingomyelin clusters in the outer leaflet and inner leaflet of the plasma membranes in human red blood cells and in human skin fibroblasts. Sphingomyelin is exclusively distributed in the outer leaflet in red blood cells, whereas the lipids form distinct clusters in the inner leaflet of human skin fibroblasts.

【References】
[1] Murate M, Abe M, Kasahara K, Iwabuchi K, Umeda M, Kobayashi T. (2015) Transbilayer distribution of lipids at nano scale. J. Cell Sci. 128, 1627-1638.