Transmembrane phospholipid distribution revealed by freeze-fracture replica labeling

Abstract

We propose the use of membrane splitting by freeze-fracture for differential phospholipid analysis of protoplasmic and exoplasmic membrane leaflets (halves). Unfixed cells or tissues are quick-frozen, freeze-fractured, and platinum-carbon (Pt/C) shadowed. The Pt/C replicas are then treated with 2.5% sodium dodecyl sulfate (SDS) to solubilize unfractured membranes and to release cytoplasm or contents. While the detergent dissolves unfractured membranes, it would not extract lipids from split membranes, as their apolar domains are stabilized by their Pt/C replicas. After washing, the Pt/C replicas, along with attached protoplasmic and exoplasmic membrane halves, are processed for immunocytochemical labeling of phospholipids with antibody, followed by electron microscopic observation. Here, we present the application of the SDS-digested freeze-fracture replica labeling (SDS-FRL) technique to the transmembrane distribution of a major membrane phospholipid, phosphatidylcholine (PC), in various cell and intracellular membranes. Immunogold labeling revealed that PC is exclusively localized on the exoplasmic membrane halves of the plasma membranes, and the intracellular membranes of various organelles, e.g. nuclei, mitochondria, endoplasmic reticulum, secretory granules, and disc membranes of photoreceptor cells. One exception to this general scheme was the plasma membrane forming the myelin sheath of neurons and the Ca(2+)-treated erythrocyte membranes. In these cell membranes, roughly equal amounts of immunogold particles for PC were seen on each outer and inner membrane half, implying a symmetrical transmembrane distribution of PC. Initial screening suggests that the SDS-FRL technique allows in situ analysis of the transmembrane distribution of membrane lipids, and at the same time opens up the possibility of labeling membranes such as intracellular membranes not normally accessible to cytochemical labels without the distortion potentially associated with membrane isolation procedures.