The mystery of phospholipid flip-flop in biogenic membranes.

Phospholipid flip-flop is required for bilayer assembly and the maintenance of biogenic (self-synthesizing) membranes such as the eukaryotic endoplasmic reticulum and the bacterial cytoplasmic membrane. Due to the membrane topology of phospholipid biosynthesis, newly synthesized phospholipids are initially located in the cytoplasmic leaflet of biogenic membranes and must be translocated to the exoplasmic leaflet to give uniform bilayer growth. It is clear from many studies that phospholipid flip-flop in biogenic membranes occurs very rapidly, within a period of a few minutes. These studies also reveal that phospholipid translocation in biogenic membranes occurs bi-directionally, independently of the phospholipid head group, via a facilitated diffusion process in the absence of metabolic energy input, and that this type of transport requires specific membrane proteins. These translocators have been termed biogenic membrane flippases, and they differ from metabolic energy-dependent transporters (ABC transporters and MDR proteins). No biogenic membrane flippases have been characterized. This review briefly discusses the importance of biogenic membrane flippases, the various assay methods used for measuring the rate of phospholipid flip-flop, and the progress that has been made towards identifying these proteins.