What is proper role of phospholipid translocators?

Krishan k jakhad

The PM is characterized by a strict lipid asymmetry in which phosphatidylcholine (PC) and sphingomyelin (SM) are mainly localized in the extracellular leaflet, whereas phosphatidylethanolamine (PE) and phosphatidylserine (PS) are exclusively present in the cytoplasmic leaflet. Maintenance of the lipid asymmetry in the PM is an energy-dependent process that is important for a variety of cell processes. For example, PS exposure on the cell surface leaflet after certain stimuli has been associated with apoptosis and senescence. flippase and floppase terms that designate lipid translocating enzymes. Maintenance of membrane asymmetry in the resting state cannot be mediated by passive lipid flip-flop alone, since spontaneous translocation of phospholipids across the bilayer has been postulated to be extremely slow (10−15 s−1), on average a single lipid flip-flop event every 24 h. The slow pace of this translocation is due to the high energy barrier imposed by the resistance to the passage of polar head groups through the hydrophobic core but also to the additional resistance imposed by the increased lateral tension developed in the receiving monolayer upon insertion of the new phospholipids. Therefore, asymmetric membranes cannot be maintained by passive flip-flops. Establishment and maintenance of lipid asymmetry involves several different membrane proteins. These lipid translocators include ATP-dependent flippases and floppases and energy-independent scramblases....catalyze lipid transfer towards the inward monolayer (flippase) or towards the outward monolayer (floppases). Flippases and floppases are required to equalize the number of lipids at both sides of the membrane when a new membrane/organelle is being generated, lipid synthesis taking place only on one side of the bilayer. Thus flippases and floppases help make the membrane more symmetric. Moreover in some cases they operate in the opposite way, generating lipid asymmetry.