A New Determination Of The Transbilayer Distribution Of Plasma Membrane Cholesterol

The transbilayer distribution of plasma membrane cholesterol remains uncertain despite repeated analysis. We propose a new mechanism driving cholesterol sidedness: sterols form simple stoichiometric associations with phospholipids. Our model postulates that the phospholipids in the plasma membrane bilayer are fully complexed with cholesterol. The cholesterol in each leaflet is then the product of the abundance of its phospholipid and its sterol stoichiometry. Notably, lipid affinities are not relevant. Applying literature values for the composition, abundance and sterol stoichiometry of the phospholipid in each leaflet, the model predicts that two-thirds of the cholesterol in the human erythrocyte membrane bilayer is located in its outer leaflet, an exofacial to endofacial ratio of 2:1. The model also predicts that the overall cholesterol content of the bilayer is [~]0.75 mole/mole phospholipid, in agreement with literature values. Furthermore, our analysis suggests that the areas of the two membrane leaflets are about the same. The concordance of prediction with observation validates the model and the values used for the parameters. The sterol in the exofacial leaflet of the plasma membrane of any cell is predicted to exceed that on its contralateral side when its phospholipids have a higher sterol stoichiometry and are fully complexed. SynopsisWe propose that the transbilayer distribution of cholesterol in the plasma membrane bilayer is determined by its complexation with the phospholipids in the two leaflets. Because the complexes are homeostatically filled to stoichiometric equivalence, leaflet cholesterol is given by the abundance of its phospholipids multiplied by its sterol stoichiometry. The model predicts that two-thirds of the cholesterol in the human erythrocyte membrane bilayer resides in the outer leaflet. It also predicts the cholesterol content of the bilayer as a whole.