Fig. 4 ∣. Cholesterol accessibility in cellular membranes.

a, Calculated chemical activity (related to the chemical potential) of cholesterol as a function of membrane cholesterol content (expressed as the mole fraction of cholesterol relative to total lipids). Shown are chemical activities for binary membrane mixtures of cholesterol and phospholipid assuming no interactions (blue curve) or a complex containing one cholesterol and two phospholipid molecules (pink curve, derived from a regular solution free energy calculation¹⁰⁰). At concentrations below the equivalence point, cholesterol is sequestered by phospholipids. Above the equivalence point, cholesterol exceeds the sequestering capacity of phospholipids, and its accessibility sharply rises. b, Three pools of cholesterol in plasma membranes, along with toxin-based probes that can be used to detect and manipulate the sphingomyelin (SM)-sequestered (OlyA) and accessible (ALOD4 or PFO) pools. c, Schematic showing the three cholesterol pools in unperturbed plasma membranes or after cholesterol loading or SM depletion.