| Exocytosis is a critical cellular process enabling key cellular activities such as exo- and endocrine functions and synaptic transmission. In turn, exocytosis is defined by the calcium-triggered membrane fusion step of release. To effect release of soluble vesicular contents first requires the complete merger of two apposed, amphipathic, lipid bilayers. In model lipid systems, this fusion step is strongly dependent on the specific composition of the hydrated bilayer, requiring membrane components of defined intrinsic curvature. Herein we demonstrate in a well characterized, biological membrane fusion system of isolated cortical vesicles, that the native fusion reaction depends on cholesterol in two distinct roles. First, as a native membrane component of high negative curvature, cholesterol lowers energy barriers to the formation of high curvature lipidic fusion intermediates, defining the ability of vesicles to undergo fusion. Second, cholesterol acts as a membrane organizer to integrate the functions of specific proteins and lipids at the active fusion site, dependent on the integrity of cholesterol-enriched microdomains. Altering the stability of such microdomains affects the fusion process indirectly, by diminishing the efficiency of membrane fusion while leaving the inherent ability of vesicles to fuse intact. The role of cholesterol in regulating the efficiency is quite specific, as it cannot be replaced by structurally similar sterols. |
