Fusion of Peroxisome and Lipid Droplet Membranes: Expansion of a π-Shaped Structure

Classical theory of fusion considers the fusion of bilayer membranes as a unification of the material of the membranes themselves and the water volumes surrounded by them. It has been shown that membrane fusion is accompanied by significant deformation of lipid monolayers. The optimal trajectory of the process passes through several intermediate structures characterized by local minima of the free energy of the system; the minima are separated by energy barriers. The key fusion intermediate is stalk, where the contacting membrane monolayers have already fused, but the distal monolayers have not yet, and hemifusion diaphragm, a structure with an extended lipid bilayer formed by two distal monolayers of merging membranes located in the center between the radially displaced fused contact monolayers. In this work, we consider fusion of a bilayer membrane and a lipid monolayer located at the water–triolein interface from the standpoint of the classical theory of fusion. An intermediate π-shaped structure, formed as a result of a lipid droplet monolayer and a peroxisome bilayer fusion, was considered, and the dependence of its energy on the geometric parameters and elastic characteristics of the system was analyzed. In particular, it was shown that the π‑shaped structure is similar to the hemifusion diaphragm of the classical theory of bilayer membrane fusion: an increase in the radial dimensions of both structures becomes more energetically favorable with a decrease in the spontaneous curvature of the membrane monolayers. This result is consistent with the available experimental data on the fusion of lipid droplets with peroxisomes.