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

The classical theory of fusion considers the fusion of bilayer membranes as the union 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 via several intermediate structures characterized by local minima of the free energy of the system; these minima are separated by energy barriers. The key fusion intermediates are the so-called stalk, with the contacting membrane monolayers already fused but the distal monolayers not yet fused, and the 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 paper we consider the problem of the 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 pi-shaped structure, formed as a result of the fusion of a lipid droplet monolayer and a peroxisome bilayer, was considered, and the dependence of its energy on the geometric and elastic parameters of the system was analyzed. In particular, it was shown that the pi-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.