Figure 2.

Schematic representation of formation models corresponding to the three types of bacterial intracellular neo-membranes. These three models are based on different membrane curvature-acting properties of the overexpressed membrane proteins and the surrounding lipids. Type I (connected tubules). (a,b) The initial membrane budding step (curvature initiation) results from the local assembly of curvogenic lipids (I), which is accompanied by the segregation of the overexpressed membrane protein (rectangles) at the level of the “peripheral ring” of the nascent curved membrane patch, due to the tropism of this protein for a 1D (one-dimensional) curvature. (c) Further local self-assembly of the overexpressed membrane protein tends to progressively elongate this peripheral membrane zone, stabilizing it in a cylindrical shape (curvature propagation). (d) This membrane protein self-assembly finally leads to the formation of a tubular membrane structure, still connected to the cytoplasmic membrane, with a rather low lipid-to-protein ratio (compared to the cytoplasmic membrane). (e) Subsequently, protein–protein interactions mediated by the hydrosoluble domains of the overexpressed membrane protein can promote stacking of such intracellular tubules (when present in high amounts). Type II (heterogeneous saccules). (a,b) The initial membrane budding step (curvature initiation) results from the local co-assembly of curvogenic lipids (I) and the 2D-curvophilic overexpressed membrane protein (trapezoids), this protein not being sufficient alone to create by itself a driving force that is able to bend the cytoplasmic membrane. (c) Further local co-assembly tends to quasi-spherically extend the nascent membrane patch (curvature propagation). (d) The more or less random clustering of the overexpressed membrane protein, associated with curvo-acting lipids (in particular with domains of different leaflet asymmetry), proceeds to promote the growth of an irregular membrane structure. (e) Subsequently, this membrane structure tends to eventually fission from the cytoplasmic membrane, due to energetic constraints, to finally give intracellular membrane saccules and cisternae of different sizes and shapes, with a rather high lipid-to-protein ratio (compared to the cytoplasmic membrane); stacking of these membranes can occur via interactions between the hydrosoluble domains of the overexpressed membrane protein (when present in high amounts). Type III (homogenous vesicles). (a,b) The initial membrane budding step (curvature initiation) results from local self-association of the 2D-curvogenic overexpressed membrane protein (triangles). (c) Further local self-assembly of the overexpressed membrane protein, associated with curvophilic lipids (not represented), leads to the formation of a hemispherical membrane (curvature propagation). (d) This membrane protein self-assembly progressively proceeds to build a quasi-spherical structure appending to the cytoplasmic membrane. (e) Subsequently, this membrane structure fissions from the cytoplasmic membrane due to energetic constraints (line tension at the level of the fission pore), to finally give intracellular spherical vesicles of homogenous size, with a rather high lipid-to-protein ratio compared to the cytoplasmic membrane (proteins not represented).