The contributions made by protein monomers can be greatly enhanced by oligomerization, which also stabilizes membrane curvature ( Fig. 2 B) or the asymmetric insertion of hydrophobic protein domains, e.g., hairpins ( Fig. Asymmetry within the bilayer is also introduced by the intrinsic shape of protein membrane-spanning segments ( Fig. In model membranes, lipids of the same kind tend to cluster together, and protein transmembrane domains prefer to accumulate specific lipids as a lipid coat. The shape of the individual lipids gives a spontaneous curvature to the membrane ( Fig. One way of altering the curvature of the membrane is by modifying the local lipid composition, whether it be the lipid headgroup, tail, or cholesterol enrichment. It is helpful to consider membrane-intrinsic forces that act by introducing local asymmetry to the bilayer distinct from membrane-extrinsic forces that are contributed by peripheral interacting proteins acting outside the lipid bilayer itself. The local restriction of curvature to specific areas implies lateral compartmentalization within the fluid mosaic membrane ( Singer, 1972 Kusumi et al., 2011). Membranes can be positively curved (toward the cytoplasm) or negatively curved (away from the cytoplasm), and their deformability varies depending on the tension in the membrane ( McMahon and Gallop, 2005). Protein mechanisms range from different types of binding interaction to oligomerization processes and mechanochemical ATP and GTPases. The energy needed is provided by modification of the lipid composition or from the membrane-associated proteins that make up ∼50% of the membrane surface. Pure lipid bilayers remain flat, and stimulating them to curve requires energy ( Helfrich, 1973 Helfrich and Jakobsson, 1990). Furthermore, expanding the surface area of membranes for reactions is achieved by sculpting folds and tubules into intracellular organelles.
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The exchange of molecules and signals across this barrier requires many processes to bend, invaginate, protrude, fuse, and break membranes. Membranes are the frontier between the inside and outside of cells and separate diverse intracellular compartments while being hubs of signaling activity.