Cartoon summary of the known mechanisms of membrane curvature induction. (a) Lipid composition corresponds to the scenario in which different sizes of the lipid headgroups or tails cause membrane curvature. Larger headgroups (green) or smaller tails (coloured in yellow) lead to a positive curvature; larger tails (cyan) in turn lead to negative curvatures. (b) Scaffolding involves peripheral proteins (pink) imposing their shape onto the membrane. (c) Helix insertion corresponds to peripheral proteins inserting amphipathic helices (blue) into the middle of the lipid bilayer and causes stress. (d) Membrane proteins are usually transmembrane proteins (green), such as ion channels or receptors that have an intrinsic conical shape. (e) Often, peripheral proteins (blue) collaborate using oligomerization to induce functional curvatures. BAR proteins are frequently reported to tubulate membranes bothin vitroandin vivoand are believed to be responsible for forming the neck of a budding vesicle as shown in the cartoon. (f) Finally, other macroscopic cellular events such as actin polymerization (red) also contribute to macroscopic membrane curvatures, as shown.