FIGURE 1.

Nomenclature of leaflets (monolayers) during different kinds of membrane fission reaction. Only membranes decorated with fission-inducing proteins (yellow circles) are considered. When membrane fission takes place, separation of proximal leaflets is followed by separation of distal leaflets. During normal topology fission (membrane-enclosed compartment buds toward the cytoplasm), distal leaflet is cytoplasmic, whereas proximal leaflet is exoplasmic. During reverse topology fission (membrane-enclosed compartment buds away from the cytoplasm), distal leaflet is exoplasmic, and proximal leaflet is cytoplasmic. During both normal and reverse topology fission reactions, disjoint or joint union of membrane-enclosed compartments can form. If disjoint union forms, outer leaflet is distal, and inner leaflet is proximal. When joint union forms, outer leaflet is proximal, whereas inner leaflet is distal. Fission-inducing proteins are usually present on the cytoplasmic side of the membrane.