FIGURE 2.

Mechanisms of membrane fission. Membrane fission mechanisms can be classified as active, that require consumption of cellular energy by nucleoside triphosphate (ATP or GTP) hydrolysis (upper panel; blue line) or passive that do not require the direct use of energy (lower panel; green line). Each of these above-mentioned categories can be further divided into two types: normal topology, when the vesicle buds toward the cytoplasm (left panel; purple dashed line) and reverse topology, when the vesicle buds away from the cytoplasm (right panel; dark green dashed line). Consequently, membrane fission mechanisms are divided into four classes: Class I, active mechanism with normal topology; Class II, passive mechanism with normal topology; Class III, active mechanism with reverse topology; Class IV, passive mechanism with reverse topology. The details are in the text.