FIGURE 1.

Schematic representation of fusion according to the stalk theory (2,3). The monolayers of the bilayers are depicted as slabs. All structures are axially symmetric and appear in the cross section that contains their vertical axes (dotted lines). The first structure to form that bridges two opposed membranes is the stalk (A). For sufficiently negative values of J_s (3), or in the presence of sufficient membrane tension, the stalk can expand radially to form a hemifusion diaphragm (B), which contains a disk of planar bilayer membrane in the center. Fusion occurs when a pore forms within this single bilayer (C). The axis of the pore is depicted as the shorter vertical dashed line. The edge of the bilayer pore is unstable, and the system can lower its free energy by forming a catenoidal fusion pore (D).