Membrane mobility agents. IV. The mechanism of particle-cell and cell-cell fusion

Fusion processes are classified according to type and size of the fusing elements:I, particle-particle, vesicle-vesicle; II, vesicle-cell, particle-cell; III, cell-cell. Fusion may be direct (original elements merge) or indirect (transfer of components from one element to another). Membrane mobility agents like A₂C (structural formula below) {A figure is presented} form small particles on dispersion in aqueous solutions, particles which can be made fluorescent by the addition of fluorescent probes, the Flomols (fluorescent probes of mobility in membranes). Fusions of these particles with cells (particle-cell fusion) and of cells with cells induced by membrane mobility agents (cell-cell fusion) fulfill the criteria for direct fusion. The following stages in the overall process have been identified, using A₂C and Flomol F20C in the fusion of hen red cells: 1, Approach (of particle to cell); 2, sticking (of particle to cell); 3, local entry (of particle contents into cell membrane); 4, membrane spread (of particle contents from original entry point throughout the rest of the cell membrane); 5, rounding (from ovals to cups to spheres). (These steps of particle-cell interaction are succeeded by those for cell-cell interaction.) 6, approach (of one rounded cell to another rounded cell); 7, sticking (cell to cell); 8, fusion (of two cells); 9, sticking (of binucleated cell to additional cells); 10, fusion (to multinucleated cells). After contact, sticking is achieved through "hydrophobic bonding". It is proposed that the critical step which follows sticking and commits the elements irreversibly is the microfusion between the outer surface layers of the elements. Microfusion (for particle-cell and cell-cell) results from the permeation of water into the isolated surface contact region after sticking, followed by a rearrangement of the molecules in that contact region into inverted micelles. The latter arrangement constitutes a region of instability which is resolved by fusion of the outer surface elements (see diagrams). The entry of membrane mobility agent into the membrane and the spread of the reagent along with the motion of membrane components follows. The consequent alterations in membrane properties lead to a spherical cell, a morphological state with the minimum area. Two phase temperature effects on the fusion process, as shown by experiments at different temperatures and by temperature jump, imply the presence of a barrier within the membrane more complex than phospholipids, possibly involving intramembranous particles and/or cytoskeletal elements which must be moved to form a "clear region" for lipid bilayer contact. The applicability of the overall scheme to physiological processes like neurotransmitter release and mucocyst release, as well as parallels to "Sendai virus" (HVJ, or hemagglutinating virus of Japan)-induced fusion are discussed. The contribution of other factors (Dextran, ph, Ca²⁺) to the progress of fusion is also examined.