Fusion in Biological and Model Membranes: Similarities and Differences

Abstract

Fusion is an exotic phenomenon, attractive to study, poorly understood (cf. this volume). This chapter will focus on measurements of single fusion events in two systems with similar geometry phospholipid vesicles to planar phospholipid membranes and secretory granules to plasma membranes in exocytosis. Large expanses of phospholipid bilayer are joined together in both. Observation of particle-free pre-fusion zones has suggested that the role of protein in cell membrane fusion is to get out of the way, and allow lipid bilayer to contact and fuse (Ornberg & Reese, 1981; Chandler, 1987). This hypothesis suggests that the fundamental molecular event is the same in lipid and biological membrane fusion. However, in examining the details of the fusion reaction on the scale of tens of angstroms, we see major differences between phospholipid bilayer fusion and exocytotic fusion. The stable, hydrated pore which in exocytosis develops between the inside of the vesicle and the extracellular space is not usually detected in bilayer fusion. Osmotic swelling of the vesicle does not precede exocytotic fusion although it appears to drive phospholipid bilayer fusion. Lipid membranes fusion is irreversible; cell membranes seem to occasionally exhibit a reversible step before fusing. These observations suggest that some of the components of cell membranes other than lipids play crucial roles in the control, catalysis and molecular mechanism of cell membrane fusion.