Liposomes as tools for elucidating the mechanisms of membrane fusion

Membrane fusion is a central biophysical and biochemical reaction in numerous biological processes. Exocytosis involves the fusion of the secretory vesicle membrane with the plasma membrane in diverse biological systems, including neurotransmitter release at the neuromuscular junction, histamine release from mast cells, chromaffin granule extrusion from adrenal medullary cells, trichocyst discharge in Paramecium, endotoxin-induced degranulation in Limulus amebocytes, and the cortical reaction in sea urchin eggs.^1-4 The first stage in the formation of an endocytotic vesicle is the fusion of apposed regions of the invaginated plasma membrane. Later in the endocytotic pathway, endosomes and phagosomes fuse with lysosomes. Receptor recycling back to the plasma membrane proceeds through pinching off of receptor-containing vesicles from the compartment of uncoupling of receptor and ligand. Transport of newly synthesized membrane or secretory proteins from the endoplasmic reticulum to the Golgi apparatus, within the cis, medial and trans regions of the Golgi is thought to be mediated by transport vesicles that bud off from one compartment and fuse with another.^5-7.