membranfusion

Membranfusion is the process by which two separate phospholipid bilayers merge to form a single continuous membrane, allowing the mixing of their luminal or cytosolic contents. It underpins many cellular activities, including vesicle trafficking, fertilization, and the entry of enveloped viruses, as well as various laboratory and therapeutic applications involving liposomes and artificial membranes.

The fusion process proceeds through multiple steps and is driven by specialized proteins and biophysical forces. In cells, SNARE proteins assemble into a four-helix bundle that brings a vesicle membrane (v-SNARE) into close proximity with a target membrane, priming the membranes for fusion. A rise in intracellular calcium often triggers the final fusion step, with proteins such as synaptotagmin acting as calcium sensors. In many viruses, fusion proteins undergo large conformational changes to insert a fusion peptide into the host membrane and pull the two membranes together, forming a fusion pore.

Biophysical intermediates include hemifusion, where only the outer leaflets merge, followed by pore formation that creates an aqueous channel between compartments. The lipid composition of membranes—curvature tendencies, cholesterol content, and the presence of specific lipids—affects fusion efficiency. Accessory proteins and cytoskeletal elements can regulate docking, priming, and pore expansion.

Biological contexts include exocytosis of neurotransmitters and hormones, vesicle trafficking through the Golgi and endosomes, and fertilization in which sperm and egg membranes fuse. In biotechnology, liposome fusion is exploited for drug delivery and gene transfer, and fusion inhibitors are used as antiviral therapies to block entry of enveloped viruses.

Researchers study membrane fusion using model membranes, reconstituted proteoliposomes, and single-vesicle assays, employing fluorescence resonance energy transfer, content release assays, and electron microscopy to monitor lipid mixing, pore formation, and fusion efficiency.