semifusion

Semifusion is a term used in membrane biology to describe an intermediate state in the process by which two lipid bilayers merge. In some studies it denotes a transitional configuration between hemifusion, where the outer leaflets have merged, and full fusion, where a complete fusion pore forms and lumen contents can mix. However, semifusion is not universally defined, and some researchers use it as a synonym for hemifusion or as a broader category of partial fusion states.

Mechanistically, membrane fusion proceeds through stages that can include docking, lipid rearrangement, and pore formation. In frameworks that distinguish semifusion, the idea is that the membranes have partially merged or are in a metastable configuration where lipid continuity or limited exchange occurs, but a stable, open fusion pore is absent or highly restricted. Semifusion thus refers to a state that lies between initial hemifusion and completed fusion, rather than a final outcome.

Evidence for semifusion is primarily inferential, arising from experiments that detect partial lipid mixing without sustained content transfer, or from transient, small-diameter pores in single-vesicle assays. Such observations can be consistent with intermediate states, though they are often interpreted as variants of hemifusion or fleeting fusion events.

The concept is relevant to understanding the energetics and regulation of exocytosis, endocytosis, and viral entry, where fusion intermediates influence the timing and efficiency of cargo release. Terminology varies, and because semifusion is not universally standardized, researchers may use it differently or favor the more established term hemifusion.