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J. Cell Biol.,
Volume 140, Number 6, March 23, 1998 1369-1382

* Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National
Institutes of Health, Bethesda, Maryland 20892-1855; and The mechanism of bilayer unification in biological fusion is unclear. We reversibly arrested hemagglutinin (HA)-mediated cell-cell fusion right before fusion pore opening. A low-pH conformation of HA was
required to form this intermediate and to ensure fusion beyond it. We present evidence indicating that outer
monolayers of the fusing membranes were merged and
continuous in this intermediate, but HA restricted lipid
mixing. Depending on the surface density of HA and
the membrane lipid composition, this restricted hemifusion intermediate either transformed into a fusion pore
or expanded into an unrestricted hemifusion, without
pores but with unrestricted lipid mixing. Our results
suggest that restriction of lipid flux by a ring of activated HA is necessary for successful fusion, during
which a lipidic fusion pore develops in a local and transient hemifusion diaphragm.
Laboratory of Bioelectrochemistry, A.N. Frumkin Institute of
Electrochemistry, Russian Academy of Science, Moscow, 117071, Russia
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