Membrane hemifusion is a stable intermediate of exocytosis.
Membrane fusion during exocytosis requires that two initially distinct bilayers pass through a hemifused intermediate in which the proximal monolayers are shared. Passage through this intermediate is an essential step in the process of secretion, but is difficult to observe directly in vivo. Here we study membrane fusion in the sea urchin egg, in which thousands of homogeneous cortical granules are associated with the plasma membrane prior to fertilization. Using fluorescence redistribution after photobleaching, we find that these granules are stably hemifused to the plasma membrane, sharing a cytoplasmic-facing monolayer. Furthermore, we find that the proteins implicated in the fusion process-the vesicle-associated proteins VAMP/synaptobrevin, synaptotagmin, and Rab3-are each immobile within the granule membrane. Thus, these secretory granules are tethered to their target plasma membrane by a static, catalytic fusion complex that maintains a hemifused membrane intermediate.
Wong
Julian L
JL
Department of Molecular Biology, Cellular Biology, and Biochemistry, Brown University, Providence, RI 02912, USA.
Koppel
Dennis E
DE
Cowan
Ann E
AE
Wessel
Gary M
GM
D000818:Animals; D015550:Cell Degranulation; D002462:Cell Membrane / Q000502:physiology*; D005089:Exocytosis; D008053:Lipid Mobilization; D008561:Membrane Fusion; D008856:Microscopy, Fluorescence; D050600:SNARE Proteins / Q000378:metabolism / Q000502:physiology; D047270:Strongylocentrotus purpuratus / Q000378:metabolism / Q000502:physiology*; D033921:Vesicular Transport Proteins / Q000378:metabolism
D050600:SNARE Proteins; D033921:Vesicular Transport Proteins