Phosphatidylinositol 4,5-bisphosphate clusters act as molecular beacons for vesicle recruitment

• DOI: 10.1038/nsmb.2570
• Publication Date: 2013-05-12T19:29:52Z
• Authors: Alf Honigmann; Geert van den Bog...; Emilio Iraheta; H Jelger Risselada; Dragomir Milovanovic; Veronika Mueller; Stefan Müllar; Ulf Diederichsen; Dirk Fasshauer; Helmut Grubmüller; Stefan W Hell; Christian Eggeling; Karin Kühnel; Reinhard Jahn
• Keywords: Synaptotagmin 1; Vesicle fusion; STX1A; Syntaxin 3; SNAP25

Synaptic-vesicle exocytosis is mediated by the vesicular Ca(2+) sensor synaptotagmin-1. Synaptotagmin-1 interacts with the SNARE protein syntaxin-1A and acidic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PIP2). However, it is unclear how these interactions contribute to triggering membrane fusion. Using PC12 cells from Rattus norvegicus and artificial supported bilayers, we show that synaptotagmin-1 interacts with the polybasic linker region of syntaxin-1A independent of Ca(2+) through PIP2. This interaction allows both Ca(2+)-binding sites of synaptotagmin-1 to bind to phosphatidylserine in the vesicle membrane upon Ca(2+) triggering. We determined the crystal structure of the C2B domain of synaptotagmin-1 bound to phosphoserine, allowing development of a high-resolution model of synaptotagmin bridging two different membranes. Our results suggest that PIP2 clusters organized by syntaxin-1 act as molecular beacons for vesicle docking, with the subsequent Ca(2+) influx bringing the vesicle membrane close enough for membrane fusion.