Cis- and trans-membrane interactions of synaptotagmin-1.
In neurotransmission synaptotagmin-1 tethers synaptic vesicles to the presynaptic plasma membrane by binding to acidic membrane lipids and SNAREs and promotes rapid SNARE-mediated fusion upon Ca(2+) triggering. However, recent studies suggested that upon membrane contact synaptotagmin may not only bind in trans to the target membrane but also in cis to its own membrane. Using a sensitive membrane tethering assay we have now dissected the structural requirements and concentration ranges for Ca(2+)-dependent and -independent cis-binding and trans-tethering in the presence and absence of acidic phospholipids and SNAREs. Using variants of membrane-anchored synaptotagmin in which the Ca(2+)-binding sites in the C2 domains and a basic cluster involved in membrane binding were disrupted we show that Ca(2+)-dependent cis-binding prevents trans-interactions if the cis-membrane contains 12-20% anionic phospholipids. Similarly, no trans-interactions were observable using soluble C2AB-domain fragments at comparable concentrations. At saturating concentrations, however, tethering was observed with soluble C2AB domains, probably due to crowding on the vesicle surface and competition for binding sites. We conclude that trans-interactions of synaptotagmin considered to be essential for its function are controlled by a delicate balance between cis- and trans-binding, which may play an important modulatory role in synaptic transmission.
Research Group Biomolecular Spectroscopy and Single-Molecule Detection and Department of Neurobiology,  Max Planck Institute for Biophysical Chemistry, 37077 Göttingen,  Germany.
Wensi
W
Vennekate
Sabrina
S
Schröder
Chao-Chen
CC
Lin
Geert
G
van den Bogaart
Matthias
M
Grunwald
Reinhard
R
Jahn
Peter Jomo
PJ
Walla
D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D008081:Liposomes / Q000378:metabolism; D017729:Presynaptic Terminals / Q000378:metabolism*; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D051381:Rats; D050600:SNARE Proteins / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D009435:Synaptic Transmission / Q000502:physiology*; D013572:Synaptic Vesicles / Q000378:metabolism*; D050863:Synaptotagmin I / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
D008081:Liposomes; D050600:SNARE Proteins; D050863:Synaptotagmin I; C494312:Syt1 protein, rat; D002118:Calcium