Ca(2+)-dependent and -independent activities of neural and non-neural synaptotagmins.
Synaptotagmins (Syts) are brain-specific Ca2+/phospholipid-binding proteins. In hippocampal synapses, Syt I is essential for fast Ca(2+)-dependent synaptic vesicle exocytosis but not for Ca(2+)-independent exocytosis. In vertebrates and invertebrates, Syt may therefore participate in Ca(2+)-dependent synaptic membrane fusion, either by serving as the Ca2+ sensor in the last step of fast Ca(2+)-triggered neurotransmitter release, or by collaborating with an additional Ca2+ sensor. While Syt I binds Ca2+ (refs 10, 11), its phospholipid binding is triggered at lower calcium concentrations (EC50 = 3-6 microM) than those required for exocytosis. Furthermore, Syts bind clathrin-AP2 with high affinity, indicating that they may play a general role in endocytosis rather than being confined to a specialized function in regulated exocytosis. Here we resolve this apparent contradiction by describing four Syts, three of which (Syt VI, VII and VIII) are widely expressed in non-neural tissues. All Syts tested share a common domain structure, with a cytoplasmic region composed of two C2 domains that interacts with clathrin-AP2 (Kd = 0.1-1.0 nM) and with neural and non-neural syntaxins. The first C2 domains of Syt I, II, III, V and VII, but not of IV, VI or VIII, bind phospholipids with a similar Ca(2+)-concentration dependence (EC50 = 3-6 microM). The same C2 domains also bind syntaxin as a function of Ca2+ but the Ca(2+)-concentration dependence of Syt I, II and V (> 200 microM) differs from that of Syt III and VII (< 10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)
Li
C
C
Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas 75235, USA.
Ullrich
B
B
Zhang
J Z
JZ
Anderson
R G
RG
Brose
N
N
Südhof
T C
TC
D000595:Amino Acid Sequence; D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins; D002460:Cell Line; D003001:Cloning, Molecular; D005089:Exocytosis; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009474:Neurons / Q000378:metabolism*; D011485:Protein Binding; D050765:Qa-SNARE Proteins; D050683:R-SNARE Proteins; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D013569:Synapses / Q000378:metabolism; D050825:Synaptosomal-Associated Protein 25; D050857:Synaptotagmins
D002135:Calcium-Binding Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D050765:Qa-SNARE Proteins; D050683:R-SNARE Proteins; D011994:Recombinant Proteins; C494288:SYT3 protein, human; C494298:SYT6 protein, human; C494305:SYT8 protein, human; C494209:Snap25 protein, mouse; C494210:Snap25 protein, rat; D050825:Synaptosomal-Associated Protein 25; D050857:Synaptotagmins; D002118:Calcium