Ca2+-dependent and -independent activities of neural and non-neural synaptotagmins

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Abstract
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)