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

Abstract

SYNAPTOTAGMINS (Syts) are brain-specific Ca²⁺/phospholipid-binding proteins(1-5). In hippocampal synapses, Syt I is essential for fast Ca²⁺-dependent synaptic vesicle exocytosis but not for Ca²⁺-independent exocytosis(3). In vertebrates and invertebrates(6-9), Syt may therefore participate in Ca²⁺-dependent synaptic membrane fusion, either by serving as the Ca²⁺ sensor in the last step of fast Ca²⁺-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 (EC(50)=3-6 mu M) than those required for exocytosis(12). Furthermore, Syts bind clathrin-AP2 with high affinity, indicating that they may play a general role in endocytosis(4,5) rather than being confined to a specialized function in regulated exocytosis(3). Here me 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 C-2 domains that interacts with clathrin-AP2 (K-d=0.1-1.0 nM) and with neural and non-neural syntaxins. The first C-2 domains of Syt I, II, III, V and VII, but not of IV, VI or VIII, bind phospholipids with a similar Ca²⁺-concentration dependence (EC(50)=3-6 mu M). The same C-2 domains also bind syntaxin as a function of Ca²⁺ but the Ca²⁺-concentration dependence of Syt I, II, and V (>200 mu M) differs from that of Syt III and VII (<10 mu M), Syts therefore appear to be ubiquitous proteins with a role in exocytosis mediated by syntaxin binding. The Ca²⁺ levels needed to trigger syntaxin binding by the different Syts suggest that they play distinct roles in membrane fusion; the level required by Syt I approximates those required for synaptic exocytosis.