CA2-DEPENDENT AND CA2+-INDEPENDENT ACTIVITIES OF NEURAL AND NONNEURALSYNAPTOTAGMINS()

SYNAPTOTAGMINS (Syts) are brain-specific Ca2+/phospholipid-binding pro teins(1-5). In hippocampal synapses, Syt I is essential for fast Ca2+- dependent synaptic vesicle exocytosis but not for Ca2+-independent exo cytosis(3). In vertebrates and invertebrates(6-9), Syt may therefore p articipate in Ca2+-dependent synaptic membrane fusion, either by servi ng as the Ca2+ sensor in the last step of fast Ca2+-triggered neurotra nsmitter release, or by collaborating with an additional Ca2+ sensor. While Syt I binds Ca2+ (refs 10, 11), its phospholipid binding is trig gered at lower calcium concentrations (EC(50)=3-6 mu M) than those req uired for exocytosis(12). Furthermore, Syts bind clathrin-AP2 with hig h affinity, indicating that they may play a general role in endocytosi s(4,5) rather than being confined to a specialized function in regulat ed exocytosis(3). Here me resolve this apparent contradiction by descr ibing four Syts, three of which (Syt VI, VII and VIII) are widely expr essed in non-neural tissues. All Syts tested share a common domain str ucture, with a cytoplasmic region composed of two C-2 domains that int eracts with clathrin-AP2 (K-d=0.1-1.0 nM) and with neural and non-neur al 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+-concentr ation dependence (EC(50)=3-6 mu M). The same C-2 domains also bind syn taxin as a function of Ca2+ but the Ca2+-concentration dependence of S yt 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 tri gger syntaxin binding by the different Syts suggest that they play dis tinct roles in membrane fusion; the level required by Syt I approximat es those required for synaptic exocytosis.