Functions and modes of action of proteins with C2 domains in Ca^<2+>-dependent

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The C2 domain has first been identified in a conventional type of protein kinase C.The C2 domain interacts with Ca^<2+> and phospholipids.The interactions of protein kinase C with these factors through the C2 domain are essential for its activation.Protein kinase C has one C2 domain, but proteins having two C2-like domains have recently identified.Accumulating evidence suggests that proteins having two C2-like domains are implicated in Ca_<+1> -dependent neurotrans- mitter release.These include synaptotagmin, Munc13, rabphilin-3A, and Doc2.Of these proteins, rabphilin-3A and Doc2 were discovered in our laboratory.During this support from 1997 to 1998, we have focused on studying the functions and modes of actions of rabphilin-3A and Doc2 in neurotransmitter release.The results obtained are as follows :(1) Electrophysiological study using the squid giant axon system indicates that rabphilin-3A is involved in Ca^<2+> -dependent neurotransmitter release.(2) We have found that Doc2 directly interacts with Munc13, which is localized at the presynaptic plasma membrane and is implicated in Ca^<2+> -dependent neuro- transmitter release.We have found that the region localized near the N-terminus of Doc2, named Mid (Munc13-interacting domain), directly binds to a region between the two C2-like domains of Munc13, named Did (Doc2-interacting domain).The Doc2-Munc13 interaction is enhanced by the binding of diacylglycerol or phorbol ester to the C1-like domain of Munc13.(3) Electrophysiological study using cultured rat superior cervical ganglion neurons indicates that the Doc2-Munc13 interaction is involved in Ca^<2+> -dependent neurotransmitter release.Electrophysiological study using the CA1 region of hippocampal slices of Doc2 null mutant mice indicates that Doc2 is not essential for basal neurotransmission but is involved in short-term plasticity and long-term potentiation.