Synaptotagmin V: a novel synaptotagmin isoform expressed in rat brain

Phospholipid composition dependence of Ca2+-dependent phospholipid binding to the C2A domain of synaptotagmin IV

Synaptotagmins I and II are Ca2+- and phospholipid-binding proteins of synaptic vesicles that may function as Ca2+ receptors for neurotransmitter release via their first C2 domains. Herein, we describe the phospholipid binding properties of C2A domains of multiple synaptotagmins (II-VI). We demonstrate that all synaptotagmins can bind negatively charged phospholipids (phosphatidylserine (PS) and phosphatidylinositol (PI)) in a Ca2+-dependent manner, although it was previously reported that synaptotagmins IV and VI do not bind phospholipids. The Ca2+-dependent interaction of the C2A domain of synaptotagmin IV with PS was found to have two components with EC50 values of approximately 5 and 120 microM free Ca2+ and exhibited positive cooperativity (Hill coefficient of approximately 2 for both components). This value is lower than that of the C2A domain of synaptotagmin II (Hill coefficient of approximately 3). All other isoforms bound PS with high affinity (EC50 of 0.3-1 microM free Ca2+; Hill coefficient of 3-3.5). In addition, the C2A domain of synaptotagmin IV cannot bind liposomes consisting of PS (or PI) and phosphatidylcholine, PC (or phosphatidylethanolamine, PE) (1:1, w/w), indicating that the binding to negatively charged phospholipids is inhibited by the presence of PC or PE. In contrast, other isoforms bound all of the liposomes, which include either PS or PI, in a Ca2+-dependent manner. Mutational analysis indicated that this phospholipid composition-dependent Ca2+ binding of synaptotagmin IV results in the substitution of Asp for Ser at position 244. The cytoplasmic domain of synaptotagmin IV also shows this unique phospholipid binding. However, it binds PS with a positive cooperativity and an affinity similar to those of the C2A domains of other isoforms. Our results suggest that synaptotagmin IV is also a potential Ca2+ sensor for neurotransmitter release.

Distinct Ca(2+)-dependent properties of the first and second C2-domains of synaptotagmin I

Synaptotagmin 1 (SytI) is a synaptic vesicle protein that binds Ca2+ and is essential for fast, Ca(2+)-dependent neurotransmitter release in the hippocampus, suggesting that it serves as a Ca2+ sensor for exocytosis. Although SytI has two cytoplasmic C2-domains, only the first C2-domain was shown to exhibit Ca2+ regulation; it binds phospholipids and syntaxin in a Ca(2+)-dependent manner. By contrast, the second C2-domain is inactive in these assays and only binds putative interacting molecules in a Ca(2+)-independent manner. We have now discovered in a yeast two-hybrid screen for SytI-interacting molecules that the C2-domains of SytI interact with themselves. Using immobilized recombinant C2-domains from SytI and SytII, we found that only the second but not the first C2-domains of these synaptotagmins are capable of affinity-purifying native rat brain SytI and that this binding is Ca(2+)-dependent, suggesting that only the second C2-domain is capable of a Ca(2+)-triggered self-association. A relatively high Ca2+ concentration (> 100 microM) is required for binding in the presence of Mg2+; Sr2+ and Ba2+ but not Mg2+ can substitute for Ca2+. Our data suggest that the second C2-domain of SytI is also a Ca(2+)-regulated domain similar to the first C2-domain but with distinct binding activities.

The high-affinity binding of Clostridium botulinum type B neurotoxin to synaptotagmin II associated with gangliosides GT1b/GD1a

125I-labeled botulinum type B neurotoxin was shown to bind specifically to recombinant rat synaptotagmins I and II. Binding required reconstitution of the recombinant proteins with gangliosides GT1b/GD1a. Scatchard plot analyses revealed a single class of binding site with dissociation constants of 0.23 and 2.3 nM for synaptotagmin II and synaptotagmin I, respectively, values very similar to those of the high- (0.4 nM) and low-affinity (4.1 nM) binding sites in synaptosomes. The high-affinity binding of neurotoxin to synaptosomes was specifically inhibited by a monoclonal antibody recognizing with the amino-terminal region of synaptotagmin II. These results suggest that this region of synaptotagmin II participates in the formation of the high-affinity toxin binding site by associating with specific gangliosides.

Identification of a novel protein containing two C2 domains selectively expressed in the rat brain and kidney

We have isolated and characterized a rat brain cDNA clone which encodes a new protein of 474 amino acids in length which contains two C2 domains structurally homologous to those present in synaptotagmins. The overall amino acid identity in C2 domains between this protein and the synaptotagmins is 36-44%. This protein also contains 3 putative consensus sequences for phosphorylation by cAMP-dependent protein kinase. RNA blot hybridization revealed a 3.0 kb transcript abundantly expressed only in the rat brain and the kidney. Thus, we called this brain/kidney protein (B/K). In situ hybridization and Northern blot analyses showed that the B/K transcript was found in forebrain including the olfactory bulb, cerebral cortex, hippocampus, and hypothalamus. In the kidney, high levels of B/K transcript were expressed in the papillary region of the inner medulla, the inner stripe of the outer medulla and the cortex. The selective expression in forebrain and kidney suggests that B/K may be involved in similar cAMP-dependent processes at these very different sites.

Antibodies to recombinant synaptotagmin and calcium channel subtypes in Lambert-Eaton myasthenic syndrome

Several proteins have been postulated as possible targets of immune attack in Lambert-Eaton myasthenic syndrome (LEMS). Heterogeneity of autoantibodies in sera from 20 LEMS patients was studied by comparing their reactivity to synaptotagmin, a synaptic vesicle protein, and voltage-gated calcium channels (VGCCs). Six patients' sera (1 with small cell lung carcinoma (SCLC)) contained antibodies specifically recognizing the recombinant synaptotagmin on immunoblots. Thirteen (11 with SCLC) and 16 (11 with SCLC and 1 with poorly differentiated cell carcinoma in the lung) patients' sera immunoprecipitated omega-conotoxin GVIA-labeled N-type and omega-conotoxin MVIIC-labeled Q-type VGCCs, respectively. Three of 6 synaptotagmin-positive sera had cross-reactivity with N and/or Q subtypes of VGCC; the remaining 3 showed no cross-reactivity with VGCCs. Results indicate that LEMS sera are heterogeneous in the spectrum of containing antibodies, and suggest that this heterogeneity reflects the immune response to various synaptic proteins including not only multiple VGCCs but also synaptosecretory complex proteins.

Distinct Ca2+ and Sr2+ binding properties of synaptotagmins. Definition of candidate Ca2+ sensors for the fast and slow components of neurotransmitter release

Ca(2+)-dependent neurotransmitter release consists of at least two components: a major fast component that is insensitive to Sr2+ and a minor slow component that is potentiated by Sr2+ (Goda, Y., and Stevens, C. F. (1994) Proc. Natl. Acad. U. S. A. 91, 12942-12946). These results suggest that at least two Ca2+ sensors act in synaptic vesicle fusion with distinct Ca2+ and Sr2+ binding properties. We have now investigated the relative Ca2+ and Sr2+ binding activities of synaptotagmins to evaluate their potential roles as Ca2+ sensors for the fast and slow components. Our results demonstrate that the first C2 domains of synaptotagmins I, II, III, V, and VII have very similar Ca2+ requirements for phospholipid binding (range of EC50 = 2.6 microM to 5.0 microM), but distinct Sr2+ requirements (EC50 range = 23 microM to 133 microM); synaptotagmins I and II had the lowest Sr2+ affinity, and synaptotagmin III the highest Sr2+ affinity. Purified synaptotagmin I from bovine brain exhibited similar properties as its recombinant first C2 domain, suggesting that the first C2 domain fully accounts for its Ca(2+)-dependent phospholipid binding properties. Sr2+ was unable to trigger syntaxin binding by synaptotagmin I at all concentrations tested, whereas it was effective for synaptotagmin III. These results suggest that different C2 domains have distinct Sr2+ binding properties. They support the hypothesis that synaptotagmins localized on the same vesicle perform distinct functions, with synaptotagmins I and II serving as candidate Ca2+ sensors for the fast component in release and synaptotagmin III for the slow component.