Distinct self-oligomerization activities of synaptotagmin family. Unique calcium-dependent oligomerization properties of synaptotagmin VII

Mechanism of the SDS-resistant synaptotagmin clustering mediated by the cysteine cluster at the interface between the transmembrane and spacer domains

Synaptotagmin I (Syt I), a proposed major Ca(2+) sensor in the central nervous system, has been hypothesized as functioning in an oligomerized state during neurotransmitter release. We previously showed that Syts I, II, VII, and VIII form a stable SDS-resistant, beta-mercaptoethanol-insensitive, and Ca(2+)-independent oligomer surrounding the transmembrane domain (Fukuda, M., and Mikoshiba, K. (2000) J. Biol. Chem. 275, 28180-28185), but little is known about the molecular mechanism of the Ca(2+)-independent oligomerization by the synaptotagmin family. In this study, we analyzed the Ca(2+)-independent oligomerization properties of Syt I and found that it shows two distinct forms of self-oligomerization activity: stable SDS-resistant self-oligomerization activity and relatively unstable SDS-sensitive self-oligomerization activity. The former was found to be mediated by a post-translationally modified (i.e. fatty-acylated) cysteine (Cys) cluster (Cys-74, Cys-75, Cys-77, Cys-79, and Cys-82) at the interface between the transmembrane and spacer domains of Syt I. We also show that the number of Cys residues at the interface between the transmembrane and spacer domains determines the SDS- resistant oligomerizing capacity of each synaptotagmin isoform: Syt II, which contains seven Cys residues, showed the strongest SDS-resistant oligomerizing activity in the synaptotagmin family, whereas Syt XII, which has no Cys residues, did not form any SDS-resistant oligomers. The latter SDS-sensitive self-oligomerization of Syt I is mediated by the spacer domain, because deletion of the whole spacer domain, including the Cys cluster, abolished it, whereas a Syt I(CA) mutant carrying Cys to Ala substitutions still exhibited self-oligomerization. Based on these results, we propose that the oligomerization of the synaptotagmin family is regulated by two distinct mechanisms: the stable SDS-resistant oligomerization is mediated by the modified Cys cluster, whereas the relatively unstable (SDS-sensitive) oligomerization is mediated by the environment of the spacer domain.

The C2B domain of synaptotagmin I is a Ca2+-binding module

Synaptotagmin I is a synaptic vesicle protein that contains two C(2) domains and acts as a Ca(2+) sensor in neurotransmitter release. The Ca(2+)-binding properties of the synaptotagmin I C(2)A domain have been well characterized, but those of the C(2)B domain are unclear. The C(2)B domain was previously found to pull down synaptotagmin I from brain homogenates in a Ca(2+)-dependent manner, leading to an attractive model whereby Ca(2+)-dependent multimerization of synaptotagmin I via the C(2)B domain participates in fusion pore formation. However, contradictory results have been described in studies of Ca(2+)-dependent C(2)B domain dimerization, as well as in analyses of other C(2)B domain interactions. To shed light on these issues, the C(2)B domain has now been studied using biophysical techniques. The recombinant C(2)B domain expressed as a GST fusion protein and isolated by affinity chromatography contains tightly bound bacterial contaminants despite being electrophoretically pure. The contaminants bind to a polybasic sequence that has been previously implicated in several C(2)B domain interactions, including Ca(2+)-dependent dimerization. NMR experiments show that the pure recombinant C(2)B domain binds Ca(2+) directly but does not dimerize upon Ca(2+) binding. In contrast, a cytoplasmic fragment of native synaptotagmin I from brain homogenates, which includes the C(2)A and C(2)B domains, participates in a high molecular weight complex as a function of Ca(2+). These results show that the recombinant C(2)B domain of synaptotagmin I is a monomeric, autonomously folded Ca(2+)-binding module and suggest that a potential function of synaptotagmin I multimerization in fusion pore formation does not involve a direct interaction between C(2)B domains or requires a posttranslational modification.

Synaptotagmin VI participates in the acrosome reaction of human spermatozoa

Acrosomal exocytosis is a calcium-dependent secretion event causing the release of the acrosomal contents and the loss of the membranes surrounding the acrosome. The synaptotagmins are a family of calcium-binding proteins that participate in the exocytosis of synaptic vesicles. The ubiquitous synaptotagmin VI isoform was found in human sperm cells by Western blot analysis. Immunocytochemistry at the optical and electron microscopy levels localized the protein to the outer acrosomal membrane. Calcium-triggered acrosomal exocytosis in permeabilized sperm cells was abrogated by a specific anti-synaptotagmin VI antibody, indicating that the protein is required for the process. Moreover, a recombinant fusion protein between glutathione S-transferase and the two calcium and phospholipid binding domains of synaptotagmin VI completely inhibited calcium-triggered exocytosis. Interestingly, phorbol ester-dependent in vitro phosphorylation of this recombinant protein abolished its inhibitory effect. We previously showed that, in permeabilized spermatozoa, addition of active Rab3A triggers acrosomal exocytosis at very low calcium concentration. Rab3A-promoted exocytosis was inhibited by the cytosolic domain of synaptotagmin VI and by the anti-synaptotagmin VI antibody, indicating that synaptotagmin is also necessary for Rab-mediated acrosomal content release. In conclusion, the results strongly indicate that synaptotagmin VI is a key component of the secretory machinery involved in acrosomal exocytosis.

Ca2+ binding to alpha-synuclein regulates ligand binding and oligomerization

alpha-Synuclein is a protein normally involved in presynaptic vesicle homeostasis. It participates in the development of Parkinson's disease, in which the nerve cell lesions, Lewy bodies, accumulate alpha-synuclein filaments. The synaptic neurotransmitter release is primarily dependent on Ca(2+)-regulated processes. A microdialysis technique was applied showing that alpha-synuclein binds Ca(2+) with an IC(50) of about 2-300 microm and in a reaction uninhibited by a 50-fold excess of Mg(2+). The Ca(2+)-binding site consists of a novel C-terminally localized acidic 32-amino acid domain also present in the homologue beta-synuclein, as shown by Ca(2+) binding to truncated recombinant and synthetic alpha-synuclein peptides. Ca(2+) binding affects the functional properties of alpha-synuclein. First, the ligand binding of (125)I-labeled bovine microtubule-associated protein 1A is stimulated by Ca(2+) ions in the 1-500 microm range and is dependent on an intact Ca(2+) binding site in alpha-synuclein. Second, the Ca(2+) binding stimulates the proportion of (125)I-alpha-synuclein-containing oligomers. This suggests that Ca(2+) ions may both participate in normal alpha-synuclein functions in the nerve terminal and exercise pathological effects involved in the formation of Lewy bodies.

Mechanism of the calcium-dependent multimerization of synaptotagmin VII mediated by its first and second C2 domains

The Ca(2+)-dependent oligomerization activity of the second C2 (C2B) domain of synaptotagmin I (Syt I) has been hypothesized to regulate neurotransmitter release. We previously showed that the cytoplasmic domains of several other Syt isoforms also show Ca(2+)-dependent oligomerization activity (Fukuda, M., and Mikoshiba, K. (2000) J. Biol. Chem. 275, 28180-28185), but little is known about the involvement of their C2 domains in Ca(2+)-dependent oligomerization. In this study, we analyzed the Ca(2+)-dependent oligomerization properties of the first (C2A) and the second C2 (C2B) domains of Syt VII. Unlike Syt I, both C2 domains of Syt VII contribute to Ca(2+)-dependent homo- and hetero-oligomerization with other isoforms. For instance, the Syt VII C2A domain Ca(2+)-dependently binds itself and the C2A domain of Syt VI but not its C2B domain, whereas the Syt VII C2B domain Ca(2+)-dependently binds itself and the C2B domain of Syt II but not its C2A domain. In addition, we showed by gel filtration that a single Syt VII C2 domain is sufficient to form a Ca(2+)-dependent multimer of very high molecular weight. Because of this "two handed" structure, the Syt VII cytoplasmic domain has been found to show the strongest Ca(2+)-dependent multimerization activity in the Syt family. We also identified Asn-328 in the C2B domain as a crucial residue for the efficient Ca(2+)-dependent switch for multimerization by site-directed mutagenesis. Our results suggest that Syt VII is a specific isoform that can cluster different Syt isoforms with two hands in response to Ca(2+).

The C2A domain of double C2 protein gamma contains a functional nuclear localization signal

The C2 domain was originally defined as a homologous domain to the C2 regulatory region of Ca2+ -dependent protein kinase C and has been identified in more than 50 different signaling molecules. The original C2 domain of protein kinase Calpha functions as a Ca2+ binding module, and the Ca2+ binding to the C2 domain allows translocation of proteins to phospholipid membranes. By contrast, however, some C2 domains do not exhibit Ca2+ binding activity because of amino acid substitutions at Ca2+ -binding sites, and their physiological meanings remain largely unknown. In this study, we discovered an unexpected function of the Ca2+ -independent C2A domain of double C2 protein gamma (Doc2gamma) in nuclear localization. Deletion and mutation analyses revealed that the putative Ca2+ binding loop 3 of Doc2gamma contains six Arg residues ((177)RLRRRRR(183)) and that this basic cluster is both necessary and sufficient for nuclear localization of Doc2gamma. Because of the presence of the basic cluster, the C2A domain of Doc2gamma did not show Ca2+ -dependent phospholipid binding activity. Our findings indicate that by changing the nature of the putative Ca2+ binding loops the C2 domain has more diversified function in cellular signaling than a simple Ca2+ binding motif.

The Exocytosis-regulatory protein synaptotagmin VII mediates cell invasion by Trypanosoma cruzi

The intracellular protozoan parasite Trypanosoma cruzi causes Chagas' disease, which affects millions of people in Latin America. T. cruzi enters a large number of cell types by an unusual mechanism that involves Ca(2+)-triggered fusion of lysosomes with the plasma membrane. Here we show that synaptotagmin VII (Syt VII), a ubiquitously expressed synaptotagmin isoform that regulates exocytosis of lysosomes, is localized on the membranes of intracellular vacuoles containing T. cruzi. Antibodies against the C(2)A domain of Syt VII or recombinant peptides including this domain inhibit cell entry by T. cruzi, but not by Toxoplasma gondii or Salmonella typhimurium. The C(2)A domains of other ubiquitously expressed synaptotagmin isoforms have no effect on T. cruzi invasion, and mutation of critical residues on Syt VII C(2)A abolish its inhibitory activity. These findings indicate that T. cruzi exploits the Syt VII-dependent, Ca(2+)-regulated lysosomal exocytic pathway for invading host cells.

A unique spacer domain of synaptotagmin IV is essential for Golgi localization

Synaptotagmin (Syt) family members consist of six separate domains: a short amino terminus, a single transmembrane domain, a spacer domain, a C2A domain, a C2B domain and a short carboxyl (C) terminus. Despite sharing the same domain structures, several synaptotagmin isoforms show distinct subcellular localization. Syt IV is mainly localized at the Golgi, while Syt I, a possible Ca(2+)-sensor for secretory vesicles, is localized at dense-core vesicles and synaptic-like microvesicles in PC12 cells. In this study, we sought to identify the region responsible for the Golgi localization of Syt IV by immunocytochemical and biochemical analyses as a means of defining the distinct subcellular localization of the synaptotagmin family. We found that the unique C-terminus of the spacer domain (amino acid residues 73-144) between the transmembrane domain and the C2A domain is essential for the Golgi localization of Syt IV. In addition, the short C-terminus is probably involved in proper folding of the protein, especially the C2B domain. Without the C-terminus, Syt IVdeltaC proteins are not targeted to the Golgi and seem to colocalize with an endoplasmic reticulum (ER) marker (i.e. induce crystalloid ER-like structures). On the basis of these results, we propose that the divergent spacer domain among synaptotagmin isoforms may contain certain signals that determine the final destination of each isoform.

Expression and localisation of synaptotagmin isoforms in endocrine (β)-cells: their function in insulin exocytosis

Exocytosis of insulin containing Large Dense Core Vesicles (LDCVs) from pancreatic (β)-cells and derived cell lines is mainly controlled by Ca(2+). Several lines of evidence have demonstrated a role of the Ca(2+)- and phospholipid-binding protein synaptotagmin (syt) in this event. Synaptotagmins form a large protein family with distinct affinities for Ca(2+) determined by their two C(2) domains (C(2)A/B). Except for the well-characterized isoforms I and II, their role is still unclear. We have used here insulin-secreting cells as a model system for LDCV exocytosis to gain insight into the function of synaptotagmins. Immunocytochemical analysis revealed that of the candidate Ca(2+) sensors in LDCV exocytosis, syt III was not expressed in primary (β)-cells, whereas syt IV was only found adjacent to the TGN. However, syt V-VIII isoforms were expressed at different levels in various insulin-secreting cells and in pancreatic islet preparations. In streptolysin-O permeabilized primary (β)-cells the introduction of recombinant peptides (100 nM) corresponding to the C(2) domains of syt V, VII and VIII, but not of syt III, IV or VI, inhibited Ca(2+)-evoked insulin exocytosis by 30% without altering GTP*S-induced release. Our observations demonstrate that syt III and IV are not involved in the exocytosis of LDCVs from primary (β)-cells whereas V, VII and VIII may mediate Ca(2+)-regulation of exocytosis.

Synaptotagmin-like protein 1-3: a novel family of C-terminal-type tandem C2 proteins

Synaptotagmins (Syt), rabphilin-3A, and Doc2 belong to a family of carboxyl terminal type (C-type) tandem C2 proteins and are thought to be involved in vesicular trafficking. We have cloned and characterized a novel family of C-type tandem C2 proteins, designated Slp1-3 (synaptotagmin-like protein 1-3). The Slp1-3 C2 domains show high homology to granuphilin-a C2 domains, but the amino-terminal domain of Slp1-3 does not contain any known protein motifs or a transmembrane domain. A subcellular fractionation study indicated that Slp1-3 proteins are peripheral membrane proteins. Phospholipid binding experiments indicated that Slp3 is a Ca(2+)-dependent isoform, but Slp1 and Slp2 are Ca(2+)-independent isoforms, because only the Slp3 C2A domain showed Ca(2+)-dependent phospholipid binding activity. The C-terminus of Slp1-3 also bound neurexin Ialpha in vitro, in the same manner as Syt family proteins, which may be important for the membrane association of Slp1-3. In addition, Slp family proteins are differentially distributed in different mouse tissues and at different developmental stages.

Characterization of KIAA1427 protein as an atypical synaptotagmin (Syt XIII)

Synaptotagmin (Syt) belongs to a family of type-I membrane proteins and is a protein that consists of a short extracellular N-terminus, a single transmembrane domain, two C2 domains and a short C-terminus. Here, we cloned and characterized a mouse orthologue of human KIAA1427 protein as an atypical Syt (named Syt XIII). Subcellular fractionation and antibody-uptake experiments indicate that Syt XIII is indeed a type-I membrane protein, but, unlike other Syt isoforms, lacks an N-terminal extracellular domain. Syt XIII C2 domains show relatively little similarity to Syt I (less than 35% identity at the amino acid level), and lack key amino acids responsible for Ca2+ binding. Because of these substitutions, the Syt XIII C2 domains did not show Ca2+-dependent phospholipid-binding activity, and Syt XIII is thus classified as a Ca2+ -independent isoform. By contrast, the Syt XIII C-terminal domain is highly homologous with other Syt isoforms and can function as a common receptor for neurexin Ialpha in vitro. Since Syt XIII is expressed in various tissues outside the brain, Syt XIII may be involved in constitutive vesicle transport.

Role of the conserved WHXL motif in the C terminus of synaptotagmin in synaptic vesicle docking

Synaptotagmin (Syt) I, an abundant synaptic vesicle protein, consists of one transmembrane region, two C2 domains, and a short C terminus. This protein is essential for both synaptic vesicle exocytosis and endocytosis via its C2 domains. Although the short C terminus is highly conserved among the Syt family and across species, little is known about the exact role of the conserved C terminus of Syt I. In this paper, we report a function of the Syt I C terminus in synaptic vesicle docking at the active zones. Presynaptic injection of a peptide corresponding to the C-terminal 21 amino acids of Syt I (named Syt-C) into the squid giant synapse blocked synaptic transmission without affecting the presynaptic action potential or the presynaptic Ca(2+) currents. The same procedure repeated with a mutant C-terminal peptide (Syt-CM) had no effect on synaptic transmission. Repetitive presynaptic stimulation with Syt-C produced a rapid decrease in the amplitude of the postsynaptic potentials as the synaptic block progressed, indicating that the peptide interferes with the docking step rather than the fusion step of synaptic vesicles. Electron microscopy of the synapses injected with the Syt-C peptide showed a marked decrease in the number of docked synaptic vesicles at the active zones, as compared with controls. These results indicate that Syt I is a multifunctional protein that is involved in at least three steps of synaptic vesicle cycle: docking, fusion, and reuptake of synaptic vesicles.

Protein-protein interactions in intracellular membrane fusion

The fusion of intracellular vesicles with their target membranes is an essential feature of the compartmental structure of eukaryotic cells. This process requires proteins that dictate the targeting of a vesicle to the correct cellular location, mediate bilayer fusion and, in some systems, regulate the precise time at which fusion occurs. Recent biophysical and structural studies of these proteins have begun to provide a foundation for understanding their functions at a molecular level.

Drosophila AD3 mutation of synaptotagmin impairs calcium-dependent self-oligomerization activity

Genetic analysis of a Drosophila synaptotagmin (Syt) I mutant (AD3) has revealed that Tyr-334 within the C2B domain is essential for efficient Ca(2+)-dependent neurotransmitter release. However, little is known as to why a missense mutation (Tyr-334-Asn) disrupts the function of the C2B domain at the molecular level. Here, we present evidence that a Tyr-312 to Asn substitution in mouse Syt II, which corresponds to the Drosophila AD3 mutation, completely impairs Ca(2+)-dependent self-oligomerization activity mediated by the C2B domain but allows partial interaction with wild-type proteins in a Ca(2+)-dependent manner. This observation is consistent with the fact that the AD3 allele is homozygous lethal but complements another mutant phenotype. We also showed that the Ca(2+)-dependent C2B self-oligomerization is inhibited by inositol 1,3,4, 5-tetrakisphosphate, a potent inhibitor of neurotransmitter release. All of these findings strongly support the idea that self-oligomerization of Syt I or II is essential for neurotransmitter release in vivo.