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

Synaptotagmin 1 oligomerization via the juxtamembrane linker regulates spontaneous and evoked neurotransmitter release

Synaptotagmin 1 (syt1) is a synaptic vesicle (SV) protein that is rapidly activated by Ca²⁺ influx into presynaptic nerve terminals, triggering SV exocytosis. Syt1 also inhibits exocytosis, prior to Ca²⁺ influx, and thus helps synchronize evoked exocytosis upon Ca²⁺ binding. Herein, we identified a cluster of lysine residues, in the oft-ignored juxtamembrane linker region of syt1, that governs homo-multimerization in an anionic lipid-dependent manner. Neutralization of this positively charged region abolished syt1 self-association on phospholipid bilayers in vitro. Subsequently, in neurons, we found mutations that disrupted syt1 self-association were correlated with defects in clamping spontaneous SV release and in triggering and synchronizing evoked exocytosis. Thus, syt1 regulates SV exocytosis as an oligomer via charged residues in the juxtamembrane linker.

Synaptotagmin 1 (syt1) is a Ca²⁺ sensor that regulates synaptic vesicle exocytosis. Cell-based experiments suggest that syt1 functions as a multimer; however, biochemical and electron microscopy studies have yielded contradictory findings regarding putative self-association. Here, we performed dynamic light scattering on syt1 in solution, followed by electron microscopy, and we used atomic force microscopy to study syt1 self-association on supported lipid bilayers under aqueous conditions. Ring-like multimers were clearly observed. Multimerization was enhanced by Ca²⁺ and required anionic phospholipids. Large ring-like structures (∼180 nm) were reduced to smaller rings (∼30 nm) upon neutralization of a cluster of juxtamembrane lysine residues; further substitution of residues in the second C2-domain completely abolished self-association. When expressed in neurons, syt1 mutants with graded reductions in self-association activity exhibited concomitant reductions in 1) clamping spontaneous release and 2) triggering and synchronizing evoked release. Thus, the juxtamembrane linker of syt1 plays a crucial role in exocytosis by mediating multimerization.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

The diversity of synaptotagmin isoforms

The synaptotagmin family of molecules is known for regulating calcium-dependent membrane fusion events. Mice and humans express 17 synaptotagmin isoforms, where most studies have focused on isoforms 1, 2, and 7, which are involved in synaptic vesicle exocytosis. Recent work has highlighted how brain function relies on additional isoforms, with roles in postsynaptic receptor endocytosis, vesicle trafficking, membrane repair, synaptic plasticity, and protection against neurodegeneration, for example, in addition to the traditional concept of synaptotagmin-mediated neurotransmitter release - in neurons as well as glia, and at different timepoints. In fact, it is not uncommon for the same isoform to feature several splice isoforms, form homo- and heterodimers, and function in different subcellular locations and cell types. This review aims to highlight the diversity of synaptotagmins, offers a concise summary of key findings on all isoforms, and discusses different ways of grouping these.

The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis

MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis.

Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.

A synaptotagmin suppressor screen indicates SNARE binding controls the timing and Ca2+ cooperativity of vesicle fusion

The synaptic vesicle Ca²⁺ sensor Synaptotagmin binds Ca²⁺ through its two C2 domains to trigger membrane interactions. Beyond membrane insertion by the C2 domains, other requirements for Synaptotagmin activity are still being elucidated. To identify key residues within Synaptotagmin required for vesicle cycling, we took advantage of observations that mutations in the C2B domain Ca²⁺-binding pocket dominantly disrupt release from invertebrates to humans. We performed an intragenic screen for suppressors of lethality induced by expression of Synaptotagmin C2B Ca²⁺-binding mutants in Drosophila. This screen uncovered essential residues within Synaptotagmin that suggest a structural basis for several activities required for fusion, including a C2B surface implicated in SNARE complex interaction that is required for rapid synchronization and Ca²⁺ cooperativity of vesicle release. Using electrophysiological, morphological and computational characterization of these mutants, we propose a sequence of molecular interactions mediated by Synaptotagmin that promote Ca²⁺ activation of the synaptic vesicle fusion machinery.

Putting a brake on synaptic vesicle endocytosis

In chemical synapses, action potentials evoke synaptic vesicle fusion with the presynaptic membrane at the active zone to release neurotransmitter. Synaptic vesicle endocytosis (SVE) then follows exocytosis to recapture vesicle proteins and lipid components for recycling and the maintenance of membrane homeostasis. Therefore, SVE plays an essential role during neurotransmission and is one of the most precisely regulated biological processes. Four modes of SVE have been characterized and both positive and negative regulators have been identified. However, our understanding of SVE regulation remains unclear, especially the identity of negative regulators and their mechanisms of action. Here, we review the current knowledge of proteins that function as inhibitors of SVE and their modes of action in different forms of endocytosis. We also propose possible physiological roles of such negative regulation. We believe that a better understanding of SVE regulation, especially the inhibitory mechanisms, will shed light on neurotransmission in health and disease.

The juxtamembrane linker of full-length synaptotagmin 1 controls oligomerization and calcium-dependent membrane binding

Synaptotagmin 1 (Syt1) is the calcium sensor for synchronous neurotransmitter release. The two C2 domains of Syt1, which may mediate fusion by bridging the vesicle and plasma membranes, are connected to the vesicle membrane by a 60-residue linker. Here, we use site-directed spin labeling and a novel total internal reflection fluorescence vesicle binding assay to characterize the juxtamembrane linker and to test the ability of reconstituted full-length Syt1 to interact with opposing membrane surfaces. EPR spectroscopy demonstrates that the majority of the linker interacts with the membrane interface, thereby limiting the extension of the C2A and C2B domains into the cytoplasm. Pulse dipolar EPR spectroscopy provides evidence that purified full-length Syt1 is oligomerized in the membrane, and mutagenesis indicates that a glycine zipper/GXXXG motif within the linker helps mediate oligomerization. The total internal reflection fluorescence-based vesicle binding assay demonstrates that full-length Syt1 that is reconstituted into supported lipid bilayers will capture vesicles containing negatively charged lipid in a Ca(2+)-dependent manner. Moreover, the rate of vesicle capture increases with Syt1 density, and mutations in the GXXXG motif that inhibit oligomerization of Syt1 reduce the rate of vesicle capture. This work demonstrates that modifications within the 60-residue linker modulate both the oligomerization of Syt1 and its ability to interact with opposing bilayers. In addition to controlling its activity, the oligomerization of Syt1 may play a role in organizing proteins within the active zone of membrane fusion.

Developmental expression and subcellular distribution of synaptotagmin 11 in rat hippocampus

Synaptotagmins are required for Ca(2+)-dependent membrane-trafficking in either neuronal synaptic vesicles or cellular membranes. Previous reports suggested that the synaptotagmin 11 (syt11) gene is involved in the development of schizophrenia based on the genomic analysis of patients. Parkin protein binds to the C2 domains of Syt11 which leads to the polyubiquitination of Syt11. However, where and how Syt11 performs its role in the brain is largely unknown. Here, we report that Syt11 is expressed mainly in the brain. In addition, exogenously expressed Syt11 in HEK293 cells can form higher molecular weight complex via its transmembrane domain. Also, Syt11 is targeted to both dendrite and axon compartments. Immunocytochemistry showed that Syt11 is juxtaposed to postsynaptic markers in both excitatory and inhibitory synapses. Both neuroligin 1 and 2, which are postsynaptic cell adhesion molecules and differentially induce excitatory and inhibitory presynapses, respectively, recruit Syt11 in neuron coculture. Immunogold electron microscopy analysis revealed that Syt11 exists mainly in presynaptic neurotransmitter vesicles and plasma membrane, and rarely in postsynaptic sites. These results suggest that Syt11 may contribute to the regulation of neurotransmitter release in the excitatory and inhibitory presynapses, and postsynapse-targeted membrane trafficking in dendrites.

Release mode of large and small dense-core vesicles specified by different synaptotagmin isoforms in PC12 cells

Different synaptotagmin isoforms (syt I, VII, and IX) sort to populations of dense-core vesicles with different sizes. These isoforms differ in their sensitivities to divalent cations and trigger different modes of exocytosis. Exocytosis triggered by these isoforms also differs in its sensitivity to inhibition by another isoform, syt IV.

Many cells release multiple substances in different proportions according to the specific character of a stimulus. PC12 cells, a model neuroendocrine cell line, express multiple isoforms of the exocytotic Ca²⁺ sensor synaptotagmin. We show that these isoforms sort to populations of dense-core vesicles that differ in size. These synaptotagmins differ in their Ca²⁺ sensitivities, their preference for full fusion or kiss-and-run, and their sensitivity to inhibition by synaptotagmin IV. In PC12 cells, vesicles that harbor these different synaptotagmin isoforms can be preferentially triggered to fuse by different forms of stimulation. The mode of fusion is specified by the synaptotagmin isoform activated, and because kiss-and-run exocytosis can filter small molecules through a size-limiting fusion pore, the activation of isoforms that favor kiss-and-run will select smaller molecules over larger molecules packaged in the same vesicle. Thus synaptotagmin isoforms can provide multiple levels of control in the release of different molecules from the same cell.

Palmitoylation-dependent association with CD63 targets the Ca2+ sensor synaptotagmin VII to lysosomes

Posttranslational lipid modifications promote association of Syt VII with the tetraspanin CD63, determining its exit from the Golgi and targeting to lysosomes.

Syt VII is a Ca²⁺ sensor that regulates lysosome exocytosis and plasma membrane repair. Because it lacks motifs that mediate lysosomal targeting, it is unclear how Syt VII traffics to these organelles. In this paper, we show that mutations or inhibitors that abolish palmitoylation disrupt Syt VII targeting to lysosomes, causing its retention in the Golgi complex. In macrophages, Syt VII is translocated simultaneously with the lysosomal tetraspanin CD63 from tubular lysosomes to nascent phagosomes in a Ca²⁺-dependent process that facilitates particle uptake. Mutations in Syt VII palmitoylation sites block trafficking of Syt VII, but not CD63, to lysosomes and phagosomes, whereas tyrosine replacement in the lysosomal targeting motif of CD63 causes both proteins to accumulate on the plasma membrane. Complexes of CD63 and Syt VII are detected only when Syt VII palmitoylation sites are intact. These findings identify palmitoylation-dependent association with the tetraspanin CD63 as the mechanism by which Syt VII is targeted to lysosomes.

Cyclic AMP-mediated endocytosis of intestinal epithelial NHE3 requires binding to synaptotagmin 1

The apical membrane Na(+)-H(+) exchanger (NHE)3 is regulated by cAMP-dependent phosphorylation, which inhibits its activity through membrane endocytosis. The clathrin complex adaptor protein synaptotagmin 1 (Syt 1) appears to be essential to this process, but little is known about its expression in intestinal epithelial cells or interaction with NHE3. The intestinal epithelial expression and apical location of Syt 1 were determined by Syt 1 mRNA profiling and immunolocalization. Tandem mass spectrometry was used for protein identification. Bis(sulfosuccinimidyl) suberate (BS(3)) cross linking suggested that NHE3 and Syt 1 were in a membrane complex following cAMP stimulation of Caco2BBE (Brush Border Expressions) cells. To investigate the regulation of NHE3 appearance in a Syt 1-containing membrane compartment, doxycycline-inducible hemaglutinin (HA)-tagged NHE3 was expressed in Caco2BBE cells. HA-NHE3 correctly targeted to the apical membrane, where, upon cAMP stimulation, it was internalized with a Syt 1-containing compartment. Site-directed mutagenesis of NHE3 showed that serine 605 (S605) was pivotal to NHE3 and Syt 1 association and internalization. Direct Syt 1 interaction with NHE3 was suggested by fluorescence resonance energy transfer (FRET) analysis. The physiological role of S552 was less clear. By FRET, this serine residue appeared to be involved in cAMP-induced Syt 1 binding of NHE3. However, when HA-tagged NHE3 S552A was expressed in Caco2 cells, the mutated construct was not inserted into the apical membrane. We conclude that intestinal epithelial Syt 1 plays an important role in cAMP-stimulated endocytosis of apical NHE3 through cAMP-dependent phosphorylation of S605 that is required for NHE3 and Syt 1 association.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

Synaptotagmin VII modulates the kinetics of dense-core vesicle exocytosis in PC12 cells

In our previous study, we showed that PC12 cell lines stably expressing synaptotagmin (Syt) VII have greater ability to release hormones Ca(2+)-dependently than the original PC12 cells. However, the precise molecular mechanism of the enhancement of hormone secretion by Syt VII has never been elucidated. In this study, we established a PC12 cell line that stably expresses Syt VII-green fluorescent protein (Syt VII-GFP) or its Ca(2+)-binding-site-deficient mutant (D172N/D303N substitutions; Syt VII-DN-GFP), and examined the effect of Syt VII-GFP expression on the kinetics of dense-core vesicle exocytosis by total internal reflection fluorescence (TIRF) microscopy. Both Syt VII-GFP and Syt VII-DN-GFP co-localized well with dense-core vesicle markers, monomeric red fluorescent protein (mRFP)-tagged neuropeptide Y (NPY-mRFP) and cyan fluorescent protein (CFP)-tagged tissue plasminogen activator (tPA-CFP). Expression of Syt VII-GFP enhanced the number of dense-core vesicle exocytotic events, whereas expression of Syt VII-DN-GFP or knockdown of Syt VII-GFP with specific small interfering RNA (siRNA) attenuated the number of exocytotic events. Monitoring individual tPA-CFP release events revealed that "full release" events are increased in Syt VII-GFP-expressing cells, but not in Syt VII-DN-GFP-expressing or Syt VII-silenced cells. Our data indicate that Syt VII modulates the kinetics of Ca(2+)-dependent dense-core vesicle exocytosis in neuroendocrine PC12 cells, possibly by modulating fusion pore opening.

Distinct Rab27A binding affinities of Slp2-a and Slac2-a/melanophilin: Hierarchy of Rab27A effectors

The small GTPase Rab27A has recently been shown to regulate melanosome transport in mammalian skin melanocytes through sequentially interacting with two Rab27A effectors, Slac2-a/melanophilin and Slp2-a. Although Slac2-a and Slp2-a contain a similar N-terminal Rab27A-binding domain (named SHD, Slp homology domain), nothing is known about the functional differences between the Slac2-a SHD and Slp2-a SHD. In this study, the Rab27A-binding affinity of ten putative Rab27A effector proteins has been investigated. It has been found that they could be classified into a low-affinity group (e.g., Slac2-a) and a high-affinity group (e.g., Slp2-a and Slp4-a) based on their Rab27A-binding affinity. Kinetic analysis of the GTP-Rab27A-binding to the SHD of Slp2-a, Slp4-a, and Slac2-a by surface plasmon resonance further indicated that the kinetic parameters of Rab27A for the Slp2-a SHD, Slp4-a SHD, and Slac2-a SHD consisted of a fast association rate constant (3.35 x 10(4), 2.06 x 10(4), and 2.11 x 10(4) M(-1) s(-1), respectively) and a slow dissociation rate constant (4.48 x 10(-4), 3.96 x 10(-4), and 2.37 x 10(-3) s(-1) respectively), and their equilibrium dissociation constants were determined to be 13.4, 19.2, and 112 nM, respectively. Our data suggest that distinct Rab27A binding activities of Slac2-a and Slp2-a ensure the order (or hierarchy) of Rab27A effectors that sequentially function in melanosome transport in melanocytes.

There's more to life than neurotransmission: the regulation of exocytosis by synaptotagmin VII

Among the 16 known vertebrate synaptotagmins, only Syt I, IV and VII are also present in C. elegans and Drosophila, suggesting that these isoforms play especially important roles in vivo. Extensive evidence indicates that Syt I is a synaptic vesicle Ca(2+) sensor essential for rapid neurotransmitter release. It has been suggested that the ubiquitously expressed Syt VII also regulates synaptic vesicle exocytosis, despite its presence in several tissues in addition to the brain. Here, we discuss recent genetic and biochemical evidence that does not support this view. Syt VII null mutants do not have a neurological phenotype, and the protein is found on the membrane of lysosomes and some non-synaptic secretory granules, where it regulates Ca(2+)-triggered exocytosis and plasma membrane repair.

Slp4-a/granuphilin-a interacts with syntaxin-2/3 in a Munc18-2-dependent manner

Slp4-a/granuphilin-a was originally described as a protein specifically associated with insulin-containing granules in pancreatic beta-cells, but it was subsequently found to be present on amylase-containing granules in parotid acinar cells. Although Slp4-a has been suggested to control insulin secretion through interaction with syntaxin-1a and/or Munc18-1, nothing is known about the binding partner(s) of Slp4-a during amylase release from parotid acinar cells, which do not endogenously express either syntaxin-1a or Munc18-1. In this study we systematically investigated the interaction between syntaxin-1-5 and Munc18-1-3 by co-immunoprecipitation assay using COS-7 cells and discovered that Slp4-a interacts with a closed conformation of syntaxin-2/3 in a Munc18-2-dependent manner, whereas Munc18-2 itself hardly interacts with Slp4-a at all. By contrast, Slp4-a was found to strongly interact with Munc18-1 regardless of the presence of syntaxin-2/3, and syntaxin-2/3 co-immunoprecipitated with Slp4-a only in the presence of Munc18-1/2. Deletion analysis showed that the syntaxin-2/3 (or Munc18-1/2)-binding site is a linker domain of Slp4-a (amino acid residues 144-354), a previously uncharacterized region located between the N-terminal Rab27A binding domain and the C2A domain. We also found that the Slp4-a.syntaxin-2 complex is actually present in rat parotid glands and that introduction of the antibody against Slp4-a linker domain into streptolysin O-permeabilized parotid acinar cells severely attenuates isoproterenol-stimulated amylase release, possibly by disrupting the interaction between Slp4-a and syntaxin-2/3 (or Munc18-2). These results suggest that Slp4-a modulates amylase release from parotid acinar cells through interaction with syntaxin-2/3 on the apical plasma membrane.

External Ca2+ dependency of synaptic transmission in drosophila synaptotagmin I mutants

To resolve some of differences in reports on the function of Synaptotagmin I (Syt I), we re-examined synaptic transmission at the neuromuscular junction of Drosophila embryos that have mutations in the Syt I gene (syt I). Two major questions addressed were which Ca2+ binding domain, C2A or C2B, sense Ca2+ and is Syt I a negative regulator of spontaneous vesicle fusion. Synaptic currents were induced by nerve stimulation or by high K+ treatment in external solutions containing various Ca2+ concentrations. In a null allele, syt I(AD4), synchronous synaptic currents were rarely observed but not abolished. The quantal content was about 1/60 of control but increased linearly with [Ca2+](e) with a slope of 0.95 (N) in the double logarithmic plot, in contrast to 3.01 in control. The slope of 1.06 in an allele, syt I(AD1), which lacks the second Ca2+ binding domain, C2B, was not different from in syt I(AD4). In another allele, syt I(AD3), in which one amino acid in C2B is mutated, synchronous synaptic transmission was also impaired and N was 1.54, which is significantly smaller than in control. In high K+ saline, the [Ca2+](e) dependency of vesicle release in syt I(AD4) was lower than in controls, whereas that in syt I(AD3) was even lower than in syt I(AD4), suggesting that syt I(AD3) is inhibiting vesicle fusion. These findings led us to conclude that C2B, not C2A, senses Ca2+, and Syt I is a negative regulator of vesicle fusion.

Dual roles of the C2B domain of synaptotagmin I in synchronizing Ca2+-dependent neurotransmitter release

Although the vesicular protein synaptotagmin I contains two Ca2+-binding domains (C2A and C2B), Ca2+ binding to the C2B domain is more important for triggering synchronous neurotransmitter release. We have used point mutagenesis to determine the functional contributions of the five negatively charged aspartate (Asp) residues that constitute the Ca2+-binding sites in the C2B domain of synaptotagmin I. Transfecting wild-type synaptotagmin I DNA into cultured hippocampal neurons from synaptotagmin I knock-out mice rescued Ca2+-dependent synchronous transmitter release and reduced a slower, asynchronous component of release, indicating that synaptotagmin I suppresses asynchronous release. Mutating either the second or third Asp residues of the C2B domain potently inhibited the ability of synaptotagmin I to rescue synchronous release but did not change its ability to suppress asynchronous release. Synaptotagmin I with mutations in the first or fourth Asp residues of the C2B domain partially rescued synchronous release and partially suppressed asynchronous release, whereas neutralizing the fifth Asp residue had no effect on the ability of synaptotagmin I to rescue transmitter release. Thus, we conclude that the C2B domain of synaptotagmin I regulates neurotransmitter release in at least two ways. Synchronous release absolutely requires binding of Ca2+ to the second and third Asp residues in this domain. For the suppression of asynchronous release, Ca2+ binding to the C2B domain of synaptotagmin I apparently is not necessary because mutation of the second Asp residue inhibits Ca2+ binding, yet still allows this protein to suppress asynchronous release.

Vesicular reuptake inhibition by a synaptotagmin I C2B domain antibody at the squid giant synapse

Synaptotagmin (Syt) I, a ubiquitous synaptic vesicle protein, comprises a transmembrane region and two C2 domains. The C2 domains, which have been shown to be essential for both synaptic vesicle exocytosis and endocytosis, are also seen as the Ca(2+) sensors in synaptic vesicular release. In a previous study, we reported that a polyclonal antibody raised against the squid (Loligo pealei) Syt I C2B domain, while inhibiting vesicular endocytosis, was synaptic release neutral at the squid giant synapse. Recent reports concerning the C2B requirements for synaptic release prompted us to readdress the role of C2B in squid giant synapse function. Presynaptic injection of another anti-Syt I-C2B antibody (using recombinant whole C2B domain expressed in mammalian cell culture as an antigen) into the presynaptic terminal reproduced our previous results, i.e., reduction of vesicular endocytosis without affecting synaptic release. This set of results addresses the issue of the geometrical arrangement of the Ca(2+) sensor, allowing the C2B domain antibody to restrict Ca(2+)-dependent C2B self-oligomerization without modifying the Ca(2+)-dependent release process.

Synaptotagmin VII Is Targeted to Dense-core Vesicles and Regulates Their Ca2+-dependent Exocytosis in PC12 Cells

It has recently been proposed that synaptotagmin (Syt) VII functions as a plasma membrane Ca2+ sensor for dense-core vesicle exocytosis in PC12 cells based on the results of transient overexpression studies using green fluorescent protein (GFP)-tagged Syt VII; however, the precise subcellular localization of Syt VII is still a matter of controversy (plasma membrane versus secretory granules). In this study we established a PC12 cell line "stably expressing" the Syt VII-GFP molecule and demonstrated by immunocytochemical and immunoelectron microscopic analyses that the Syt VII-GFP protein is localized on dense-core vesicles as well as in other intracellular membranous structures, such as the trans-Golgi network and lysosomes. Syt VII-GFP forms a complex with endogenous Syts I and IX, but not with Syt IV, and it colocalize well with Syts I and IX in the cellular processes (where dense-core vesicles are accumulated) in the PC12 cell line. We further demonstrated by an N-terminal antibody-uptake experiment that Syt VII-GFP-containing dense-core vesicles undergo Ca2+ -dependent exocytosis, the same as endogenous Syt IX-containing vesicles. Moreover, silencing of Syt VII-GFP with specific small interfering RNA dramatically reduced high KCl-dependent neuropeptide Y secretion from the stable PC12 cell line (approximately 60% of the control cells), whereas the same small interfering RNA had little effect on neuropeptide Y secretion from the wild-type PC12 cells (approximately 85-90% of the control cells), indicating that the level of endogenous expression of Syt VII molecules must be low. Our results indicate that the targeting of Syt VII-GFP molecules to specific membrane compartment(s) is affected by the transfection method (transient expression versus stable expression) and suggested that Syt VII molecule on dense-core vesicles functions as a vesicular Ca2+ sensor for exocytosis in endocrine cells.

Molecular regulation of membrane resealing in 3T3 fibroblasts

Membrane resealing in mammalian cells after injury depends on Ca(2+)-dependent fusion of intracellular vesicles with the plasma membrane. When cells are wounded twice, the subsequent resealing is generally faster. Physiological and biochemical studies have shown the initiation of two different repair signaling pathways, which are termed facilitated and potentiated responses. The facilitated response is dependent on the generation and recruitment of new vesicles, whereas the potentiated response is not. Here, we report that the two responses can be differentially defined molecularly. Using recombinant fragments of synaptobrevin-2 and synaptotagmin C2 domains we were able to dissociate the molecular requirements of vesicle exocytosis for initial membrane resealing and the facilitated and potentiated responses. The initial resealing response was blocked by fragments of synaptobrevin-2 and the C2B domain of synaptotagmin VII. Both the facilitated and potentiated responses were also blocked by the C2B domain of synaptotagmin VII. Although the initial resealing response was not blocked by the C2AB domain of synaptotagmin I or the C2A domain of synaptotagmin VII, recruitment of new vesicles for the facilitated response was inhibited. We also used Ca2+ binding mutant studies to show that the effects of synaptotagmins on membrane resealing are Ca(2+)-dependent. The pattern of inhibition by synaptotagmin C2 fragments that we observed cannot be used to specify a vesicle compartment, such as lysosomes, in membrane repair.

High metal concentrations are required for self-association of synaptotagmin II

Several members of the synaptotagmin (syt) family of vesicle proteins have been proposed to act as Ca2+ sensors on synaptic vesicles. The mechanism by which calcium activates this class of proteins has been the subject of controversy, yet relatively few detailed biophysical studies have been reported on how isoforms other than syt I respond to divalent metal ions. Here, we report a series of studies on the response of syt II to a wide range of metal ions. Analytical ultracentrifugation studies demonstrate that Ca2+ induces protein dimerization upon exposure to 5 mM Ca2+. Whereas Ba2+, Mg2+, or Sr2+ do not potentiate self-association as strongly as Ca2+, Pb2+ triggers self-association of syt II at concentrations as low as 10 microM. Partial proteolysis studies suggest that the various divalent metals cause different changes in the conformation of the protein. The high calcium concentrations required for self-association of syt II suggest that the oligomerized state of this protein is not a critical intermediate in vesicle fusion; however, low-affinity calcium sites on syt II may play a critical role in buffering calcium at the presynaptic active zone. In addition, the high propensity of lead to oligomerize syt II offers a possible molecular explanation for how lead interferes with calcium-evoked neurotransmitter release.

RNA interference-mediated silencing of synaptotagmin IX, but not synaptotagmin I, inhibits dense-core vesicle exocytosis in PC12 cells

Although PC12 cells express three synaptotagmin isoforms (Syts I, IV and IX), all of which have been proposed to regulate dense-core vesicle exocytosis, it remains unknown which of the Sytisoforms acts as the major Ca2+ sensor for dense-core vesicle exocytosis. In the present study, it has been shown by immunoaffinity purification and immunocytochemistry that Syts I and IX, but not Syt IV, are present on the same secretory vesicles in PC12 cells. Silencing of Syt IX with specific small interfering RNA significantly reduced high KCl-dependent neuropeptide Y secretion from PC12 cells, whereas silencing of Syt I with specific small interfering RNA had no significant effect. The results indicate that Syts I and IX are not functionally equivalent and that Syt IX, and not Syt I, is indispensable for the regulation of Ca2+-dependent dense-core vesicle exocytosis in PC12 cells.

Presynaptic trafficking of synaptotagmin I is regulated by protein palmitoylation

Protein palmitoylation plays a critical role in sorting and targeting of several proteins to pre- and postsynaptic sites. In this study, we have analyzed the role of palmitoylation in trafficking of synaptotagmin I and its modulation by synaptic activity. We found that palmitoylation of N-terminal cysteines contributed to sorting of synaptotagmin I to an intracellular vesicular compartment at the presynaptic terminal. Presynaptic targeting is a unique feature of N-terminal sequences of synaptotagmin I because the palmitoylated N terminus of synaptotagmin VII failed to localize to presynaptic sites. We also found that palmitate was stably associated with both synaptotagmin I and SNAP-25 and that rapid neuronal depolarization did not affect palmitate turnover on these proteins. However, long-term treatment with drugs that either block synaptic activity or disrupt SNARE complex assembly modulated palmitoylation and accumulation of synaptotagmin I at presynaptic sites. We conclude that palmitoylation is involved in trafficking of specific elements involved in transmitter release and that distinct mechanisms regulate addition and removal of palmitate on select neuronal proteins.

Alternative splicing in the first alpha-helical region of the Rab-binding domain of Rim regulates Rab3A binding activity: is Rim a Rab3 effector protein during evolution?

Rim1 and Rim2 were originally described as specific Rab3A-effector proteins involved in the regulation of secretory vesicle exocytosis. The putative Rab3A-binding domain (RBD) of Rim consists of two alpha-helical regions (named RBD1 and RBD2) separated by two zinc finger motifs. Although alternative splicing in the RBD1 of Rim is known to produce long and short forms of RBD (named Rim1 and Rim1Delta56-105, and Rim2(+40A) and Rim2, respectively), with the long form of Rim1 and short form of Rim2 being dominant in mouse brain, the physiological significance of the alternative splicing in RBD1 has never been elucidated. In the present study I discovered that alternative splicing in Rim RBD1 alters Rab3A binding affinity to Rims, and found that insertion of 40 amino acids into the RBD1 of Rim2 (i.e. Rim2(+40A)) dramatically reduced its Rab3A binding activity (more than a 50-fold decrease in affinity). Similarly, Rim1Delta56-105 exhibited higher affinity binding to Rab3A than the long form of Rim1. Expression of the short forms of the Rim RBD in PC12 cells co-localized well with endogenous Rab3A, whereas expression of the long forms of the Rim RBD in PC12 cells resulted in cytoplasimc and nuclear localization. Moreover, I found that Caenorhabditis elegans Rim/UNC-10 (ce-Rim) and Drosophila Rim (dm-Rim) do not interact with ce-Rab3 and dm-Rab3, respectively, indicating that the Rab3-effector function of Rim has not been retained during evolution. Based on these findings, I propose that the Rab3A-effector function of Rim during secretory vesicle exocytosis is limited to the short form of the mammalian Rim RBD alone.

Rabphilin and Noc2 are recruited to dense-core vesicles through specific interaction with Rab27A in PC12 cells

Rabphilin and Noc2 were originally described as Rab3A effector proteins involved in the regulation of secretory vesicle exocytosis, however, recently both proteins have been shown to bind Rab27A in vitro in preference to Rab3A (Fukuda, M. (2003) J. Biol. Chem. 278, 15373-15380), suggesting that Rab3A is not their major ligand in vivo. In the present study we showed by means of deletion and mutation analyses that rabphilin and Noc2 are recruited to dense-core vesicles through specific interaction with Rab27A, not with Rab3A, in PC12 cells. Rab3A binding-defective mutants of rabphilin(E50A) and Noc2(E51A) were still localized in the distal portion of the neurites (where dense-core vesicles had accumulated) in nerve growth factor-differentiated PC12 cells, the same as the wild-type proteins, whereas Rab27A binding-defective mutants of rabphilin(E50A/I54A) and Noc2(E51A/I55A) were present throughout the cytosol. We further showed that expression of the wild-type or the E50A mutant of rabphilin-RBD, but not the E50A/I54A mutant of rabphilin-RBD, significantly inhibited high KCl-dependent neuropeptide Y secretion by PC12 cells. We also found that rabphilin and its binding partner, Rab27 have been highly conserved during evolution (from nematoda to humans) and that Caenorhabditis elegans and Drosophila rabphilin (ce/dm-rabphilin) specifically interact with ce/dm-Rab27, but not with ce/dm-Rab3 or ce/dm-Rab8, suggesting that rabphilin functions as a Rab27 effector across phylogeny. Based on these findings, we propose that the N-terminal Rab binding domain of rabphilin and Noc2 be referred to as "RBD27 (Rab binding domain for Rab27)", the same as the synaptotagmin-like protein homology domain (SHD) of Slac2-a/melanophilin.

Missense mutations in the globular tail of myosin-Va indilutemice partially impair binding of Slac2-a/melanophilin

The well-known coat-color mutant mouse dilute exhibits a defect in melanosome transport, and although various mutations in the myosin-Va gene, which encodes an actin-based motor protein, have been identified in dilute mice, why missense mutations in the globular tail of myosin-Va, a putative cargo-binding site, cause the dilute phenotype (i.e. lighter coat color) has never been elucidated. In this study we discovered that missense mutations (I1510N, M1513K and D1519G) in the globular tail (GT) of myosin-Va partially impair the binding of Slac2-a/melanophilin, a linker protein between myosin-Va and Rab27A on the melanosome. The myosin-Va-GT-binding site in Slac2-a was mapped to the region (amino acids 147-240) adjacent to the N-terminal Rab27A-binding site, but it is distinct from the myosin-Va-exon-F-binding site (amino acids 320-406). The myosin-Va-GT·Slac2-a interaction was much weaker than the myosin-Va-exon-F·Slac2-a interaction. The missense mutations in the GT found in dilute mice abrogated only the myosin-Va-GT·Slac2-a interaction and had no effect on the myosin-Va-exon-F·Slac2-a interaction. We further showed that expression of green fluorescence protein-tagged Slac2-a lacking the myosin-Va-GT-binding site (ΔGT), but not the wild-type Slac2-a, severely inhibits melanosome transport in melan-a cells, especially at the melanosome transfer step from microtubles to actin filaments (i.e. perinuclear aggregation of melanosomes). On the basis of our findings, we propose that myosin-Va interacts with Slac2-a·Rab27A complex on the melanosome via two distinct domains, both of which are essential for melanosome transport in melanocytes.

Synaptotagmin IX, a possible linker between the perinuclear endocytic recycling compartment and the microtubules

The pericentriolar endocytic recycling compartment (ERC) is involved in receptor and lipid recycling as well as in the delivery of internalized cargo from early endosomes to the trans Golgi network (TGN). We show that synaptotagmin (Syt) IX, a member of the Syt family of proteins, localizes to the ERC and is required for export from the ERC to the cell surface. We demonstrate that rat basophilic leukemia (RBL-2H3) mast cells endogenously express Syt IX mRNA and protein. Localization studies employing fractionation on linear sucrose gradients combined with confocal microscopy by indirect immunofluorescence or stable expression of a Syt IX-green fluorescent fusion protein demonstrate that Syt IX colocalizes with internalized transferrin (Tfn) and with Rab 11 at the perinuclear ERC. Syt IX also colocalizes with tubulin at the microtubules organizing center (MTOC) and remains associated with tubulin clusters formed in taxol-treated cells. Moreover, Syt IX coimmunoprecipitates with tubulin from intact RBL cells, and chimeric fusion proteins comprising either the C2A or the C2B domain of Syt IX are able to pull down tubulin from RBL cell lysates. To study the functional role of Syt IX, we have stably transfected RBL cells with Syt IX sense or antisense cDNA and monitored the routes of Tfn internalization and recycling in cells that overexpress (RBL-Syt IX+) or display substantially reduced (<90%) levels of Syt IX (RBL-Syt IX-). In these cells, Tfn binding and internalization into early endosomes and the ERC are unaltered. However, recycling from the ERC to the cell surface is significantly slowed down in the RBL-Syt IX- cells. These results therefore indicate that Syt IX is involved in regulating transport from the ERC to the cell surface, and suggest that it may play a role in linking vesicles that exit the ERC with the microtubules network.

Synaptotagmins I and II mediate entry of botulinum neurotoxin B into cells

Botulinum neurotoxins (BoNTs) cause botulism by entering neurons and cleaving proteins that mediate neurotransmitter release; disruption of exocytosis results in paralysis and death. The receptors for BoNTs are thought to be composed of both proteins and gangliosides; however, protein components that mediate toxin entry have not been identified. Using gain-of-function and loss-of-function approaches, we report here that the secretory vesicle proteins, synaptotagmins (syts) I and II, mediate the entry of BoNT/B (but not BoNT/A or E) into PC12 cells. Further, we demonstrate that BoNT/B entry into PC12 cells and rat diaphragm motor nerve terminals was activity dependent and can be blocked using fragments of syt II that contain the BoNT/B-binding domain. Finally, we show that syt II fragments, in conjunction with gangliosides, neutralized BoNT/B in intact mice. These findings establish that syts I and II can function as protein receptors for BoNT/B.

The actin-binding domain of Slac2-a/melanophilin is required for melanosome distribution in melanocytes

Melanosomes containing melanin pigments are transported from the cell body of melanocytes to the tips of their dendrites by a combination of microtubule- and actin-dependent machinery. Three proteins, Rab27A, myosin Va, and Slac2-a/melanophilin (a linker protein between Rab27A and myosin Va), are known to be essential for proper actin-based melanosome transport in melanocytes. Although Slac2-a directly interacts with Rab27A and myosin Va via its N-terminal region (amino acids 1 to 146) and the middle region (amino acids 241 to 405), respectively, the functional importance of the putative actin-binding domain of the Slac2-a C terminus (amino acids 401 to 590) in melanosome transport has never been elucidated. In this study we showed that formation of a tripartite protein complex between Rab27A, Slac2-a, and myosin Va alone is insufficient for peripheral distribution of melanosomes in melanocytes and that the C-terminal actin-binding domain of Slac2-a is also required for proper melanosome transport. When a Slac2-a deletion mutant (DeltaABD) or point mutant (KA) that lacks actin-binding ability was expressed in melanocytes, the Slac2-a mutants induced melanosome accumulation in the perinuclear region, possibly by a dominant negative effect, the same as the Rab27A-binding-defective mutant of Slac2-a or the myosin Va-binding-defective mutant. Our findings indicate that Slac2-a organizes actin-based melanosome transport in cooperation with Rab27A, myosin Va, and actin.

Identification of synaptotagmin effectors via acute inhibition of secretion from cracked PC12 cells

The synaptotagmins (syts) are a family of membrane proteins proposed to regulate membrane traffic in neuronal and nonneuronal cells. In neurons, the Ca2+-sensing ability of syt I is critical for fusion of docked synaptic vesicles with the plasma membrane in response to stimulation. Several putative Ca2+-syt effectors have been identified, but in most cases the functional significance of these interactions remains unknown. Here, we have used recombinant C2 domains derived from the cytoplasmic domains of syts I-XI to interfere with endogenous syt-effector interactions during Ca2+-triggered exocytosis from cracked PC12 cells. Inhibition was closely correlated with syntaxin-SNAP-25 and phosphatidylinositol 4,5-bisphosphate (PIP2)-binding activity. Moreover, we measured the expression levels of endogenous syts in PC12 cells; the major isoforms are I and IX, with trace levels of VII. As expected, if syts I and IX function as Ca2+ sensors, fragments from these isoforms blocked secretion. These data suggest that syts trigger fusion via their Ca2+-regulated interactions with t-SNAREs and PIP2, target molecules known to play critical roles in exocytosis.

Molecular cloning and characterization of human, rat, and mouse synaptotagmin XV

Synaptotagmin (Syt) constitutes a large family of putative membrane trafficking proteins that share a short extracellular domain, a single N-terminal transmembrane domain, and C-terminal tandem C2 domains. In this study, I identified and characterized a novel member of the Syt family (named Syt XV-a) in the mouse, the rat, and humans. Although Syt XV-a protein has a short hydrophobic region at the very end of the N terminus (i.e., lacks a putative extracellular domain), biochemical and cellular analyses have indicated that the short hydrophobic region (amino acids 5-22) is sufficient for producing type I membrane topology in cultured cells, the same as in other Syt family proteins. Unlike other Syt isoforms, however, the mouse and human Syt XV have an alternative splicing isoform that lacks the C-terminal portion of the C2B domain (named Syt XV-b). Since the expression of Syt XV-a/b mRNA was mainly found in non-neuronal tissues (e.g., lung and testis) and Syt XV-a C2 domains lack Ca(2+)-dependent phospholipid binding activity, Syt XV-a is classified as a non-neuronal, Ca(2+)-independent Syt.

Slp4-a/granuphilin-a inhibits dense-core vesicle exocytosis through interaction with the GDP-bound form of Rab27A in PC12 cells

Slp4-a (synaptotagmin-like protein 4-a)/granuphilin-a is specifically localized on dense-core vesicles in PC12 cells and negatively controls dense-core vesicle exocytosis through specific interaction with Rab27A via the N-terminal Slp homology domain (SHD) (Fukuda, M., Kanno, E., Saegusa, C., Ogata, Y., and Kuroda, T. S. (2002) J. Biol. Chem. 277, 39673-39678). However, the mechanism of the inhibition by Slp4-a has never been elucidated at the molecular level and is still a matter of controversy. In this study, I discovered an unexpected biochemical property of Slp4-a, that Slp4-a, but not other Rab27 effectors reported thus far, is capable of interacting with both Rab27A(T23N), a dominant negative form that mimics the GDP-bound form, and Rab27A(Q78L), a dominant active form that mimics the GTP-bound form, whereas Slp4-a specifically recognizes the GTP-bound form of Rab3A and Rab8A and does not recognize their GDP-bound form. I show by deletion and mutation analyses that the TGDWFY sequence in SHD2 is essential for Rab27A(T23N) binding, whereas SHD1 is involved in Rab27A(Q78L) binding. I further show by immunoprecipitation and cotransfection assays that Munc18-1, but not syntaxin IA, directly interacts with the C-terminal domain of Slp4-a in a Rab27A-independent manner. Expression of Slp4-a mutants that lack Rab27A(T23N) binding activity (i.e. specific binding to Rab27A(Q78L)) completely reverses the inhibitory effect of the wild-type Slp4-a on high KCl-dependent neuropeptide Y secretion in PC12 cells. The results strongly indicate that interaction of Slp4-a with the GDP-bound form of Rab27A, not with syntaxin IA or Munc18-1, is the primary reason that Slp4-a expression inhibits dense core vesicle exocytosis in PC12 cells.

Distinct Rab binding specificity of Rim1, Rim2, rabphilin, and Noc2. Identification of a critical determinant of Rab3A/Rab27A recognition by Rim2

Rabphilin, Rim, and Noc2 have generally been believed to be the Rab3 isoform (Rab3A/B/C/D)-specific effectors that regulate secretory vesicle exocytosis in neurons and in some endocrine cells. The results of recent genetic analysis of rabphilin knock-out animals, however, strongly refute this notion, because there are no obvious genetic interactions between Rab3 and rabphilin in nematoda (Staunton, J., Ganetzky, B., and Nonet, M. L. (2001) J. Neurosci. 21, 9255-9264), suggesting that Rab3 is not a major ligand of rabphilin in vivo. In this study, I tested the interaction of rabphilin, Rim1, Rim2, and Noc2 with 42 different Rab proteins by cotransfection assay and found differences in rabphilin, Rim1, Rim2, and Noc2 binding to several Rab proteins that belong to the Rab functional group III (Rab3A/B/C/D, Rab26, Rab27A/B, and Rab37) and/or VIII (Rab8A and Rab10). Rim1 interacts with Rab3A/B/C/D, Rab10, Rab26, and Rab37; Rim2 interacts with Rab3A/B/C/D and Rab8A; and rabphilin and Noc2 interact with Rab3A/B/C/D, Rab8A, and Rab27A/B. By contrast, the synaptotagmin-like protein homology domain of Slp homologue lacking C2 domains-a (Slac2-a)/melanophilin specifically recognizes Rab27A/B but not other Rabs. I also found that alternative splicing events in the first alpha-helical region (alpha(1)) of the Rab binding domain of Rim1 alter the Rab binding specificity of Rim1. Site-directed mutagenesis and chimeric analyses of Rim2 and Slac2-a indicate that the acidic cluster (Glu-50, Glu-51, and Glu-52) in the alpha(1) region of the Rab binding domain of Rim2, which is not conserved in the synaptotagmin-like pro tein homology domain of Slac2-a, is a critical determinant of Rab3A recognition. Based on these results, I propose that Rim, rabphilin, and Noc2 function differently in concert with functional group III and/or VIII Rab proteins, including Rab3 isoforms.

Visualization of synaptotagmin I oligomers assembled onto lipid monolayers

Neuronal exocytosis is mediated by Ca(2+)-triggered rearrangements between proteins and lipids that result in the opening and dilation of fusion pores. Synaptotagmin I (syt I) is a Ca(2+)-sensing protein proposed to regulate fusion pore dynamics via Ca(2+)-promoted binding of its cytoplasmic domain (C2A-C2B) to effector molecules, including anionic phospholipids and other copies of syt. Functional studies indicate that Ca(2+)-triggered oligomerization of syt is a critical step in excitation-secretion coupling; however, this activity has recently been called into question. Here, we show that Ca(2+) does not drive the oligomerization of C2A-C2B in solution. However, analysis of Ca(2+).C2A-C2B bound to lipid monolayers, using electron microscopy, revealed the formation of ring-like heptameric oligomers that are approximately 11 nm long and approximately 11 nm in diameter. In some cases, C2A-C2B also assembled into long filaments. Oligomerization, but not membrane binding, was disrupted by neutralization of two lysine residues (K326,327) within the C2B domain of syt. These data indicate that Ca(2+) first drives C2A-C2B.membrane interactions, resulting in conformational changes that trigger a subsequent C2B-mediated oligomerization step. Ca(2+)-mediated rearrangements between syt subunits may regulate the opening or dilation kinetics of fusion pores or may play a role in endocytosis after fusion.

Biochemical and functional characterization of the Ebola virus VP24 protein: implications for a role in virus assembly and budding

The VP24 protein of Ebola virus is believed to be a secondary matrix protein and minor component of virions. In contrast, the VP40 protein of Ebola virus is the primary matrix protein and the most abundant virion component. The structure and function of VP40 have been well characterized; however, virtually nothing is known regarding the structure and function of VP24. Wild-type and mutant forms of VP24 were expressed in mammalian cells to gain a better understanding of the biochemical and functional nature of this viral protein. Results from these experiments demonstrated that (i) VP24 localizes to the plasma membrane and perinuclear region in both transfected and Ebola virus-infected cells, (ii) VP24 associates strongly with lipid membranes, (iii) VP24 does not contain N-linked sugars when expressed alone in mammalian cells, (iv) VP24 can oligomerize when expressed alone in mammalian cells, (v) progressive deletions at the N terminus of VP24 resulted in a decrease in oligomer formation and a concomitant increase in the formation of high-molecular-weight aggregates, and (vi) VP24 was present in trypsin-resistant virus like particles released into the media covering VP24-transfected cells. These data indicate that VP24 possesses structural features commonly associated with viral matrix proteins and that VP24 may have a role in virus assembly and budding.

Nerve growth factor-dependent sorting of synaptotagmin IV protein to mature dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells

Synaptotagmin IV (Syt IV) is a fourth member of the Syt family and has been shown to regulate some forms of memory and learning by analysis of Syt IV null mutant mice (Ferguson, G. D., Anagnostaras, S. G., Silva, A. J., and Herschman, H. R. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 5598-5603). However, the involvement of Syt IV protein in vesicular trafficking and even its localization in secretory vesicles are still matters of controversy. Here we present several lines of evidence showing that the Syt IV protein in PC12 cells is normally localized in the Golgi or immature vesicles at the cell periphery and is sorted to fusion-competent mature dense-core vesicles in response to short nerve growth factor (NGF) stimulation. (i) In undifferentiated PC12 cells, Syt IV protein is mainly localized in the Golgi and small amounts are also present at the cell periphery, but according to the results of an immunocytochemical analysis, they do not colocalize with conventional secretory vesicle markers (Syt I, Syt IX, Rab3A, Rab27A, vesicle-associated membrane protein 2, and synaptophysin) at all. By contrast, limited colocalization of Syt IV protein with dense-core vesicle markers is found in the distal parts of the neurites of NGF-differentiated PC12 cells. (ii) Immunoelectron microscopy with highly specific anti-Syt IV antibody revealed that the Syt IV protein in undifferentiated PC12 cells is mainly present on the Golgi membranes and immature secretory vesicles, whereas after NGF stimulation Syt IV protein is also present on the mature dense-core vesicles. (iii) An N-terminal antibody-uptake experiment indicated that Syt IV-containing vesicles in the neurites of NGF-differentiated PC12 cells undergo Ca(2+)-dependent exocytosis, whereas no uptake of the anti-Syt IV-N antibody was observed in undifferentiated PC12 cells. Our results suggest that Syt IV is a stimulus (e.g. NGF)-dependent regulator for exocytosis of dense-core vesicles.

Synaptotagmin-like protein (Slp) homology domain 1 of Slac2-a/melanophilin is a critical determinant of GTP-dependent specific binding to Rab27A

The N-terminal synaptotagmin-like protein (Slp) homology domain (SHD) of the Slp and Slac2 families has recently been identified as a specific Rab27A-binding domain (Kuroda, T. S., Fukuda, M., Ariga, H., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 9212-9218; Fukuda, M., Kuroda, T. S., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 12432-12436). The SHD consists of two conserved alpha-helical regions (SHD1 and SHD2) that are often separated by two zinc finger motifs. However, the structural basis of Rab27A recognition by the SHD (i.e. involvement of each region (SHD1, zinc finger motifs, and SHD2) in Rab27A recognition and critical residue(s) for Rab27A/SHD interaction) had never been elucidated. In this study, systematic deletion analysis and Ala-based site-directed mutagenesis showed that SHD1 of Slac2-a/melanophilin alone is both necessary and sufficient for high affinity specific recognition of the GTP-bound form of Rab27A. By contrast, the zinc finger motifs and SHD2 are not an autonomous Rab27A-binding site and seem to be important for stabilization of the structure of the SHD or higher affinity Rab27A binding. In addition, chimeric analysis of Rab3A and Rab27A showed that the specific sequence of the switch II region of Rab27 isoforms (especially Leu-84, Phe-88, and Asp-91 of Rab27A), which is not conserved in the Rab3 or Rab8 isoforms, is essential for recognition by the Slac2-a SHD. Based on these findings, I propose that SHD1 of the Slp and Slac2 families be referred to as RBD27 (Rab-binding domain specific for Rab27 isoforms).

Vesicle-associated membrane protein-2/synaptobrevin binding to synaptotagmin I promotes O-glycosylation of synaptotagmin I

Synaptotagmin I (Syt I), an evolutionarily conserved integral membrane protein of synaptic vesicles, is now known to regulate Ca2+-dependent neurotransmitter release. Syt I protein should undergo several post-translational modifications before maturation and subsequent functioning on synaptic vesicles (e.g. N-glycosylation and fatty acylation in vertebrate Syt I), because the apparent molecular weight of Syt I on synaptic vesicles (mature form, 65,000) was much higher than the calculated molecular weight (47,400) predicted from the cDNA sequences both in vertebrates and invertebrates. Common post-translational modification(s) of Syt I conserved across phylogeny, however, have never been elucidated. In the present study, I discovered that dithreonine residues (Thr-15 and Thr-16) at the intravesicular domain of mouse Syt I are post-translationally modified by a complex form of O-linked sugar (i.e. the addition of sialic acids) in PC12 cells and that the O-glycosylation of Syt I in COS-7 cells depends on the coexpression of vesicle-associated membrane protein-2 (VAMP-2)/synaptobrevin. I also showed that a transmembrane domain of Syt I directly interacts with isolated VAMP-2, but not VAMP-2, in the heterotrimeric SNARE (SNAP receptor) complex (vesicle SNARE, VAMP-2, and two target SNAREs, syntaxin IA and SNAP-25). Since di-Thr or di-Ser residues are often found at the intravesicular domain of invertebrate Syt I, and VAMP-dependent O-glycosylation was also observed in squid Syt expressed in COS-7 cells, I propose that VAMP-dependent O-glycosylation of Syt I is a common modification during evolution and may have important role(s) in synaptic vesicle trafficking.

Role of synaptotagmin in Ca2+-triggered exocytosis

The Ca(2+)-binding synaptic-vesicle protein synaptotagmin I has attracted considerable interest as a potential Ca(2+) sensor that regulates exocytosis from neurons and neuroendocrine cells. Recent studies have shed new light on the structure, biochemical/biophysical properties and function of synaptotagmin, and the emerging view is that it plays an important role in both exocytosis and endocytosis. At least a dozen additional isoforms exist, some of which are expressed outside of the nervous system, suggesting that synaptotagmins might regulate membrane traffic in a variety of cell types. Here we provide an overview of the members of this gene family, with particular emphasis on the question of whether and how synaptotagmin I functions during the final stages of membrane fusion: does it regulate the Ca(2+)-triggered opening and dilation of fusion pores?

The calcium-binding loops of the tandem C2 domains of synaptotagmin VII cooperatively mediate calcium-dependent oligomerization

Synaptotagmin VII (Syt VII), a proposed regulator for Ca2+-dependent exocytosis, showed a robust Ca2+-dependent oligomerization property via its two C2 domains (Fukuda, M., and Mikoshiba, K. (2001) J. Biol. Chem. 276, 27670-27676), but little is known about its structure or the critical residues directly involved in the oligomerization interface. In this study, site-directed mutagenesis and chimeric analysis between Syt I and Syt VII showed that three Asp residues in Ca2+-binding loop 1 or 3 (Asp-172, Asp-303, and Asp-357) are crucial to robust Ca(2+)-dependent oligomerization. Unlike Syt I, however, the polybasic sequence in the beta4 strands of the C2 structures (so-called "C2 effector domain") is not involved in the Ca2+-dependent oligomerization of Syt VII. The results also showed that the Ca2+-binding loops of the two C2 domains cooperatively mediate Syt VII oligomerization (i.e. the presence of redundant Ca2+-binding site(s)) as well as the importance of Ca2+-dependent oligomerization of Syt VII in Ca2+-regulated secretion. Expression of wild-type tandem C2 domains of Syt VII in PC12 cells inhibited Ca2+-dependent neuropeptide Y release, whereas mutant fragments lacking Ca2+-dependent oligomerization activity had no effect. Finally, rotary-shadowing electron microscopy showed that the Ca2+-dependent oligomer of Syt VII is "a large linear structure," not an irregular aggregate. By contrast, in the absence of Ca2+ Syt VII molecules were observed to form a globular structure. Based on these results, we suggest that the linear Ca2+-dependent oligomer may be aligned at the fusion site between vesicles and plasma membrane and modulate Ca2+-regulated exocytosis by opening or dilating fusion pores.

Synaptotagmin V is targeted to dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells

Synaptotagmins (Syts) III, V, VI, and X are classified as a subclass of Syt, based on their sequence similarities and biochemical properties (Ibata, K., Fukuda, M., and Mikoshiba, K. (1998) J. Biol. Chem. 273, 12267-12273; Fukuda, M., Kanno, E., and Mikoshiba, K. (1999) J. Biol. Chem. 274, 31421-31427). Although they have been suggested to be involved in vesicular trafficking, as in the role of the Syt I isoform in synaptic vesicle exocytosis, their exact functions remain to be clarified, and even their precise subcellular localization is still a matter of controversy. In this study, we established rat pheochromocytoma (PC12) cell lines that stably express Syts III-, V-, VI-, and X-GFP (green fluorescence protein) fusion proteins, respectively, to determine their precise subcellular localizations. Surprisingly, Syts III-, V-, VI-, and X-GFP proteins were found to be targeted to specific organelles: Syt III-GFP to near the plasma membrane, Syt V-GFP to dense-core vesicles, Syt VI-GFP to endoplasmic reticulum-like structures, and Syt X-GFP to vesicles (other than dense-core vesicles) present in cytoplasm. We showed that Syt V-containing vesicles at the neurites of PC12 cells were processed to exocytosis in a Ca2+-dependent manner. Immunohistochemical analysis further showed that endogenous Syt V was also localized on dense-core vesicles in the mouse brain and specifically expressed in glucagon-positive alpha-cells in mouse pancreatic islets, but not in beta- or delta-cells. Based on these results, we propose that Syt V is a dense-core vesicle-specific Syt isoform that controls a specific type of Ca2+-regulated secretion.

Alternative splicing isoforms of synaptotagmin VII in the mouse, rat and human

Synaptotagmin VII (Syt VII) has been proposed to regulate several different types of Ca2+-dependent exocytosis, but its subcellular localization (lysosome or plasma membrane) and the number of alternative splicing isoforms of Syt VII (single or multiple forms) are matters of controversy. In the present study, we show by reverse transcriptase-PCR analysis that mouse Syt VII has one major isoform (Syt VIIalpha), the original Syt VII, and two minor isoforms (Syt VIIbeta and Syt VIIgamma), which contain unique insertions (of 44 and 116 amino acids respectively) in the spacer domain between the transmembrane and C2 domains of Syt VIIalpha. Similar results were obtained with respect to rat and human Syt VII mRNA expression. An antibody against the N-terminal domain of mouse Syt VII [anti-(Syt VII-N)], which specifically recognized recombinant Syt VII but not other Syt isoforms expressed in COS-7 cells, recognized two major, closely co-migrating bands (p58 and p60) and minor bands of approx. 65 kDa in mouse brain. Immunoaffinity purification of proteins that bind the anti-(Syt VII-N) antibody, and peptide sequence analysis revealed that: (i) the major p58 and p60 bands are identified as adenylate cyclase-associated protein 2; (ii) actin-binding protein is localized at the plasma membrane; and (iii) Syt VIIalpha (65 kDa) is the major Syt VII isoform, but with a much lower expression level than previously thought. It was also shown that FLAG-Syt VII-green-fluorescence-protein fusion protein stably expressed in PC12 cells is localized in the perinuclear region (co-localization with TGN38 protein, even after brefeldin A treatment) and in the tips of neurites (co-localization with Syt I), and not in the plasma membrane.

Synaptotagmin-like protein 5: a novel Rab27A effector with C-terminal tandem C2 domains

Synaptotagmin-like proteins 1-4 (Slp1-4) are new members of the carboxyl-terminal-type (C-type) tandem C2 proteins and are classified as a subfamily distinct from the synaptotagmin and the Doc2 families, because the Slp family contains a unique homology domain at the amino terminus, referred to as the Slp homology domain (SHD). We previously showed that the SHD functions as a binding site for Rab27A, which is associated with human hemophagocytic syndrome (Griscelli syndrome) [J. Biol. Chem. 277 (2002) 9212; J. Biol. Chem. 277 (2002) 12432]. In the present study, we identified a novel member of the Slp family, Slp5. The same as other Slp family members, the SHD of Slp5 preferentially interacted with the GTP-bound form of Rab27A and marginally with Rab3A and Rab6A, both in vitro and in intact cells, but not with other Rabs tested (Rab1, Rab2, Rab4A, Rab5A, Rab7, Rab8, Rab9, Rab10, Rab11A, Rab17, Rab18, Rab20, Rab22, Rab23, Rab25, Rab28, and Rab37). However, unlike other members of the Slp family, expression of Slp5 mRNA was highly restricted to human placenta and liver. Expression of Slp5 protein and in vivo association of Slp5 with Rab27A in the mouse liver were further confirmed by immunoprecipitation. The results suggest that Slp5 might be involved in Rab27A-dependent membrane trafficking in specific tissues.

Slac2-a/melanophilin, the missing link between Rab27 and myosin Va: implications of a tripartite protein complex for melanosome transport

Myosin Va is a member of the unconventional class V myosin family, and a mutation in the myosin Va gene causes pigment granule transport defects in human Griscelli syndrome and dilute mice. How myosin Va recognizes its cargo (i.e. melanosomes), however, has remained undetermined over the past decade. In this study, we discovered Slac2-a/melanophilin to be the "missing link" between myosin Va and GTP-Rab27A present in the melanosome. Deletion analysis and site-directed mutagenesis showed that the N-terminal Slp (synaptotagmin-like protein) homology domain of Slac2-a specifically binds Rab27A/B isoforms and that the C-terminal half directly binds the globular tail of myosin Va. The tripartite protein complex (Rab27A.Slac2-a.myosin Va) in melanoma cells was further confirmed by immunoprecipitation. The discovery that myosin Va indirectly recognizes its cargo through Slac2-a, a novel Rab27A/B effector, should shed light on molecular recognition of its specific cargo by class V myosin.

The Slp homology domain of synaptotagmin-like proteins 1-4 and Slac2 functions as a novel Rab27A binding domain

rab27A, which encodes a small GTP-binding protein, was recently identified as a gene in which mutations caused human hemophagocytic syndrome (Griscelli syndrome) and ashen mice, which exhibit defects in melanosome transport as well as in regulated granule exocytosis in cytotoxic T lymphocytes. However, little is known about the molecular mechanism of Rab27A-dependent membrane trafficking or the specific effector molecules of Rab27A. In this study, we discovered that the Slp (synaptotagmin-like protein) homology domain (SHD) of Slp1--3 and Slac2-a/b specifically and directly binds the GTP-bound form of Rab27A both in vitro and in intact cells but not of the other Rabs tested (Rab1, Rab2, Rab3A, Rab4, Rab5A, Rab6A, Rab7, Rab8, Rab9, Rab10, Rab11A, Rab17, Rab18, Rab20, Rab22, Rab23, Rab25, Rab28, and Rab37). Immunocytochemical analysis revealed that Slp2 (or Slp1) colocalized with Rab27A in the melanosomes of melanoma cells. Slp2 and Rab27A were distributed to the periphery of the cells (especially at the dendritic tips) in the wild-type melanoma cells, whereas they accumulated in the perinuclear region in the melanosome transport-defective cells (S91/Cloudman). These results strongly indicated that the SHD of Slp1--3 and Slac2 functions as an in vivo Rab27A binding domain.

Formation of crystalloid endoplasmic reticulum induced by expression of synaptotagmin lacking the conserved WHXL motif in the C terminus. Structural importance of the WHXL motif in the C2B domain

Synaptotagmin (Syt) is a family of type I membrane proteins that consists of a single transmembrane domain, a spacer domain, two Ca(2+)-binding C2 domains, and a short C terminus. We recently showed that deletion of the short C terminus (17 amino acids) of Syt IV prevented the Golgi localization of Syt IV proteins in PC12 cells and induced granular structures of various sizes in the cell body by an unknown mechanism (Fukuda, M., Ibata, K., and Mikoshiba, K. (2001) J. Neurochem. 77, 730-740). In this study we showed by electron microscopy that these structures are crystalloid endoplasmic reticulum (ER), analyzed the mechanism of its induction, and demonstrated that: (a) mutation or deletion of the evolutionarily conserved WHXL motif in the C terminus of the synaptotagmin family (Syt DeltaC) destabilizes the C2B domain structure (i.e. causes misfolding of the protein), probably by disrupting the formation of stable anti-parallel beta-sheets between the beta-1 and beta-8 strands of the C2B domain; (b) the resulting malfolded proteins accumulate in the ER rather than being transported to other membrane structures (e.g. the Golgi apparatus), with the malfolded proteins also inducing the expression of BiP (immunoglobulin binding protein), one of the ER stress proteins; and (c) the ERs in which the Syt DeltaC proteins have accumulated associate with each other as a result of oligomerization capacity of the synaptotagmin family, because the Syt IDeltaC mutant, which lacks oligomerization activity, cannot induce crystalloid ER. Our findings indicate that the conserved WHXL motif is important not only for protein interaction site but for proper folding of the C2B domain.