Solution structures of the Ca2+-free and Ca2+-bound C2A domain of synaptotagmin I: does Ca2+ induce a conformational change?

Dynamic binding mode of a Synaptotagmin-1-SNARE complex in solution

Rapid neurotransmitter release depends on the Ca²⁺-sensor Synaptotagmin-1 and the SNARE complex formed by synaptobrevin, syntaxin-1 and SNAP-25. How Synaptotagmin-1 triggers release remains unclear, in part because elucidating high-resolution structures of Synaptotagmin-1-SNARE complexes has been challenging. An NMR approach based on lanthanide-induced pseudocontact shifts now reveals a dynamic binding mode where basic residues in the concave side of the Synaptotagmin-1 C₂B domain β-sandwich interact with a polyacidic region of the SNARE complex formed by syntaxin-1 and SNAP-25. The physiological relevance of this dynamic structural model is supported by mutations in basic residues of Synaptotagmin-1 that markedly impair SNARE-complex binding in vitro and Synaptotagmin-1 function in neurons. Mutations with milder effects on binding have correspondingly milder effects on Synaptotagmin-1 function. Our results support a model whereby their dynamic interaction facilitates cooperation between synaptotagmin-1 and the SNAREs in inducing membrane fusion.

The molecular machinery of neurotransmitter release (Nobel lecture)

The most important property of synaptic transmission is its speed, which is crucial for the overall workings of the brain. In his Nobel Lecture, T. C. Südhof explains how the synaptic vesicle and the plasma membrane undergo rapid fusion during neurotransmitter release and how this process is spatially organized, such that opening of Ca(2+) -channels allows rapid translation of the entering Ca(2+) signal into a fusion event.

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

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.

Cellular and molecular interactions of phosphoinositides and peripheral proteins

Anionic lipids act as signals for the recruitment of proteins containing cationic clusters to biological membranes. A family of anionic lipids known as the phosphoinositides (PIPs) are low in abundance, yet play a critical role in recruitment of peripheral proteins to the membrane interface. PIPs are mono-, bis-, or trisphosphorylated derivatives of phosphatidylinositol (PI) yielding seven species with different structure and anionic charge. The differential spatial distribution and temporal appearance of PIPs is key to their role in communicating information to target proteins. Selective recognition of PIPs came into play with the discovery that the substrate of protein kinase C termed pleckstrin possessed the first PIP binding region termed the pleckstrin homology (PH) domain. Since the discovery of the PH domain, more than ten PIP binding domains have been identified including PH, ENTH, FYVE, PX, and C2 domains. Representative examples of each of these domains have been thoroughly characterized to understand how they coordinate PIP headgroups in membranes, translocate to specific membrane docking sites in the cell, and function to regulate the activity of their full-length proteins. In addition, a number of novel mechanisms of PIP-mediated membrane association have emerged, such as coincidence detection-specificity for two distinct lipid headgroups. Other PIP-binding domains may also harbor selectivity for a membrane physical property such as charge or membrane curvature. This review summarizes the current understanding of the cellular distribution of PIPs and their molecular interaction with peripheral proteins.

Calcium regulates molecular interactions of otoferlin with soluble NSF attachment protein receptor (SNARE) proteins required for hair cell exocytosis

Mutations in otoferlin, a C2 domain-containing ferlin family protein, cause non-syndromic hearing loss in humans (DFNB9 deafness). Furthermore, transmitter secretion of cochlear inner hair cells is compromised in mice lacking otoferlin. In the present study, we show that the C2F domain of otoferlin directly binds calcium (KD = 267 μM) with diminished binding in a pachanga (D1767G) C2F mouse mutation. Calcium was found to differentially regulate binding of otoferlin C2 domains to target SNARE (t-SNARE) proteins and phospholipids. C2D-F domains interact with the syntaxin-1 t-SNARE motif with maximum binding within the range of 20-50 μM Ca(2+). At 20 μM Ca(2+), the dissociation rate was substantially lower, indicating increased binding (KD = ∼10(-9)) compared with 0 μM Ca(2+) (KD = ∼10(-8)), suggesting a calcium-mediated stabilization of the C2 domain·t-SNARE complex. C2A and C2B interactions with t-SNAREs were insensitive to calcium. The C2F domain directly binds the t-SNARE SNAP-25 maximally at 100 μM and with reduction at 0 μM Ca(2+), a pattern repeated for C2F domain interactions with phosphatidylinositol 4,5-bisphosphate. In contrast, C2F did not bind the vesicle SNARE protein synaptobrevin-1 (VAMP-1). Moreover, an antibody targeting otoferlin immunoprecipitated syntaxin-1 and SNAP-25 but not synaptobrevin-1. As opposed to an increase in binding with increased calcium, interactions between otoferlin C2F domain and intramolecular C2 domains occurred in the absence of calcium, consistent with intra-C2 domain interactions forming a "closed" tertiary structure at low calcium that "opens" as calcium increases. These results suggest a direct role for otoferlin in exocytosis and modulation of calcium-dependent membrane fusion.

Signaling through C2 domains: more than one lipid target

C2 domains are membrane-binding modules that share a common overall fold: a single compact Greek-key motif organized as an eight-stranded anti-parallel β-sandwich consisting of a pair of four-stranded β-sheets. A myriad of studies have demonstrated that in spite of sharing the common structural β-sandwich core, slight variations in the residues located in the interconnecting loops confer C2 domains with functional abilities to respond to different Ca(2+) concentrations and lipids, and to signal through protein-protein interactions as well. This review summarizes the main structural and functional findings on Ca(2+) and lipid interactions by C2 domains, including the discovery of the phosphoinositide-binding site located in the β3-β4 strands. The wide variety of functions, together with the different Ca(2+) and lipid affinities of these domains, converts this superfamily into a crucial player in many functions in the cell and more to be discovered. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Alternate splicing of dysferlin C2A confers Ca²⁺-dependent and Ca²⁺-independent binding for membrane repair

Dysferlin plays a critical role in the Ca²⁺-dependent repair of microlesions that occur in the muscle sarcolemma. Of the seven C2 domains in dysferlin, only C2A is reported to bind both Ca²⁺ and phospholipid, thus acting as a key sensor in membrane repair. Dysferlin C2A exists as two isoforms, the "canonical" C2A and C2A variant 1 (C2Av1). Interestingly, these isoforms have markedly different responses to Ca²⁺ and phospholipid. Structural and thermodynamic analyses are consistent with the canonical C2A domain as a Ca²⁺-dependent, phospholipid-binding domain, whereas C2Av1 would likely be Ca²⁺-independent under physiological conditions. Additionally, both isoforms display remarkably low free energies of stability, indicative of a highly flexible structure. The inverted ligand preference and flexibility for both C2A isoforms suggest the capability for both constitutive and Ca²⁺-regulated effector interactions, an activity that would be essential in its role as a mediator of membrane repair.

The C2B domain is the primary Ca2+ sensor in DOC2B: a structural and functional analysis

DOC2B (double-C2 domain) protein is thought to be a high-affinity Ca(2+) sensor for spontaneous and asynchronous neurotransmitter release. To elucidate the molecular features underlying its physiological role, we determined the crystal structures of its isolated C2A and C2B domains and examined their Ca(2+)-binding properties. We further characterized the solution structure of the tandem domains (C2AB) using small-angle X-ray scattering. In parallel, we tested structure-function correlates with live cell imaging tools. We found that, despite striking structural similarity, C2B binds Ca(2+) with considerably higher affinity than C2A. The C2AB solution structure is best modeled as two domains with a highly flexible orientation and no difference in the presence or absence of Ca(2+). In addition, kinetic studies of C2AB demonstrate that, in the presence of unilamellar vesicles, Ca(2+) binding is stabilized, as reflected by the ~10-fold slower rate of Ca(2+) dissociation than in the absence of vesicles. In cells, isolated C2B translocates to the plasma membrane (PM) with an EC50 of 400 nM while the C2A does not translocate at submicromolar Ca(2+) concentrations, supporting the biochemical observations. Nevertheless, C2AB translocates to the PM with an ~2-fold lower EC50 and to a greater extent than C2B. Our results, together with previous studies, reveal that the C2B is the primary Ca(2+) sensing unit in DOC2B, whereas C2A enhances the interaction of C2AB with the PM.

Prevalent mechanism of membrane bridging by synaptotagmin-1

Synaptotagmin-1 functions as a Ca(2+) sensor in neurotransmitter release through its two C2 domains (the C2A and C2B domain). The ability of synaptotagmin-1 to bridge two membranes is likely crucial for its function, enabling cooperation with the soluble N-ethylmaleimide sensitive factor adaptor protein receptors (SNAREs) in membrane fusion, but two bridging mechanisms have been proposed. A highly soluble synaptotagmin-1 fragment containing both domains (C2AB) was shown to bind simultaneously to two membranes via the Ca(2+)-binding loops at the top of both domains and basic residues at the bottom of the C2B domain (direct bridging mechanism). In contrast, a longer fragment including a linker sequence (lnC2AB) was found to aggregate in solution and was proposed to bridge membranes through trans interactions between lnC2AB oligomers bound to each membrane via the Ca(2+)-binding loops, with no contact of the bottom of the C2B domain with the membranes. We now show that lnC2AB containing impurities indeed aggregates in solution, but properly purified lnC2AB is highly soluble. Moreover, cryo-EM images reveal that a majority of lnC2AB molecules bridge membranes directly. Fluorescence spectroscopy indicates that the bottom of the C2B domain contacts the membrane in a sizeable population of molecules of both membrane-bound C2AB and membrane-bound lnC2AB. NMR data on nanodiscs show that a fraction of C2AB molecules bind to membranes with antiparallel orientations of the C2 domains. Together with previous studies, these results show that direct bridging constitutes the prevalent mechanism of membrane bridging by both C2AB and lnC2AB, suggesting that this mechanism underlies the function of synaptotagmin-1 in neurotransmitter release.

Subtle Interplay between synaptotagmin and complexin binding to the SNARE complex

Ca²⁺-triggered neurotransmitter release depends on the formation of SNARE complexes that bring the synaptic vesicle and plasma membranes together, on the Ca²⁺ sensor synaptotagmin-1 and on complexins, which play active and inhibitory roles. Release of the complexin inhibitory activity by binding of synaptotagmin-1 to the SNARE complex, causing complexin displacement, was proposed to trigger exocytosis. However, the validity of this model was questioned based on the observation of simultaneous binding of complexin-I and a fragment containing the synaptotagmin-1 C2 domains (C2AB) to membrane-anchored SNARE complex. Using diverse biophysical techniques, here we show that C2AB and complexin-I do not bind to each other but can indeed bind simultaneously to the SNARE complex in solution. Hence, the SNARE complex contains separate binding sites for both proteins. However, total internal reflection fluorescence microscopy experiments show that C2AB can displace a complexin-I fragment containing its central SNARE-binding helix and an inhibitory helix (Cpx26-83) from membrane-anchored SNARE complex under equilibrium conditions. Interestingly, full-length complexin-I binds more tightly to membrane-anchored SNARE complex than Cpx26-83, and it is not displaced by C2AB. These results show that interactions of N- and/or C-terminal sequences of complexin-I with the SNARE complex and/or phospholipids increase the affinity of complexin-I for the SNARE complex, hindering dissociation induced by C2AB. We propose a model whereby binding of synaptotagmin-1 to the SNARE complex directly or indirectly causes a rearrangement of the complexin-I inhibitory helix without inducing complexin-I dissociation, thus relieving the inhibitory activity and enabling cooperation between synaptotagmin-1 and complexin-I in triggering release.

A structure-based protocol for learning the family-specific mechanisms of membrane-binding domains

MOTIVATION

Peripheral membrane-targeting domain (MTD) families, such as C1-, C2- and PH domains, play a key role in signal transduction and membrane trafficking by dynamically translocating their parent proteins to specific plasma membranes when changes in lipid composition occur. It is, however, difficult to determine the subset of domains within families displaying this property, as sequence motifs signifying the membrane binding properties are not well defined. For this reason, procedures based on sequence similarity alone are often insufficient in computational identification of MTDs within families (yielding less than 65% accuracy even with a sequence identity of 70%).

RESULTS

We present a machine learning protocol for determining membrane-targeting properties achieving 85-90% accuracy in separating binding and non-binding domains within families. Our model is based on features from both sequence and structure, thereby incorporation statistics obtained from the entire domain family and domain-specific physical quantities such as surface electrostatics. In addition, by using the enriched rules in alternating decision tree classifiers, we are able to determine the meaning of the assigned function labels in terms of biological mechanisms.

CONCLUSIONS

The high accuracy of the learned models and good agreement between the rules discovered using the ADtree classifier and mechanisms reported in the literature reflect the value of machine learning protocols in both prediction and biological knowledge discovery. Our protocol can thus potentially be used as a general function annotation and knowledge mining tool for other protein domains.

AVAILABILITY

metador.bioengr.uic.edu

CONTACT

huilu@uic.edu.

Analysis of SNARE complex/synaptotagmin-1 interactions by one-dimensional NMR spectroscopy

Neurotransmitter release depends critically on the Ca(2+) sensor synaptotagmin-1 and the SNARE proteins syntaxin-1, synaptobrevin, and SNAP-25, which mediate membrane fusion by forming tight SNARE complexes that bridge the synaptic vesicle and plasma membranes. Interactions between the SNARE complex and the two C2 domains of synaptotagmin-1 (the C2A and C2B domains) are believed to play a key role in coupling Ca(2+) sensing to membrane fusion, but the nature of these interactions is unclear, in part because of a paucity of data obtained by quantitative biophysical methods. Here we have analyzed synaptotagmin-1/SNARE complex interactions by monitoring the decrease in the intensities of one-dimensional (13)C-edited (1)H NMR spectra of (13)C-labeled fragments of synaptotagmin-1 upon binding to unlabeled SNARE complex. Our results indicate that there is a primary binding mode between synaptotagmin-1 and the SNARE complex that involves a polybasic region in the C2B domain and has a sub-micromolar affinity. Our NMR data, combined with precipitation assays, show that there are additional SNARE complex/synaptotagmin-1 interactions that lead to aggregation and that involve in part two arginines at the bottom of the C2B domain. Overall, this study shows the importance of disentangling the contributions of different types of interactions to SNARE complex/synaptotagmin-1 binding and illustrates the usefulness of one-dimensional NMR methods to analyze intricate protein interactions.

Doc2b promotes GLUT4 exocytosis by activating the SNARE-mediated fusion reaction in a calcium- and membrane bending-dependent manner

Reconstitution of GLUT4 vesicle fusion in a defined fusion system shows that the C2-domain factor Doc2b activates the SNARE-dependent fusion reaction by a calcium- and membrane bending–dependent mechanism. Of importance, certain features of Doc2b function appear to be distinct from how synaptotagmin-1 promotes synaptic release.

The glucose transporter GLUT4 plays a central role in maintaining body glucose homeostasis. On insulin stimulation, GLUT4-containing vesicles fuse with the plasma membrane, relocating GLUT4 from intracellular reservoirs to the cell surface to uptake excess blood glucose. The GLUT4 vesicle fusion reaction requires soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) as the core fusion engine and a group of regulatory proteins. In particular, the soluble C2-domain factor Doc2b plays a key role in GLUT4 vesicle fusion, but its molecular mechanism has been unclear. Here we reconstituted the SNARE-dependent GLUT4 vesicle fusion in a defined proteoliposome fusion system. We observed that Doc2b binds to GLUT4 exocytic SNAREs and potently accelerates the fusion kinetics in the presence of Ca²⁺. The stimulatory activity of Doc2b requires intact Ca²⁺-binding sites on both the C2A and C2B domains. Using electron microscopy, we observed that Doc2b strongly bends the membrane bilayer, and this membrane-bending activity is essential to the stimulatory function of Doc2b in fusion. These results demonstrate that Doc2b promotes GLUT4 exocytosis by accelerating the SNARE-dependent fusion reaction by a Ca²⁺- and membrane bending–dependent mechanism. Of importance, certain features of Doc2b function appear to be distinct from how synaptotagmin-1 promotes synaptic neurotransmitter release, suggesting that exocytic Ca²⁺ sensors may possess divergent mechanisms in regulating vesicle fusion.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Hydrophobic contributions to the membrane docking of synaptotagmin 7 C2A domain: mechanistic contrast between isoforms 1 and 7

Synaptotagmin (Syt) triggers Ca(2+)-dependent membrane fusion via its tandem C2 domains, C2A and C2B. The 17 known human isoforms are active in different secretory cell types, including neurons (Syt1 and others) and pancreatic β cells (Syt7 and others). Here, quantitative fluorescence measurements reveal notable differences in the membrane docking mechanisms of Syt1 C2A and Syt7 C2A to vesicles comprised of physiological lipid mixtures. In agreement with previous studies, the Ca(2+) sensitivity of membrane binding is much higher for Syt7 C2A. We report here for the first time that this increased sensitivity is due to the slower target membrane dissociation of Syt7 C2A. Association and dissociation rate constants for Syt7 C2A are found to be ~2-fold and ~60-fold slower than Syt1 C2A, respectively. Furthermore, the membrane dissociation of Syt7 C2A but not Syt1 C2A is slowed by Na(2)SO(4) and trehalose, solutes that enhance the hydrophobic effect. Overall, the simplest model consistent with these findings proposes that Syt7 C2A first docks electrostatically to the target membrane surface and then inserts into the bilayer via a slow hydrophobic mechanism. In contrast, the membrane docking of Syt1 C2A is known to be predominantly electrostatic. Thus, these two highly homologous domains exhibit distinct mechanisms of membrane binding correlated with their known differences in function.

Mechanism for calcium ion sensing by the C2A domain of synaptotagmin I

The C2A domain is one of two calcium ion (Ca(2+))- and membrane-binding domains within synaptotagmin I (Syt I), the identified Ca(2+) sensor for regulated exocytosis of neurotransmitter. We propose that the mechanistic basis for C2A's response to Ca(2+) and cellular function stems from marginal stability and ligand-induced redistributions of protein conformers. To test this hypothesis, we used a combination of calorimetric and fluorescence techniques. We measured free energies of stability by globally fitting differential scanning calorimetry and fluorescence lifetime spectroscopy denaturation data, and found that C2A is weakly stable. Additionally, using partition functions in a fluorescence resonance energy transfer approach, we found that the Ca(2+)- and membrane-binding sites of C2A exhibit weak cooperative linkage. Lastly, a dye-release assay revealed that the Ca(2+)- and membrane-bound conformer subset of C2A promote membrane disruption. We discuss how these phenomena may lead to both cooperative and functional responses of Syt I.

Enlightening molecular mechanisms through study of protein interactions

The investigation of molecular mechanisms is a fascinating area of current biological research that unites efforts from scientists with very diverse expertise. This review provides a perspective on the characterization of protein interactions as a central aspect of this research. We discuss case studies on the neurotransmitter release machinery that illustrate a variety of principles and emphasize the power of combining nuclear magnetic resonance (NMR) spectroscopy with other biophysical techniques, particularly X-ray crystallography. These studies have shown that: (i) the soluble SNAP receptor (SNARE) proteins form a tight complex that brings the synaptic vesicle and plasma membranes together, which is key for membrane fusion; (ii) the SNARE syntaxin-1 adopts an autoinhibitory closed conformation; (iii) Munc18-1 plays crucial functions through interactions with closed syntaxin-1 and with the SNARE complex; (iv) Munc13s mediate the opening of syntaxin-1; (v) complexins play dual roles through distinct interactions with the SNARE complex; (vi) synaptotagmin-1 acts a Ca(2+) sensor, interacting simultaneously with the membranes and the SNAREs; and (vii) a Munc13 homodimer to Munc13-RIM heterodimer switch modulates neurotransmitter release. Overall, this research underlines the complexities involved in elucidating molecular mechanisms and how these mechanisms can depend critically on an interplay between strong and weak protein interactions.

Munc13-4 reconstitutes calcium-dependent SNARE-mediated membrane fusion

Munc13-4 is a Ca²⁺-dependent membrane- and SNARE-binding protein that promotes membrane fusion.

Munc13-4 is a widely expressed member of the CAPS/Munc13 protein family proposed to function in priming secretory granules for exocytosis. Munc13-4 contains N- and C-terminal C2 domains (C2A and C2B) predicted to bind Ca²⁺, but Ca²⁺-dependent regulation of Munc13-4 activity has not been described. The C2 domains bracket a predicted SNARE-binding domain, but whether Munc13-4 interacts with SNARE proteins is unknown. We report that Munc13-4 bound Ca²⁺ and restored Ca²⁺-dependent granule exocytosis to permeable cells (platelets, mast, and neuroendocrine cells) dependent on putative Ca²⁺-binding residues in C2A and C2B. Munc13-4 exhibited Ca²⁺-stimulated SNARE interactions dependent on C2A and Ca²⁺-dependent membrane binding dependent on C2B. In an apparent coupling of membrane and SNARE binding, Munc13-4 stimulated SNARE-dependent liposome fusion dependent on putative Ca²⁺-binding residues in both C2A and C2B domains. Munc13-4 is the first priming factor shown to promote Ca²⁺-dependent SNARE complex formation and SNARE-mediated liposome fusion. These properties of Munc13-4 suggest its function as a Ca²⁺ sensor at rate-limiting priming steps in granule exocytosis.

Synergistic effect of Pb(2+) and phosphatidylinositol 4,5-bisphosphate on C2 domain-membrane interactions

Ca(2+)-responsive C2 domains are peripheral membrane modules that target their host proteins to anionic membranes upon binding Ca(2+) ions. Several C2 domain-containing proteins, such as protein kinase C isoenzymes (PKCs), have been identified as molecular targets of Pb(2+), a known environmental toxin. We demonstrated previously that the C2 domain from PKCα (C2α) binds Pb(2+) with high affinity and undergoes membrane insertion in the Pb(2+)-complexed form. The objective of this work was to determine the effect of phosphatidylinositol 4,5-bisphosphate (PIP(2)) on the C2α-Pb(2+) interactions. Using nuclear magnetic resonance (NMR) experiments, we show that Pb(2+) and PIP(2) synergistically enhance each other's affinity for C2α. Moreover, the affinity of C2α for PIP(2) increases upon progressive saturation of the metal-binding sites. Combining the NMR data with the results of protein-to-membrane Förster resonance energy transfer and vesicle sedimentation experiments, we demonstrate that PIP(2) can influence two aspects of C2α-Pb(2+)-membrane interactions: the affinity of C2α for Pb(2+) and the association of Pb(2+) with the anionic sites on the membrane. Both factors may contribute to the toxic effect of Pb(2+) resulting from the aberrant modulation of PKCα activity. Finally, we propose a mechanism for Pb(2+) outcompeting Ca(2+) from membrane-bound C2α.

Calcium sensing in exocytosis

Neurotransmitters, neuropeptides and hormones are released through regulated exocytosis of synaptic vesicles and large dense core vesicles. This complex and highly regulated process is orchestrated by SNAREs and their associated proteins. The triggering signal for regulated exocytosis is usually an increase in intracellular calcium levels. Besides the triggering role, calcium signaling modulates the precise amount and kinetics of vesicle release. Thus, it is a central question to understand the molecular machineries responsible for calcium sensing in exocytosis. Here we provide an overview of our current understanding of calcium sensing in neurotransmitter release and hormone secretion.

A conformational switch in complexin is required for synaptotagmin to trigger synaptic fusion

The crystal structure of complexin bound to a prefusion SNAREpin mimetic shows that the accessory helix extends away from the SNAREpin in an 'open' conformation, binding another SNAREpin and inhibiting its assembly, to clamp fusion. In contrast, the accessory helix in the postfusion complex parallels the SNARE complex in a 'closed' conformation. Here we use targeted mutations, FRET spectroscopy and a functional assay that reconstitutes Ca(2+)-triggered exocytosis to show that the conformational switch from open to closed in complexin is needed for synaptotagmin-Ca(2+) to trigger fusion. Triggering fusion requires the zippering of three crucial aspartate residues in the switch region (residues 64-68) of v-SNARE. Conformational switching in complexin is integral to clamp release and is probably triggered when its accessory helix is released from its trans-binding to the neighboring SNAREpin, allowing the v-SNARE to complete zippering and open a fusion pore.

Pb2+ as modulator of protein-membrane interactions

Lead is a potent environmental toxin that mimics the effects of divalent metal ions, such as zinc and calcium, in the context of specific molecular targets and signaling processes. The molecular mechanism of lead toxicity remains poorly understood. The objective of this work was to characterize the effect of Pb(2+) on the structure and membrane-binding properties of C2α. C2α is a peripheral membrane-binding domain of Protein Kinase Cα (PKCα), which is a well-documented molecular target of lead. Using NMR and isothermal titration calorimetry (ITC) techniques, we established that C2α binds Pb(2+) with higher affinity than its natural cofactor, Ca(2+). To gain insight into the coordination geometry of protein-bound Pb(2+), we determined the crystal structures of apo and Pb(2+)-bound C2α at 1.9 and 1.5 Å resolution, respectively. A comparison of these structures revealed that the metal-binding site is not preorganized and that rotation of the oxygen-donating side chains is required for the metal coordination to occur. Remarkably, we found that holodirected and hemidirected coordination geometries for the two Pb(2+) ions coexist within a single protein molecule. Using protein-to-membrane Förster resonance energy transfer (FRET) spectroscopy, we demonstrated that Pb(2+) displaces Ca(2+) from C2α in the presence of lipid membranes through the high-affinity interaction with the membrane-unbound C2α. In addition, Pb(2+) associates with phosphatidylserine-containing membranes and thereby competes with C2α for the membrane-binding sites. This process can contribute to the inhibitory effect of Pb(2+) on the PKCα activity.

Evaluation of chemotherapy response in VX2 rabbit lung cancer with 18F-labeled C2A domain of synaptotagmin I

The C2A domain of synaptotagmin I can target apoptotic cells by binding to exposed anionic phospholipids. The goal of this study was to synthesize and develop (18)F-labeled C2A-glutathione-S-transferase (GST) as a molecular imaging probe for the detection of apoptosis and to assess the response of paclitaxel chemotherapy in VX2 rabbit lung cancer.

METHODS

(18)F-C2A-GST was prepared by labeling C2A-GST with N-succinimidyl 4-(18)F-fluorobenzoate ((18)F-SFB). (18)F-C2A-GST was confirmed by high-performance liquid chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis. The binding of (18)F-C2A-GST toward apoptosis was validated in vitro using camptothecin-induced Jurkat cells. Biodistribution of (18)F-C2A-GST was determined in mice by a dissection method and small-animal PET. Single-dose paclitaxel was used to induce apoptosis in rabbits bearing VX2 tumors (n = 6), and 2 VX2 rabbits without treatment served as control. (18)F-C2A-GST PET was performed before and at 72 h after therapy, and (18)F-FDG PET/CT was also performed before treatment. To confirm the presence of apoptosis, tumor tissue was analyzed and activated caspase-3 was measured.

RESULTS

(18)F-C2A-GST was obtained with more than 95% radiochemical purity and was stable for 4 h after formulation. (18)F-C2A-GST bound apoptotic cells specifically. Biodistribution in mice showed that (18)F-C2A-GST mainly excreted from the kidneys and rapidly cleared from blood and nonspecific organs. High focal uptake of (18)F-C2A-GST in the tumor area was determined after therapy, whereas no significant uptake before therapy was found in the tumor with (18)F-FDG-avid foci. The maximum standardized uptake value after therapy was 0.47 ± 0.28, significantly higher than that in the control (0.009 ± 0.001; P < 0.001). The apoptotic index was 79.81% ± 8.73% in the therapy group, significantly higher than that in the control (5.03% ± 0.81%; P < 0.001). Activated caspase-3 after paclitaxel treatment increased to 69.55% ± 16.27% and was significantly higher than that in the control (12.26% ± 5.39%; P < 0.001).

CONCLUSION

(18)F-C2A-GST was easily synthesized by conjugation with (18)F-SFB and manifested a favorable biodistribution. Our results demonstrated the feasibility of (18)F-C2A-GST for the early detection of apoptosis after chemotherapy in a VX2 lung cancer model that could imitate the human lung cancer initiation, development, and progress.

Synaptotagmin 1 and SNAREs form a complex that is structurally heterogeneous

Synaptotagmin 1 (syt1) functions as a Ca(2+)-sensor for neuronal exocytosis. Here, site-directed spin labeling was used to examine the complex formed between a soluble fragment of syt1, which contains its two C2 domains, and the neuronal core soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. Changes in electron paramagnetic resonance lineshape and accessibility for spin-labeled syt1 mutants indicate that in solution, the assembled core SNARE complex contacts syt1 in several regions. For the C2B domain, contact occurs in the polybasic face and sites opposite the Ca(2+)-binding loops. For the C2A domain, contact is seen with the SNARE complex in a region near loop 2. Double electron-electron resonance was used to estimate distances between the two C2 domains of syt1. These distances have broad distributions in solution, which do not significantly change when syt1 is fully associated with the core SNARE complex. The broad distance distributions indicate that syt1 is structurally heterogeneous when bound to the SNAREs and does not assume a well-defined structure. Simulated annealing using electron paramagnetic resonance-derived distance restraints produces a family of syt1 structures where the Ca(2+)-binding regions of each domain face in roughly opposite directions. The results suggest that when associated with the SNAREs, syt1 is configured to bind opposing bilayers, but that the syt1/SNARE complex samples multiple conformational states.

Otoferlin is a calcium sensor that directly regulates SNARE-mediated membrane fusion

Otoferlin is a large multi-C2 domain protein proposed to act as a calcium sensor that regulates synaptic vesicle exocytosis in cochlear hair cells. Although mutations in otoferlin have been associated with deafness, its contribution to neurotransmitter release is unresolved. Using recombinant proteins, we demonstrate that five of the six C2 domains of otoferlin sense calcium with apparent dissociation constants that ranged from 13-25 µM; in the presence of membranes, these apparent affinities increase by up to sevenfold. Using a reconstituted membrane fusion assay, we found that five of the six C2 domains of otoferlin stimulate membrane fusion in a calcium-dependent manner. We also demonstrate that a calcium binding-deficient form of the C2C domain is incapable of stimulating membrane fusion, further underscoring the importance of calcium for the protein's function. These results demonstrate for the first time that otoferlin is a calcium sensor that can directly regulate soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor-mediated membrane fusion reactions.

Tomosyn inhibits synaptotagmin-1-mediated step of Ca2+-dependent neurotransmitter release through its N-terminal WD40 repeats

Neurotransmitter release is triggered by Ca(2+) binding to a low affinity Ca(2+) sensor, mostly synaptotagmin-1, which catalyzes SNARE-mediated synaptic vesicle fusion. Tomosyn negatively regulates Ca(2+)-dependent neurotransmitter release by sequestering target SNAREs through the C-terminal VAMP-like domain. In addition to the C terminus, the N-terminal WD40 repeats of tomosyn also have potent inhibitory activity toward Ca(2+)-dependent neurotransmitter release, although the molecular mechanism underlying this effect remains elusive. Here, we show that through its N-terminal WD40 repeats tomosyn directly binds to synaptotagmin-1 in a Ca(2+)-dependent manner. The N-terminal WD40 repeats impaired the activities of synaptotagmin-1 to promote SNARE complex-mediated membrane fusion and to bend the lipid bilayers. Decreased acetylcholine release from N-terminal WD40 repeat-microinjected superior cervical ganglion neurons was relieved by microinjection of the cytoplasmic domain of synaptotagmin-1. These results indicate that, upon direct binding, the N-terminal WD40 repeats negatively regulate the synaptotagmin-1-mediated step of Ca(2+)-dependent neurotransmitter release. Furthermore, we show that synaptotagmin-1 binding enhances the target SNARE-sequestering activity of tomosyn. These results suggest that the interplay between tomosyn and synaptotagmin-1 underlies inhibitory control of Ca(2+)-dependent neurotransmitter release.

Structural and mutational analysis of functional differentiation between synaptotagmins-1 and -7

Synaptotagmins are known to mediate diverse forms of Ca²⁺-triggered exocytosis through their C₂ domains, but the principles underlying functional differentiation among them are unclear. Synaptotagmin-1 functions as a Ca²⁺ sensor in neurotransmitter release at central nervous system synapses, but synaptotagmin-7 does not, and yet both isoforms act as Ca²⁺ sensors in chromaffin cells. To shed light into this apparent paradox, we have performed rescue experiments in neurons from synaptotagmin-1 knockout mice using a chimera that contains the synaptotagmin-1 sequence with its C₂B domain replaced by the synaptotagmin-7 C₂B domain (Syt1/7). Rescue was not achieved either with the WT Syt1/7 chimera or with nine mutants where residues that are distinct in synaptotagmin-7 were restored to those present in synaptotagmin-1. To investigate whether these results arise because of unique conformational features of the synaptotagmin-7 C₂B domain, we determined its crystal structure at 1.44 Å resolution. The synaptotagmin-7 C₂B domain structure is very similar to that of the synaptotagmin-1 C₂B domain and contains three Ca²⁺-binding sites. Two of the Ca²⁺-binding sites of the synaptotagmin-7 C₂B domain are also present in the synaptotagmin-1 C₂B domain and have analogous ligands to those determined for the latter by NMR spectroscopy, suggesting that a discrepancy observed in a crystal structure of the synaptotagmin-1 C₂B domain arose from crystal contacts. Overall, our results suggest that functional differentiation in synaptotagmins arises in part from subtle sequence changes that yield dramatic functional differences.

Rat and Drosophila synaptotagmin 4 have opposite effects during SNARE-catalyzed membrane fusion

Synaptotagmins (Syt) are a large family of proteins that regulate membrane traffic in neurons and other cell types. One isoform that has received considerable attention is SYT4, with apparently contradictory reports concerning the function of this isoform in fruit flies and mice. SYT4 was reported to function as a negative regulator of neurotrophin secretion in mouse neurons and as a positive regulator of secretion of a yet to be identified growth factor from muscle cells in flies. Here, we have directly compared the biochemical and functional properties of rat and fly SYT4. We report that rat SYT4 inhibited SNARE-catalyzed membrane fusion in both the absence and presence of Ca²⁺. In marked contrast, fly SYT4 stimulated SNARE-mediated membrane fusion in response to Ca²⁺. Analysis of chimeric molecules, isolated C2 domains, and point mutants revealed that the C2B domain of the fly protein senses Ca²⁺ and is sufficient to stimulate fusion. Rat SYT4 was able to stimulate fusion in response to Ca²⁺ when the conserved Asp-to-Ser Ca²⁺ ligand substitution in its C2A domain was reversed. In summary, rat SYT4 serves as an inhibitory isoform, whereas fly SYT4 is a bona fide Ca²⁺ sensor capable of coupling Ca²⁺ to membrane fusion.

The diversity of calcium sensor proteins in the regulation of neuronal function

Calcium signaling in neurons as in other cell types mediates changes in gene expression, cell growth, development, survival, and cell death. However, neuronal Ca(2+) signaling processes have become adapted to modulate the function of other important pathways including axon outgrowth and changes in synaptic strength. Ca(2+) plays a key role as the trigger for fast neurotransmitter release. The ubiquitous Ca(2+) sensor calmodulin is involved in various aspects of neuronal regulation. The mechanisms by which changes in intracellular Ca(2+) concentration in neurons can bring about such diverse responses has, however, become a topic of widespread interest that has recently focused on the roles of specialized neuronal Ca(2+) sensors. In this article, we summarize synaptotagmins in neurotransmitter release, the neuronal roles of calmodulin, and the functional significance of the NCS and the CaBP/calneuron protein families of neuronal Ca(2+) sensors.

Regulation of exocytosis and fusion pores by synaptotagmin-effector interactions

Synaptotagmin isoforms and mutants altered fusion event frequency and fusion pore transitions. These effects showed a strong correlation with PS binding, but not with SNARE binding. Synaptotagmin-PS interaction thus function in two distinct kinetic steps in Ca2+ triggered exocytosis, and stabilize open fusion pores.

Synaptotagmin (syt) serves as a Ca²⁺ sensor in the release of neurotransmitters and hormones. This function depends on the ability of syt to interact with other molecules. Syt binds to phosphatidylserine (PS)-containing lipid bilayers as well as to soluble N-ethylmaleimide sensitive factor receptors (SNAREs) and promotes SNARE assembly. All these interactions are regulated by Ca²⁺, but their specific roles in distinct kinetic steps of exocytosis are not well understood. To explore these questions we used amperometry recording from PC12 cells to investigate the kinetics of exocytosis. Syt isoforms and syt I mutants were overexpressed to perturb syt-PS and syt-SNARE interactions to varying degrees and evaluate the effects on fusion event frequency and the rates of fusion pore transitions. Syt I produced more rapid dilation of fusion pores than syt VII or syt IX, consistent with its role in synchronous synaptic release. Stronger syt-PS interactions were accompanied by a higher frequency of fusion events and more stable fusion pores. By contrast, syt-SNARE interactions and syt-induced SNARE assembly were uncorrelated with rates of exocytosis. This associates the syt-PS interaction with two distinct kinetic steps in Ca²⁺ triggered exocytosis and supports a role for the syt-PS interaction in stabilizing open fusion pores.

Single-molecule FRET-derived model of the synaptotagmin 1-SNARE fusion complex

Synchronous neurotransmission is triggered when Ca(2+) binds to synaptotagmin 1 (Syt1), a synaptic-vesicle protein that interacts with SNAREs and membranes. We used single-molecule fluorescence resonance energy transfer (FRET) between synaptotagmin's two C2 domains to determine that their conformation consists of multiple states with occasional transitions, consistent with domains in random relative motion. SNARE binding results in narrower intrasynaptotagmin FRET distributions and less frequent transitions between states. We obtained an experimentally determined model of the elusive Syt1-SNARE complex using a multibody docking approach with 34 FRET-derived distances as restraints. The Ca(2+)-binding loops point away from the SNARE complex, so they may interact with the same membrane. The loop arrangement is similar to that of the crystal structure of SNARE-induced Ca(2+)-bound Syt3, suggesting a common mechanism by which the interaction between synaptotagmins and SNAREs aids in Ca(2+)-triggered fusion.

Munc13 C2B domain is an activity-dependent Ca2+ regulator of synaptic exocytosis

Munc13 is a multidomain protein present in presynaptic active zones that mediates the priming and plasticity of synaptic vesicle exocytosis, but the mechanisms involved remain unclear. Here we use biophysical, biochemical and electrophysiological approaches to show that the central C(2)B domain of Munc13 functions as a Ca(2+) regulator of short-term synaptic plasticity. The crystal structure of the C(2)B domain revealed an unusual Ca(2+)-binding site with an amphipathic alpha-helix. This configuration confers onto the C(2)B domain unique Ca(2+)-dependent phospholipid-binding properties that favor phosphatidylinositolphosphates. A mutation that inactivated Ca(2+)-dependent phospholipid binding to the C(2)B domain did not alter neurotransmitter release evoked by isolated action potentials, but it did depress release evoked by action-potential trains. In contrast, a mutation that increased Ca(2+)-dependent phosphatidylinositolbisphosphate binding to the C(2)B domain enhanced release evoked by isolated action potentials and by action-potential trains. Our data suggest that, during repeated action potentials, Ca(2+) and phosphatidylinositolphosphate binding to the Munc13 C(2)B domain potentiate synaptic vesicle exocytosis, thereby offsetting synaptic depression induced by vesicle depletion.

Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion

Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.

Synaptotagmin-mediated bending of the target membrane is a critical step in Ca(2+)-regulated fusion

Decades ago it was proposed that exocytosis involves invagination of the target membrane, resulting in a highly localized site of contact between the bilayers destined to fuse. The vesicle protein synaptotagmin-I (syt) bends membranes in response to Ca(2+), but whether this drives localized invagination of the target membrane to accelerate fusion has not been determined. Previous studies relied on reconstituted vesicles that were already highly curved and used mutations in syt that were not selective for membrane-bending activity. Here, we directly address this question by utilizing vesicles with different degrees of curvature. A tubulation-defective syt mutant was able to promote fusion between highly curved SNARE-bearing liposomes but exhibited a marked loss of activity when the membranes were relatively flat. Moreover, bending of flat membranes by adding an N-BAR domain rescued the function of the tubulation-deficient syt mutant. Hence, syt-mediated membrane bending is a critical step in membrane fusion.

The Ca2+ affinity of synaptotagmin 1 is markedly increased by a specific interaction of its C2B domain with phosphatidylinositol 4,5-bisphosphate

The synaptic vesicle protein synaptotagmin 1 is thought to convey the calcium signal onto the core secretory machinery. Its cytosolic portion mainly consists of two C2 domains, which upon calcium binding are enabled to bind to acidic lipid bilayers. Despite major advances in recent years, it is still debated how synaptotagmin controls the process of neurotransmitter release. In particular, there is disagreement with respect to its calcium binding properties and lipid preferences. To investigate how the presence of membranes influences the calcium affinity of synaptotagmin, we have now measured these properties under equilibrium conditions using isothermal titration calorimetry and fluorescence resonance energy transfer. Our data demonstrate that the acidic phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), but not phosphatidylserine, markedly increases the calcium sensitivity of synaptotagmin. PI(4,5)P2 binding is confined to the C2B domain but is not affected significantly by mutations of a lysine-rich patch. Together, our findings lend support to the view that synaptotagmin functions by binding in a trans configuration whereby the C2A domain binds to the synaptic vesicle and the C2B binds to the PI(4,5)P2-enriched plasma membrane.

Solution and membrane-bound conformations of the tandem C2A and C2B domains of synaptotagmin 1: Evidence for bilayer bridging

Synaptotagmin 1 (syt1) is a synaptic vesicle membrane protein that functions as the Ca(2)(+) sensor in neuronal exocytosis. Here, site-directed spin labeling was used to generate models for the solution and membrane-bound structures of a soluble fragment of syt1 containing its two C2 domains, C2A and C2B. In solution, distance restraints between the two C2 domains of syt1 were measured using double electron-electron resonance and used in a simulated annealing routine to generate models for the structure of the tandem C2A-C2B fragment. The data indicate that the two C2 domains are flexibly linked and do not interact with each other in solution, with or without Ca(2+). However, the favored orientation is one where the Ca(2+)-binding loops are oriented in opposite directions. A similar approach was taken for membrane-associated C2A-C2B, combining both distances and bilayer depth restraints with simulated annealing. The restraints can only be satisfied if the Ca(2+) and membrane-binding surfaces of the domains are oriented in opposite directions so that C2A and C2B are docked to opposing bilayers. The result suggests that syt1 functions to bridge across the vesicle and plasma membrane surfaces in a Ca(2+)-dependent manner.

Conformation and membrane position of the region linking the two C2 domains in synaptotagmin 1 by site-directed spin labeling

Synaptotagmin 1 (syt1) is an integral membrane protein localized on the synaptic vesicle that acts as the Ca(2+) sensor for neuronal exocytosis. Synaptotagmin 1 contains two C2 domains, C2A and C2B, which bind Ca(2+) ions, membranes, and SNAREs. Here, site-directed spin labeling (SDSL) was used to determine the position and dynamics of the region that links the two C2 domains in a water soluble construct encompassing the two C2 domains (syt1C2AB). An analysis of the EPR line shapes from this region indicates that the linker is flexible and unstructured when syt1 is in solution or bound to lipid bilayers. The nanosecond dynamics of the linker does not change, in the presence or absence of Ca(2+), suggesting that there is no Ca(2+)-dependent intramolecular association between the two domains. When syt1C2AB is membrane-bound, the position of the linker relative to the membrane interface was determined by measuring parameters for the collision of the spin-labeled syt1C2AB mutants with both soluble and membrane-bound Ni(II) chelates. These data indicate that the linker does not penetrate the membrane surface but lies approximately 7-10 A from the bilayer surface. In addition, the linker remains flexible when syt1C2AB binds to the SNARE complex, indicating that direct interactions between this linker and the SNAREs do not mediate association. These data suggest that the two C2 domains of syt1 interact independently on the membrane interface, or when bound to SNAREs.

The Janus-faced nature of the C(2)B domain is fundamental for synaptotagmin-1 function

Synaptotagmin-1 functions as a Ca2+ sensor in neurotransmitter release and was proposed to act on both the synaptic vesicle and plasma membranes through interactions involving the Ca2+ binding top loops of its C(2) domains and the Ca2+-independent bottom face of the C(2)B domain. However, the functional importance of the C(2)B domain bottom face is unclear. We now show that mutating two conserved arginine residues at the C(2)B domain bottom face practically abolishes synchronous release in hippocampal neurons. Reconstitution experiments reveal that Ca2+-synaptotagmin-1 can dramatically stimulate the rate of SNARE-dependent lipid mixing, and that the two-arginine mutation strongly impairs this activity. These results demonstrate that synaptotagmin-1 function depends crucially on the bottom face of the C(2)B domain and strongly support the notion that synaptotagmin-1 triggers membrane fusion and neurotransmitter release by bringing the vesicle and plasma membranes together, much like the SNAREs do but in a Ca2+-dependent manner.

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.

Ca2+-dependent, phospholipid-binding residues of synaptotagmin are critical for excitation-secretion coupling in vivo

Synaptotagmin I is the Ca(2+) sensor for fast, synchronous release of neurotransmitter; however, the molecular interactions that couple Ca(2+) binding to membrane fusion remain unclear. The structure of synaptotagmin is dominated by two C(2) domains that interact with negatively charged membranes after binding Ca(2+). In vitro work has implicated a conserved basic residue at the tip of loop 3 of the Ca(2+)-binding pocket in both C(2) domains in coordinating this electrostatic interaction with anionic membranes. Although results from cultured cells suggest that the basic residue of the C(2)A domain is functionally significant, such studies provide contradictory results regarding the importance of the C(2)B basic residue during vesicle fusion. To directly test the functional significance of each of these residues at an intact synapse in vivo, we neutralized either the C(2)A or the C(2)B basic residue and assessed synaptic transmission at the Drosophila neuromuscular junction. The conserved basic residues at the tip of the Ca(2+)-binding pocket of both the C(2)A and C(2)B domains mediate Ca(2+)-dependent interactions with anionic membranes and are required for efficient evoked transmitter release. Our results directly support the hypothesis that the interactions between synaptotagmin and the presynaptic membrane, which are mediated by the basic residues at the tip of both the C(2)A and C(2)B Ca(2+)-binding pockets, are critical for coupling Ca(2+) influx with vesicle fusion during synaptic transmission in vivo. Our model for synaptotagmin's direct role in coupling Ca(2+) binding to vesicle fusion incorporates this finding with results from multiple in vitro and in vivo studies.

Synaptic vesicle fusion

The core of the neurotransmitter release machinery is formed by SNARE complexes, which bring the vesicle and plasma membranes together and are key for fusion, and by Munc18-1, which controls SNARE-complex formation and may also have a direct role in fusion. In addition, SNARE complex assembly is likely orchestrated by Munc13s and RIMs, active-zone proteins that function in vesicle priming and diverse forms of presynaptic plasticity. Synaptotagmin-1 mediates triggering of release by Ca²⁺, probably through interactions with SNAREs and both membranes, as well as through a tight interplay with complexins. Elucidation of the release mechanism will require a full understanding of the network of interactions among all these proteins and the membranes.