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

Cellular membranes and lipid-binding domains as attractive targets for drug development

Interdisciplinary research focused on biological membranes has revealed them as signaling and trafficking platforms for processes fundamental to life. Biomembranes harbor receptors, ion channels, lipid domains, lipid signals, and scaffolding complexes, which function to maintain cellular growth, metabolism, and homeostasis. Moreover, abnormalities in lipid metabolism attributed to genetic changes among other causes are often associated with diseases such as cancer, arthritis and diabetes. Thus, there is a need to comprehensively understand molecular events occurring within and on membranes as a means of grasping disease etiology and identifying viable targets for drug development. A rapidly expanding field in the last decade has centered on understanding membrane recruitment of peripheral proteins. This class of proteins reversibly interacts with specific lipids in a spatial and temporal fashion in crucial biological processes. Typically, recruitment of peripheral proteins to the different cellular sites is mediated by one or more modular lipid-binding domains through specific lipid recognition. Structural, computational, and experimental studies of these lipid-binding domains have demonstrated how they specifically recognize their cognate lipids and achieve subcellular localization. However, the mechanisms by which these modular domains and their host proteins are recruited to and interact with various cell membranes often vary drastically due to differences in lipid affinity, specificity, penetration as well as protein-protein and intramolecular interactions. As there is still a paucity of predictive data for peripheral protein function, these enzymes are often rigorously studied to characterize their lipid-dependent properties. This review summarizes recent progress in our understanding of how peripheral proteins are recruited to biomembranes and highlights avenues to exploit in drug development targeted at cellular membranes and/or lipid-binding proteins.

NMR analysis of the closed conformation of syntaxin-1

The Sec1/Munc18 (SM) protein Munc18-1 and the SNAREs syntaxin-1, SNAP-25 and synaptobrevin form the core of the membrane fusion machinery that triggers neurotransmitter release. Munc18-1 binds to syntaxin-1 folded into a closed conformation and to the SNARE complex formed by the three SNAREs, which involves an open syntaxin-1 conformation. The former interaction is likely specialized for neurotransmitter release, whereas SM protein/SNARE complex interactions are likely key for all types of intracellular membrane fusion. It is currently unclear whether the closed conformation is highly or only marginally populated in isolated syntaxin-1, and whether Munc18-1 stabilizes the close conformation or helps to open it to facilitate SNARE complex formation. A detailed NMR analysis now suggests that the closed conformation is almost quantitatively populated in isolated syntaxin-1 in the absence of oligomerization, and indicates that its structure is very similar to that observed previously in the crystal structure of the Munc18-1/syntaxin-1 complex. Moreover, we demonstrate that Munc18-1 binding prevents opening of the syntaxin-1 closed conformation. These results support a model whereby the closed conformation constitutes a key intrinsic property of isolated syntaxin-1 and Munc18-1 binding stabilizes this conformation; in this model, Munc18-1 plays in addition an active role in downstream events after another factor(s) helps to open the syntaxin-1 conformation.

Structure and dynamics of Ca2+-binding domain 1 of the Na+/Ca2+ exchanger in the presence and in the absence of Ca2+

The Na(+)/Ca(2+) exchanger is the major exporter of Ca(2+) across the cell membrane of cardiomyocytes. The activity of the exchanger is regulated by a large intracellular loop that contains two Ca(2+)-binding domains, calcium-binding domain (CBD) 1 and CBD2. CBD1 binds Ca(2+) with much higher affinity than CBD2 and is considered to be the primary Ca(2+) sensor. The effect of Ca(2+) on the structure and dynamics of CBD1 has been characterized by NMR spectroscopy using chemical shifts, residual dipolar couplings, and spin relaxation. Residual dipolar couplings are used in a new way for residue selection in the determination of the anisotropic rotational diffusion tensor from spin relaxation data. The results provide a highly consistent description across these complementary data sets and show that Ca(2+) binding is accompanied by a selective conformational change among the binding site residues. Residues that exhibit a significant conformational change are also sites of altered dynamics. In particular, Ca(2+) binding restricts the mobility of the major acidic segment and affects the dynamics of several nearby binding loops. These observations indicate that Ca(2+) elicits a local transition to a well-ordered coordination geometry in the CBD1-binding site.

Tag team action at the synapse

Communication between neurons relies on chemical synapses and the release of neurotransmitters into the synaptic cleft. Neurotransmitter release is an exquisitely regulated membrane fusion event that requires the linking of an electrical nerve stimulus to Ca(2+) influx, which leads to the fusion of neurotransmitter-filled vesicles with the cell membrane. The timing of neurotransmitter release is controlled through the regulation of the soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) proteins-the core of the membrane fusion machinery. Assembly of the fusion-competent SNARE complex is regulated by several neuronal proteins, including complexin and the Ca(2+)-sensor synaptotagmin. Both complexin and synaptotagmin bind directly to SNAREs, but their mechanism of action has so far remained unclear. Recent studies revealed that synaptotagmin-Ca(2+) and complexin collaborate to regulate membrane fusion. These compelling new results provide a molecular mechanistic insight into the functions of both proteins: complexin 'clamps' the SNARE complex in a pre-fusion intermediate, which is then released by the action of Ca(2+)-bound synaptotagmin to trigger rapid fusion.

Molecular tools for cell and systems biology

The sequencing of the genomes of key organisms and the subsequent identification of genes merely leads us to the next real challenge in modern biology-revealing the precise functions of these genes. Further, detailed knowledge of how the products of these genes behave in space and time is required, including their interactions with other molecules. In order to tackle these considerable tasks, a large and continuously expanding toolbox is required to probe the functions of proteins on a cellular level. Here, the currently available tools are described and future developments are projected. There is no doubt that only the close interplay between the life science disciplines in addition to advances in engineering will be able to meet the challenge.

Structure of human synaptotagmin 1 C2AB in the absence of Ca2+ reveals a novel domain association

Release of neurotransmitter from synaptic vesicles requires the Ca2+/phospholipid-binding protein synaptotagmin 1. There is considerable evidence that cooperation between the tandem C2 domains of synaptotagmin is a requirement of regulated exocytosis; however, high-resolution structural evidence for this interaction has been lacking. The 2.7 A crystal structure of the cytosolic domains of human synaptotagmin 1 in the absence of Ca2+ reveals a novel closed conformation of the protein. The shared interface between C2A and C2B is stabilized by a network of interactions between residues on the C-terminal alpha-helix of the C2B domain and residues on loops 1-3 of the Ca2+-binding region of C2A. These interactions alter the overall shape of the Ca2+-binding pocket of C2A, but not that of C2B. Thus, synaptotagmin 1 C2A-C2B may utilize a novel regulatory mechanism whereby one C2 domain could regulate the other until an appropriate triggering event decouples them.

Synaptotagmin C2A loop 2 mediates Ca2+-dependent SNARE interactions essential for Ca2+-triggered vesicle exocytosis

Synaptotagmins contain tandem C2 domains and function as Ca(2+) sensors for vesicle exocytosis but the mechanism for coupling Ca(2+) rises to membrane fusion remains undefined. Synaptotagmins bind SNAREs, essential components of the membrane fusion machinery, but the role of these interactions in Ca(2+)-triggered vesicle exocytosis has not been directly assessed. We identified sites on synaptotagmin-1 that mediate Ca(2+)-dependent SNAP25 binding by zero-length cross-linking. Mutation of these sites in C2A and C2B eliminated Ca(2+)-dependent synaptotagmin-1 binding to SNAREs without affecting Ca(2+)-dependent membrane binding. The mutants failed to confer Ca(2+) regulation on SNARE-dependent liposome fusion and failed to restore Ca(2+)-triggered vesicle exocytosis in synaptotagmin-deficient PC12 cells. The results provide direct evidence that Ca(2+)-dependent SNARE binding by synaptotagmin is essential for Ca(2+)-triggered vesicle exocytosis and that Ca(2+)-dependent membrane binding by itself is insufficient to trigger fusion. A structure-based model of the SNARE-binding surface of C2A provided a new view of how Ca(2+)-dependent SNARE and membrane binding occur simultaneously.

Relevance of partially structured states in the non-classical secretion of acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) is a signal peptide-less protein that is secreted into the extracellular compartment as part of a multiprotein release complex, consisting of aFGF, S100A13 (a calcium binding protein), and a 40 kDa (p40) form of synaptotagmin (Syt1), a protein that participates in the docking of a variety of secretory vesicles. p40 Syt1, and specifically its C2A domain, is believed to play a major role in the non-classical secretion of the aFGF release complex mediated by the interaction of aFGF and p40 Syt1with the phospholipids of the cell membrane inner leaflet. In the present study, we investigate the structural characteristics of aFGF and the C2A domain of p40 Syt1 under acidic conditions, using a variety of biophysical techniques including multidimensional NMR spectroscopy. Urea-induced equilibrium unfolding (at pH 3.4) of both aFGF and the C2A domain are non-cooperative and proceed with the accumulation of stable intermediate states. 1-Anilino-8-napthalene sulfonate (ANS) binding and size-exclusion chromatography results suggest that both aFGF and the C2A domain exist as partially structured states under acidic conditions (pH 3.4). Limited trypsin digestion analysis and 1H-15N chemical shift perturbation data reveal that the flexibility of certain portions of the protein backbone is increased in the partially structured state(s) of aFGF. The residues that are perturbed in the partially structured state(s) in aFGF are mostly located at the N- and C-terminal ends of the protein. In marked contrast, most of the interactions stabilizing the native secondary structure are preserved in the partially structured state of the C2A domain. Isothermal titration calorimetry data indicate that the binding affinity between aFGF and the C2A domain is significantly enhanced at pH 3.4. In addition, both aFGF and the C2A domain exhibit much higher lipid binding affinity in their partially structured states. The translocation of the multiprotein FGF release complex across the membrane appears to be facilitated by the formation of partially structured states of aFGF and the C2A domain of p40 Syt1.

Structural analysis of conserved oligomeric Golgi complex subunit 2

The conserved oligomeric Golgi (COG) complex is strongly implicated in retrograde vesicular trafficking within the Golgi apparatus. Although its mechanism of action is poorly understood, it has been proposed to function by mediating the initial physical contact between transport vesicles and their membrane targets. An analogous role in tethering vesicles has been suggested for at least six additional large multisubunit complexes, including the exocyst, a complex essential for trafficking to the plasma membrane. Here we report the solution structure of a large portion of yeast Cog2p, one of eight subunits composing the COG complex. The structure reveals a six-helix bundle with few conserved surface features but a general resemblance to recently determined crystal structures of four different exocyst subunits. This finding provides the first structural evidence that COG, like the exocyst and potentially other tethering complexes, is constructed from helical bundles. These structures may represent platforms for interaction with other trafficking proteins including SNAREs (soluble N-ethylmaleimide factor attachment protein receptors) and Rabs.

How synaptotagmin promotes membrane fusion

Synaptic vesicles loaded with neurotransmitters are exocytosed in a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent manner after presynaptic depolarization induces calcium ion (Ca2+) influx. The Ca2+ sensor required for fast fusion is synaptotagmin-1. The activation energy of bilayer-bilayer fusion is very high (approximately 40 k(B)T). We found that, in response to Ca2+ binding, synaptotagmin-1 could promote SNARE-mediated fusion by lowering this activation barrier by inducing high positive curvature in target membranes on C2-domain membrane insertion. Thus, synaptotagmin-1 triggers the fusion of docked vesicles by local Ca2+-dependent buckling of the plasma membrane together with the zippering of SNAREs. This mechanism may be widely used in membrane fusion.

The cooperative response of synaptotagmin I C2A. A hypothesis for a Ca2+-driven molecular hammer

In the current understanding of exocytosis at the nerve terminal, the C2 domain of synaptotagmin (C2A) is presumed to bind Ca2+ and the membrane in a stepwise fashion: cation then membrane as cation increases the affinity of protein for membrane. Fluorescence spectroscopy data were gathered over a variety of lipid and Ca2+ concentrations, enabling the rigorous application of microscopic binding models derived from partition functions to differentiate between Ca2+ and phosphatidylserine contributions to binding. The data presented here are in variance with previously published models, which were based on the Hill approximation. Rather, the data are consistent with two forms of cooperativity that modulate the responsiveness of C2A: in Ca2+ binding to a network of three cation sites and in interaction with the membrane surface. We suggest synaptotagmin I C2A is preassociated with the synaptic vesicle membrane or nerve terminal. In this state, upon Ca2+ influx the protein will bind the three Ca2+ ions immediately and with high cooperativity. Thus, membrane association creates a high-affinity Ca2+ switch that is the basis for the role of synaptotagmin I in Ca2+-regulated exocytosis. Based on this model, we discuss the implications of protein-induced phosphatidylserine demixing to the exocytotic process.

The cPLA2 C2alpha domain in solution: structure and dynamics of its Ca2+-activated and cation-free states

Cytosolic phospholipase A2 is involved in several signal transduction pathways where it catalyses release of arachidonic acid from intracellular lipid membranes. Its membrane insertion is facilitated by its independently folding C2alpha domain, which is activated by the binding of two intracellular Ca2+ ions. However, the details of its membrane-insertion mechanism, including its Ca2+-activation mechanism, are not understood. There are several unresolved issues, including the following. There are two experimentally resolved structures of the Ca2+-activated state of its isolated C2alpha domain, one determined using x-ray crystallography and the other determined using NMR spectroscopy, which differ from each other significantly in the spatial region that inserts into the membrane. This by itself adds to ambiguities associated with investigations targeting its mechanism of membrane insertion. Furthermore, there is no experimentally determined structure of its cation-free state, which hinders investigations associated with its cation-activation mechanism. In this work, we generate several unrestrained molecular dynamics trajectories of its isolated C2alpha domain in solution (equivalent to approximately 60 ns) and investigate these issues. Our main results are as follows: a), the Ca2+ coordination scheme of the domain is consistent with the x-ray structure and with previous mutagenesis studies; b), the helical segment of the Ca2+-binding loop, CBL-I, undergoes nanosecond timescale flexing (but not an unwinding), as can be inferred from physiological temperature NMR data and in contrast to low temperature x-ray data; and c), removal of the two activating Ca2+ ions from their binding pockets does not alter the backbone structure of the domain, a result consistent with electron paramagnetic resonance data.

A quaternary SNARE-synaptotagmin-Ca2+-phospholipid complex in neurotransmitter release

The function of synaptotagmin as a Ca(2+) sensor in neurotransmitter release involves Ca(2+)-dependent phospholipid binding to its two C(2) domains, but this activity alone does not explain why Ca(2+) binding to the C(2)B domain is more critical for release than Ca(2+) binding to the C(2)A domain. Synaptotagmin also binds to SNARE complexes, which are central components of the membrane fusion machinery, and displaces complexins from the SNAREs. However, it is unclear how phospholipid binding to synaptotagmin is coupled to SNARE binding and complexin displacement. Using supported lipid bilayers deposited within microfluidic channels, we now show that Ca(2+) induces simultaneous binding of synaptotagmin to phospholipid membranes and SNARE complexes, resulting in an intimate quaternary complex that we name SSCAP complex. Mutagenesis experiments show that Ca(2+) binding to the C(2)B domain is critical for SSCAP complex formation and displacement of complexin, providing a clear rationale for the preponderant role of the C(2)B domain in release. This and other correlations between the effects of mutations on SSCAP complex formation and their functional effects in vivo suggest a key role for this complex in release. We propose a model whereby the highly positive electrostatic potential at the tip of the SSCAP complex helps to induce membrane fusion during release.

Purification, crystallization and X-ray diffraction analysis of human synaptotagmin 1 C2A-C2B

Synaptotagmin acts as the Ca(2+) sensor for neuronal exocytosis. The cytosolic domain of human synaptotagmin 1 is composed of tandem C2 domains: C2A and C2B. These C2 domains modulate the interaction of synaptotagmin with the phospholipid bilayer of the presynaptic terminus and effector proteins such as the SNARE complex. Human synaptotagmin C2A-C2B has been expressed as a glutathione-S-transferase fusion protein in Escherichia coli. The purification, crystallization and preliminary X-ray analysis of this protein are reported here. The crystals diffract to 2.7 A and belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 82.37, b = 86.31, c = 140.2 A. From self-rotation function analysis, there are two molecules in the asymmetric unit. The structure determination of the protein using this data is ongoing.

Unraveling the mechanisms of synaptotagmin and SNARE function in neurotransmitter release

SNARE proteins and synaptotagmin are key components of the complex machinery that controls Ca(2+)-triggered neurotransmitter release but their mechanisms of action are under debate. Recent research has shed light on which biochemical and/or biophysical properties underlie SNARE and synaptotagmin function. SNARE proteins most likely have a role in membrane fusion owing to their ability to bring the synaptic vesicle and plasma membranes together and to perturb lipid bilayers through their transmembrane regions. Synaptotagmin acts as a Ca(2+) sensor and might cooperate with the SNAREs in accelerating fusion by binding simultaneously to the two membranes. However, recent research has strongly challenged the validity of models proposing that the SNAREs (with or without synaptotagmin) constitute "minimal membrane fusion machineries" and has emphasized the essential nature of other proteins for exocytosis. Understanding the functions of these proteins will be crucial to reach a faithful description of the mechanisms of membrane fusion and neurotransmitter release.

DOC2A and DOC2B are sensors for neuronal activity with unique calcium-dependent and kinetic properties

Elevation of the intracellular calcium concentration ([Ca2+]i) to levels below 1 microm alters synaptic transmission and induces short-term plasticity. To identify calcium sensors involved in this signalling, we investigated soluble C2 domain-containing proteins and found that both DOC2A and DOC2B are modulated by submicromolar calcium levels. Fluorescent-tagged DOC2A and DOC2B translocated to plasma membranes after [Ca2+]i elevation. DOC2B translocation preceded DOC2A translocation in cells co-expressing both isoforms. Half-maximal translocation occurred at 450 and 175 nm[Ca2+]i for DOC2A and DOC2B, respectively. This large difference in calcium sensitivity was accompanied by a modest kinetic difference (halftimes, respectively, 2.6 and 2.0 s). The calcium sensitivity of DOC2 isoforms can be explained by predicted topologies of their C2A domains. Consistently, neutralization of aspartates D218 and D220 in DOC2B changed its calcium affinity. In neurones, both DOC2 isoforms were reversibly recruited to the plasma membrane during trains of action potentials. Consistent with its higher calcium sensitivity, DOC2B translocated at lower depolarization frequencies. Styryl dye uptake experiments in hippocampal neurones suggest that the overexpression of mutated DOC2B alters the synaptic activity. We conclude that both DOC2A and DOC2B are regulated by neuronal activity, and hypothesize that their calcium-dependent translocation may regulate synaptic activity.

Close membrane-membrane proximity induced by Ca2+-dependent multivalent binding of synaptotagmin-1 to phospholipids

Synaptotagmin acts as a Ca(2+) sensor in neurotransmitter release through its two C(2) domains. Ca(2+)-dependent phospholipid binding is key for synaptotagmin function, but it is unclear how this activity cooperates with the SNARE complex involved in release or why Ca(2+) binding to the C(2)B domain is more crucial for release than Ca(2+) binding to the C(2)A domain. Here we show that Ca(2+) induces high-affinity simultaneous binding of synaptotagmin to two membranes, bringing them into close proximity. The synaptotagmin C(2)B domain is sufficient for this ability, which arises from the abundance of basic residues around its surface. We propose a model wherein synaptotagmin cooperates with the SNAREs in bringing the synaptic vesicle and plasma membranes together and accelerates membrane fusion through the highly positive electrostatic potential of its C(2)B domain.

Structural and functional diversity generated by alternative mRNA splicing

Alternative mRNA splicing is becoming increasingly recognized as an important mechanism for the generation of structural and functional diversity in proteins. Recent estimations predict that approximately 50% of all eukaryotic proteins can be alternatively spliced. Several lines of evidence suggest that alternative mRNA splicing results in small changes in protein structure and is likely to fine-tune the function and specificity of the affected protein. However, knowledge of how alternative splicing regulates cellular processes on the molecular level is still limited. It is only recently that structures of alternatively spliced proteins have been solved. These studies have shown that alternative splicing affects the structure not only in the vicinity of the splice site but also at long distance.

Membrane-Protein Interactions in Cell Signaling and Membrane Trafficking

Research in the past decade has revealed that many cytosolic proteins are recruited to different cellular membranes to form protein-protein and lipid-protein interactions during cell signaling and membrane trafficking. Membrane recruitment of these peripheral proteins is mediated by a growing number of modular membrane-targeting domains, including C1, C2, PH, FYVE, PX, ENTH, ANTH, BAR, FERM, and tubby domains, that recognize specific lipid molecules in the membranes. Structural studies of these membrane-targeting domains demonstrate how they specifically recognize their cognate lipid ligands. However, the mechanisms by which these domains and their host proteins are recruited to and interact with various cell membranes are only beginning to unravel with recent computational studies, in vitro membrane binding studies using model membranes, and cellular translocation studies using fluorescent protein-tagged proteins. This review summarizes the recent progress in our understanding of how the kinetics and energetics of membrane-protein interactions are regulated during the cellular membrane targeting and activation of peripheral proteins.

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.

Single-molecule studies of synaptotagmin and complexin binding to the SNARE complex

The assembly of multiprotein complexes at the membrane interface governs many signaling processes in cells. However, very few methods exist for obtaining biophysical information about protein complex formation at the membrane. We used single molecule fluorescence resonance energy transfer to study complexin and synaptotagmin interactions with the SNARE complex in deposited lipid bilayers. Using total internal reflectance microscopy, individual binding events at the membrane could be resolved despite an excess of unbound protein in solution. Fluorescence resonance energy transfer (FRET)-efficiency derived distances for the complexin-SNARE interaction were consistent with the crystal structure of the complexin-SNARE complex. The unstructured N-terminal region of complexin showed broad distributions of FRET efficiencies to the SNARE complex, suggesting that information on conformational variability can be obtained from FRET efficiency distributions. The low-affinity interaction of synaptotagmin with the SNARE complex changed dramatically upon addition of Ca2+ with high FRET efficiency interactions appearing between the C2B domain and linker domains of synaptotagmin and the membrane proximal portion of the SNARE complex. These results demonstrate that single molecule FRET can be used as a "spectroscopic ruler" to simultaneously gain structural and kinetic information about transient multiprotein complexes at the membrane interface.

Ca2+ activation of the cPLA2 C2 domain: ordered binding of two Ca2+ ions with positive cooperativity

During Ca(2+) activation, the Ca(2+)-binding sites of C2 domains typically bind multiple Ca(2+) ions in close proximity. These binding events exhibit positive cooperativity, despite the strong charge repulsion between the adjacent divalent cations. Using both experimental and computational approaches, the present study probes the detailed mechanisms of Ca(2+) activation and positive cooperativity for the C2 domain of cytosolic phospholipase A(2), which binds two Ca(2+) ions in sites I and II, separated by only 4.1 A. First, each of the five coordinating side chains in the Ca(2+)-binding cleft is individually mutated and the effect on Ca(2+)-binding affinity and cooperativity is measured. The results identify Asp 43 as the major contributor to Ca(2+) affinity, while the two coordinating side chains that provide bridging coordination to both Ca(2+) ions, Asp 43 and Asp 40, are observed to make the largest contributions to positive cooperativity. Electrostatic calculations reveal that Asp 43 possesses the highest pseudo-pK(a) of the coordinating acidic residues, as well as the highest general cation affinity, due to its relatively buried location within 3.5 A of seven protein oxygens with full or partial negative charges. These calculations therefore explain the greater importance of Asp 43 in defining the Ca(2+) affinity. Overall, the experimental and computational results support an activation model in which the first Ca(2+) ion binds usually to site I, thereby preordering both bridging side chains Asp 40 and 43, and partially or fully deprotonating the three coordinating Asp residues. This initial binding event prepares the conformation and protonation state of the remaining site for Ca(2+) binding, enabling the second Ca(2+) ion to bind with higher affinity than the first as required for positive cooperativity.

Three-dimensional Structure of the rSly1 N-terminal Domain Reveals a Conformational Change Induced by Binding to Syntaxin 5

Sec1/Mun18-like (SM) proteins and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) play central roles in intracellular membrane fusion. Diverse modes of interaction between SM proteins and SNAREs from the syntaxin family have been described. However, the observation that the N-terminal domains of Sly1 and Vps45, the SM proteins involved in traffic at the endoplasmic reticulum, the Golgi, the trans-Golgi network and the endosomes, bind to similar N-terminal sequences of their cognate syntaxins suggested a unifying theme for SM protein/SNARE interactions in most internal membrane compartments. To further understand this mechanism of SM protein/SNARE coupling, we have elucidated the structure in solution of the isolated N-terminal domain of rat Sly1 (rSly1N) and analyzed its complex with an N-terminal peptide of rat syntaxin 5 by NMR spectroscopy. Comparison with the crystal structure of a complex between Sly1p and Sed5p, their yeast homologues, shows that syntaxin 5 binding requires a striking conformational change involving a two-residue shift in the register of the C-terminal beta-strand of rSly1N. This conformational change is likely to induce a significant alteration in the overall shape of full-length rSly1 and may be critical for its function. Sequence analyses indicate that this conformational change is conserved in the Sly1 family but not in other SM proteins, and that the four families represented by the four SM proteins found in yeast (Sec1p, Sly1p, Vps45p and Vps33p) diverged early in evolution. These results suggest that there are marked distinctions between the mechanisms of action of each of the four families of SM proteins, which may have arisen from different regulatory requirements of traffic in their corresponding membrane compartments.

Calcium-dependent plasma membrane binding and cell lysis by perforin are mediated through its C2 domain: A critical role for aspartate residues 429, 435, 483, and 485 but not 491

The lymphocyte pore-forming protein perforin is essential for maintaining immune homeostasis and for effective defense against intracellular pathogens. To date, there have been no reported structure-function studies to substantiate the function of any putative domains of perforin, which have been postulated totally on primary sequence similarities with domains in other proteins. In this report, we have used recently developed modalities for expressing full-length perforin and robust functional assays to investigate one of the hallmarks of perforin function: its absolute dependence on calcium for lipid binding and cell lysis. We provide, for the first time, experimental evidence that the predicted C-terminal C2 motif constitutes a functional domain that is responsible for membrane binding of perforin. Whereas conserved aspartate residues at positions 429, 435, 483, and 485 were essential for calcium-dependent plasma membrane binding and cell lysis, the contribution of Asp-491 was limited. Finally, after experimentally verifying an optimized three-dimensional model, we have made predictions on the impact of two inherited perforin mutations of the C2 domain on calcium-dependent lipid binding and cell lysis.

Solution structure of a novel calcium binding protein, MTH1880, from Methanobacterium thermoautotrophicum

MTH1880 is a hypothetical protein from Methanobacterium thermoautotrophicum, a target organism of structural genomics. The solution structure determined by NMR spectroscopy demonstrates a typical alpha + beta-fold found in many proteins with different functions. The molecular surface of the protein reveals a small, highly acidic pocket comprising loop B (Asp36, Asp37, Asp38), the end of beta2 (Glu39), and loop D (Ser57, Ser58, Ser61), indicating that the protein would have a possible cation binding site. The NMR resonances of several amino acids within the acidic binding pocket in MTH1880, shifted upon addition of calcium ion. This calcium binding motif and overall topology of MTH1880 differ from those of other calcium binding proteins. MTH1880 did not show a calcium-induced conformational change typical of calcium sensor proteins. Therefore, we propose that the MTH1880 protein contains a novel motif for calcium-specific binding, and may function as a calcium buffering protein.

WNK1 Phosphorylates Synaptotagmin 2 and Modulates Its Membrane Binding

WNK (with no lysine [K]) protein kinases were named for their unique active site organization. Mutations in WNK1 and WNK4 cause a familial form of hypertension by undefined mechanisms. Here, we report that WNK1 selectively binds to and phosphorylates synaptotagmin 2 (Syt2) within its calcium binding C2 domains. Endogenous WNK1 and Syt2 coimmunoprecipitate and colocalize on a subset of secretory granules in INS-1 cells. Phosphorylation by WNK1 increases the amount of Ca2+ required for Syt2 binding to phospholipid vesicles; mutation of threonine 202, a WNK1 phosphorylation site, partially prevents this change. These findings suggest that phosphorylation of Syts by WNK1 can regulate Ca2+ sensing and the subsequent Ca2+-dependent interactions mediated by Syt C2 domains. These findings provide a biochemical mechanism that could lead to the retention or insertion of proteins in the plasma membrane. Interruption of this regulatory pathway may disturb membrane events that regulate ion balance.

Crystallographic identification of Ca2+ and Sr2+ coordination sites in synaptotagmin I C2B domain

Synaptotagmin I has two tandem Ca(2+)-binding C(2) domains, which are essential for fast synchronous synaptic transmission in the central nervous system. We have solved four crystal structures of the C(2)B domain, one of them in the cation-free form at 1.50 A resolution, two in the Ca(2+)-bound form at 1.04 A (two bound Ca(2+) ions) and 1.65 A (three bound Ca(2+) ions) resolution and one in the Sr(2+)-bound form at 1.18 A (one bound Sr(2+) ion) resolution. The side chains of four highly conserved aspartic acids (D303, D309, D363, and D365) and two main chain oxygens (M302:O and Y364:O), together with water molecules, are in direct contact with two bound Ca(2+) ions (sites 1 and 2). At higher Ca(2+) concentrations, the side chain of N333 rotates and cooperates with D309 to generate a third Ca(2+) coordination site (site 3). Divalent cation binding sites 1 and 2 in the C(2)B domain were previously identified from NMR NOE patterns and titration studies, supplemented by site-directed mutation analysis. One difference between the crystal and NMR studies involves D371, which is not involved in coordination with any of the identified Ca(2+) sites in the crystal structures, while it is coordinated to Ca(2+) in site 2 in the NMR structure. In the presence of Sr(2+), which is also capable of triggering exocytosis, but with lower efficiency, only one cation binding site (site 1) was occupied in the crystallographic structure.

Structural basis for the evolutionary inactivation of Ca2+ binding to synaptotagmin 4

The neuronal protein synaptotagmin 1 functions as a Ca(2+) sensor in exocytosis via two Ca(2+)-binding C(2) domains. The very similar synaptotagmin 4, which includes all the predicted Ca(2+)-binding residues in the C(2)B domain but not in the C(2)A domain, is also thought to function as a neuronal Ca(2+) sensor. Here we show that, unexpectedly, both C(2) domains of fly synaptotagmin 4 exhibit Ca(2+)-dependent phospholipid binding, whereas neither C(2) domain of rat synaptotagmin 4 binds Ca(2+) or phospholipids efficiently. Crystallography reveals that changes in the orientations of critical Ca(2+) ligands, and perhaps their flexibility, render the rat synaptotagmin 4 C(2)B domain unable to form full Ca(2+)-binding sites. These results indicate that synaptotagmin 4 is a Ca(2+) sensor in the fly but not in the rat, that the Ca(2+)-binding properties of C(2) domains cannot be reliably predicted from sequence analyses, and that proteins clearly identified as orthologs may nevertheless have markedly different functional properties.

Membrane-docking loops of the cPLA2 C2 domain: detailed structural analysis of the protein-membrane interface via site-directed spin-labeling

C2 domains are protein modules found in numerous eukaryotic signaling proteins, where their function is to target the protein to cell membranes in response to a Ca(2+) signal. Currently, the structure of the interface formed between the protein and the phospholipid bilayer is inaccessible to high-resolution structure determination, but EPR site-directed spin-labeling can provide a detailed medium-resolution view of this interface. To apply this approach to the C2 domain of cytosolic phospholipase A(2) (cPLA(2)), single cysteines were introduced at all 27 positions in the three Ca(2+)-binding loops and labeled with a methanethiosulfonate spin-label. Altogether, 24 of the 27 spin-labeled domains retained Ca(2+)-activated phospholipid binding. EPR spectra of these 24 labeled domains obtained in the presence and absence of Ca(2+) indicate that Ca(2+) binding triggers subtle changes in the dynamics of two localized regions within the Ca(2+)-binding loops: one face of the loop 1 helix and the junction between loops 1 and 2. However, no significant changes in loop structure were detected upon Ca(2+) binding, nor upon Ca(2+)-triggered docking to membranes. EPR depth parameters measured in the membrane-docked state allow determination of the penetration depth of each residue with respect to the membrane surface. Analysis of these depth parameters, using an improved, generalizable geometric approach, provides the most accurate picture of penetration depth and angular orientation currently available for a membrane-docked peripheral protein. Finally, the observation that Ca(2+) binding does not trigger large rearrangements of the membrane-docking loops favors the electrostatic switch model for Ca(2+) activation and disfavors, or places strong constraints on, the conformational switch model.

The calcium binding loops of the cytosolic phospholipase A2 C2 domain specify targeting to Golgi and ER in live cells

Translocation of cytosolic phospholipase A2 (cPLA2) to Golgi and ER in response to intracellular calcium mobilization is regulated by its calcium-dependent lipid-binding, or C2, domain. Although well studied in vitro, the biochemical characteristics of the cPLA2C2 domain offer no predictive value in determining its intracellular targeting. To understand the molecular basis for cPLA2C2 targeting in vivo, the intracellular targets of the synaptotagmin 1 C2A (Syt1C2A) and protein kinase Calpha C2 (PKCalphaC2) domains were identified in Madin-Darby canine kidney cells and compared with that of hybrid C2 domains containing the calcium binding loops from cPLA2C2 on Syt1C2A and PKCalphaC2 domain backbones. In response to an intracellular calcium increase, PKCalphaC2 targeted plasma membrane regions rich in phosphatidylinositol-4,5-bisphosphate, and Syt1C2A displayed a biphasic targeting pattern, first targeting phosphatidylinositol-4,5-bisphosphate-rich regions in the plasma membrane and then the trans-Golgi network. In contrast, the Syt1C2A/cPLA2C2 and PKCalphaC2/cPLA2C2 hybrids targeted Golgi/ER and colocalized with cPLA2C2. The electrostatic properties of these hybrids suggested that the membrane binding mechanism was similar to cPLA2C2, but not PKCalphaC2 or Syt1C2A. These results suggest that primarily calcium binding loops 1 and 3 encode structural information specifying Golgi/ER targeting of cPLA2C2 and the hybrid domains.

PIP2 increases the speed of response of synaptotagmin and steers its membrane-penetration activity toward the plasma membrane

Synaptotagmin-1 (syt), the putative Ca2+ sensor for exocytosis, is anchored to the membrane of secretory organelles. Its cytoplasmic domain is composed of two Ca2+-sensing modules, C2A and C2B. Syt binds phosphatidylinositol 4,5-bisphosphate (PIP2), a plasma membrane lipid with an essential role in exocytosis and endocytosis. We resolved two modes of PIP2 binding that are mediated by distinct surfaces on the C2B domain of syt. A novel Ca2+-independent mode of binding predisposes syt to penetrate PIP2-harboring target membranes in response to Ca2+ with submillisecond kinetics. Thus, PIP2 increases the speed of response of syt and steers its membrane-penetration activity toward the plasma membrane. We propose that syt-PIP2 interactions are involved in exocytosis by facilitating the close apposition of the vesicle and target membrane on rapid time scales in response to Ca2+.

A conformational switch in the Piccolo C2A domain regulated by alternative splicing

C2 domains are widespread Ca2+-binding modules. The active zone protein Piccolo (also known as Aczonin) contains an unusual C2A domain that exhibits a low affinity for Ca2+, a Ca2+-induced conformational change and Ca2+-dependent dimerization. We show here that removal of a nine-residue sequence by alternative splicing increases the Ca2+ affinity, abolishes the conformational change and abrogates dimerization of the Piccolo C2A domain. The NMR structure of the Ca2+-free long variant provides a structural basis for these different properties of the two splice forms, showing that the nine-residue sequence forms a beta-strand otherwise occupied by a nonspliced sequence. Consequently, Ca2+-binding to the long Piccolo C2A domain requires a marked rearrangement of secondary structure that cannot occur for the short variant. These results reveal a novel mechanism of action of C2 domains and uncover a structural principle that may underlie the alteration of protein function by short alternatively spliced sequences.

Mutations in the effector binding loops in the C2A and C2B domains of synaptotagmin I disrupt exocytosis in a nonadditive manner

The secretory vesicle protein synaptotagmin I (syt) plays a critical role in Ca2+-triggered exocytosis. Its cytoplasmic domain is composed of tandem C2 domains, C2A and C2B; each C2 domain binds Ca2+. Upon binding Ca2+, positively charged residues within the Ca2+-binding loops are thought to interact with negatively charged phospholipids in the target membrane to mediate docking of the cytoplasmic domain of syt onto lipid bilayers. The C2 domains of syt also interact with syntaxin and SNAP-25, two components of a conserved membrane fusion complex. Here, we have neutralized single positively charged residues at the membrane-binding interface of C2A (R233Q) and C2B (K366Q). Either of these mutations shifted the Ca2+ requirements for syt-liposome interactions from approximately 20 to approximately 40 microm Ca2+. Kinetic analysis revealed that the reduction in Ca2+-sensing activity was associated with a decrease in affinity for membranes. These mutations did not affect sytsyntaxin interactions but resulted in an approximately 50% loss in SNAP-25 binding activity, suggesting that these residues lie at an interface between membranes and SNAP-25. Expression of full-length versions of syt that harbored these mutations reduced the rate of exocytosis in PC12 cells. In both biochemical and functional assays, effects of the R233Q and K366Q mutations were not additive, indicating that mutations in one domain affect the activity of the adjacent domain. These findings indicate that the tandem C2 domains of syt cooperate with one another to trigger release via loop-mediated electrostatic interactions with effector molecules.

Membrane fusion: a structural perspective on the interplay of lipids and proteins

The fusion of biological membranes is governed by the carefully orchestrated interplay of membrane proteins and lipids. Recently determined structures of fusion proteins, individual domains of fusion proteins and their complexes with regulatory proteins and membrane lipids have yielded much suggestive insight into how viral and intracellular membrane fusion might proceed. These structures may be combined with new knowledge on the fusion of pure lipid bilayer membranes in an attempt to begin to piece together the complex puzzle of how biological membrane fusion machines operate on membranes.

Synaptotagmin I, a Ca2+ sensor for neurotransmitter release

The discovery that Ca(2+) triggers rapid neurotransmitter release has prompted the search for the Ca(2+) sensor. There is now general agreement that the vesicle-associated Ca(2+)-binding protein, synaptotagmin I, is required for the tight temporal coupling between Ca(2+) influx and synaptic vesicle fusion. However, the precise mechanism of Ca(2+)-sensing by synaptotagmin I is still under debate despite intensive investigation using genetic, biochemical and electrophysiological tools. Here, we discuss many of the genetic manipulations from the past few years that have shed light on the Ca(2+)-sensing function of synaptotagmin I. We also present our view as to how the Ca(2+) signal is translated rapidly into membrane fusion at fast chemical synapses.

The synaptotagmin C2A domain is part of the calcium sensor controlling fast synaptic transmission

Synaptotagmin is a synaptic vesicle protein that has been proposed to be the calcium sensor responsible for fast neurotransmitter release at synapses. Synaptotagmin's two C2 domains, C2A and C2B, each provide a calcium binding pocket lined with negative charges contributed by five conserved aspartates. We find that even when all of C2A's conserved aspartates are neutralized by replacement with asparagines, neurotransmitter release still occurs at hippocampal synapses in culture. Because exocytosis continues to be dependent on extracellular calcium concentration, the C2A domain cannot represent the entire calcium sensor. C2A does appear to be part of the calcium sensor, however, because substitution of D232 alters the calcium dependence of release, perhaps by reducing the number of calcium ions that must bind to trigger exocytosis. We conclude that neutralization of the negative charge at D232 by coordination of a calcium ion is necessary--but not sufficient--for fast neurotransmission at mammalian CNS synapses.

Endocytosis of synaptotagmin 1 is mediated by a novel, tryptophan-containing motif

The rate at which a membrane protein is internalized from the plasma membrane can be regulated by revealing a latent internalization signal in response to an appropriate stimulus. Internalization of the synaptic vesicle membrane protein, synaptotagmin 1, is controlled by two distinct regions of its intracytoplasmic C2B domain, an internalization signal present in the 29 carboxyterminal (CT) amino acids and a separate regulatory region. We have now characterized the internalization motif by mutagenesis and found that it involves an essential tryptophan in the last beta strand of the C2B domain, a region that is distinct from the AP2-binding site previously described. Internalization through the tryptophan-based motif is sensitive to eps15 and dynamin mutants and is therefore likely to be clathrin mediated. A tryptophan-to-phenylalanine mutation had no effect on internalization of the CT domain alone, but completely inhibited endocytosis of the folded C2B domain. This result suggests that recognition of sorting motifs can be influenced by their structural context. We conclude that endocytosis of synaptotagmin 1 requires a novel type of internalization signal that is subject to regulation by the rest of the C2B domain.

The molecular basis of differential subcellular localization of C2 domains of protein kinase C-alpha and group IVa cytosolic phospholipase A2

The C2 domain is a Ca(2+)-dependent membrane-targeting module found in many cellular proteins involved in signal transduction or membrane trafficking. C2 domains are unique among membrane targeting domains in that they show a wide range of lipid selectivity for the major components of cell membranes, including phosphatidylserine and phosphatidylcholine. To understand how C2 domains show diverse lipid selectivity and how this functional diversity affects their subcellular targeting behaviors, we measured the binding of the C2 domains of group IVa cytosolic phospholipase A(2) (cPLA(2)) and protein kinase C-alpha (PKC-alpha) to vesicles that model cell membranes they are targeted to, and we monitored their subcellular targeting in living cells. The surface plasmon resonance analysis indicates that the PKC-alpha C2 domain strongly prefers the cytoplasmic plasma membrane mimic to the nuclear membrane mimic due to high phosphatidylserine content in the former and that Asn(189) plays a key role in this specificity. In contrast, the cPLA(2) C2 domain has specificity for the nuclear membrane mimic over the cytoplasmic plasma membrane mimic due to high phosphatidylcholine content in the former and aromatic and hydrophobic residues in the calcium binding loops of the cPLA(2) C2 domain are important for its lipid specificity. The subcellular localization of enhanced green fluorescent protein-tagged C2 domains and mutants transfected into HEK293 cells showed that the subcellular localization of the C2 domains is consistent with their lipid specificity and could be tailored by altering their in vitro lipid specificity. The relative cell membrane translocation rate of selected C2 domains was also consistent with their relative affinity for model membranes. Together, these results suggest that biophysical principles that govern the in vitro membrane binding of C2 domains can account for most of their subcellular targeting properties.

The C2A domain of synaptotagmin alters the kinetics of voltage-gated Ca2+ channels Ca(v)1.2 (Lc-type) and Ca(v)2.3 (R-type)

Biochemical and genetic studies implicate synaptotagmin (Syt 1) as a Ca2+ sensor for neuronal and neuroendocrine neurosecretion. Calcium binding to Syt 1 occurs through two cytoplasmic repeats termed the C2A and C2B domains. In addition, the C2A domain of Syt 1 has calcium-independent properties required for neurotransmitter release. For example, mutation of a polylysine motif (residues 189-192) reverses the inhibitory effect of injected recombinant Syt 1 C2A fragment on neurotransmitter release from PC12 cells. Here we examined the requirement of the C2A polylysine motif for Syt 1 interaction with the cardiac Cav1.2 (L-type) and the neuronal Cav2.3 (R-type) voltage-gated Ca2+ channels, two channels required for neurotransmission. We find that the C2A polylysine motif presents a critical interaction surface with Cav1.2 and Cav2.3 since truncated Syt 1 containing a mutated motif (Syt 1*1-264) was ineffective at modifying the channel kinetics. Mutating the polylysine motif also abolished C2A binding to Lc753-893, the cytosolic interacting domain of Syt 1 at Cav1.2 1 subunit. Syt 1 and Syt 1* harboring the mutation at the KKKK motif modified channel activation, while Syt 1* only partially reversed the syntaxin 1A effects on channel activity. This mutation would interfere with the assembly of Syt 1/channel/syntaxin into an exocytotic unit. The functional interaction of the C2A polylysine domain with Cav1.2 and Cav2.3 is consistent with tethering of the secretory vesicle to the Ca2+ channel. It indicates that calcium-independent properties of Syt 1 regulate voltage-gated Ca2+ channels and contribute to the molecular events underlying transmitter release.

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.

C2 domain of protein kinase C alpha: elucidation of the membrane docking surface by site-directed fluorescence and spin labeling

The C2 domain is a conserved signaling motif that triggers membrane docking in a Ca(2+)-dependent manner, but the membrane docking surfaces of many C2 domains have not yet been identified. Two extreme models can be proposed for the docking of the protein kinase C alpha (PKC alpha) C2 domain to membranes. In the parallel model, the membrane-docking surface includes the Ca(2+) binding loops and an anion binding site on beta-strands 3-4, such that the beta-strands are oriented parallel to the membrane. In the perpendicular model, the docking surface is localized to the Ca(2+) binding loops and the beta-strands are oriented perpendicular to the membrane surface. The present study utilizes site-directed fluorescence and spin-labeling to map out the membrane docking surface of the PKC alpha C2 domain. Single cysteine residues were engineered into 18 locations scattered over all regions of the protein surface, and were used as attachment sites for spectroscopic probes. The environmentally sensitive fluorescein probe identified positions where Ca(2+) activation or membrane docking trigger measurable fluorescence changes. Ca(2+) binding was found to initiate a global conformational change, while membrane docking triggered the largest fluorescein environmental changes at labeling positions on the three Ca(2+) binding loops (CBL), thereby localizing these loops to the membrane docking surface. Complementary EPR power saturation measurements were carried out using a nitroxide spin probe to determine a membrane depth parameter, Phi, for each spin-labeled mutant. Positive membrane depth parameters indicative of membrane insertion were found for three positions, all located on the Ca(2+) binding loops: N189 on CBL 1, and both R249 and R252 on CBL 3. In addition, EPR power saturation revealed that five positions near the anion binding site are partially protected from collisions with an aqueous paramagnetic probe, indicating that the anion binding site lies at or near the surface of the headgroup layer. Together, the fluorescence and EPR results indicate that the Ca(2+) first and third Ca(2+) binding loops insert directly into the lipid headgroup region of the membrane, and that the anion binding site on beta-strands 3-4 lies near the headgroups. The data support a model in which the beta-strands are tilted toward the parallel orientation relative to the membrane surface.

Structure/function analysis of Ca2+ binding to the C2A domain of synaptotagmin 1

Synaptotagmin 1, a Ca2+ sensor for fast synaptic vesicle exocytosis, contains two C2 domains that form Ca2+-dependent complexes with phospholipids. To examine the functional importance of Ca2+ binding to the C2A domain of synaptotagmin 1, we studied two C2A domain mutations, D232N and D238N, using recombinant proteins and knock-in mice. Both mutations severely decreased intrinsic Ca2+ binding and Ca2+-dependent phospholipid binding by the isolated C2A domain. Both mutations, however, did not alter the apparent Ca2+ affinity of the double C2 domain fragment, although both decreased the tightness of the Ca2+/phospholipid/double C2 domain complex. When introduced into the endogenous synaptotagmin 1 gene in mice, the D232N and D238N mutations had no apparent effect on morbidity and mortality and caused no detectable alteration in the Ca2+-dependent properties of synaptotagmin 1. Electrophysiological recordings of cultured hippocampal neurons from knock-in mice revealed that neither mutation induced major changes in synaptic transmission. The D232N mutation, however, caused increased synaptic depression during repetitive stimulation, whereas the D238N mutation did not exhibit this phenotype. Our data indicate that Ca2+ binding to the C2A domain of synaptotagmin 1 may be important but not essential, consistent with the finding that the two C2 domains cooperate and may be partially redundant in Ca2+-dependent phospholipid binding. Moreover, although the apparent Ca2+ affinity of the synaptotagmin 1/phospholipid complex is critical, the tightness of the Ca2+/phospholipid complex is not. Our data also demonstrate that subtle changes in the biochemical properties of synaptotagmin 1 can result in significant alterations in synaptic responses.

C2 domains of protein kinase C isoforms alpha, beta, and gamma: activation parameters and calcium stoichiometries of the membrane-bound state

The independently folding C2 domain motif serves as a Ca(2+)-dependent membrane docking trigger in a large number of Ca(2+) signaling pathways. A comparison was initiated between three closely related C2 domains from the conventional protein kinase C subfamily (cPKC, isoforms alpha, beta, and gamma). The results reveal that these C2 domain isoforms exhibit some similarities but are specialized in important ways, including different Ca(2+) stoichiometries. In the absence of membranes, Ca(2+) affinities of the isolated C2 domains are similar (2-fold difference) while Hill coefficients reveal cooperative Ca(2+) binding for the PKC beta C2 domain but not for the PKC alpha or PKC gamma C2 domain (H = 2.3 +/- 0.1 for PKC beta, 0.9 +/- 0.1 for PKC alpha, and 0.9 +/- 0.1 for PKC gamma). When phosphatidylserine-containing membranes are present, Ca(2+) affinities range from the sub-micromolar to the micromolar (7-fold difference) ([Ca(2+)](1/2) = 0.7 +/- 0.1 microM for PKC gamma, 1.4 +/- 0.1 microM for PKC alpha, and 5.0 +/- 0.2 microM for PKC beta), and cooperative Ca(2+) binding is observed for all three C2 domains (Hill coefficients equal 1.8 +/- 0.1 for PKC beta, 1.3 +/- 0.1 for PKC alpha, and 1.4 +/- 0.1 for PKC gamma). The large effects of membranes are consistent with a coupled Ca(2+) and membrane binding equilibrium, and with a direct role of the phospholipid in stabilizing bound Ca(2+). The net negative charge of the phospholipid is more important to membrane affinity than its headgroup structure, although a slight preference for phosphatidylserine is observed over other anionic phospholipids. The Ca(2+) stoichiometries of the membrane-bound C2 domains are detectably different. PKC beta and PKC gamma each bind three Ca(2+) ions in the membrane-associated state; membrane-bound PKC alpha binds two Ca(2+) ions, and a third binds weakly or not at all under physiological conditions. Overall, the results indicate that conventional PKC C2 domains first bind a subset of the final Ca(2+) ions in solution, and then associate weakly with the membrane and bind additional Ca(2+) ions to yield a stronger membrane interaction in the fully assembled tertiary complex. The full complement of Ca(2+) ions is needed for tight binding to the membrane. Thus, even though the three C2 domains are 64% identical, differences in Ca(2+) affinity, stoichiometry, and cooperativity are observed, demonstrating that these closely related C2 domains are specialized for their individual functions and contexts.

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?

NMR measurement of the off rate from the first calcium-binding site of the synaptotagmin I C2A domain

The off rate from the first calcium-binding site of the C2A domain of synaptotagmin I, a putative calcium receptor in neurotransmitter release, has been determined by 15N-nuclear magnetic resonance relaxation dispersion measurements. The exchange rate was obtained by fitting the dependence of the transverse relaxation rates on the interval between 180 degrees pulses in relaxation-compensated CPMG experiments at 3.2 microM calcium concentration. The measured k(ex) is 2.0x10(3) x s(-1). The calcium on rate of 3.5+/-1x10(7) x s(-1), determined from the measured off rate and the dissociation constant (5.3x10(-5) M), is close to the diffusion limit. These results are consistent with the proposed role of synaptotagmin I as a calcium sensor in release, but suggest that additional factors may help to accelerate the diffusion of Ca2+ to the sensor.

Membrane targeting of C2 domains of phospholipase C-delta isoforms

The C2 domain is a Ca(2+)-dependent membrane-targeting module found in many cellular proteins involved in signal transduction or membrane trafficking. To understand the mechanisms by which the C2 domain mediates the membrane targeting of PLC-delta isoforms, we measured the in vitro membrane binding of the C2 domains of PLC-delta1, -delta3, and -delta4 by surface plasmon resonance and monolayer techniques and their subcellular localization by time-lapse confocal microscopy. The membrane binding of the PLC-delta1-C2 is driven by nonspecific electrostatic interactions between the Ca(2+)-induced cationic surface of protein and the anionic membrane and specific interactions involving Ca(2+), Asn(647), and phosphatidylserine (PS). The PS selectivity of PLC-delta1-C2 governs its specific Ca(2+)-dependent subcellular targeting to the plasma membrane. The membrane binding of the PLC-delta3-C2 also involves Ca(2+)-induced nonspecific electrostatic interactions and PS coordination, and the latter leads to specific subcellular targeting to the plasma membrane. In contrast to PLC-delta1-C2 and PLC-delta3-C2, PLC-delta4-C2 has significant Ca(2+)-independent membrane affinity and no PS selectivity due to the presence of cationic residues in the Ca(2+)-binding loops and the substitution of Ser for the Ca(2+)-coordinating Asp in position 717. Consequently, PLC-delta4-C2 exhibits unique pre-localization to the plasma membrane prior to Ca(2+) import and non-selective Ca(2+)-mediated targeting to various cellular membranes, suggesting that PLC-delta4 might have a novel regulatory mechanism. Together, these results establish the C2 domains of PLC-delta isoforms as Ca(2+)-dependent membrane targeting domains that have distinct membrane binding properties that control their subcellular localization behaviors.

Role of electrostatic and hydrophobic interactions in Ca(2+)-dependent phospholipid binding by the C(2)A-domain from synaptotagmin I

Most C(2)-domains bind to phospholipid bilayers as a function of Ca(2+). Although phospholipid binding is central for the normal functions of C(2)-domain proteins, the precise mechanism of phospholipid binding is unclear. One of the key questions is whether phospholipid binding by C(2)-domains is primarily governed by electrostatic or hydrophobic interactions. We have now examined this question for the C(2)A-domain of synaptotagmin I, a membrane protein of secretory vesicles with an essential function in Ca(2+)-triggered exocytosis. Our results confirm previous data showing that Ca(2+)-dependent phospholipid binding by the synaptotagmin C(2)A-domain is exquisitely sensitive to ionic strength, suggesting an essential role for electrostatic interactions. However, we find that hydrophobic interactions mediated by exposed residues in the Ca(2+)-binding loops of the C(2)A-domain, in particular methionine 173, are also essential for tight phospholipid binding. Furthermore, we demonstrate that the apparent Ca(2+) affinity of the C(2)A-domain is determined not only by electrostatic interactions as shown previously, but also by hydrophobic interactions. Together these data indicate that phospholipid binding by the C(2)A-domain, although triggered by an electrostatic Ca(2+)-dependent switch, is stabilized by a hydrophobic mechanism. As a result, Ca(2+)-dependent phospholipid binding proceeds by a multimodal mechanism that mirrors the amphipathic nature of the phospholipid bilayer. The complex phospholipid binding mode of synaptotagmins may be important for its role in regulated exocytosis of secretory granules and synaptic vesicles.

Cation charge and size selectivity of the C2 domain of cytosolic phospholipase A(2)

C2 domains regulate numerous eukaryotic signaling proteins by docking to target membranes upon binding Ca(2+). Effective activation of the C2 domain by intracellular Ca(2+) signals requires high Ca(2+) selectivity to exclude the prevalent physiological metal ions K(+), Na(+), and Mg(2+). The cooperative binding of two Ca(2+) ions to the C2 domain of cytosolic phospholipase A(2) (cPLA(2)-alpha) induces docking to phosphatidylcholine (PC) membranes. The ionic charge and size selectivities of this C2 domain were probed with representative mono-, di-, and trivalent spherical metal cations. Physiological concentrations of monovalent cations and Mg(2+) failed to bind to the domain and to induce docking to PC membranes. Superphysiological concentrations of Mg(2+) did bind but still failed to induce membrane docking. In contrast, Ca(2+), Sr(2+), and Ba(2+) bound to the domain in the low micromolar range, induced electrophoretic mobility shifts in native polyacrylamide gels, stabilized the domain against thermal denaturation, and induced docking to PC membranes. In the absence of membranes, the degree of apparent positive cooperativity in binding of Ca(2+), Sr(2+), and Ba(2+) decreased with increasing cation size, suggesting that the C2 domain binds two Ca(2+) or Sr(2+) ions, but only one Ba(2+) ion. These stoichiometries were correlated with the abilities of the ions to drive membrane docking, such that micromolar concentrations of Ca(2+) and Sr(2+) triggered docking while even millimolar concentrations of Ba(2+) yielded poor docking efficiency. The simplest explanation is that two bound divalent cations are required for stable membrane association. The physiological Ca(2+) ion triggered membrane docking at 20-fold lower concentrations than Sr(2+), due to both the higher Ca(2+) affinity of the free domain and the higher affinity of the Ca(2+)-loaded domain for membranes. Kinetic studies indicated that Ca(2+) ions bound to the free domain are retained at least 5-fold longer than Sr(2+) ions. Moreover, the Ca(2+)-loaded domain remained bound to membranes 2-fold longer than the Sr(2+)-loaded domain. For both Ca(2+) and Sr(2+), the two bound metal ions dissociate from the protein-membrane complex in two kinetically resolvable steps. Finally, representative trivalent lanthanide ions bound to the domain with high affinity and positive cooperativity, and induced docking to PC membranes. Overall, the results demonstrate that both cation charge and size constraints contribute to the high Ca(2+) selectivity of the C2 domain and suggest that formation of a cPLA(2)-alpha C2 domain-membrane complex requires two bound multivalent metal ions. These features are proposed to stem from the unique structural features of the metal ion-binding site in the C2 domain.