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Synaptic vesicle fusion.

Rizo J, Rosenmund C - Nat. Struct. Mol. Biol. (2008)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Texas Southwestern Medical Center, 6000 Harry Hines Boulevard, Dallas, Texas 75390, USA. jose@arnie.swmed.edu

ABSTRACT
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.

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Related in: MedlinePlus

Syntaptotagmin-1 and its coupling to SNAREs and membranes. (a) Domain structure of synaptotagmin-1, with the number of residues indicated in the top right corner. TM, transmembrane. (b) Model of the SSCAP complex built from the structures of the SNARE complex11 and the Ca2+-bound synaptotagmin-1 C2 domains81,82 and a mutagenesis analysis of SSCAP complex formation92. Orange spheres, Ca2+ ions; dashed black curve, the linker between the syntaxin-1 SNARE motif and transmembrane region. Note that it is uncertain whether the synaptotagmin-1 Ca2+-binding loops bind to the plasma membrane or the synaptic vesicle membrane. (c) Crystal structure of the tandem synaptotagmin-1 C2 domains in the absence of Ca2+; the structure involves an antiparallel interaction between the two domains that needs to be disrupted to allow Ca2+ binding to the C2A domain109. (d) Model of how the synaptotagmin-1 C2B domain could cooperate with the SNAREs in triggering membrane fusion upon Ca2+ influx by binding to both membranes and the C terminus of the SNARE complex. Each orange circle represents the two Ca2+ ions bound to the C2B domain. The + and − signs illustrate the electrostatic charge distribution of the C2B domain and the SNARE complex. The model assumes that synaptotagmin-1 interacts with the SNARE complex through the polybasic region on the side of the C2B domain, as in b. This interaction is weak in solution but is likely to be strengthened by colocalization of synaptotagmin-1 and the SNARE complex on one membrane, which at the same time may increase binding specificity by disfavoring irrelevant interactions existing in solution between these highly charged molecules92. The C2A domain is not shown in this model for simplicity, but could play a related role. (e) Model of how the Ca2+ binding loops of the synaptotagmin-1 C2 domains (only the C2B domain is shown for simplicity) could help to cause membrane fusion by inducing positive curvature on the plasma membrane96. (f) Two diagrams showing the types of curvature involved in membrane bending and illustrating that such bending requires both positive and negative curvature.
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Figure 3: Syntaptotagmin-1 and its coupling to SNAREs and membranes. (a) Domain structure of synaptotagmin-1, with the number of residues indicated in the top right corner. TM, transmembrane. (b) Model of the SSCAP complex built from the structures of the SNARE complex11 and the Ca2+-bound synaptotagmin-1 C2 domains81,82 and a mutagenesis analysis of SSCAP complex formation92. Orange spheres, Ca2+ ions; dashed black curve, the linker between the syntaxin-1 SNARE motif and transmembrane region. Note that it is uncertain whether the synaptotagmin-1 Ca2+-binding loops bind to the plasma membrane or the synaptic vesicle membrane. (c) Crystal structure of the tandem synaptotagmin-1 C2 domains in the absence of Ca2+; the structure involves an antiparallel interaction between the two domains that needs to be disrupted to allow Ca2+ binding to the C2A domain109. (d) Model of how the synaptotagmin-1 C2B domain could cooperate with the SNAREs in triggering membrane fusion upon Ca2+ influx by binding to both membranes and the C terminus of the SNARE complex. Each orange circle represents the two Ca2+ ions bound to the C2B domain. The + and − signs illustrate the electrostatic charge distribution of the C2B domain and the SNARE complex. The model assumes that synaptotagmin-1 interacts with the SNARE complex through the polybasic region on the side of the C2B domain, as in b. This interaction is weak in solution but is likely to be strengthened by colocalization of synaptotagmin-1 and the SNARE complex on one membrane, which at the same time may increase binding specificity by disfavoring irrelevant interactions existing in solution between these highly charged molecules92. The C2A domain is not shown in this model for simplicity, but could play a related role. (e) Model of how the Ca2+ binding loops of the synaptotagmin-1 C2 domains (only the C2B domain is shown for simplicity) could help to cause membrane fusion by inducing positive curvature on the plasma membrane96. (f) Two diagrams showing the types of curvature involved in membrane bending and illustrating that such bending requires both positive and negative curvature.

Mentions: Key for the Ca2+-triggering step of release are synaptotagmin-1 and complexins1,3,4,7 (Figs. 3 and 4). Synaptotagmin-1 is a synaptic vesicle protein with two C2 domains, the C2A and C2B domains, that adopt similar β-sandwich structures and bind three and two Ca2+ ions, respectively, through loops at the top of the sandwich79–82 (Fig. 3a,b). These top loops also mediate Ca2+-dependent phospholipid binding to both C2 domains82–84. Mutations that decrease or increase the apparent Ca2+ affinity of synaptotagmin-1 lead to parallel changes in the Ca2+ sensitivity of release85,86, showing that synaptotagmin-1 acts as a Ca2+ sensor in release and that Ca2+-dependent phospholipid binding to both C2 domains is key for this function. However, mutations in the Ca2+-binding ligands of the C2B domain impair release much more severely than analogous mutations in the C2A domain87,88, indicating that Ca2+ binding to the C2B domain is more critical for release. Among several potential explanations for these findings4,7, particularly attractive is the observation that the C2B domain mediates simultaneous binding to two membranes through its Ca2+-binding loops and the abundant basic residues around its surface, thus showing that the C2B domain can bring two membranes together as the SNAREs do, but through a Ca2+-dependent mechanism89 (Fig. 3d).


Synaptic vesicle fusion.

Rizo J, Rosenmund C - Nat. Struct. Mol. Biol. (2008)

Syntaptotagmin-1 and its coupling to SNAREs and membranes. (a) Domain structure of synaptotagmin-1, with the number of residues indicated in the top right corner. TM, transmembrane. (b) Model of the SSCAP complex built from the structures of the SNARE complex11 and the Ca2+-bound synaptotagmin-1 C2 domains81,82 and a mutagenesis analysis of SSCAP complex formation92. Orange spheres, Ca2+ ions; dashed black curve, the linker between the syntaxin-1 SNARE motif and transmembrane region. Note that it is uncertain whether the synaptotagmin-1 Ca2+-binding loops bind to the plasma membrane or the synaptic vesicle membrane. (c) Crystal structure of the tandem synaptotagmin-1 C2 domains in the absence of Ca2+; the structure involves an antiparallel interaction between the two domains that needs to be disrupted to allow Ca2+ binding to the C2A domain109. (d) Model of how the synaptotagmin-1 C2B domain could cooperate with the SNAREs in triggering membrane fusion upon Ca2+ influx by binding to both membranes and the C terminus of the SNARE complex. Each orange circle represents the two Ca2+ ions bound to the C2B domain. The + and − signs illustrate the electrostatic charge distribution of the C2B domain and the SNARE complex. The model assumes that synaptotagmin-1 interacts with the SNARE complex through the polybasic region on the side of the C2B domain, as in b. This interaction is weak in solution but is likely to be strengthened by colocalization of synaptotagmin-1 and the SNARE complex on one membrane, which at the same time may increase binding specificity by disfavoring irrelevant interactions existing in solution between these highly charged molecules92. The C2A domain is not shown in this model for simplicity, but could play a related role. (e) Model of how the Ca2+ binding loops of the synaptotagmin-1 C2 domains (only the C2B domain is shown for simplicity) could help to cause membrane fusion by inducing positive curvature on the plasma membrane96. (f) Two diagrams showing the types of curvature involved in membrane bending and illustrating that such bending requires both positive and negative curvature.
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Related In: Results  -  Collection

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Figure 3: Syntaptotagmin-1 and its coupling to SNAREs and membranes. (a) Domain structure of synaptotagmin-1, with the number of residues indicated in the top right corner. TM, transmembrane. (b) Model of the SSCAP complex built from the structures of the SNARE complex11 and the Ca2+-bound synaptotagmin-1 C2 domains81,82 and a mutagenesis analysis of SSCAP complex formation92. Orange spheres, Ca2+ ions; dashed black curve, the linker between the syntaxin-1 SNARE motif and transmembrane region. Note that it is uncertain whether the synaptotagmin-1 Ca2+-binding loops bind to the plasma membrane or the synaptic vesicle membrane. (c) Crystal structure of the tandem synaptotagmin-1 C2 domains in the absence of Ca2+; the structure involves an antiparallel interaction between the two domains that needs to be disrupted to allow Ca2+ binding to the C2A domain109. (d) Model of how the synaptotagmin-1 C2B domain could cooperate with the SNAREs in triggering membrane fusion upon Ca2+ influx by binding to both membranes and the C terminus of the SNARE complex. Each orange circle represents the two Ca2+ ions bound to the C2B domain. The + and − signs illustrate the electrostatic charge distribution of the C2B domain and the SNARE complex. The model assumes that synaptotagmin-1 interacts with the SNARE complex through the polybasic region on the side of the C2B domain, as in b. This interaction is weak in solution but is likely to be strengthened by colocalization of synaptotagmin-1 and the SNARE complex on one membrane, which at the same time may increase binding specificity by disfavoring irrelevant interactions existing in solution between these highly charged molecules92. The C2A domain is not shown in this model for simplicity, but could play a related role. (e) Model of how the Ca2+ binding loops of the synaptotagmin-1 C2 domains (only the C2B domain is shown for simplicity) could help to cause membrane fusion by inducing positive curvature on the plasma membrane96. (f) Two diagrams showing the types of curvature involved in membrane bending and illustrating that such bending requires both positive and negative curvature.
Mentions: Key for the Ca2+-triggering step of release are synaptotagmin-1 and complexins1,3,4,7 (Figs. 3 and 4). Synaptotagmin-1 is a synaptic vesicle protein with two C2 domains, the C2A and C2B domains, that adopt similar β-sandwich structures and bind three and two Ca2+ ions, respectively, through loops at the top of the sandwich79–82 (Fig. 3a,b). These top loops also mediate Ca2+-dependent phospholipid binding to both C2 domains82–84. Mutations that decrease or increase the apparent Ca2+ affinity of synaptotagmin-1 lead to parallel changes in the Ca2+ sensitivity of release85,86, showing that synaptotagmin-1 acts as a Ca2+ sensor in release and that Ca2+-dependent phospholipid binding to both C2 domains is key for this function. However, mutations in the Ca2+-binding ligands of the C2B domain impair release much more severely than analogous mutations in the C2A domain87,88, indicating that Ca2+ binding to the C2B domain is more critical for release. Among several potential explanations for these findings4,7, particularly attractive is the observation that the C2B domain mediates simultaneous binding to two membranes through its Ca2+-binding loops and the abundant basic residues around its surface, thus showing that the C2B domain can bring two membranes together as the SNAREs do, but through a Ca2+-dependent mechanism89 (Fig. 3d).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry, University of Texas Southwestern Medical Center, 6000 Harry Hines Boulevard, Dallas, Texas 75390, USA. jose@arnie.swmed.edu

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus