Zero ionic layer

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Zero ionic layer is the main site of interaction in the core SNARE complex. Dipole-dipole interactions take place between 3 glutamine (Q) residues and 1 arginine (R) residue exposed in this layer. Despite that, the majority of the SNARE complex is hydrophobic because of the leucine zipper. [1] Extensively studied layers within the SNARE alpha-helical bundle are designated from "-7" to "+8". Zero ionic layer is at the center of the bundle, and thus designated as "0" layer. [2]

Contents

Structure

SNARE complex zoomed in to the zero ionic layer level on Pymol simulator 3-8 3Q-1R zero ionic layer structure on Pymol.jpg
SNARE complex zoomed in to the zero ionic layer level on Pymol simulator

SNARE complex is a bundle formed by 4 alpha-helical proteins, including vesicle-associated synaptobrevin and cell-membrane-associated syntaxin and SNAP. [3] When the bundle is viewed on the side, for every alpha-helical turn, the alpha-carbons from each helix form a plane, which is thus designated as a "layer". Along the helical bundle from N-terminus to C-terminus, layers are designated from "-7" to "+8" respectively. "0" layer (i.e. zero ionic layer) is at the center of the helical bundle. [2] [4]

The zero ionic layer is an ionic domain within the otherwise largely hydrophobic alpha-helical complex (SNARE complex) . It is stabilized by attractive forces(dipole-dipole interactions) between three partially negatively charged carbonyl groups of glutamine residues and a positively charged arginine. [5] Specifically, these interacting groups include Q226 on Syntaxin, Q53 on SNAP-25 (Sn1), Q174 on SNAP-25 (Sn2) and R56 on Synaptobrevin (v-SNARE). [1]

The 4 amino acids are asymmetrically arranged in the layer, as shown in the picture. However, their intensive interactions ensure the layer's stability: the arginine side chain end lies in the center of the asymmetry and amino groups form hydrogen bonds with the three glutamine residues. Thus, steric and electrostatic fit is well established. [6]

Function and research interest

SNARE proteins are a family of a proteins that are located in cell membranes to mediate any secretory pathways. [7] The complex is formed during exocytosis, a process where the vesicles inside the cell fuse with the cell membrane to secrete molecules into the extracellular space. [3] [8]

The zero ionic layer of the SNARE complex is at special interest to scientists studying SNARE because of its three characteristics. Firstly, it is the only hydrophilic region in the entire hydrophobic SNARE complex; secondly, unlike most of the other layers, it displays asymmetry; thirdly, the 3Q:1R arrangement is found in almost all of the SNARE superfamily among eukaryotic cells. [6] [4] These unique aspects imply its importance to eukaryotic organisms in general. However, the exact and functions of zero ionic layer is still under investigation. [6] [2]

Previous studies have focused on how mutations in this layer would affect the functionality of SNARE complex in secretory pathways. Even though the exact mechanism still awaits further investigation, these studies have revealed that the integrity of zero ionic layer is not essential to the proper alignment during complex formation, but it is essential to the disassociation of SNARE complex and the recycling of its 4 constituent alpha-helical proteins after exocytosis. [1] [6]

An ATPase (NSF) together with a cofactor (α-SNAP) facilitates the breakdown of the SNARE complex after the completion of exocytosis. [9] Studies have suggested that, during the disassociation process, the NSF/α-SNAP complex acts specifically on the zero ionic layer, particularly, the glutamine residue (Q226) in Syntaxin. The glutamine residue transmits the conformational change of NSF/α-SNAP complex to the SNARE complex in order to disrupt and thus disassociate the SNARE complex at the zero ionic layer. [1] [6] More specifically, even though the ionic layer is buried within the hydrophobic complex for the most part, during disassociation, NSF/α-SNAP complex may disturb the hydrophobic shielding and thus let water molecules into the core. This exposure of other hydrophilic molecules disturb the original hydrogen bonding equilibrium and thus facilitate disassembly of the alpha-helical bundle. [4]

Mutation and alternation

In studies that use exocytotic SNAREs of yeast as models, a mutation from glutamine to arginine in the zero ionic layer leads to yeast cells that have deficient growth and protein secretion ability. However, a mutation from arginine to glutamine in this layer leads to yeast cells that are functionally wild-type. [6] In the mutation where all four amino acids in the zero ionic layer are glutamine residues, the cells still exhibit normal secretory ability, but defects may become pronounced when there are other mutations. [10]

Complementary mutations, where a glutamine to arginine mutation is paired with an arginine to glutamine mutation in the zero ionic layer, have resulted in functionally wild-type yeast cells too, according to their secretory ability. [11]

These mutation studies have been done to study the role of the four amino acids in zero ionic layer. Underlying mechanisms of why these mutations would lead to certain results are not well discussed. In general, the glutamine residues in this layer are of critical importance to the functionality of mutated strains. As long as the glutamine is intact or compensated in someway during mutation, functionality of SNARE complex will be retained. [6] [10] [11]

Related Research Articles

<i>N</i>-ethylmaleimide sensitive fusion protein Protein-coding gene in Homo sapiens

N-ethylmaleimide-sensitive factor, also known as NSF or N-ethylmaleimide sensitive fusion proteins, is an enzyme which in humans is encoded by the NSF gene.

<span class="mw-page-title-main">SNARE protein</span> Protein family

SNARE proteins – "SNAPREceptors" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells, and some numbers in plants. The primary role of SNARE proteins is to mediate the fusion of vesicles with the target membrane; this notably mediates exocytosis, but can also mediate the fusion of vesicles with membrane-bound compartments. The best studied SNAREs are those that mediate the release of synaptic vesicles containing neurotransmitters in neurons. These neuronal SNAREs are the targets of the neurotoxins responsible for botulism and tetanus produced by certain bacteria.

<span class="mw-page-title-main">Synaptobrevin</span>

Synaptobrevins are small integral membrane proteins of secretory vesicles with molecular weight of 18 kilodalton (kDa) that are part of the vesicle-associated membrane protein (VAMP) family.

<span class="mw-page-title-main">SNAP25</span> Protein-coding gene in the species Homo sapiens

Synaptosomal-Associated Protein, 25kDa (SNAP-25) is a Target Soluble NSF (N-ethylmaleimide-sensitive factor) Attachment Protein Receptor (t-SNARE) protein encoded by the SNAP25 gene found on chromosome 20p12.2 in humans. SNAP-25 is a component of the trans-SNARE complex, which accounts for membrane fusion specificity and directly executes fusion by forming a tight complex that brings the synaptic vesicle and plasma membranes together.

<span class="mw-page-title-main">Complexin</span>

Complexin (also known as synaphin) refers to a one of a small set of eukaryotic cytoplasmic neuronal proteins which binds to the SNARE protein complex (SNAREpin) with a high affinity. These are called synaphin 1 and 2. In the presence of Ca2+, the transport vesicle protein synaptotagmin displaces complexin, allowing the SNARE protein complex to bind the transport vesicle to the presynaptic membrane.

<span class="mw-page-title-main">STX1A</span> Protein-coding gene in the species Homo sapiens

Syntaxin-1A is a protein that in humans is encoded by the STX1A gene.

<span class="mw-page-title-main">SNAP23</span> Protein-coding gene in the species Homo sapiens

Synaptosomal-associated protein 23 is a protein that in humans is encoded by the SNAP23 gene. Two alternative transcript variants encoding different protein isoforms have been described for this gene.

<span class="mw-page-title-main">STX4</span> Protein-coding gene in the species Homo sapiens

Syntaxin-4 is a protein that in humans is encoded by the STX4 gene.

<span class="mw-page-title-main">VAMP2</span> Protein-coding gene in the species Homo sapiens

Vesicle-associated membrane protein 2 (VAMP2) is a protein that in humans is encoded by the VAMP2 gene.

<span class="mw-page-title-main">STX6</span> Protein-coding gene in the species Homo sapiens

Syntaxin-6 is a protein that in humans is encoded by the STX6 gene.

<span class="mw-page-title-main">NAPA (gene)</span> Protein-coding gene in the species Homo sapiens

N-ethylmaleimide-sensitive factor Attachment Protein Alpha, also known as SNAP-α, is a SNAP protein that is involved in the intra-cellular trafficking and fusing of vesicles to target membranes in cells.

<span class="mw-page-title-main">VAMP3</span> Protein-coding gene in the species Homo sapiens

Vesicle-associated membrane protein 3 is a protein that in humans is encoded by the VAMP3 gene.

<span class="mw-page-title-main">STX2</span> Protein-coding gene in the species Homo sapiens

Syntaxin-2, also known as epimorphin, is a protein that in humans is encoded by the STX2 gene.

<span class="mw-page-title-main">YKT6</span> Protein-coding gene in humans

Synaptobrevin homolog YKT6 is a protein that in humans is encoded by the YKT6 gene.

<span class="mw-page-title-main">Vesicle-associated membrane protein 8</span> Protein-coding gene in the species Homo sapiens

Vesicle-associated membrane protein 8 is a protein that in humans is encoded by the VAMP8 gene.

<span class="mw-page-title-main">VAMP1</span> Protein-coding gene in the species Homo sapiens

Vesicle-associated membrane protein 1 (VAMP1) is a protein that in humans is encoded by the VAMP1 gene.

<span class="mw-page-title-main">Syntaxin 3</span> Protein-coding gene in the species Homo sapiens

Syntaxin 3, also known as STX3, is a protein which in humans is encoded by the STX3 gene.

<span class="mw-page-title-main">Syntaxin</span> Group of proteins

Syntaxins are a family of membrane integrated Q-SNARE proteins participating in exocytosis.

Munc-18 proteins are the mammalian homologue of UNC-18 and are a member of the Sec1/Munc18-like (SM) protein family. Munc-18 proteins have been identified as essential components of the synaptic vesicle fusion protein complex and are crucial for the regulated exocytosis of neurons and neuroendocrine cells.

<span class="mw-page-title-main">Soluble NSF attachment protein</span> Protein family

Soluble N-ethylmaleimide-Sensitive Factor Attachment Proteins are a family of cytosolic adaptor proteins involved in vesicular fusion at membranes during intracellular transport and exocytosis. SNAPs interact with proteins of the SNARE complex and NSF to play a key role in recycling the components of the fusion complex. SNAPs are involved in the priming of the vesicle fusion complex during assembly, as well as in the disassembly following a vesicle fusion event. Following membrane fusion, the tethering SNARE proteins complex disassembles in response to steric changes originating from the ATPase NSF. The energy provided by NSF is transferred throughout the SNARE complex and SNAP, allowing the proteins to untangle, and recycled for future fusion events. Mammals have three SNAP genes: α-SNAP, β-SNAP, and γ-SNAP. α- and γ-SNAP are expressed throughout the body, while β-SNAP is specific to the brain. The yeast homolog of the human SNAP is Sec17, the structural diagram of which is included on this page.

References

  1. 1 2 3 4 Scales, Suzie J.; Yoo, Bryan Y.; Scheller, Richard H. (2001-12-04). "The ionic layer is required for efficient dissociation of the SNARE complex by α-SNAP and NSF". Proceedings of the National Academy of Sciences. 98 (25): 14262–14267. doi: 10.1073/pnas.251547598 . ISSN   0027-8424. PMC   64670 . PMID   11762430.
  2. 1 2 3 Fasshauer, D.; Sutton, R. B.; Brunger, A. T.; Jahn, R. (1998-12-22). "Conserved structural features of the synaptic fusion complex: SNARE proteins reclassified as Q- and R-SNAREs". Proceedings of the National Academy of Sciences of the United States of America. 95 (26): 15781–15786. doi: 10.1073/pnas.95.26.15781 . ISSN   0027-8424. PMC   28121 . PMID   9861047.
  3. 1 2 Hanson, P. I.; Heuser, J. E.; Jahn, R. (June 1997). "Neurotransmitter release - four years of SNARE complexes". Current Opinion in Neurobiology. 7 (3): 310–315. doi: 10.1016/s0959-4388(97)80057-8 . ISSN   0959-4388. PMID   9232812. S2CID   35713372.
  4. 1 2 3 Sutton, R. B.; Fasshauer, D.; Jahn, R.; Brunger, A. T. (1998-09-24). "Crystal structure of a SNARE complex involved in synaptic exocytosis at 2.4 A resolution". Nature. 395 (6700): 347–353. doi:10.1038/26412. ISSN   0028-0836. PMID   9759724. S2CID   1815214.
  5. ↑ McMahon, Harvey T.; Südhof, Thomas C. (1995-02-03). "Synaptic Core Complex of Synaptobrevin, Syntaxin, and SNAP25 Forms High Affinity -SNAP Binding Site". Journal of Biological Chemistry. 270 (5): 2213–2217. doi: 10.1074/jbc.270.5.2213 . ISSN   0021-9258. PMID   7836452.
  6. 1 2 3 4 5 6 7 Ossig, Rainer; Schmitt, Hans Dieter; Groot, Bert de; Riedel, Dietmar; Keränen, Sirkka; Ronne, Hans; Grubmüller, Helmut; Jahn, Reinhard (2000-11-15). "Exocytosis requires asymmetry in the central layer of the SNARE complex". The EMBO Journal. 19 (22): 6000–6010. doi:10.1093/emboj/19.22.6000. ISSN   0261-4189. PMC   305815 . PMID   11080147.
  7. ↑ Götte, M (1998). "A new beat for the SNARE drum". Trends in Cell Biology. 8 (6): 215–218. doi:10.1016/s0962-8924(98)01272-0. PMID   9695844.
  8. ↑ Söllner, Thomas; Whiteheart, Sidney W.; Brunner, Michael; Erdjument-Bromage, Hediye; Geromanos, Scott; Tempst, Paul; Rothman, James E. (March 1993). "SNAP receptors implicated in vesicle targeting and fusion". Nature. 362 (6418): 318–324. doi:10.1038/362318a0. ISSN   1476-4687. PMID   8455717. S2CID   4366054.
  9. ↑ Chang, Sunghoe; Girod, Romain; Morimoto, Takako; O’Donoghue, Michael; Popov, Sergey (1998). "Constitutive Secretion of Exogenous Neurotransmitter by Nonneuronal Cells: Implications for Neuronal Secretion". Biophysical Journal. 75 (3): 1354–1364. doi:10.1016/s0006-3495(98)74053-6. PMC   1299809 . PMID   9726936.
  10. 1 2 Katz, L.; Brennwald, P. (November 2000). "Testing the 3Q:1R "rule": mutational analysis of the ionic "zero" layer in the yeast exocytic SNARE complex reveals no requirement for arginine". Molecular Biology of the Cell. 11 (11): 3849–3858. doi:10.1091/mbc.11.11.3849. ISSN   1059-1524. PMC   15041 . PMID   11071911.
  11. 1 2 Graf, Carmen T.; Riedel, Dietmar; Schmitt, Hans Dieter; Jahn, Reinhard (2005-05-01). "Identification of Functionally Interacting SNAREs by Using Complementary Substitutions in the Conserved '0' Layer". Molecular Biology of the Cell. 16 (5): 2263–2274. doi:10.1091/mbc.e04-09-0830. ISSN   1059-1524. PMC   1087233 . PMID   15728725.
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