Syntaxin

Botulinum neurotoxins (A-G), tetanus toxin Synaptic peptides a) Synapto-brevin b) Syntaxin c) SNAP25 Zinc dependent endoprotease Cleavage of synaptic peptides Inhibition of transmitter release (tetanus, botulism)... 

The light chains of the clostridial neurotoxins are metalloproteases with exclusive specificity for neuronal SNAREs. TeNT, BoNTs B,D,F, and G cleave synapto-brevin, BoNTs A and E SNAP-25, and BoNT/Cl syntaxin, and to a lesser extent also SNAP-25. Cleavage of any of the SNAREs causes complete and irreversible block of synaptic transmission. 

A second mechanism that impinges on the localization of transporters is through the association with proteins, the most prominent example being syntaxin. Syntaxin is a t-SNARE protein necessary for the fusion of vesicles with the plasma membrane (see the chapter on exocytosis). On the cell surface syntaxin consistently stabilizes the localization of GABA, noradrenaline, glycine, and 5HT transporters the PKCa isoform can sever the interaction with syntaxin suggesting a general mechanism for transporter internalization. 

Much evidence supports a role for these proteins in exocytosis. For instance, injection of recombinant SNAP into the squid giant axon increases vesicular exocytosis. Also, membrane SNAP-25 and syntaxin are both targets for botulinum toxin while the vesicule protein, synaptobrevin, is a target for tetanus and botulinum toxins both these toxins are well known for disrupting transmitter release. 

SATA has been used to form conjugates with avidin or steptavidin with excellent retention of activity (Chapter 23, Section 3.1). It also has been used in the formation of a therapeutically useful toxin conjugate with recombinant CD4 (Ghetie et al., 1990), to study syntaxin proteins (Amessou et al., 2007), to prepare bispecific antibodies (Lindorfer et al., 2001), and to make a unique polylysine conjugate as a vehicle for drug delivery (Sakharov et al., 2001). 

Amessou, M., Fradagrada, A., Falguieres, T., Lord, J.M., Smith, D.C., Roberts, L.M., Lamaze, C., and Johannes, L. (2007) Syntaxin 16 and syntaxin 5 are required for efficient retrograde transport of several exogenous and endogenous cargo proteins./. Cell Sci. 120, 1457-1468. 

Synaptotagmins Membrane proteins with at least 15 isoforms that contain C2 domains bind Ca2+ and phospholipids and interact with neurexins, AP2 and syntaxins. Synaptotagmins 1 and 2 may function as Ca2+ sensors in fast Ca2+-dependent neurotransmitter release... 

Neurexins a protein kinase. A component of active zones that interact with RIM, syntaxin and other proteins. Cell surface proteins with more than 1,000 isoforms generated by alternative splicing from three genes. Neurexins include one of the receptors for aratrotoxin and may function in cell-cell recognition between neurons. 

SNAP-25 Syntaxins Paimitoylated peripheral membrane protein that is cleaved by botulinum toxins A and E and binds to syntaxins. Ubiquitous membrane proteins that are cleaved by botulinum toxin Cl and bind to synaptotagmins, SNAP-25,... 

SuEE, S., Misea, S., Saidi, L. F., and Hurley, J. H., Structure of the GAT domain of human GGAl a syntaxin amino-terminal domain fold in an endosomal trafScking adaptor, Proc. Natl. Acad. Sci. USA, 2003, 100, 4451. 

All botulin neurotoxins act in a similar way. They only differ in the amino-acid sequence of some protein parts (Prabakaran et al., 2001). Botulism symptoms are provoked both by oral ingestion and parenteral injection. Botulin toxin is not inactivated by enzymes present in the gastrointestinal tracts. Foodborne BoNT penetrates the intestinal barrier, presumably due to transcytosis. It is then transported to neuromuscular junctions within the bloodstream and blocks the secretion of the neurotransmitter acetylcholine. This results in muscle limpness and palsy caused by selective hydrolysis of soluble A-ethylmalemide-sensitive factor activating (SNARE) proteins which participate in fusion of synaptic vesicles with presynaptic plasma membrane. SNARE proteins include vesicle-associated membrane protein (VAMP), synaptobrevin, syntaxin, and synaptosomal associated protein of 25 kDa (SNAP-25). Their degradation is responsible for neuromuscular palsy due to blocks in acetylcholine transmission from synaptic terminals. In humans, palsy caused by BoNT/A lasts four to six months. 

Schiavo, G., Shone, C.C., Bennett, M.K., Scheller, R.H. and Montecucco, C.M., Botulinum neurotoxin type C cleaves a single Lys-Ala bond within the carboxyl-terminal region of syntaxins, /. Biol Chem., 270, 10566-10570, 1995. 

Darios, F. and Davletov, B. (2006). Omega-3 and omega-6 fatty acids stimulate cell membrane expansion by acting on syntaxin 3. Nature 440, 813-817. 

Other proteins are also needed. All cell fusion processes seem to require regulatory proteins that are essential to neurotransmission in the nematode C. elegans. Two of these are encoded by the nematode genes unc-13 and unc-18. The corresponding mammalian proteins munc-13 and munc-18 interact with syntaxin and are essential for exocytosis of synaptic vesicles.572 575 An ATPase is also needed for correct functioning of the SNARE complex574 as are other additional proteins.570... 

Shao X, Li C, Fernandez I, Zhang X, Sudhof TC, Rizo J. 1997. Synaptotagmin-syntaxin interaction the C2 domain as a Ca2+-dependent electrostatic switch. Neuron 18(1) 133—142. 

Sutton KG, McRory JE, Guthrie H, Murphy TH, Snutch TP (1999) P/Q-type calcium channels mediate the activity-dependent feedback of syntaxin-lA. Nature 401 800-804. 

Synaptic exocytosis involves three SNARE proteins the R-SNARE synaptobrevin/VAMP (isoforms 1 and 2) on the vesicle, and the Q-SNAREs syntaxin (isoforms 1 and 2) and SNAP-25 on the plasma membrane (Figure 4). Since SNAP-25 has two SNARE-motifs, synaptobrevin, syntaxin, and SNAP-25 together have four SNARE-motifs. Synaptobrevins and SNAP-25 are relatively simple SNARE proteins that are composed of little else besides SNARE motifs and membrane-attachment sequences (a transmembrane region for synaptobrevin, and a cysteine-rich palmitoylated sequence for SNAP-25). Syntaxins, in contrast, are complex proteins. The N-terminal two-thirds of syntaxins include a separate, autonomously folded domain (the so-called Habc-domain), while the C-terminal third is composed of a SNARE motif and transmembrane region just like synaptobrevin. 

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