Syntaxins (syntaxin isotypes 1-4) are membrane integrated Q-SNARE proteins participating in exocytosis. Syntaxins possess a single transmembrane domain known as H3. The C-terminal H3 domain binds to both synaptobrevin and SNAP-25 forming the core SNARE complex. The N-terminal Habc domain is formed by 3 $\backslash alpha$-helices and when collapsed onto its own H3 helix forms an inactive "closed" syntaxin conformation. The "open" syntaxin conformation is aided by nSec1, that activates syntaxin by detaching the Habc domain from the H3 helix. This interaction is inhibited by phosphorylation of nSec1, suggesting a role for protein modification in the fine tuning of vesicle docking and vesicle fusion.

Syntaxins bind synaptotagmin in a calcium-dependent fashion and interact with voltage-gated calcium channels (VGCCs) via the C-terminal H3 domain. Direct syntaxin-VGCC interaction is a suitable molecular mechanism for proximity between the fusion machinery and the gates of $Ca^\{2+\}$ entry during depolarization of the presynaptic axonal boutons.

In vitro syntaxin per se is sufficient to drive spontaneous calcium independent fusion of synaptic vesicles containing v-SNAREs. This is consistent with a circular array of 5-8 syntaxin molecules forming the fusion pore of exocytosis.

References

• Han X, Wang CT, Bai J, Chapman ER, Jackson MB (2004). Transmembrane Segments of Syntaxin Line the Fusion Pore of Ca2+-Triggered Exocytosis. Science 304: 289-292.

• Woodbury DJ, Rognlien K. (2000). The t-SNARE syntaxin is sufficient for spontaneous fusion of synaptic vesivles to planar membranes. Cell Biology International 24(11): 809-818.

Proteins | Syntaxin