Syntaxin

Syntaxins ("syntaxin isotypes 1-4") are membrane integrated Q-SNARE proteins participating in exocytosis.

Domains

Syntaxins possess a single C-terminal transmembrane domain, a SNARE domain (known as H3), and an N-terminal regulatory domain (Habc).
* The SNARE (H3) domain binds to both synaptobrevin and SNAP-25 forming the core SNARE complex. Formation of this incredibly stable SNARE core complex is believed to generate the free energy required to initiate fusion between the vesicle membrane and plasma membrane.
* The N-terminal Habc domain is formed by 3 $alpha$-helices and when collapsed onto its own H3 helix forms an inactive "closed" syntaxin conformation. This closed conformation of syntaxin is believed to be stabilized by binding of nSec1 (Munc18), although more recent data suggests that nSec1 may bind to other conformations of syntaxin, as well. The "open" syntaxin conformation is the conformation that is competent to form into SNARE core complexes.

Binding

Syntaxins bind synaptotagmin in a calcium-dependent fashion and interact with voltage dependent calcium and potassium channels via the C-terminal H3 domain. Direct syntaxin-channel 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. Interestingly a N-terminal cleavage product of Syntaxin 3 (STX3) interacts very strong with RanBP5 and might act in nuclear signaling. The helices might play an important role in this aspect. (STX3 is the only syntaxin which contains a cleavage site)Recently published data show that alternative spliced Syntaxin 1 (STX1B) which lacks the transmembrane domain localizes in the nuclei (Nuclear localization of a novel human syntaxin 1B isoform. Pereira et al., Gene 2008) similar like STX3.

Function

"In vitro" syntaxin per se is sufficient to drive spontaneous calcium independent fusion of synaptic vesicles containing v-SNAREs.cite journal
author = Woodbury DJ, Rognlien K
url = http://pdbio.byu.edu/Faculty/djw82/Woodbury%20Rognlien%202000%20CBI%20color.pdf
title = The t-SNARE syntaxin is sufficient for spontaneous fusion of synaptic vesicles to planar membranes
journal = Cell Biology International
volume = 24
issue = 11
pages = 809–818
year = 2000
doi = 10.1006/cbir.2000.0631] More recent and somewhat controversial amperometric data suggest that the transmembrane domain of Syntaxin1A may form part of the fusion pore of exocytosis.cite journal
author = Han X, Wang CT, Bai J, Chapman ER, Jackson MB
url = http://www.physiology.wisc.edu/pubs/Han_et_al._Science_2004.pdf
title = Transmembrane Segments of Syntaxin Line the Fusion Pore of Ca2+-Triggered Exocytosis
journal = Science
volume = 304
issue =
pages = 289–292
year = 2004
doi = 10.1126/science.1095801
pmid = 15016962]

Genes

* STX1A, STX1B, STX2, STX3, STX4, STX5, STX6, STX7, STX8, STX10, STX11, STX12, STX16, STX17, STX18, STX19

References and notes

External links

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