About STX / Syntaxin:
Syntaxin is a protein that in humans is encoded by the STXN gene. It functions as an essential part of the membrane dynamics and fusion processes within cells, including neurotransmitter release from synapses. Syntaxin also plays a role in mitosis (cell division). The syntaxin molecule has three domains: SNARE proteins for vesicle docking and fusion, a coiled-coil region containing the SNARE motifs which facilitate binding to SNAP-25 (another protein), and an N-terminal amphipathic helix that binds membrane phospholipids.
Syntaxin facilitates neurotransmitter release from presynaptic neurons or cells by allowing the fusion of neurotransmitter-containing vesicles with the plasma membrane.
Syntaxin is typically assembled as a four-helix bundle. It forms a coiled-coil structure consisting of an amphipathic alpha-helix, which binds to phospholipids in cell membranes via its lipid anchor sequence (VFGXG). This helix is connected to the SNARE complex by an extended coil region (RX-L/F-Y) that forms a coiled-coil with another syntaxin molecule. The long alpha-helices of Syntaxin are held together in the bundle by hydrophobic interactions between the nonpolar side chains on each helical turn.
Syntaxin interacts with SNAP-25 via the amphipathic helices that bind to membrane phospholipids and thus allows for the association of these two proteins in creating a fusion pore used to allow neurotransmitters to pass through the plasma membrane into the synaptic cleft. It also binds to N-ethylmaleimide sensitive fusion protein (NSF), a ubiquitin ligase that catalyzes the transfer of ubiquitin to SNAP-25 and Syntaxin. This modification of these proteins is necessary for their role in neurotransmitter release and membrane fusion.