Syntaxin-1A modulates vesicle fusion in mammalian neurons via juxtamembrane domain dependent palmitoylation of its transmembrane domain
SNAREs are undoubtedly one of the core elements of synaptic transmission. On the contrary to the well characterized function of their SNARE domains bringing the plasma and vesicular membranes together, the level of contribution of their juxtamembrane domain (JMD) and the transmembrane domain (TMD) to the vesicle fusion is still under debate. To elucidate this issue, we analyzed three groups of STX1A mutations: 1) elongation of STX1As JMD by three amino acid insertions in the junction of SNARE-JMD or JMD-TMD; 2) charge reversal mutations in STX1As JMD; and 3) palmitoylation deficiency mutations in STX1As TMD. We found that both JMD elongations and charge reversal mutations have position dependent differential effects on Ca2+-evoked and spontaneous neurotransmitter release in cultured murine hippocampal neurons. Importantly, we show that STX1As JMD regulates the palmitoylation of STX1As TMD and loss of STX1A palmitoylation either through charge reversal mutation K260E or by loss of TMD cysteines particularly inhibits spontaneous vesicle fusion. Interestingly, the retinal ribbon specific STX3B has a glutamate in the position corresponding to the K260E mutation in STX1A and mutating it with E259K acts as a molecular on-switch. Furthermore, palmitoylation of post-synaptic STX3A can be induced by the exchange of its JMD with STX1As JMD together with the incorporation of two cysteines into its TMD. Forced palmitoylation of STX3A dramatically enhances spontaneous vesicle fusion suggesting that STX1A regulates spontaneous release through two distinct mechanisms: one through the C-terminal half of its SNARE domain and the other through the palmitoylation of its TMD.