Syntaxin clusters at secretory granules in a munc18-bound conformation

Abstract

Syntaxin (stx)-1 is an integral plasma membrane protein that is crucial for two distinct steps of regulated exocytosis, docking of secretory granules at the plasma membrane and membrane fusion. During docking, stx1 clusters at the granule docking site, together with the S/M protein munc18. Here we determined features of stx1 that contribute to its clustering at granules. In live insulin-secreting cells, stx1 and stx3 (but not stx4 or stx11) accumulated at docked granules, and stx1 (but not stx4) rescued docking in cells expressing botulinum neurotoxin-C. Using a series of stx1 deletion mutants and stx1/4 chimeras, we found that all four helical domains (Ha, Hb, Hc, SNARE) and the short N-terminal peptide contribute to recruitment to granules. However, only the Hc domain confers specificity, and it must be derived from stx1 for recruitment to occur. Point mutations in the Hc or the N-terminal peptide designed to interfere with binding to munc18-1 prevent stx1 from clustering at granules, and a mutant munc18 deficient in binding to stx1 does not cluster at granules. We conclude that stx1 is recruited to the docking site in a munc18-1-bound conformation, providing a rationale for the requirement for both proteins for granule docking.

Simultaneous Release of Multiple Vesicles from Rods Involves Synaptic Ribbons and Syntaxin 3B

“…Similarly, retinal ribbon synapses of bipolar and photoreceptor cells use syntaxin 3B rather than the syntaxin 1 isoform used by most neurons (39,40). Although syntaxin is principally expressed on target membranes, it can also be found on vesicular membranes (41)(42)(43), including those of ribbon synapses (44). The close proximity of vesicles tethered next to one another on a synaptic ribbon (45) could potentially facilitate vesiclevesicle fusion.…”

Molecular Evolution and Characterization of Fish Stathmin Genes

“…The release of glutamate from neurons in syt knock-out mice decreased significantly, suggesting that the release of presynaptic neurotransmitters plays an important function [ 51 ]. Stx regulates exocytosis, membrane fusion, and the docking of secretory particles on the plasma membrane [ 52 ]. Snap25 is a presynaptic membrane protein specifically expressed in neuron cells [ 53 ].…”

Isolation and Proteomics of the Insulin Secretory Granule

“…Only 1–2% of total ISG content is released upon a single glucose stimulation [ 12 ]. Plasma membrane proximity [ 12 , 13 ] and docking [ 14 , 15 ] have long been suggested to contribute to an ISG’s secretory capacity. More recently, ISG motility [ 16 ] and age [ 16 , 17 , 18 ] have also been shown to significantly contribute to an ISG’s propensity for translocation to the plasma membrane and its necessity for docking [ 16 , 18 ].…”