Munc18a clusters SNARE-bearing liposomes prior to trans-SNARE zippering

Abstract

Sec1-Munc18 (SM) proteins cooperate with SNAREs {SNAP [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein] receptors} to mediate membrane fusion in eukaryotic cells. Studies of Munc18a/Munc18-1/Stxbp1 in neurotransmission suggest that SM proteins accelerate fusion kinetics primarily by activating the partially zippered trans-SNARE complex. However, accumulating evidence has argued for additional roles for SM proteins in earlier steps in the fusion cascade. Here we investigate the function of Munc18a in reconstituted exocytic reactions mediated by neuronal and non-neuronal SNAREs. We show that Munc18a plays a direct role in promoting proteoliposome clustering, underlying vesicle docking during exocytosis. In the three different fusion reactions examined, Munc18a-dependent clustering requires an intact N-terminal peptide (N-peptide) motif in syntaxin that mediates the binary interaction between syntaxin and Munc18a. Importantly, clustering is preserved under inhibitory conditions that abolish both trans-SNARE complex formation and lipid mixing, indicating that Munc18a promotes membrane clustering in a step that is independent of trans-SNARE zippering and activation.

Gatekeeper helix activates Golgi SM protein Sly1 and directly mediates close-range vesicle tethering

“…In an interesting parallel, an ALPS-like domain within the HOPS subunit Vps41 was proposed to select high-curvature endocytic vesicles for docking and fusion (Cabrera et al, 2010). We also note that Munc18-1, despite lacking the Sly1-specific regulatory loop, is 430 reported to tether vesicles in a reaction that requires at least the Qa-SNARE N-peptide and the R-SNARE on the opposing membrane (Arnold et al, 2017;Tareste et al, 2008). It is not clear whether Munc18-1 mediated tethering entails a direct interaction between Munc18-1 and the vesicle bilayer.…”

Synapse-specific trapping of Syntaxin1a into nanoclusters by the general anesthetic isoflurane

“…Decreases in diffusion could be due some interaction with the Sx1a/Rop complex (Han et al, 2010) whereas clustering of Sx1a could occur post-recruitment to an active zone, without Rop involvement. It is for example understood that Munc18a chaperones Sx1a to the plasma membrane, before both proteins terminate their journey to active zones (Arnold et al, 2017; Kasula et al, 2016; Lai et al, 2017; Shu et al, 2019; Yang et al, 2022).…”