Reconciling the regulatory role of Munc18 proteins in SNARE-complex assembly

Abstract

Membrane fusion is essential for human health, playing a vital role in processes as diverse as neurotransmission and blood glucose control. Two protein families are key: (1) the Sec1p/Munc18 (SM) and (2) the soluble N-ethylmaleimidesensitive attachment protein receptor (SNARE) proteins. Whilst the essential nature of these proteins is irrefutable, their exact regulatory roles in membrane fusion remain controversial. In particular, whether SM proteins promote and/or inhibit the SNARE-complex formation required for membrane fusion is not resolved. Crystal structures of SM proteins alone and in complex with their cognate SNARE proteins have provided some insight, however, these structures lack the transmembrane spanning regions of the SNARE proteins and may not accurately reflect the native state. Here, we review the literature surrounding the regulatory role of mammalian Munc18 SM proteins required for exocytosis in eukaryotes. Our analysis suggests that the conflicting roles reported for these SM proteins may reflect differences in experimental design. SNARE proteins appear to require C-terminal immobilization or anchoring, for example through a transmembrane domain, to form a functional fusion complex in the presence of Munc18 proteins.

Granulopoiesis and granules of human neutrophils

“…Such flickers invariable end in no‐fusion, indicating that a substance needed to complete fusion is lacking . Such could be one of the proteins known to assist the SNARE complex in mediating fusion as discussed later. However, it has not been possible to identify the substance required, nor whether this is a specific problem for any particular granule subset.…”

“…The MUNC‐18 family regulates SNARE dependent fusion, but whether inhibitory to or supportive of fusion, has not been entirely clear. Consensus seems to dictate that the three known Munc‐18s are essential for fusion . Munc‐2 and Munc‐3 are present in peroxidase negative and peroxidase positive granules of neutrophils, respectively , but Munc‐18s are not known to dictate specificity of the interaction of the fusing partners.…”

The nuts and bolts of the platelet release reaction

“…In mammals, the seven SM proteins act as syntaxin chaperones, targeting and directing the t-SNARES to form specific SNARE complexes [50, 51]. Platelets contain VPS33a and 33b, which are involved in dense and α-granule biogenesis, respectively [52, 53].…”

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

“…In fact, immunoprecipitation experiments and direct binding assays all appear to have excluded Munc18a as a likely partner for syntaxin4 (41, 65), despite the fact that Munc18a shows (slightly) stronger affinity for the N-peptide of syntaxin4 than that of syntaxin1 (66). Since detergent lysates or soluble domains of syntaxin4 were used in the majority of the binding studies, which might introduce unexpected artifacts (25), we decided to use full-length syntaxin4 anchored in lipid bilayers as an alternative platform in our binding assay. We incubated His 6 -tagged Munc18a with liposomes bearing either syntaxin3 or syntaxin4 overnight at 4°C.…”