Functionally and Spatially Distinct Modes of munc18-Syntaxin 1 Interaction

Colin Rickman,Claire N Medine,Axel Bergmann,Rory R Duncan

doi:10.1074/jbc.m700227200

Colin Rickman, Claire N Medine + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m700227200

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Abstract

Eukaryotic membrane trafficking is a conserved process under tight temporal and spatial regulation in which the fusion of membranes is driven by the formation of the ternary SNARE complex. Syntaxin 1a, a core component of the exocytic SNARE complex in neurons and neuroendocrine cells, is regulated directly by munc18-1, its cognate Sec1p/munc18 (SM) protein. SM proteins show remarkable structural conservation throughout evolution, indicating a common binding mechanism and function. However, SM proteins possess disparate binding mechanisms and regulatory effects with munc18-1, the major brain isoform, classed as atypical in both its binding specificity and its mode. We now show that munc18-1 interacts with syntaxin 1a through two mechanistically distinct modes of binding, both in vitro and in living cells, in contrast to current models. Furthermore, these functionally divergent interactions occur at distinct cellular locations. These findings provide a molecular explanation for the multiple, spatially distinct roles of munc18-1.

Highlights

Nary SNARE complex [8, 9]
Using quantitative colocalization and fluorescence lifetime imaging microscopy (FLIM), we show that these two binding modes of syntaxin1 and munc18-1 are spatially segregated in living cells
There is a high degree of similarity between the syntaxin homologues throughout the sequence corresponding to the SNARE helix and head domain of syntaxin

Summary

Introduction

Nary SNARE complex [8, 9]. Syntaxin 1 can act as a molecular switch, adopting two structurally distinct forms [10]. Using quantitative colocalization and fluorescence lifetime imaging microscopy (FLIM), we show that these two binding modes of syntaxin1 and munc18-1 are spatially segregated in living cells. This is the first demonstration that the major neuronal and neuroendocrine SNARE, syntaxin 1, and its cognate SM protein, munc18-1, can interact through dual modes of binding.

Results

Conclusion