Munc18-1 prevents the formation of ectopic SNARE complexes in living cells.
Membrane trafficking in eukaryotic cells must be strictly regulated both temporally and spatially. The assembly at the plasma membrane of the ternary SNARE complex, formed between syntaxin1a, SNAP-25 and VAMP, is essential for efficient exocytotic membrane fusion. These exocytotic SNAREs are known to be highly promiscuous in their interactions with other non-cognate SNAREs. It is therefore an important cellular requirement to traffic exocytotic SNARE proteins through the endoplasmic reticulum and Golgi complex while avoiding ectopic interactions between SNARE proteins. Here, we show that syntaxin1a traffics in an inactive form to the plasma membrane, requiring a closed-form interaction, but not N-terminal binding, with munc18-1. If syntaxin is permitted to interact with SNAP-25, both proteins fail to traffic to the plasma membrane, becoming trapped in intracellular compartments. The munc18-1-syntaxin interactions must form before syntaxin encounters SNAP-25 in the Golgi complex, preventing the formation of intracellular exocytotic SNARE complexes there. Upon delivery to the plasma membrane, most SNARE clusters in resting cells do not produce detectable FRET between t-SNARE proteins. These observations highlight the crucial role that munc18-1 plays in trafficking syntaxin through the secretory pathway.
Centre for Integrative Physiology, University of Edinburgh, George Square, Edinburgh EH8 9XD, UK.
Claire N
CN
Medine
Colin
C
Rickman
Luke H
LH
Chamberlain
Rory R
RR
Duncan
D000818:Animals; D002460:Cell Line; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism*; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D051379:Mice; D018613:Microscopy, Confocal; D046912:Multiprotein Complexes / Q000378:metabolism*; D051938:Munc18 Proteins / Q000378:metabolism*; D021381:Protein Transport; D050765:Qa-SNARE Proteins / Q000378:metabolism*; D051381:Rats; D050600:SNARE Proteins / Q000378:metabolism*; D050825:Synaptosomal-Associated Protein 25 / Q000378:metabolism
D046912:Multiprotein Complexes; D051938:Munc18 Proteins; D050765:Qa-SNARE Proteins; D050600:SNARE Proteins; D050825:Synaptosomal-Associated Protein 25