Chaperoning SNARE Folding and Assembly

Abstract

SNARE proteins and Sec1/Munc18 (SM) proteins constitute the core molecular engine that drives nearly all intracellular membrane fusion and exocytosis. While SNAREs are known to couple their folding and assembly to membrane fusion, the physiological pathways of SNARE assembly and the mechanistic roles of SM proteins have long been enigmatic. Here, we review recent advances in understanding the SNARE–SM fusion machinery with an emphasis on biochemical and biophysical studies of proteins that mediate synaptic vesicle fusion. We begin by discussing the energetics, pathways, and kinetics of SNARE folding and assembly in vitro. Then, we describe diverse interactions between SM and SNARE proteins and their potential impact on SNARE assembly in vivo. Recent work provides strong support for the idea that SM proteins function as chaperones, their essential role being to enable fast, accurate SNARE assembly. Finally, we review the evidence that SM proteins collaborate with other SNARE chaperones, especially Munc13-1, and briefly discuss some roles of SNARE and SM protein deficiencies in human disease. Expected final online publication date for the Annual Review of Biochemistry, Volume 90 is June 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling

“…Unc18 is universally required for eukaryotic membrane fusion (Südhof and Rothman, 2009), while Unc13 functions only in regulated secretion (Aravamudan et al, 1999;. Both proteins act by controlling Syx1 availability and chaperoning SNARE complex assembly (Zhang and Hughson, 2021). Syx1 contains four α-helical domains with only the most C-terminal helix (termed the H3 domain) participating in SNARE complex formation (Fernandez et al, 1998;Sutton et al, 1998;Wu et al, 1999).…”

Single-Molecule Manipulation of Macromolecules on Membranes Using High-Resolution Optical Tweezers

“…This motivated us to examine the lateral mobility of integral membrane proteins in the lipid bilayers coated on silica beads using fluorescence recovery after photobleaching (FRAP). To this end, we chose VAMP2, a SNARE protein of 116 amino acids in length with a single C-terminal transmembrane domain, as a representative for integral membrane proteins (3). We labeled VAMP2 with the Alexa Fluor dye and reconstituted the protein into the bilayer on the surface of a silica bead µm in diameter.…”

“…Numerous biological processes on membranes involve complex protein-protein and proteinmembrane interactions that are further regulated by mechanical force. These processes include membrane protein folding (1,2), membrane fusion or lipid exchange (3)(4)(5), cell migration (6), immune responses (7), mechanosensation (8)(9)(10), and mechanotransduction (1,5,(8)(9)(10)(11). These interactions are difficult to study using traditional experimental approaches based on a large number molecules due to ensemble averaging or lack of mechanical force (3)(4)(5)(12)(13)(14).…”

“…These processes include membrane protein folding (1,2), membrane fusion or lipid exchange (3)(4)(5), cell migration (6), immune responses (7), mechanosensation (8)(9)(10), and mechanotransduction (1,5,(8)(9)(10)(11). These interactions are difficult to study using traditional experimental approaches based on a large number molecules due to ensemble averaging or lack of mechanical force (3)(4)(5)(12)(13)(14). Singlemolecule force spectroscopy, including atomic force microscopy (AFM), optical tweezers, and magnetic tweezers, has widely been applied to study dynamics of soluble proteins (15)(16)(17)(18)(19)(20)(21)(22).…”

Epileptic Phenotypes Associated With SNAREs and Related Synaptic Vesicle Exocytosis Machinery

“…The core SNARE machinery consists of VAMP2 (synaptobrevin-2), the only vesicular binding SNARE (v-SNARE) of the complex and a combination of target membrane SNARE proteins (t-SNAREs): syntaxin1-A (STX1A) and synaptosomal-associated protein 25 kD (SNAP25) ( 9 ). The assembly of the SNARE machinery is carefully arranged by the assembly complex, which is composed of MUNC18-1 and MUNC13-1 and plays an essential role in the fusion of synaptic vesicles ( 10 ). Pathogenic biallelic and monoallelic variants disrupting proteins of the SNARE complex and associated regulators are a known cause for neurodevelopmental disorders consisting of an overlapping phenotype of developmental delay (DD), intellectual disability (ID), movement disorders, and epilepsy.…”