SNARE chaperone Sly1 directly mediates close-range vesicle tethering.

The essential Golgi protein Sly1 is a member of the Sec1/mammalian Unc-18 (SM) family of SNARE chaperones. Sly1 was originally identified through remarkable gain-of-function alleles that bypass requirements for diverse vesicle tethering factors. Employing genetic analyses and chemically defined reconstitutions of ER-Golgi fusion, we discovered that a loop conserved among Sly1 family members is not only autoinhibitory but also acts as a positive effector. An amphipathic lipid packing sensor (ALPS)-like helix within the loop directly binds high-curvature membranes. Membrane binding is required for relief of Sly1 autoinhibition and also allows Sly1 to directly tether incoming vesicles to the Qa-SNARE on the target organelle. The SLY1-20 mutation bypasses requirements for diverse tethering factors but loses this ability if the tethering activity is impaired. We propose that long-range tethers, including Golgins and multisubunit tethering complexes, hand off vesicles to Sly1, which then tethers at close range to initiate trans-SNARE complex assembly and fusion in the early secretory pathway.

Spatially regulated activation of membrane fusogenic peptides with chaperone-like ionic copolymers.

Controlled or targeted membrane lysis induced by cascades of assembly and activation of biomolecules on membrane surfaces is important in programmed cell death and host defense systems. In a previous study, we reported that an ionic graft copolymer with a polycation backbone and water-soluble graft chains, poly(allylamine)-graft-dextran (PAA-g-Dex) chaperoned folding and assembly of E5, a membrane-destructive peptide derived from influenza hemagglutinin, to its increase membrane-disruptive activity. In this study, we modified the copolymer with long acyl chains, which resulted in delivery of the copolymer to membrane surfaces of liposomes and living cells. The liposomes with PAA-g-Dex functionalized with stearic acid (PAA-g-Dex-SA) on their surfaces underwent vesicle-to-sheet conversion upon addition of E5, whereas control liposomes did not. E5 also induced selective lysis of cells incubated with PAA-g-Dex-SA. The spatially specific activation of E5 on target membrane surfaces driven by self-assembly of copolymer and activation of E5 should find application in lipid-based delivery devices and cell-based therapeutics.

Cationic Copolymer-Chaperoned 2D-3D Reversible Conversion of Lipid Membranes.

Nanosheets have thicknesses on the order of nanometers and planar dimensions in the micrometer range. Nanomaterials that are capable of converting reversibly between 2D nanosheets and 3D structures in response to specific triggers can enable construction of nanodevices. Supra-molecular lipid nanosheets and their triggered conversions to 3D structures including vesicles and cups are reported. They are produced from lipid vesicles upon addition of amphiphilic peptides and cationic copolymers that act as peptide chaperones. By regulation of the chaperoning activity of the copolymer, 2D to 3D conversions are reversibly triggered, allowing tuning of lipid bilayer structures and functionalities.