Gatekeeper helix activates Golgi SM protein Sly1 and directly mediates close-range vesicle tethering

The essential Golgi protein Sly1 is a member of the SM (Sec1/mammalian Unc-18) family of SNARE chaperones. Sly1 was originally identified through 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 helix within the loop directly binds high-curvature membranes; membrane binding is required for relief of Sly1 autoinhibition and allows Sly1 to directly tether incoming vesicles to the Qa-SNARE on the target organelle. The SLY1-20 allele bypasses requirements for diverse tethering factors but loses this functionality if Sly1 membrane binding 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 activate SNARE assembly and fusion in the early secretory pathway.