Structural Interactome of the Yeast Dsl1 Complex

Abstract

Multisubunit tethering complexes (MTCs) are essential positive regulators of membrane fusion at all eukaryotic organelles in the secretory and endolysosomal system. Although this family of protein complexes is incredibly diverse, all MTCs have affinity for receptors on two different membranes, leading to the hypothesis that MTCs mediate tethering, the first step in membrane fusion. Yet understanding the mechanism of these large and often highly dynamic protein complexes has been hindered by a dearth of biochemical reconstitution systems and high resolution structural information. Previously, through a combination of X-ray crystallography and negative stain electron microscopy, an atomic resolution model of 90% of the yeast Dsl1 complex was reported. This model represented a landmark for MTCs in terms of resolution and completeness. However, because structures were unavailable of Dsl1 with receptors on either the vesicle or target organelle membrane, many questions still remained as to Dsl1’s mechanism of action. In this work, we report the structure of a Dsl1 subunit bound to the COPI vesicle coat and to two ER-localized SNAREs. These structures support the hypothesis that Dsl1 acts as a vesicle tether. They also suggest novel mechanisms by which Dsl1 could sense or even compete with COPI coat assembly, as well as regulate the entry of SNAREs into the fusogenic SNARE bundle.