Double NPY motifs at the N-terminus of Sso2 synergistically bind Sec3 to promote membrane fusion
Exocytosis is an active vesicle trafficking process by which eukaryotes secrete materials to the extracellular environment and insert membrane proteins into the plasma membrane. The final step of exocytosis in yeast involves the assembly of two t-SNAREs, Sso1/2 and Sec9, with the v-SNARE, Snc1/2, on secretory vesicles. The rate-limiting step in this process is the formation of a binary complex of the two t-SNAREs. Despite a previous report of acceleration of binary complex assembly by Sec3, it remains unknown how Sso2 is efficiently recruited to the vesicle-docking site marked by Sec3. Here we report a crystal structure of the pleckstrin homology (PH) domain of Sec3 in complex with a nearly full-length version of Sso2 lacking only its C-terminal transmembrane helix. The structure shows a previously uncharacterized binding site for Sec3 at the N-terminus of Sso2, consisting of two highly conserved triple residue motifs (NPY: Asn-Pro-Tyr). We further reveal that the two NPY motifs bind Sec3 synergistically, which together with the previously reported binding interface constitute dual-site interactions between Sso2 and Sec3 to drive the fusion of secretory vesicles at target sites on the plasma membrane. SignificanceSNARE assembly, which involves one v-SNARE with two t-SNARE proteins, drives the fusion of vesicles to target compartments. The rate-limiting step in SNARE assembly is the assembly of the two t-SNARE proteins on the target membrane. Previous studies in yeast showed that Sec3, a component of the exocyst vesicle tethering complex, directly interacts with the t-SNARE protein Sso2 to promote fast assembly of an Sso2-Sec9 binary t-SNARE complex. This paper presents a new crystal structure of the Sec3 PH domain in complex with a nearly full-length version of Sso2, which reveals a previously unknown binding site for Sec3 at the N-terminus of Sso2. Our work demonstrates that the dual-site interactions between Sso2 and Sec3 plays an essential role in promoting the fusion of secretory vesicles at target sites on the plasma membrane.