Characterizing Q-SNARE Oligomerization

Abstract

The final event in vesicular trafficking is membrane fusion, facilitated by specific SNARE proteins and SM proteins, which function as SNARE chaperones. A vesicle SNARE and three unique target membrane SNAREs (Q-SNAREs) form membrane-bridging complexes via interactions between their SNARE motifs thereby driving membrane fusion. Some Q-SNAREs form self-interactions via oligomerization, which can inhibit their ability to interact with other SNARE proteins and prevent fusion events. However, the mechanism driving Q-SNARE oligomerization is relatively unknown. Interestingly, SM protein Sly1 prevents Ufe1 from oligomerizing, holding it as a monomer. Here, we show that the removal of the SNARE motif from Q-SNARE Ufe1 prevents its oligomerization. The SNARE motif of Ufe1 forms oligomers in size exclusion chromatography in a concentration-dependent manner but remains monomeric in analytical ultracentrifugation. Our findings suggest that the SNARE motif contributes to Ufe1 self-oligomerization, but additional regions outside the motif are likely necessary to stabilize these interactions. Crystals of a Sly1-Ufe1 complex lacking the SNARE motif diffracted to a resolution of 10 Å by X-ray crystallography. Future optimization of crystallization conditions or structural studies by cryo-EM may clarify how Sly1 inhibits Ufe1 oligomerization and whether this regulation depends on the SNARE motif as well as other structural features of Ufe1. Such investigations will further our understanding of how SM proteins coordinate with Q-SNAREs to promote SNARE complex assembly.