Open or Closed? SM Protein Sly1 and Qa-SNARE Ufe1, the Possible Key to Understanding SM-Qa Interactions

Abstract

Vesicle trafficking within the cell is mediated by the interaction of SNAREs that form a 4-helix complex to fuse two membranes. SNARE complex formation is regulated by the interaction of SNAREs and Sec1/Munc18-family (SM) proteins. The first step to complexing is the SM protein binding its cognate Qa SNARE. However, the two known SM:Qa-SNARE structures display conflicting characteristics with Munc18:Syntaxin1 binding the Qa-SNARE, Syntaxin1, in an autoinhibited closed conformation, and Vps45:Tlg2 binding the Qa-SNARE, Tlg2, in an open conformation. Furthermore, the open or closed conformation of the SNARE contributes to the unfurling or furling conformation of the domain 3a helical hair, the presumptive site of R-SNARE binding. To identify the default characteristics of SM:Qa-SNARE interactions, we present another SM:Qa-SNARE structure of Sly1:Ufe1. The structure of Qa-SNARE, Ufe1, contained the N-peptide and Habc domain, however, the SNARE motif was cleaved off. The structure of the SM protein, Sly1, was mostly structured except for a portion of the helical hairpin that furls and unfurls. Furthermore, binding assays of Sly1 and Ufe1 showed strong interactions with each other. Overall, our findings demonstrate that Sly1 and Ufe1 bind quickly, and the absence of the SNARE motif may cause the helical hairpin to be disordered. However, due to the absence of the SNARE motif, the default characteristics of SM:Qa-SNARE interactions remain unclear.