||In the secretory pathway of eukaryotic cells transport between membranous organelles is largely mediated by transport vesicles. One of the main unsolved questions in the field of membrane trafficking is what determines transport vesicle targeting specificity? SNAREs are thought to contribute to both vesicle-targeting specificity as well as to play a central role in the fusion of the vesicle with target organellar membranes. For SNAREs to fulfill such a role the formation of the SNARE-complex itself must be specific and an exclusive feature of the SNARE proteins themselves. In my Ph.D. studies I have investigated whether SNAREs could form compartmental-specific complexes by studying complex formation among bacterially expressed yeast Golgi / endosomal SNAREs. The results I obtained have revealed that only a select number of complexes could form in vitro and that some of these SNAREs could participate in the formation of more than one SNARE complex. Thus SNARE complex formation in vitro was selective among this group of SNARE proteins but not exclusively so, findings that where also supported through genetic studies in yeast. Nonetheless my work, together with the contributions from others, have forged the current view in the field that although some SNAREs can form selective complexes in vitro it seems unlikely that they are, in the strict sense, the sole determinants of vesicle targeting specificity in vivo. In addition, I have identified a previously overlooked and atypical yeast SNARE, Syn8p, which I have shown to play a role in endosomal sorting. I have also shown that the R-SNARE Ykt6p adopted a folded-back conformation and that this conformation of Ykt6p influenced the rate of SNARE complex formation as well as the specificity of Ykt6p-binding interactions, revealing an important regulatory role for the N-terminal "profilin-like" fold of this family of SNARE proteins.