Figure 1

SNARE proteins are key factors in vesicle trafficking in yeast. A) Current model of the role of SNARE proteins in vesicle fusion. Heterologous sets of SNARE proteins, here exemplified by the secretory set of SNAREs, assemble in a zipper-like fashion into tight complexes between two membranes, in effect initiating fusion. B) Architecture of the archetypical four-helix bundle SNARE complex structure [91]. The structure of the neuronal SNARE complex is shown as ribbon diagram on the left (blue, red and green for synaptobrevin 2, syntaxin 1a and SNAP-25a, respectively). The layers in the core are indicated by virtual bonds between the corresponding C⟨ positions. The structure of the central 0-layer is shown in detail on the right [91]. According to this unusual hydrophilic layer in the center of the complex, SNAREs have been categorized as either Q- or R-SNAREs depending on whether they contribute a conserved glutamine or arginine, respectively, to this layer. The three Q-SNAREs are further subdivided into Qa-, Qb- and Qc-SNAREs based on the other sequences of their SNARE domains. These four basic types correspond to the four different helices of the canonical SNARE complex architecture [91–94]. Hence, functional complexes are composed of four different helices, each belonging to one of the four main groups ("QabcR" composition) [21, 40, 74, 95, 96]. C) Schematic outline of the vesicle trafficking pathways and tentative assignement of the involved sets of SNARE proteins of baker's yeast. It should be kept in mind, however, that the assignment of some SNAREs to certain trafficking steps, in particular of the R-SNAREs, is still debated. Note that baker's yeast has two endosomal syntaxins, Pep12 and Vam3, that are thought to be involved in consecutive trafficking steps towards the vacuole, whereas other fungi only have one.