Fig. 8. Model for v-SNARE specificity in the different pathways to the plasma membrane of polarized epithelial cells. Upon sorting at the level of the TGN, proteins are addressed to the apical or basolateral plasma membrane via at least three different pathways: the direct apical pathway (bold arrowhead), the direct basolateral pathway (dashed arrow) and the transcytotic pathway (open arrowhead). Post-TGN carriers could contain different v-SNAREs (e.g. TI-VAMP, VAMP8, VAMP3). In this case, the specificity of fusion would arise from the pairing of each v-SNARE with the appropriate t-SNARE and from the stoichiometry of the v-SNAREs present on each vesicle (the predominant VAMP is indicated by underlining and alternatives are indicated by parentheses). The specificity of sorting could be dependent on the functional interaction between a v-SNARE and a specific adaptor (e.g. AP-1B for recycling endosomes to the basolateral plasma membrane in the direct basolateral pathway). Alternatively, each v-SNARE might be actively sorted into different post-TGN cargos (in this case, only the SNARE indicated by underlining would be present in the vesicle). T-J, tight junction.