Fig. 5. Schematic models of syntaxin distribution and polarized trafficking pathways in MDCK and RPE cells. The subcellular localizations of syntaxins are indicated as reported previously in MDCK cells (Low et al., 1996) and in the present work in RPE cells. In MDCK cells, trafficking from the TGN to the apical plasma membrane (route A) was previously shown to involve syntaxin 3 (Low et al., 1998a), whereas trafficking from the TGN to the basolateral plasma membrane (route B) was shown to involve syntaxin 4 (Lafont et al., 1999). By contrast, basolateral-to-apical transcytosis is independent of syntaxin 3 in MDCK cells (Low et al., 1998a). The absence of syntaxin 3 in RPE cells suggests that route A does not exist in this cell type but that probably an alternative route designated A₂ exists. This alternative route may depend on syntaxin 1A and/or 1B, which are expressed in RPE cells and localize to the apical plasma membrane. In RPE cells, syntaxin 2 is exclusively localized to a region below the tight junctions, in contrast to its distribution along both plasma membrane domains in MDCK cells. Syntaxin 4 localizes all along the lateral membrane in MDCK cells, and post-Golgi transport vesicles carrying basolateral cargo can fuse along the entire lateral membrane (Kreitzer et al., submitted). By contrast, in RPE cells syntaxin 4 localizes to the same narrow band underneath the tight junctions as does syntaxin 2. In addition, syntaxin 4 localizes to the basal membrane. This suggests that syntaxin 4 may serve two different trafficking pathways that lead to either location (designated B₁ and B₂). These syntaxin-4-dependent routes may originate from the TGN and/or endosomes.

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