Roles of the cytoplasmic and transmembrane domains of syntaxins in intracellular localization and trafficking.

Syntaxins are target-soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (t-SNAREs) involved in docking and fusion of vesicles in exocytosis and endocytosis. Many syntaxin isoforms have been isolated, and each one displays a distinct intracellular localization pattern. However, the signals that drive the specific intracellular localization of syntaxins are poorly understood. In this study, we used indirect immunofluorescence analysis to examine the localization of syntaxin chimeras, each containing a syntaxin transmembrane domain fused to a cytoplasmic domain derived from a different syntaxin. We show that the cytoplasmic domains of syntaxins 5, 6, 7 and 8 have important effects on intracellular localization. We also demonstrate that the transmembrane domain of syntaxin 5 is sufficient to localize the chimera to the compartment expected for wild-type syntaxin 5. Additionally, we find that syntaxins 6, 7 and 8, but not syntaxin 5, are present at the plasma membrane, and that these syntaxins cycle through the plasma membrane by virtue of their cytoplasmic domains. Finally, we find that di-leucine-based motifs in the cytoplasmic domains of syntaxins 7 and 8 are necessary for their intracellular localization and trafficking via distinct transport pathways. Combined, these results suggest that both the cytoplasmic and the transmembrane domains play important roles in intracellular localization and trafficking of syntaxins.