The SNARE Machinery Is Involved in Apical Plasma Membrane Trafficking in MDCK Cells

We have investigated the controversial involvement of components of the SNARE (soluble N-ethyl maleimide-sensitive factor [NSF] attachment protein [SNAP] receptor) machinery in membrane traffic to the apical plasma membrane of polarized epithelial (MDCK) cells. Overexpression of syntaxin 3, but not of syntaxins 2 or 4, caused an inhibition of TGN to apical transport and apical recycling, and leads to an accumulation of small vesicles underneath the apical plasma membrane. All other tested transport steps were unaffected by syntaxin 3 overexpression. Botulinum neurotoxin E, which cleaves SNAP-23, and antibodies against alpha-SNAP inhibit both TGN to apical and basolateral transport in a reconstituted in vitro system. In contrast, we find no evidence for an involvement of N-ethyl maleimide-sensitive factor in TGN to apical transport, whereas basolateral transport is NSF-dependent. We conclude that syntaxin 3, SNAP-23, and alpha-SNAP are involved in apical membrane fusion. These results demonstrate that vesicle fusion with the apical plasma membrane does not use a mechanism that is entirely unrelated to other cellular membrane fusion events, but uses isoforms of components of the SNARE machinery, which suggests that they play a role in providing specificity to polarized membrane traffic.