Syntaxins, integral membrane proteins that are part of the ubiquitous membrane fusion machinery, are thought to act as target membrane receptors during the process of vesicle docking and fusion. Several isoforms of the syntaxin family have been previously identified in mammalian cells, some of which are localized to the plasma membrane. We investigated the subcellular localization of these putative plasma membrane syntaxins in polarized epithelial cells, which are characterized by the presence of distinct apical and basolateral plasma membrane domains. Syntaxins 2, 3, and 4 were found to be endogenously present in Madin-Darby canine kidney cells. The localization of syntaxins 1A, 1B, 2, 3, and 4 in stably transfected Madin-Darby canine kidney cell lines was studied with confocal immunofluorescence microscopy. Each syntaxin isoform was found to have a unique pattern of localization. Syntaxins 1A and 1B were present only in intracellular structures, with little or no apparent plasma membrane staining. In contrast, syntaxin 2 was found on both the apical and basolateral surface, whereas the plasma membrane localization of syntaxins 3 and 4 were restricted to the apical or basolateral domains, respectively. Syntaxins are therefore the first known components of the plasma membrane fusion machinery that are differentially localized in polarized cells, suggesting that they may play a central role in targeting specificity.

We have analyzed conserved domains in t-SNAREs [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors in the target membrane], proteins that are believed to be involved in the fusion of transport vesicles with their target membrane. By using a sensitive computer method, the generalized profile method, we were able to identify a new homology domain that is common in the two protein families previously identified to act as t-SNAREs, the syntaxin and SNAP-25 (synaptosomeassociated protein of 25 kDa) families, which therefore constitute a new superfamily. This homology domain of approximately 60 amino acids is predicted to form a coiled-coil structure. The significance of this homology domain could be demonstrated by a partial suppression of the coiled-coil properties of the domain profile. In proteins belonging to the syntaxin family, a single homology domain is located near the transmembrane domain, whereas the members of the SNAP-25 family possess two homology domains. This domain was also identified in several proteins that have been implicated in vesicular transport but do not belong to any of the t-SNARE protein families. Several new yeast, nematode, and mammalian proteins were identified that belong to the new superfamily. The evolutionary conservation of the SNARE coiled-coil homology domain suggests that this domain has a similar function in different membrane fusion proteins.Most if not all vesicular membrane fusion events in eukaryotic cells are believed to be mediated by a conserved fusion machinery, the SNARE [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors] machinery (for review, see refs. 1-4). The components of the SNARE machinery have been identified in recent years and have been characterized biochemically, in most detail in the case of the synaptic vesicle fusion machinery. A mechanism emerges in which, in the process of vesicle docking, proteins present on the vesicle (v-SNAREs) have to bind to their counter parts on the target membrane (t-SNAREs) to form a core complex that can then recruit the soluble proteins NSF and SNAP. This so called fusion complex can then disassemble after ATP hydrolysis mediated by the ATPase NSF in a process that leads to membrane fusion and the release of the vesicle contents. t-SNAREs consist of two different families of proteins: the type II integral membrane proteins syntaxins (5) and SNAP-25 (synaptosome-associated protein of 25 kDa, a protein that is unrelated to the soluble protein SNAP), which is anchored in the plasma membrane by attached lipids and does not span the membrane (6, 7). The v-SNARE VAMP͞synaptobrevin and the t-SNAREs syntaxin and SNAP-25 can form a stoichiometric ternary complex that involves protein domains predicted to form coiled-coil domains (8-11).According to the SNARE hypothesis, a correct pairing of tand v-SNAREs is required for vesicle fusion to occur thereby providing specificity of membrane trafficking by a final proof reading mechanism (12). This hypothesis post...

Previous studies of fibroblasts have demonstrated that recycling of endocytic receptors occurs through a default mechanism of membrane-volume sorting. Epithelial cells require an additional level of polar membrane sorting, but there are conflicting models of polar sorting, some suggesting that it occurs in early endosomes, others suggesting it occurs in a specialized apical recycling endosome (ARE). The relationship between endocytic sorting to the lysosomal, recycling and transcytotic pathways in polarized cells was addressed by characterizing the endocytic itineraries of LDL, transferrin (Tf) and IgA, respectively, in polarized Madin-Darby canine kidney (MDCK) cells. Quantitative analyses of 3-dimensional images of living and fixed polarized cells demonstrate that endocytic sorting occurs sequentially. Initially internalized into lateral sorting endosomes, Tf and IgA are jointly sorted from LDL into apical and medial recycling endosomes, in a manner consistent with default sorting of membrane from volume. While Tf is recycled to the basolateral membrane from recycling endosomes, IgA is sorted to the ARE prior to apical delivery. Quantifications of the efficiency of sorting of IgA from Tf between the recycling endosomes and the ARE match biochemical measurements of transepithelial protein transport, indicating that all polar sorting occurs in this step. Unlike fibroblasts, rab11 is not associated with Tf recycling compartments in either polarized or glass-grown MDCK cells, rather it is associated with the compartments to which IgA is directed after sorting from Tf. These results complicate a suggested homology between the ARE and the fibroblast perinuclear recycling compartment and provide a framework that justifies previous conflicting models of polarized sorting.Key words: Endocytosis, endosome, epithelia, low density lipoprotein, MDCK, polarity, polymeric Ig receptor, transcytosis, transferrin Received 9 August 1999, revised and accepted for publication 26 October 1999The transport functions of an epithelium are determined by the distinct compositions of the apical and basolateral plasma membrane domains. This membrane polarity is maintained despite significant endocytic turnover, which in MadinDarby canine kidney (MDCK) cells can amount to 40% of the plasma membrane internalized per hour (1). Whereas early studies indicated that the apical and basolateral endocytic recycling pathways of MDCK cells are distinct (2,3), recent evidence indicates that the two pathways are interconnected (4 -6). With this continuous intermixing of apical and basolateral membranes, it is clear that endocytic sorting is crucial to maintaining the plasma membrane polarity of epithelial 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 α-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 α-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.