Moreover, using a comparable approach, we show that tetanus toxin-insensitive, vesicle-associated membrane protein (TI-VAMP; also called VAMP7), a recently described apical v-SNARE, is involved

Moreover, using a comparable approach, we show that tetanus toxin-insensitive, vesicle-associated membrane protein (TI-VAMP; also called VAMP7), a recently described apical v-SNARE, is involved. surface delivery of an apical reporter protein. Moreover, using a comparable approach, we show that tetanus toxin-insensitive, vesicle-associated membrane protein (TI-VAMP; also called VAMP7), a recently described apical v-SNARE, is usually involved. Furthermore, we show the presence of syntaxin 3 and TI-VAMP in isolated apical carriers. Polarized apical sorting has been postulated to be mediated by the clustering of apical proteins into dynamic sphingolipid-cholesterol rafts. We provide evidence that syntaxin 3 and TI-VAMP are raft-associated. These data support a raft-based mechanism for the sorting of not only apically destined cargo but also of SNAREs having functions in apical membrane-docking and fusion events. The main feature of polarized epithelial cells is the compartmentalization of their plasma membrane into apical and basolateral domains. These membrane domains differ both Rabbit Polyclonal to TPH2 (phospho-Ser19) in lipid and protein composition (13). To generate and maintain Parathyroid Hormone 1-34, Human this polarized membrane composition, specific vectorial flow of membrane carriers must be ensured. This implies that along the biosynthetic pathway, apically and basolaterally destined lipids and proteins must be segregated and sorted. In MadinDarby canine kidney (MDCK) cells, the first sorting event occurs in the trans-Golgi network (TGN) (1). Several signals have been identified that participate in the specific targeting of proteins to their proper location (3). A hierarchy of these signals has been proposed such that the basolateral signals are dominant over the apical ones. Moreover, it has been postulated that apically routed proteins are recruited into glycosphingolipid- and cholesterol-enriched microdomains, called rafts (4). Because of the high melting temperature (Tm) of their sphingolipids, rafts are resistant to Triton X-100 solubilization at low temperatures (5). The mechanism for rafts to coalesce into large domains upon detergent treatment has been investigated in detail. The study of the miscibility between liquid-ordered and crystalline phases exhibited that rafts exist before detergent addition (6). Recently, using crosslinking and fluorescence resonance energy transfer (FRET) techniques, the size of a single raft was estimated to less than Parathyroid Hormone 1-34, Human 70 nm (7,8). A strict correlation between raft association and apical sorting cannot be established because some basolaterally routed proteins can partition into (basolateral) rafts, e.g., caveolin-2 (9). Rafts are clearly present basolaterally. In the basolateral plasma membrane, stable assembly of rafts occurs in caveolae (9,10). The apical plasma membrane, on the other hand, has been proposed to constitute a percolating raft domain name (11). Most importantly, caveolae are not observed at the apical surface of MDCK cells (9). In the TGN, rafts, together with apical cargo, would be clustered and packaged into apical transport carriers (12). Hence, rafts would constitute platforms for apical sorting of proteins devoid of basolateral sorting signals (24). Those rafts that are not clustered by the apical sorting machinery could be envisaged to be included into basolateral transport containers either passively or as caveolae precursors. Parathyroid Hormone 1-34, Human Therefore, raft lipids are delivered from the TGN to both plasma membrane domains although their organization might differ in these domains. Because inclusion into rafts is a prerequisite for some apical proteins to be sorted properly, we asked whether apical SNAREs could be raft-associated. The SNARE proteins are known to play a general role in vesicular docking and fusion in the biosynthetic pathway and in regulated exocytosis (13). Cytosolic proteins, i.e., solubleN-ethyl maleimide-sensitive factor (NSF) and soluble NSF attachment protein (SNAP), primary SNAP receptor (SNARE) proteins located on the donor (or vesicular) and the acceptor (or target membrane) compartment for the process that leads to membrane docking and fusion (14,15). Previous results have shown that in MDCK cells the basolateral pathway uses the NSF/SNAP/SNARE mechanism, whereas the apical route has been demonstrated to be insensitive to theN-ethyl maleimide (16,17). New insights have come from the localization studies of SNAREs. For instance, the t-SNARE syntaxin 3 and the v-SNARE TI-VAMP [or VAMP7 (18)] were reported to be apically distributed whereas syntaxin 4 was found basolaterally and syntaxin 2 and SNAP-23 were observed in both compartments (1922). Moreover, the apical route was shown to be inhibited after overexpression of syntaxin 3 in MDCK cells (17). Parathyroid Hormone 1-34, Human The aim of this study was to analyze whether or not apical SNAREs are raft-associated and to analyze their involvement in apical delivery. == EXPERIMENTAL PROCEDURES == == Antibodies and Recombinant Proteins. == The production and characteristics of the anti-alpha-SNAP antibodies (3E2 and 2F10) will be described elsewhere. Affinity-purified antibodies raised in rabbits against syntaxin 3 (TG0), SNAP-23 (TG7), and TI-VAMP (also called VAMP7; ref.18) (TG11) were described previously (17). In Western blotting experiments performed on apical TGN-derived vesicles, TG16, an anti-TI-VAMP antibody prepared following the same protocol as TG11, was used. The TG15 antibody against endobrevin (23), also called VAMP8 (26), was obtained from rabbits injected.