The polarized trafficking of cargo molecules in epithelial cells involves a variety of transport routes and sorting mechanisms. Our studies concentrate on the exocytic traffic of proteins and lipids to the apical domain of the plasma membrane. Apical trafficking involves a variety of different sorting and transport mechanisms to the cell surface.

Recent studies on the apical transport of two enterocytic hydrolases revealed different platforms for trafficking. Sucrase-isomaltase (SI) travels in association with specific lipid microdomains, the so called “lipid rafts”, while lactase-phlorizin hydrolase (LPH) and the neurotrophin receptor (p75) use a different transport mechanism. Moreover, after exit from the trans-Golgi network (TGN), both proteins travel in distinct vesicle populations to the apical cell surface, which were named SAV (SI associated vesicle) and LAV (LPH associated vesicle). Interestingly, the two vesicle populations also move along distinct cytoskelettal tracks.

For the identification of vesicle specific protein components an assay has been developed that allows the immunoisolation of both, SAVs and LAVs, followed by SDS-PAGE and MALDI-TOF-TOF analysis. Here, the SAV-associated components alpha kinase 1, annexin-2 and myosin Ia could already be identified. Galectin-3, a beta-galactosyl binding lectin, is required for apical sorting of LPH and p75. Future work will concentrate on the isolation of more vesicle-associated candidates, which will help us to elucidate the underlying sorting and trafficking mechanisms.