DIFFERENTIAL LOCALIZATION OF SYNTAXIN ISOFORMS IN POLARIZED MADIN-DARBY CANINE KIDNEY-CELLS

Syntaxins, integral membrane proteins that are part of the ubiquitous membrane fusion machinery, are thought to act as target membrane recep tors during the process of vesicle docking and fusion. Several isoform s of the syntaxin family have been previously identified in mammalian cells, some of which are localized to the plasma membrane. We investig ated the subcellular localization of these putative plasma membrane sy ntaxins in polarized epithelial cells, which are characterized by the presence of distinct apical and basolateral plasma membrane domains. S yntaxins 2, 3, and 4 were found to be endogenously present in Madin-Da rby canine kidney cells. The localization of syntaxins 1A, 1B, 2, 3, a nd 4 in stably transfected Madin-Darby canine kidney cell lines was st udied with confocal immunofluorescence microscopy. Each syntaxin isofo rm was found to have a unique pattern of localization. Syntaxins 1A an d 1B were present only in intracellular structures, with little or no apparent plasma membrane staining. In contrast, syntaxin 2 was found o n both the apical and basolateral surface, whereas the plasma membrane localization of syntaxins 3 and 4 were restricted to the apical or ba solateral domains, respectively. Syntaxins are therefore the first kno wn components of the plasma membrane fusion machinery that are differe ntially localized in polarized cells, suggesting that they may play a central role in targeting specificity.