ANNEXIN XIIIB - A NOVEL EPITHELIAL SPECIFIC ANNEXIN IS IMPLICATED IN VESICULAR TRAFFIC TO THE APICAL PLASMA-MEMBRANE

The sorting of apical and basolateral proteins into vesicular carriers takes place in the trans-Golgi network (TGN) in MDCK cells. We have p reviously analyzed the protein composition of immunoisolated apical an d basolateral transport vesicles and have now identified a component t hat is highly enriched in apical vesicles. Isolation of the encoding c DNA revealed that this protein, annexin XIIIb, is a new isoform of the epithelial specific annexin XIII sub-family which includes the previo usly described intestine-specific annexin (annexin XIIIa; Wice, B. M., and J. I, Gordon. 1992. J. Cell Biol. 116:405-422). Annexin XIIIb dif fers from annexin XIIIa in that it contains a unique insert of 41 amin o acids in the NH2 terminus and is exclusively expressed in dog intest ine and kidney, Immunofluorescence microscopy demonstrated that annexi n XIIIb was localized to the apical plasma membrane and underlying pun ctate structures. Since annexins have been suggested to play a role in membrane-membrane interactions in exocytosis and endocytosis, we inve stigated whether annexin XIIIb, is involved in delivery to the apical cell surface. To this aim we used permeabilized MDCK cells and a cytos ol-dependent in vitro transport assay. Antibodies specific for annexin XIIIb significantly inhibited the transport of influenza virus hemagg lutinin from the TGN to the apical plasma membrane while the transport of vesicular stomatitis virus glycoprotein to the basolateral cell su rface was unaffected. We propose that annexin XIIIb, plays a role in v esicular transport to the apical plasma membrane in MDCK cells.