Annexin XIIIb guides raft-dependent and -independent apical traffic in MDCK cells

Epithelial cells are characterized by a polarized organization of their plasma membrane that is divided into apical and basolateral domains. This architecture is maintained by a highly specific cargo sorting machinery that efficiently delivers components to their respective membrane domains. After TGN exit apical cargo is segregated by at least two distinct sorting mechanisms into lipid raft-dependent or lipid raft-independent apical pathways. Annexin XIIIb had been shown to be a member of the lipid raft-dependent trafficking machinery. We now identify this annexin also in raft-independent apical trafficking by mass spectrometry, immunoblotting and confocal microscopy. Annexin XIIIb accumulates in endosomal organelles that are traversed by raft-dependent and raft-independent apical cargo after TGN release. Finally, a specific reduction of annexin XIIIb expression by RNA interference results in a significant decrease in the apical delivery of raft as well as non-raft apical markers. Taken together, our data suggest that annexin XIIIb plays a general role in post Golgi apical trafficking early after TGN release, before the two apical pathways are segregated.