Late endocytic compartments are major sites of annexin VI localization in NRK fibroblasts and polarized WIF-B hepatoma cells

Annexin VI is an abundant calcium- and phospholipid-binding protein whose i ntracellular distribution and function are still controversial. Using a hig hly specific antibody, we have studied the distribution of annexin VI in NR K fibroblasts and the polarized hepatic cell line WIF-B by confocal microsc opy. In NRK cells, annexin VI was almost exclusively found associated with endocytic compartments, which were defined by their ability to receive flui d-phase marker internalized from the cell surface. However, extensive coloc alization of annexin VI and the endocytic marker was only observed after ab out 45 min, indicating that annexin VI was primarily in late endocytic comp artments or (pre)lysosomes. Consistent with this, annexin VI was predominan tly seen on structures that contained the lysosomal protein lgp120, althoug h not on dense core lysosomes by electron microscopy, Two major populations of annexin VI-containing structures were present in polarized WIF-B hepato cytes, One correlated to lgp120-positive (pre)lysosomes and was still obser ved after treatment with brefeldin A (BFA), while the other appeared to be partially associated with Golgi membranes and was BFA-sensitive, The striki ng association with prelysosomal compartments in NRK and WIF-B cells sugges ts that annexin VI could play a role in fusion events in the late endocytic pathway, possibly by acting as a tether between membranes. (C) 2000 Academ ic Press.