Annexin 11 is a widely expressed calcium- and phospholipid-binding protein that resides in the nucleoplasm in many cultured cell lines. This is in contrast to its most extensively characterized in vitro ligand, the small calcium-binding protein S100A6 (calcyclin), which is concentrated in the nuclear envelope. Here we have examined the significance of the association of annexin 11 and S100A6 by asking whether circumstances exist in which the two proteins occupy the same subcellular localization. First, we show that in both A431 and vascular smooth muscle cells, elevation of intracellular Ca2+ leads to translocation of annexin 11 from the nucleus to the nuclear envelope where it co-localizes with S100A6. We also demonstrate, using fusions of annexin 11 with green fluorescent protein, that whereas the C-terminal core domain of annexin 11 is essential for Ca2+ sensitivity, the N-terminal domain is required for targeting to the nuclear envelope. Second, we show that annexin 11 relocalizes to the nuclear envelope as A431 cells transit from early to mid-prophase. In late prophase, at the time of nuclear envelope breakdown, annexin 11 and S100A6 become intensely localized with lamina-associated polypeptide 2 to folds in the nuclear envelope. From metaphase to telophase S100A6 is degraded, but in late telophase annexin 11 associates with the reforming nuclear envelope before resuming a nucleoplasmic location in interphase. These results show that co-localization of annexin 11 and S100A6 at the nuclear envelope may be regulated either by elevation of intracellular Ca2+ or by cell cycle progression and provide the first evidence that these proteins may associate in vivo.
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