Fluorescence spectroscopic studies on interactions between liver annexin VI and nucleotides--a possible role for a tryptophan residue

Annexin VI is a 68-kDa calcium-, phospholipid-, and cytoskeletal-element-binding protein, which has been implicated in various processes, including calcium release and sequestration in calcifying cartilage, in a receptor-mediated endocytosis in human fibroblasts, and in secretion from chromaffin granules. In these processes it was found that, in addition to Ca2+ and annexin, the presence of ATP is also a prerequisite. In the present report we show that annexin VI binds ATP and the binding of nucleotide to protein is accompanied by quenching of an intrinsic fluorescence of annexin VI, which was found to be specific for 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, GTP and ATP, and dependent on the annexin conformation. The nucleotide-binding site within an annexin VI molecule is likely to be close to the tryptophan-containing domain of annexin VI. We propose that ATP plays the role of a physiological ligand for annexin VI, and its binding to annexin VI may represent an alternative cellular mechanism for the regulation of annexin-membrane interactions coupled to overall energy transitions in the cell.