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-eleme nt-binding protein, which has been implicated in various processes, in cluding 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 ad dition 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 bind ing of nucleotide to protein is accompanied by quenching of an intrins ic 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 sit e within an annexin VI molecule is likely to be close to the tryptopha n-containing domain of annexin VI. We propose that ATP plays the role of a physiological ligand for annexin VI, and its binding to annexin V I may represent an alternative cellular mechanism for the regulation o f annexin-membrane interactions coupled to overall energy transitions in the cell.