Porcine liver annexin VI (AnxVI) of Mr 68.000 is an ATP-binding protein as evidenced by specific and saturable UV-dependent labelling with 8-azido-[γ-32P]ATP or the fluorescent analog of ATP, 2′-(or 3′)-O-(2,4,6-trinitrophenyl)adenosine triphosphate and by binding of AnxVI to ATP-agarose. These characteristics of purified AnxVI were used to identify and characterize preliminary nucleotide-binding domain of the protein. AnxVI labelled with 8-azido-ATP was subjected to limited proteolysis and the proteolytic fragments of AnxVI that retained the covalently-bound nucleotide were separated by means of gel electrophoresis and visualized by exposure of the gel to a phosphor storage screen. It was found that the AnxVI proteolytic fragments of M<>_r 34-36.000 and smaller retained the nucleotide. In a reciprocal experiment, AnxVI was digested with proteolytic enzymes and in an ATP eluate from an ATP-agarose column protein fragments of similar Mr to these labelled with 8-azido-ATP were identified. The extent of AnxVI labelling with 8-azido-ATP and the distribution of proteolytic fragments varied upon calcium concentration. These results lead to the conclusion that there is a nucleotide-binding domain within the AnxVI molecule that is functionally similar to the nucleotide-binding domains of other nucleotide-binding proteins. The nucleotide-binding domain is located close to the tryptophan residue 343 of AnxVI and in close vicinity to the Ca2+- and phospholipid-binding sites of the protein. This is confirmed by the observation that the tryptophan fluorescence intensity of AnxVI decreases in the presence of a fluorescence analog of ATP in a calcium-dependent manner, due to the quenching properties of the nucleotide and/or fluorescence energy transfer from AnxVI tryptophan to fluorophore. Both processes were modulated by the presence of phospholipid molecules.