Fluorescently labeled fusion protein for assaying adenosine triphosphate

A method of measuring ATP comprising at least the following steps:

• 1) contacting a subject substance to a fluorescently labelled fusion protein obtained by individually attaching to an ε protein subunit of adenosine triphosphate (ATP) synthetase two types of fluorescent substances that act as a potential donor and acceptor for a fluorescence resonance energy transfer (FRET) reaction;
• 2) measuring fluorescence emitted from the reaction of the fluorescently labeled fusion protein and the subject substance,
• wherein said ε protein is selected from one of the following (a) and (b):
• (a) a protein having an amino acid sequence including 1 to 8 substitutions in SEQ. ID. NO:
• 1;
• and
• (b) a protein having an amino acid sequence including 1 to 8 substitutions in SEQ. ID. NO:
• 2,
• wherein said ε protein in (a) or (b) has one or more hydrophobic amino acid residues substituted with hydrophilic amino acid residues, wherein the hydrophobic residue is selected from the group consisting of Val9, Val42, Phe67, and Leu 78 in SEQ ID NO:1 or SEQ ID NO:
• 2.