Provided are a transgenic non-human mammal expressing a fusion protein, wherein the fusion protein comprises an ε subunit of an ATP synthase and two distinct fluorescent proteins as a donor and an acceptor for FRET, one of the fluorescent proteins being placed at an amino terminal moiety of the ε subunit and the other being placed at a carboxyl terminal moiety of the ε subunit, and a method of screening for an agent for preventing or treating diseases in a mammal in need thereof, comprising using an above transgenic non-human mammal.
