The present invention deals with heterotrimeric AMP-activated protein kinase (AMPK) comprising a fluorophore pair wherein the conformational change can be measured by FRET. It represents an advanced tool to screen and identify AMPK interactors in vitro and in cells in vivo. Such invention can also be considered as a reporter of the cellular energy status as it allows the spatiotemporal monitoring, in situ, of fluctuations in the ratio of AMP and ADP versus ATP.