AT1.03(YEMK) is a genetically encoded fluorescent ATP sensor. It is a fusion protein that consists of a FRET (fluorescence resonance energy transfer) donor (the mseCFP cyan fluorescent protein with a truncation of the C-terminal 11 amino acids) and a FRET acceptor (a circularly permuted form of monomeric Venus carrying the A206K amino acid substitution) separated by an ATP-binding sequence (derived from the F[[0]]F[[1]]-ATP synthase ε subunit encoded by the Bacillus subtilis atpC gene); the ATP-binding sequence carries mutations that increase the affinity for ATP. Upon binding ATP, a change in conformation of the ATP-binding sequence brings the two fluorophores closer together, increasing FRET efficiency. For detailed kinetic and spectral properties, see PMID:19720993.