Theory of FRET from three oligomer types with several fluorophore stoichiometries. (A) XFP-tagged mGAT1 protomers can assemble in three possible fluorophore stoichiometries. (B) Square tetramers can have six possible fluorophore configurations. (C) There are nine possible configurations of a rhomboid tetramer. Nomenclature for panels A–C: D, donor; A, acceptor; E_xD:yA, FRET efficiency for each configuration where x and y represent the number of donor or acceptor fluorophores, respectively; R₀, the Förster distance for a CFP-YFP pair; r₁, the pair-wise FRET radius in the basic mGAT1 dimer; r₂-r₄, the three other pair-wise FRET radii present in either square tetramers or rhomboid tetramers. (D) Plot of the apparent FRET (E_App) modeled for mGAT1 dimers, square tetramers, or rhomboid tetramers with changing r₁, the pair-wise FRET radius in the basic mGAT1 dimer.