(A–C) Cells transfected with various amount of CLIP(CT)-mGlu2 and SNAP(ST)-mGlu4 were labeled with CLIP donor and CLIP acceptor (A), SNAP-donor and SNAP-Acceptor (B), or CLIP-donor and SNAP-acceptor (C). TR-FRET signals were measured and plotted as a function of the amount of mGlu2 and mGlu4 plasmid used for the transfection. TR-FRET signal intensity is color coded, high value in red while the low values are in dark blue and is used as a representation of the amount of mGlu2 homodimers (A), mGlu4 homodimers (B) and mGlu2-4 heterodimers (C). (D-F) Calcium increase in optimized conditions of expression of mGlu2-4 (green), or in cells expressing mGlu4 alone (blue) or mGlu2 alone (red), when activated by glutamate (D), LY379268 (E), or L-AP4 (F). Data are means ± SEM of three experiments performed in triplicates. Values are in percent of the glutamate effect measured in mGlu2-expressing cells (D, E) and mglu4-expressing cells (F).