PMC

Percent decrease in eCFP-Gβ1 and eYFP-μOR FRET with continuous 1 μM morphine treatment where the values were normalized to 0.038 for SKNSN and 0.036 for Neuro-2a cells.

Example of images of eCFP-Gβ1 (top left) and eYFP-μOR (top right), their raw FRET (bottom left) and their normalized FRET (bottom right) in Neuro-2a cells. Images are artificially colorized by Zeiss software.

A Change in the normalized FRET between eYFP-μOR and eCFP-δOR with 1 μM morphine treatment in Neuro-2a cells. B – Change in the molecular brightness in eYFP-μOR homomers and non-fluorescence δOR heteromers in Neuro-2a cells. C – Change in the distribution of diffusion coefficients of eGFP-Gβ1 co-expressed with μOR-δOR in Neuro-2a cells with 1 μM morphine treatment for 48 hours.

Energy minimized 3D molecular models of permutations 8 and 9 (from ). A and B - Models of permutation 8 exhibiting symmetric interaction of TM4,5 or TM1, respectively, at the heterodimeric μOR –δOR interface. C and D) Models of permutation 9 exhibiting symmetric interaction of TM4,5 or TM1, respectively, at the heterodimeric μOR –δOR interface. In all cases, μOR is shown in red and δOR is shown in blue.

Possible configurations of μOR-μOR and μOR-δOR tetramers. A- Cartoon of the possible permutations for the μOR homomeric tetramer. The compact (iv–v) and more extended (i–iii) arrangements differ in the combination of alternative dimeric interfaces within the tetramer: TM1-TM1 (▲), TM4-TM4 (■), and TM4,5-TM4,5 (●). B- Cartoon of the possible permutations for the heteromeric tetramer formed by μOR (red oval) and δOR (blue oval). Depending on the different combination of symmetric interfaces (2 for compact configurations and 3 for more extended ones) within the tetrameric arrangement, the total number of heteromeric permutations is 29 (4×2=8 compact and 7×3=21 extended configurations). Permutations 8 and 9 (boxed in the figure) are the most likely candidates for μOR-δOR complexes based on consistency with the observed μOR-μOR and μOR-δOR FRET.