Allosteric modulation of enzyme activity in M
R-luciferase fusion proteins by nanomolar concentrations of M
R blockers. CCh (square), atropine (open circle), scopolamine (filled circle) and butylscopolamine (diamond) were applied at the indicated concentrations on COS-7 cells transfected with M3R-luci (A) and construct #8 (B) and luciferase activity was determined. The luminescences without ligands were 178,680 ± 18,171 AU (M3R-luci) and 121,578 ± 12,845 AU (construct #8). Enzyme activity of the individual constructs without ligands was set at 100%. All data are given as mean ± S.E.M. of four independent experiments each performed in triplicate. Control experiments were carried out with COS-7 cells expressing luciferase alone and V2R-luci [see Additional file 3].