Comparison of G-protein selectivity of human 5-HT2C and 5-HT1A receptors

We compared the ability of human 5-HT2C and 5-HT1A receptors to couple to selected G proteins expressed in insect Sf9 cells through simultaneous infection with recombinant baculoviruses. We also examined the coupling of G proteins to these same receptors in membranes derived from the Sf9 cells using in situ reconstitution with purified G proteins. Our data show that unoccupied 5-HT2C and 5-HT1A receptors can attain an activated conformation that is stabilized by interaction with specific G proteins. While high-affinity agonist binding to the 5-HT2C receptor was increased to a greater extent by Galphaq than by Galphai2, the high-affinity agonist binding to the 5-HT1A receptor was preferentially enhanced by Galphai2 coexpression. When the two 5-HT receptors were expressed in cells also expressing G proteins, both 5-HT2C and 5-HT1A receptors appear to activate Galphai2 in preference to Galphaq. In contrast, in situ reconstitution data show that 5-HT2C receptors robustly activate Galphaq and marginally activate Galphao or Galphai, whereas 5-HT1A receptors only marginally activate Galphaq and robustly activate Galphao and Galphai. These results suggest that the overexpression of receptor and potential G-protein coupling partners in Sf9 cells may lead to erroneous conclusions as to the signaling selectivity of receptors.