THE THROMBIN RECEPTOR 2ND CYTOPLASMIC LOOP CONFERS COUPLING TO G(Q)-LIKE G-PROTEINS IN CHIMERIC RECEPTORS - ADDITIONAL EVIDENCE FOR A COMMON TRANSMEMBRANE SIGNALING AND G-PROTEIN COUPLING MECHANISM IN G-PROTEIN-COUPLED RECEPTORS

Thrombin activates human platelets and other cells in part by cleaving an unusual G protein-coupled receptor, Thrombin cleavage of this rece ptor's amino-terminal exodomain unmasks a new amino terminus, This the n binds intramolecularly to the body of the receptor to trigger transm embrane signaling and activation of G(i)-and G(q)-like G proteins, Tow ard identifying the domains responsible for thrombin receptor-G protei n interactions, we examined the signaling properties of chimeric recep tors in which thrombin receptor cytoplasmic sequences replaced the cog nate sequences in the G(s)-coupled beta(2)-adrenergic receptor (beta(2 )AR) or the G(i)-coupled dopamine D-2 receptor (D(2)R). In Xenopus ooc ytes, a chimeric beta(2)AR bearing the thrombin receptor second cytopl asmic (C2) loop gained the ability to trigger intracellular Ca2+ relea se in response to adrenergic agonist, whereas a beta(2)AR bearing the cognate C2 loop from the D(2)R did not. Similarly, in COS-7 cells, a c himeric D(2)R bearing the thrombin receptor C2 loop gained the ability to trigger phosphoinositide hydrolysis in response to dopaminergic ag onist, apparently by coupling to a G(q)-like G protein. No detectable G(s) coupling was seen. Thus, the thrombin receptor C2 loop was able t o confer G(q)-like coupling in several different receptor contexts. Th ese observations suggest that the thrombin receptor C2 loop specifies G(q) coupling by directly contacting G(q) or by contributing to a stru cture required for G(q) coupling. The ability of the thrombin receptor C2 loop to function in the context of the D(2)R and beta(2)AR strongl y suggests that the transmembrane switching and G protein activation s trategies used by the thrombin receptor must be very similar to those used by the D(2)R and beta(2)AR despite the thrombin receptor's striki ngly different liganding mechanism.