It has been proposed that, after agonist binding, the thyrotropin-releasing hormone receptor (TRHR) becomes internalised associated with Gq, as part of a TRH-TRHR-Gq ternary complex [13]. We tested this hypothesis directly by examining the intracellular distribution of the TRHR and Gq/11 after agonist binding. The localisation of the TRH-TRHR complex and Gq/11alpha was studied by the biochemical isolation of clathrin-coated vesicles (CCVs). The internalised TRH-TRHR complex was localised in CCVs. The CCVs, which had internalised [3H]MeTRH, contained 4-fold higher levels of radiolabelled ligand than did CCVs from cells incubated with [3H]MeTRH at 4 degrees C. Like the receptor-ligand (RL) complex, G11alpha also translocated to these endocytic vesicles. For example, CCVs from cells with internalised TRH-TRHR complexes contained G11alpha, whereas CCVs from cells without internalised RL complexes lacked G11alpha. We conclude that, after agonist-induced TRHR-G11alpha coupling, both the TRH-TRHR complex and G11alpha are internalised in CCVs.
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