THE THYROTROPIN-RELEASING HORMONE-RECEPTOR COMPLEX AND G(11-ALPHA) ARE BOTH INTERNALIZED INTO CLATHRIN-COATED VESICLES

It has been proposed that, after agonist binding, the thyrotropin-rele asing hormone receptor (TRHR) becomes internalised associated with G(q ), as part of a TRH-TRHR-G(q) ternary complex [13]. We tested this hyp othesis directly by examining the intracellular distribution of the TR HR and G(q/11) after agonist binding. The localisation of the TRH-TRHR complex and G(q/11 alpha) was studied by the biochemical isolation of clathrin-coated vesicles (CCVs). The internalised TRH -TRHR complex w as localised in CCVs. The CCVs, which had internalised [H-3]MeTRH, con tained 4-fold higher levels of radiolabelled ligand than did CCVs from cells incubated with [H-3]MeTRH at 4 degrees C. Like the receptor-lig and (RL) complex, G(11 alpha) also translocated to these endocytic ves icles. For example, CCVs from cells with internalised TRH-TRHR complex es contained G(11 alpha), whereas CCVs from cells without internalised RL complexes lacked G(11 alpha). We conclude that, after agonist-indu ced TRHR-G(11 alpha) coupling, both the TRH-TRHR complex and G(11 alph a) are internalised in CCVs. CELL SIGNAL 10;8:553-559, 1998. (C) 1998 Elsevier Science Inc.