ROLE OF THE GLUCAGON RECEPTOR COOH-TERMINAL DOMAIN IN GLUCAGON-MEDIATED SIGNALING AND RECEPTOR INTERNALIZATION

The binding of glucagon to its hepatic receptor is known to result in a number of effects, including the intracellular accumulation of cAMP, the mobilization of intracellular Ca2+, and the endocytosis of glucag on and its receptor into intracellular vesicles. In this study, are be gin to define the functional role of the COOH-terminal tail of the hum an glucagon receptor in glucagon-stimulated signal transduction and re ceptor internalization. We have created and expressed in Chinese hamst er ovary (CHO) cells five truncation mutants in which the COOH-termina l 24, 56, 62, 67, and 73 amino acids have been removed. Cells expressi ng relevant truncated receptors were assayed for cell surface expressi on by immunofluorescence, for Ligand-binding properties, for cAMP and Ca2+-mediated signal transduction properties, and for receptor endocyt osis. In addition, a mutant receptor containing seven serine-to-alanin e mutations in the COOH-terminal tail was studied. Our results reveal the following: 1) a region of the COOH-terminal tail that is required for proper cell surface expression, 2) the COOH-terminal 62 amino acid s, which comprise the majority of the tail, are not required for ligan d binding, cAMP accumulation, or Ca2+ mobilization, and 3) phosphoryla tion of the COOH-terminal tail is crucial for glucagon-stimulated rece ptor endocytosis.