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 glucagon and its receptor into intracellular vesicles. In this study, we begin to define the functional role of the COOH-terminal tail of the human glucagon receptor in glucagon-stimulated signal transduction and receptor internalization. We have created and expressed in Chinese hamster ovary (CHO) cells five truncation mutants in which the COOH-terminal 24, 56, 62, 67, and 73 amino acids have been removed. Cells expressing relevant truncated receptors were assayed for cell surface expression by immunofluorescence, for ligand-binding properties, for cAMP and Ca²⁺-mediated signal transduction properties, and for receptor endocytosis. In addition, a mutant receptor containing seven serine-to-alanine 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 acids, which comprise the majority of the tail, are not required for ligand binding, cAMP accumulation, or Ca²⁺ mobilization, and 3) phosphorylation of the COOH-terminal tail is crucial for glucagon-stimulated receptor endocytosis.