TRUNCATION OF THE PORCINE CALCITONIN RECEPTOR CYTOPLASMIC TAIL INHIBITS INTERNALIZATION AND SIGNAL-TRANSDUCTION BUT INCREASES RECEPTOR AFFINITY

A series of mutant porcine calcitonin receptors with progressively truncated carboxy termini have been expressed in COS and HEK 293 cells. All forms of the receptor, including those totally lacking the cytoplasmic tail, were able to bind I-125-labeled salmon calcitonin. However, removal of C-terminal domains resulted in multiple functional changes in the receptor. First, compared with the wild type receptor, affinity of binding of salmon calcitonin was increased for truncated receptors, whether determined in intact transfected cells or in cell membranes. Second, internalization of the ligand-receptor complex was greatly attenuated for mutants truncated by 44 or 83 amino acids but not for an intermediate form truncated by 63 amino acids. Third, truncation affected signal transduction, which for the porcine calcitonin receptor occurs by generation of intracellular cAMP and Ca2+. The magnitude of adenylate cyclase responses was much reduced for the same mutants defective in internalization. Under conditions where expression of each receptor form was approximately equal, the magnitude of intracellular Ca2+ responses was decreased by C-terminal truncation. These results draw attention to the functional significance of the cytoplasmic tail of the porcine calcitonin receptor and suggest intramolecular interactions between the carboxy terminus and other receptor domains and/or cellular regulatory elements.