The delta e13 isoform of the calcitonin receptor forms a six transmembrane domain receptor with dominant negative effects on the signaling of the calcitonin receptor

Like many other class B G protein-coupled receptors, there are several splice variants of the calcitonin receptor. A splice variant of the rabbit CTR (delta e13 CTR) lacks 14 amino acids of the C-terminus of the 7th transmembrane domain that are encoded by exon 13. This isoform is poorly expressed on the cell surface and fails to mobilize intracellular calcium or activate Erk. Moreover, it exerts a dominant-negative effect on CTR signaling by inhibiting the cell surface expression of the C1a isoform. Here, the structural and topological features responsible for these features of the delta e13 isoform are characterized. Using NMR-based experiments, we found that the remainder of the 7th transmembrane domain fails to insert into the lipid bilayer and the C-terminus of delta e13 is therefore extracellular. A truncation mutant lacking the cytoplasmic tail mimicked delta e13 CTR's reduced cell surface expression and its inability to mobilize intracellular calcium or activate Erk, but not its inhibition of C1a cell surface expression. In contrast, a CTR construct lacking the entire 7th transmembrane domain and C-terminal domain reduced the cell surface expression of C1a, similar to the effect of delta e13. We conclude that the cytoplasmic C-terminal tail of the calcitonin receptor is necessary for cell surface expression, mobilization of intracellular calcium, and Erk activation, while the failure of the 7th transmembrane domain to remain in the lipid bilayer is responsible for the dominant-negative effect of the delta e13 CTR isoform.