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.
