Sorting functions of the individual cytoplasmic domains of the G protein-coupled vasopressin V2 receptor in the Madin Darby canine kidney epithelial cells

Previous studies have shown that the G protein-coupled human vasopressin V-2 receptor (V-2 receptor) is expressed predominantly in the basolateral membrane of Madin Darby canine kidney type II (MDCKII) epithelial cells at steady state. Here we have assessed the influence of the individual cytoplasmic domains of the V-2 receptor on polarized sorting in MDCKII cells. The second (ICL2) and third (ICL3) intracellular loops and the C-terminal tail were fused separately to a green fluorescent protein-tagged receptor fragment comprising the first transmembrane domain and flanking regions. We show that the ICL2 domain of the V-2 receptor alone promotes basolateral cell surface expression and thus seems to contain the basolateral sorting signal of the V2 receptor. Fusion of the other cytoplasmic domains, however, does not lead to a randomized cell surface expression. The C-terminal tail of the V-2 receptor promotes apical targeting. Fusion of ICL3 leads to a receptor fragment that is retained in the endoplasmic reticulum (ER). The results are consistent with a model in which the V-2 receptor contains signals for both apical and basolateral cell surface expression, the latter being dominant. Furthermore, ICL3 may contain a retinoid X receptor ER retention signal, which is not accessible in the correctly folded full-length receptor but which is unmasked when ICL3 is fused alone.