The C-terminal Half of the α2C-adrenoceptor Determines the Receptor's Membrane Expression Level and Drug Selectivity

In tissues as well as in transfected cells, α2C-adrenoceptors show poorer expression levels compared to α2A-adrenoceptors. In order to characterize which regions of the α2C-adrenoceptor are involved in regulating the expression of binding-competent receptors at the plasma membrane, six chimeric α2A-/α2C-adrenoceptors were constructed. The wild-type α2A- and α2C-adrenoceptors and the six chimeric α2A-/α2C-adrenoceptors were transiently transfected into human embryonic kidney 293 (HEK293) cells, and the expression levels were investigated by radioligand binding. The results show that the C-terminal half of the α2C-adrenoceptor, ranging from the second extracellular loop to the C-terminus, is the main determinant of the low expression level of binding-competent α2C-adrenoceptors in HEK293 cell membranes. The so-called retention signal in the N-terminus of the α2C-adrenoceptor had a less profound effect on the expression levels of the chimeric receptors. For seven drugs competing for [(3)H]-RX821002 binding, the K i values were determined at the wild-type α2A- and α2C-adrenoceptors and at four of the chimeric α2A-/α2C-adrenoceptors. The results show that the α2C- over α2A-selectivity of spiroxatrine, spiperone, clozapine, MK912, and chlorpromazine, as well as the α2A- over α2C-selectivity of BRL44408, resides mainly in the C-terminal half of the receptors. To some extent, the region comprising the N-terminal half of the receptors contributed to the α2C-selectivity of spiperone, clozapine, and chlorpromazine.