Abstract
1. Both in mammalian tissues and in transfected cells, only low levels of alpha1D-adrenoceptors are detected in radioligand binding studies. It has been implicated that the comparatively long N-terminal tail of the alpha1D-adrenoceptor is responsible for the inefficient surface expression of the receptor. 2. In the present study, we created gene constructs for six N-terminally truncated variants of the human alpha1D-adrenoceptor. These constructs were used to transfect Neuro2A cells. We show that the density of alpha1D-adrenoceptors, observed by [3H]-prazosin binding, gradually increased with longer truncations of the N-terminus. This seems to indicate that the long N-terminal tail nonspecifically interferes with receptor translocation to the plasma membrane. 3. The addition of a 16 amino acids long signal peptide to the N-terminus of the wild-type alpha1D-adrenoceptor increased the density of receptor binding sites 10-fold in Neuro2A and COS-7 cells. This indicates that, after the addition of a signal peptide, the long N-terminal tail of the alpha1D-adrenoceptor does not interfere with proper translocation of the receptor to the plasma membrane. This, in turn, indicates that the N-terminal tail of the wild-type alpha1D-adrenoceptor, merely by its long length, hinders the first transmembrane helix of the receptor from being a signal anchor. 4. Neither the wild-type alpha1D-adrenoceptor (for which the expression level of [3H]-prazosin binding sites is low) nor the truncated alpha1D-adrenoceptor variant (for which the expression level of [3H]-prazosin binding sites is high) showed any constitutive activity in stimulating inositol phosphate accumulation. This indicates that the low expression level of [3H]-prazosin binding sites, after transfection with the wild-type alpha1D-adrenoceptor, is not caused by constitutive activity of the receptor and subsequent receptor downregulation.