Role of N-glycosylation in the expression and functional properties of human AT1 receptor

The role of N-glycosylation in the pharmacological properties and cell surface expression of ATI receptor was evaluated. Using site-directed mutagenesis, we substituted both separately and simultaneously the asparagine residues in all three putative N-linked glycosylation consensus sequences (N-X-S/T) of AT(1) receptor (positions 4, 176, and 188) with aspartic acid. Expression of these mutant receptors in COS-7 cells followed by photolabeling with [I-125]-[p-benzoyl-Phe(8)]AngII and SDS-PAGE revealed ligand-receptor complexes of four different molecular sizes, indicating that the three N-glycosylation sites are actually occupied by oligosaccharides. Binding studies showed that the affinity of each mutant receptor for [Sar(1),Ile(8)]Ang II was not significantly different from that of wild-type AT(1) receptor. Moreover, the functional properties of all mutant receptors were unaffected as evaluated by inositol phosphate production. However, the expression levels of the aglycosylated mutant were 5-fold lower than that of the wild-type AT(1) receptor. Use of green fluorescent protein-AT(1) receptor fusion proteins in studying the cellular location of the aglycosylated mutant demonstrated that it was distributed at a much higher density to the ER-Golgi complex than to the plasma membrane in HEK 293 cells. Together, these results suggest an important role of N-glycosylation in the proper trafficking of AT(1) receptor to the plasma membrane.