Biochemical and pharmacological study of N-linked glycosylation of the human serotonin 5-HT₇a receptor

The 5-hydroxytryptamine (5-HT)(7(a)) receptor is a G-protein-coupled receptor critically involved in human psychiatric and neurological disorders. In the present study, we evaluate the presence and the functional role of N-glycosylation of the human 5-HT(7) receptor. Western blot analysis of HEK293T cells transiently expressing the 5-HT(7(a)) receptor in the presence of tunicamycin gave rise to a band shift, indicating the existence of an N-glycosylated form of the 5-HT(7(a)) receptor. To further investigate this, we mutated the two predicted N-glycosylation sites (N5Q and N66Q) and compared the molecular mass of the immunoreactive bands with those of the wild-type receptor, indicating that both asparagines were N-glycosylated. The mutant receptors had the same binding affinity for [(3) H]5-CT and the same potency and efficacy with regard to 5-HT-induced activation of adenylyl cyclase. However, there was a reduction in maximal ligand binding for the single and double mutants compared to the wild-type receptor. Next, membrane labelling and immunocytochemical studies demonstrated that the N-glycosylation mutants were expressed at the cell surface. We conclude that N-glycosylation is not important for cell surface expression of the 5-HT(7) receptor.