N-glycosylation of α1D-adrenergic receptor N-terminal domain is required for correct trafficking, function, and biogenesis

G protein-coupled receptor (GPCR) biogenesis, trafficking, and function are regulated by post-translational modifications, including N -glycosylation of asparagine residues. α 1D -adrenergic receptors (α 1D -ARs) – key regulators of central and autonomic nervous system function – contain two putative N -glycosylation sites within the large N-terminal domain at N65 and N82. However, determining the glycosylation state of this receptor has proven challenging. Towards understanding the role of these putative glycosylation sites, site-directed mutagenesis and lectin affinity purification identified N65 and N82 as bona fide acceptors for N -glycans. Surprisingly, we also report that simultaneously mutating N65 and N82 causes early termination of α 1D -AR between transmembrane domain 2 and 3. Label-free dynamic mass redistribution and cell surface trafficking assays revealed that single and double glycosylation deficient mutants display limited function with impaired plasma membrane expression. Confocal microscopy imaging analysis and SNAP-tag sucrose density fractionation assays revealed the dual glycosylation mutant α 1D -AR is widely distributed throughout the cytosol and nucleus. Based on these novel findings, we propose α 1D- AR transmembrane domain 2 acts as an ER localization signal during active protein biogenesis, and that α 1D -AR N-terminal glycosylation is required for complete translation of nascent, functional receptor.