β-adrenergic receptor O-glycosylation regulates N-terminal cleavage and signaling responses in cardiomyocytes.

β-adrenergic receptors (βARs) mediate catecholamine actions in cardiomyocytes by coupling to both Gs/cAMP-dependent and Gs-independent/growth-regulatory pathways. Structural studies of the βAR define ligand-binding sites in the transmembrane helices and effector docking sites at the intracellular surface of the βAR, but the extracellular N-terminus, which is a target for post-translational modifications, typically is ignored. This study identifies βAR N-terminal O-glycosylation at Ser/Ser as a mechanism that prevents βAR N-terminal cleavage. We used an adenoviral overexpression strategy to show that both full-length/glycosylated βARs and N-terminally truncated glycosylation-defective βARs couple to cAMP and ERK-MAPK signaling pathways in cardiomyocytes. However, a glycosylation defect that results in N-terminal truncation stabilizes βARs in a conformation that is biased toward the cAMP pathway. The identification of O-glycosylation and N-terminal cleavage as novel structural determinants of βAR responsiveness in cardiomyocytes could be exploited for therapeutic advantage.