β1-adrenergic receptor O-glycosylation regulates N-terminal cleavage and signaling responses in cardiomyocytes

beta(1)-adrenergic receptors (beta(1)ARs) mediate catecholamine actions in cardiomyocytes by coupling to both Gs/cAMP-dependent and Gs-independent/growth-regulatory pathways. Structural studies of the beta(1)AR define ligand-binding sites in the transmembrane helices and effector docking sites at the intracellular surface of the beta(1)AR, but the extracellular N-terminus, which is a target for post-translational modifications, typically is ignored. This study identifies beta(1)AR N-terminal O-glycosylation at Ser(37)/Ser(41) as a mechanism that prevents beta(1)AR N-terminal cleavage. We used an adenoviral overexpression strategy to show that both full-length/glycosylated beta(1)ARs and N-terminally truncated glycosylationdefective beta(1)ARs couple to cAMP and ERK-MAPK signaling pathways in cardiomyocytes. However, a glycosylation defect that results in N-terminal truncation stabilizes beta(1)ARs in a conformation that is biased toward the cAMP pathway. The identification of O-glycosylation and N-terminal cleavage as novel structural determinants of beta(1)AR responsiveness in cardiomyocytes could be exploited for therapeutic advantage.