The serum glycoprotein leucine-rich alpha-2-glycoprotein 1 (LRG1), primarily produced by hepatocytes and neutrophils, is a multifunctional protein that modulates various signaling cascades, mainly TGF beta signaling. Serum LRG1 and neutrophil-derived LRG1 have different molecular weights due to differences in glycosylation, but the impact of the differential glycan composition in LRG1 on its cellular function is largely unknown. We previously reported that LRG1 can promote both angiogenic and neurotrophic processes under hyperglycemic conditions by interacting with LPHN2. Here, we determined the crystal structure of LRG1, identifying the horseshoe-like solenoid structure of LRG1 and its four N-glycosylation sites. In addition, our biochemical and cell-biological analyses found that the deglycosylation of LRG1, particularly the removal of glycans on N325, is critical for the high-affinity binding of LRG1 to LPHN2 and thus promotes LRG1/LPHN2-mediated angiogenic and neurotrophic processes in mouse tissue explants, even under normal glucose conditions. Moreover, the intracavernous administration of deglycosylated LRG1 in a diabetic mouse model ameliorated vascular and neurological abnormalities and restored erectile function. Collectively, these data indicate a novel role of LRG1 glycans as molecular switches that can tune the range of LRG1's cellular functions, particularly the LRG1/LPHN2 signaling axis. Diabetes: Protein structure hints at potential treatmentResolving the crystal structure of a multifunctional protein suggests possible routes to alleviating symptoms of advanced diabetes. Leucine-rich alpha-2-glycoprotein 1 (LRG1) plays both positive and negative roles in health and disease. LRG1 glycosylation, where carbohydrate-based polymers known as glycans bind to the protein, alters LRG1's properties and functions. Aberrant LRG1 glycosylation is recognised in multiple diseases, including diabetes. Jimin Yang at the Institute for Basic Science, Daejeon, South Korea, and co-workers resolved the crystal structure of LRG1, demonstrating its horseshoe shape and identifying four glycosylation sites. Removing glycans from one particular site is vital for the interaction of LRG1 with cellular receptor, LPHN2. This interaction promotes vascular and neurological functioning under high-glucose conditions. When the team used this de-glycosylated LRG1 in a diabetic mouse model, they observed a reduction in several symptoms including erectile dysfunction.