α cells secrete proglucagon peptides to regulate nutrient metabolism. Recent findings support an α cell-to-β cell axis that is mediated by paracrine signaling through the glucagon receptor and glucagon-like peptide 1 (GLP-1) receptor in β cells. To address which proglucagon peptides stimulate insulin secretion, we developed an assay to quantify levels of GLP-1(7-36)NH2. We also generated three transgenic mouse lines that allow α cell-specific, inducible deletion of the genes for the two prohormone convertase enzymes that process proglucagon . Our studies reveal that both mouse and human islets contain GLP-1(7-36)NH2, but glucagon mediates α cell-to-β cell communication in mice. However, in the absence of normal production of glucagon, α cells up-regulate prohormone convertase 1 (PC1/3) to generate GLP-1 and enhance glucose tolerance. Human islets have substantially higher levels of GLP-1 than mice, which positively correlate with rates of insulin secretion. These studies show plasticity in proglucagon processing to support α cell-to-β cell communication.