• Cryo-EM structures reveal that insulin, IGF-I, and IGF-II all induce a conserved T-shaped quaternary assembly of the insulin receptor (IR) with four ligand molecules bound at sites 1/1′ and 2/2′.
• Despite overall architectural similarity, each ligand induces distinct conformational changes in IR, with IGF-I and IGF-II exhibiting different binding sequences at site 1 and site 2 compared to insulin.
• This study presents the first cryo-EM structures of full-length IR-A in complex with IGF-I, demonstrating that IR-A can accommodate up to four IGF-I molecules.
• The findings provide a structural framework for understanding ligand-specific IR activation and cooperativity, with implications for metabolic disorders and cancer.