Structure of the insulin receptor–insulin complex by single-particle cryo-EM analysis

Cryo-electron microscopy structures of insulin in a complex with the insulin receptor define the S2 binding site on the receptor and suggest a mechanism for downstream propagation of insulin signalling. Insulin is a peptide hormone with important roles in metabolism. Recent crystal structures of insulin bound to a truncated form of the insulin receptor have provided insights into the binding mode of this complex. Giovanna Scapin and colleagues now describe the three-dimensional structure of the ectodomain of the insulin receptor in complex with an insulin dimer, solved to a resolution of 4.6 A using single-particle cryo-electron microscopy. The findings advance our structural and mechanistic knowledge about the formation of the insulin–receptor complex and receptor activation. The insulin receptor is a dimeric protein that has a crucial role in controlling glucose homeostasis, regulating lipid, protein and carbohydrate metabolism, and modulating brain neurotransmitter levels1,2. Insulin receptor dysfunction has been associated with many diseases, including diabetes, cancer and Alzheimer’s disease1,3,4. The primary sequence of the receptor has been known since the 1980s5, and is composed of an extracellular portion (the ectodomain, ECD), a single transmembrane helix and an intracellular tyrosine kinase domain. Binding of insulin to the dimeric ECD triggers auto-phosphorylation of the tyrosine kinase domain and subsequent activation of downstream signalling molecules. Biochemical and mutagenesis data have identified two putative insulin-binding sites, S1 and S26. The structures of insulin bound to an ECD fragment containing S1 and of the apo ectodomain have previously been reported7,8, but details of insulin binding to the full receptor and the signal propagation mechanism are still not understood. Here we report single-particle cryo-electron microscopy reconstructions of the 1:2 (4.3 A) and 1:1 (7.4 A) complexes of the insulin receptor ECD dimer with insulin. The symmetrical 4.3 A structure shows two insulin molecules per dimer, each bound between the leucine-rich subdomain L1 of one monomer and the first fibronectin-like domain (FnIII-1) of the other monomer, and making extensive interactions with the α-subunit C-terminal helix (α-CT helix). The 7.4 A structure has only one similarly bound insulin per receptor dimer. The structures confirm the binding interactions at S1 and define the full S2 binding site. These insulin receptor states suggest that recruitment of the α-CT helix upon binding of the first insulin changes the relative subdomain orientations and triggers downstream signal propagation.