Insulin receptor signalling has a central role in mammalian biology, regulating cellular metabolism, growth, division, differentiation and survival1,2. Insulin resistance contributes to the pathogenesis of type 2 diabetes mellitus and the onset of Alzheimer’s disease3; aberrant signalling occurs in diverse cancers, exacerbated by crosstalk with the homologous type 1 insulin-like growth factor receptor (IGF1R)4. Despite more than three decades of investigation, the three-dimensional structure of the insulin–insulin receptor complex has proved elusive, confounded by the complexity of producing the receptor protein. Here we present the first view, to our knowledge, of the interaction of insulin with its primary binding site on the insulin receptor, on the basis of four crystal structures of insulin bound to truncated insulin receptor constructs. The direct interaction of insulin with the first leucine-rich-repeat domain (L1) of insulin receptor is seen to be sparse, the hormone instead engaging the insulin receptor carboxy-terminal α-chain (αCT) segment, which is itself remodelled on the face of L1 upon insulin binding. Contact between insulin and L1 is restricted to insulin B-chain residues. The αCT segment displaces the B-chain C-terminal β-strand away from the hormone core, revealing the mechanism of a long-proposed conformational switch in insulin upon receptor engagement. This mode of hormone–receptor recognition is novel within the broader family of receptor tyrosine kinases5. We support these findings by photo-crosslinking data that place the suggested interactions into the context of the holoreceptor and by isothermal titration calorimetry data that dissect the hormone–insulin receptor interface. Together, our findings provide an explanation for a wealth of biochemical data from the insulin receptor and IGF1R systems relevant to the design of therapeutic insulin analogues.

Insulin is a key protein hormone that regulates blood glucose levels and, thus, has widespread impact on lipid and protein metabolism. Insulin action is manifested through binding of its monomeric form to the Insulin Receptor (IR). At present, however, our knowledge about the structural behavior of insulin is based upon inactive, multimeric, and storage-like states. The active monomeric structure, when in complex with the receptor, must be different as the residues crucial for the interactions are buried within the multimeric forms. Although the exact nature of the insulin's induced-fit is unknown, there is strong evidence that the C-terminal part of the B-chain is a dynamic element in insulin activation and receptor binding. Here, we present the design and analysis of highly active (200-500%) insulin analogues that are truncated at residue 26 of the B-chain (B 26 ). They show a structural convergence in the form of a new β-turn at B 24 -B 26 . We propose that the key element in insulin's transition, from an inactive to an active state, may be the formation of the β-turn at B 24 -B 26 associated with a trans to cis isomerisation at the B 25 -B 26 peptide bond. Here, this turn is achieved with N-methylated L-amino acids adjacent to the trans to cis switch at the B 25 -B 26 peptide bond or by the insertion of certain D-amino acids at B 26 . The resultant conformational changes unmask previously buried amino acids that are implicated in IR binding and provide structural details for new approaches in rational design of ligands effective in combating diabetes.β-turn | diabetes | peptide bond isomerisation | protein | structure T he peptide hormone insulin regulates blood glucose levels with a widespread impact on lipid and protein metabolism. It is a molecule of major therapeutic importance in the treatment of diabetes. The mature form of insulin is formed by two chains "A" and "B" with a B chain running from Phe B1 -Thr B30 and an A chain Gly A1 -Asn A21 , stabilized by two inter and one intra chain disulphide bonds. Insulin's metabolic actions are expressed through binding as a monomer to the insulin receptor (IR). The structure of insulin, which has been known for four decades (1), has not provided insight into the mode of receptor binding and hormone activation. This is because detailed three-dimensional knowledge of insulin's complex structural behavior is limited to its inactive storage (hexameric, dimeric) states (2-4). The NMR structures of the monomeric form of insulin facilitated by mutations (5), applications of organic co-solvents (6) or truncation of the B-chain (7) merely confirm the conformations known from the inactive forms but also indicate intrinsic mobility of the N-and C termini of the B-chain. It has also been found that the N terminus of insulin can exist in so called T (extended) or R (helical) conformations, however, their role for insulin activation is still ambiguous (3, 4).It is widely acknowledged that insulin must therefore undergo induced-fit structural changes upon binding to the IR be...

Background:The structure of the C-terminal B21-B30 chain of insulin bound to the insulin receptor remains undetermined. Results: The structures of B24-modified insulins were determined. Conclusion: The structural integrity of Phe B24 but flexibility of B25-B30 insulin residues are important for receptor binding. Significance: The knowledge of the receptor-bound structure of insulin is important for the design of new insulin receptor agonists.

The N-terminus of the B-chain of insulin may adopt two alternative conformations designated as the T- and R-states. Despite the recent structural insight into insulin–insulin receptor (IR) complexes, the physiological relevance of the T/R transition is still unclear. Hence, this study focused on the rational design, synthesis, and characterization of human insulin analogues structurally locked in expected R- or T-states. Sites B3, B5, and B8, capable of affecting the conformation of the N-terminus of the B-chain, were subjects of rational substitutions with amino acids with specific allowed and disallowed dihedral φ and ψ main-chain angles. α-Aminoisobutyric acid was systematically incorporated into positions B3, B5, and B8 for stabilization of the R-state, and N-methylalanine and d-proline amino acids were introduced at position B8 for stabilization of the T-state. IR affinities of the analogues were compared and correlated with their T/R transition ability and analyzed against their crystal and nuclear magnetic resonance structures. Our data revealed that (i) the T-like state is indeed important for the folding efficiency of (pro)insulin, (ii) the R-state is most probably incompatible with an active form of insulin, (iii) the R-state cannot be induced or stabilized by a single substitution at a specific site, and (iv) the B1–B8 segment is capable of folding into a variety of low-affinity T-like states. Therefore, we conclude that the active conformation of the N-terminus of the B-chain must be different from the “classical” T-state and that a substantial flexibility of the B1–B8 segment, where GlyB8 plays a key role, is a crucial prerequisite for an efficient insulin–IR interaction.