Colin W. Ward

How insulin engages its primary binding site on the insulin receptor.

Insulin receptor signalling has a central role in mammalian biology, regulating cellular metabolism, growth, division, differentiation and survival. Insulin resistance contributes to the pathogenesis of type 2 diabetes mellitus and the onset of Alzheimer’s disease; aberrant signalling occurs in diverse cancers, exacerbated by cross-talk with the homologous type 1 insulin-like growth factor receptor (IGF1R). 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 kinases. 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.

The first three domains of the insulin receptor differ structurally from the insulin-like growth factor 1 receptor in the regions governing ligand specificity

The insulin receptor (IR) and the type-1 insulin-like growth factor receptor (IGF1R) are homologous multidomain proteins that bind insulin and IGF with differing specificity. Here we report the crystal structure of the first three domains (L1–CR–L2) of human IR at 2.3 Å resolution and compare it with the previously determined structure of the corresponding fragment of IGF1R. The most important differences seen between the two receptors are in the two regions governing ligand specificity. The first is at the corner of the ligand-binding surface of the L1 domain, where the side chain of F39 in IR forms part of the ligand binding surface involving the second (central) β-sheet. This is very different to the location of its counterpart in IGF1R, S35, which is not involved in ligand binding. The second major difference is in the sixth module of the CR domain, where IR contains a larger loop that protrudes further into the ligand-binding pocket. This module, which governs IGF1-binding specificity, shows negligible sequence identity, significantly more α-helix, an additional disulfide bond, and opposite electrostatic potential compared to that of the IGF1R.

Ligand-induced activation of the insulin receptor: a multi-step process involving structural changes in both the ligand and the receptor.

Current models of insulin binding to the insulin receptor (IR) propose (i) that there are two binding sites on the surface of insulin which engage with two binding sites on the receptor and (ii) that ligand binding involves structural changes in both the ligand and the receptor. Many of the features of insulin binding to its receptor, namely B‐chain helix interactions with the leucine‐rich repeat domain and A‐chain residue interactions with peptide loops from another part of the receptor, are also seen in models of relaxin and insulin‐like peptide 3 binding to their receptors. We show that these principles can likely be extended to the group of mimetic peptides described by Schäffer and coworkers, which are reported to have no sequence identity with insulin. This review summarizes our current understanding of ligand‐induced activation of the IR and highlights the key issues that remain to be addressed.

Structural resolution of a tandem hormone-binding element in the insulin receptor and its implications for design of peptide agonists

The C-terminal segment of the human insulin receptor α-chain (designated αCT) is critical to insulin binding as has been previously demonstrated by alanine scanning mutagenesis and photo-cross-linking. To date no information regarding the structure of this segment within the receptor has been available. We employ here the technique of thermal-factor sharpening to enhance the interpretability of the electron-density maps associated with the earlier crystal structure of the human insulin receptor ectodomain. The αCT segment is now resolved as being engaged with the central β-sheet of the first leucine-rich repeat (L1) domain of the receptor. The segment is α-helical in conformation and extends 11 residues N-terminal of the classical αCT segment boundary originally defined by peptide mapping. This tandem structural element (αCT-L1) thus defines the intact primary insulin-binding surface of the apo-receptor. The structure, together with isothermal titration calorimetry data of mutant αCT peptides binding to an insulin minireceptor, leads to the conclusion that putative “insulin-mimetic” peptides in the literature act at least in part as mimics of the αCT segment as well as of insulin. Photo-cross-linking by novel bifunctional insulin derivatives demonstrates that the interaction of insulin with the αCT segment and the L1 domain occurs in trans, i.e., these components of the primary binding site are contributed by alternate α-chains within the insulin receptor homodimer. The tandem structural element defines a new target for the design of insulin agonists for the treatment of diabetes mellitus.

Expression of potyvirus coat protein in Escherichia coli and yeast and its assembly into virus-like particles.

When the full-length coat protein (CP) of the potyvirus, Johnsongrass mosaic virus (JGMV), was expressed in Escherichia coli or yeast, it assembled to form potyvirus-like particles. The particles were heterogeneous in length with a stacked-ring appearance and resembled JGMV particles in their flexuous morphology and width. This cell-free assembly system should permit analysis of the mechanisms of particle assembly and genome encapsidation. Two mutant forms of CP produced by site-directed mutagenesis failed to assemble into virus-like particles.

Protective hinge in insulin opens to enable its receptor engagement

Significance Insulin provides a model for analysis of protein structure and evolution. Here we describe in detail a conformational switch that enables otherwise hidden nonpolar surfaces in the hormone to engage its receptor. Whereas the classical closed conformation of insulin enables its stable storage in pancreatic β cells, its active conformation is open and susceptible to nonnative aggregation. Our findings illuminate biophysical constraints underlying the evolution of an essential signaling system and provide a structural foundation for design of therapeutic insulin analogs. Insulin provides a classical model of a globular protein, yet how the hormone changes conformation to engage its receptor has long been enigmatic. Interest has focused on the C-terminal B-chain segment, critical for protective self-assembly in β cells and receptor binding at target tissues. Insight may be obtained from truncated “microreceptors” that reconstitute the primary hormone-binding site (α-subunit domains L1 and αCT). We demonstrate that, on microreceptor binding, this segment undergoes concerted hinge-like rotation at its B20-B23 β-turn, coupling reorientation of PheB24 to a 60° rotation of the B25-B28 β-strand away from the hormone core to lie antiparallel to the receptors L1–β2 sheet. Opening of this hinge enables conserved nonpolar side chains (IleA2, ValA3, ValB12, PheB24, and PheB25) to engage the receptor. Restraining the hinge by nonstandard mutagenesis preserves native folding but blocks receptor binding, whereas its engineered opening maintains activity at the price of protein instability and nonnative aggregation. Our findings rationalize properties of clinical mutations in the insulin family and provide a previously unidentified foundation for designing therapeutic analogs. We envisage that a switch between free and receptor-bound conformations of insulin evolved as a solution to conflicting structural determinants of biosynthesis and function.

Site-directed mutagenesis of a potyvirus coat protein and its assembly in Escherichia coli.

Multiple copies of the Johnsongrass mosaic virus coat protein synthesized in Escherichia coli can readily assemble to form potyvirus-like particles. This E. coli expression system has been used to identify some of the key amino acid residues, within the core region of the coat protein, required for assembly. The two charged residues R194 and D238 previously proposed theoretically to be involved as a pair in the construction of a salt bridge crucial for the assembly process were targeted for site-directed mutagenesis. The results from our experiments suggest that the two residues are required for the assembly process but are not necessarily involved as a pair in a common salt bridge.

Higher-Resolution Structure of the Human Insulin Receptor Ectodomain: Multi-Modal Inclusion of the Insert Domain.

Insulin receptor (IR) signaling is critical to controlling nutrient uptake and metabolism. However, only a low-resolution (3.8 Å) structure currently exists for the IR ectodomain, with some segments ill-defined or unmodeled due to disorder. Here, we revise this structure using new diffraction data to 3.3 Å resolution that allow improved modeling of the N-linked glycans, the first and third fibronectin type III domains, and the insert domain. A novel haptic interactive molecular dynamics strategy was used to aid fitting to low-resolution electron density maps. The resulting model provides a foundation for investigation of structural transitions in IR upon ligand binding.

The insulin receptor changes conformation in unforeseen ways on ligand binding: Sharpening the picture of insulin receptor activation

Unraveling the molecular detail of insulin receptor activation has proved challenging, but a major advance is the recent determination of crystallographic structures of insulin in complex with its primary binding site on the receptor. The current model for insulin receptor activation is that two distinct surfaces of insulin monomer engage sequentially with two distinct binding sites on the extracellular surface of the insulin receptor, which is itself a disulfide‐linked (αβ)2 homodimer. In the process, conformational changes occur both within the hormone and the receptor, the latter resulting in the disruption of the intracellular interactions that hold the kinase domains in their basal state and in the initiation of the phosphorylation events that drive insulin signaling. The purpose of this paper is to summarize the extant structural data relating to hormone binding and how it effects receptor activation, as well as to discuss the issues that remain unresolved.

Similar but different: ligand-induced activation of the insulin and epidermal growth factor receptor families.

The insulin and epidermal growth factor receptor families are among the most intensively studied proteins in biology. They are closely related members of the receptor tyrosine kinase superfamily and deregulated signaling by members of either receptor family has been implicated in the progression of a variety of cancers. These receptors have thus emerged as validated therapeutic targets for the development of anti-tumour agents. Recent studies have revealed detail of the ligand-binding sites in the insulin receptor family, as well as detail of conformational change upon ligand binding in the epidermal growth factor receptor family. Taken together, these findings and further data relating to kinase activation highlight the fact that while the receptor families share common structural elements, the structural detail of their functioning is remarkably different.