John G. Menting

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.

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.

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.

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.

The antimalarial drug, chloroquine, interacts with lactate dehydrogenase from Plasmodium falciparum

We have previously shown that a radioiodinated photoreactive analogue of chloroquine, [125I]N-(4-(4-diethylamino-1-methylbutylamino)quinolin-6-yl) -4-azido-2-hydroxybenzamide ([125I]ASA-Q), specifically labels two proteins in Plasmodium falciparum with apparent molecular weights (Mr) of 42 and 33 kDa (Foley M, Deady LW, Ng K, Cowman AF, Tilley L. J Biol Chem 1994:269:6955-6961). We now report the identification of the 33 kDa protein. The 33 kDa protein was purified from Plasmodium falciparum using photoaffinity labeling with [125I]ASA-Q to monitor the enrichment process. N-terminal sequence analysis of the purified protein revealed exact identity of the first 35 amino acids with P. falciparum lactate dehydrogenase (PfLDH). The plasmodial enzyme was cloned and expressed in E. coli and the recombinant protein used to produce a rabbit antiserum. Immunoprecipitation using affinity-purified anti-PfLDH antibodies confirmed the identity of the 33 kDa CQ-binding protein. The enzyme activity of purified PfLDH was not significantly affected by chloroquine indicating that PfLDH is not a direct target of CQ. PfLDH was, however, shown to be exquisitely sensitive to inhibition by free heme and chloroquine protected against this inhibitory effect.

Solution structure of ectodomains of the insulin receptor family: the ectodomain of the type 1 insulin-like growth factor receptor displays asymmetry of ligand binding accompanied by limited conformational change.

The insulin receptor (IR) and the homologous Type 1 insulin-like growth factor receptor (IGF-1R) are cell-surface tyrosine kinase receptors that effect signaling within the respective pathways of glucose metabolism and normal human growth. While ligand binding to these receptors is assumed to result in a structural transition within the receptor ectodomain that then effects signal transduction across the cell membrane, little is known about the molecular detail of these events. Presented here are small-angle X-ray scattering data obtained from the IR and IGF-1R ectodomains in solution. We show that, in solution, the ectodomains of IR and IGF-1R have a domain disposition that is very similar to that seen in the crystal structure of the ectodomain of IR, despite the constituent domains being in relatively sparse contact and potentially mobile. We also show that the IGF-1R ectodomain is capable of binding up to three molecules of IGF-1 in solution, with surprisingly little apparent change in relative domain disposition compared to the apo form. While the observed 3:1 ligand-binding stoichiometry appears to contradict earlier explanations of the absence of a bell-shaped dose-response curve for IGF-1R in ligand displacement assays, it is readily understood in the context of the harmonic oscillator model of the negative cooperativity of ligand binding to IGF-1R. Taken together, our findings suggest that the structural movements within these receptors upon ligand binding are small and are possibly limited to local rotation of domains.

Structural Congruency of Ligand Binding to the Insulin and Insulin/Type 1 Insulin-like Growth Factor Hybrid Receptors.

The homodimeric insulin and type 1 insulin-like growth factor receptors (IR and IGF-1R) share a common architecture and each can bind all three ligands within the family: insulin and insulin-like growth factors I and II (IGF-I and IFG-II). The receptor monomers also assemble as heterodimers, the primary ligand-binding sites of which each comprise the first leucine-rich repeat domain (L1) of one receptor type and an α-chain C-terminal segment (αCT) of the second receptor type. We present here crystal structures of IGF-I bound to such a hybrid primary binding site and of a ligand-free version of an IR αCT peptide bound to an IR L1 plus cysteine-rich domain construct (IR310.T). These structures, refined at 3.0-Å resolution, prove congruent to respective existing structures of insulin-complexed IR310.T and the intact apo-IR ectodomain. As such, they provide key missing links in the emerging, but sparse, repertoire of structures defining the receptor family.

A minimized human insulin-receptor-binding motif revealed in a Conus geographus venom insulin

Insulins in the venom of certain fish-hunting cone snails facilitate prey capture by rapidly inducing hypoglycemic shock. One such insulin, Conus geographus G1 (Con-Ins G1), is the smallest known insulin found in nature and lacks the C-terminal segment of the B chain that, in human insulin, mediates engagement of the insulin receptor and assembly of the hormones hexameric storage form. Removal of this segment (residues B23–B30) in human insulin results in substantial loss of receptor affinity. Here, we found that Con-Ins G1 is monomeric, strongly binds the human insulin receptor and activates receptor signaling. Con-Ins G1 thus is a naturally occurring B-chain-minimized mimetic of human insulin. Our crystal structure of Con-Ins G1 reveals a tertiary structure highly similar to that of human insulin and indicates how Con-Ins G1s lack of an equivalent to the key receptor-engaging residue PheB24 is mitigated. These findings may facilitate efforts to design ultrarapid-acting therapeutic insulins.

Aromatic Anchor at an Invariant Hormone-Receptor Interface FUNCTION OF INSULIN RESIDUE B24 WITH APPLICATION TO PROTEIN DESIGN

Background: Invariant insulin residue PheB24 (a site of diabetes-associated mutation) contacts the insulin receptor. Results: Hormonal function requires hydrophobicity rather than aromaticity at this site. Conclusion: The B24 side chain provides a nonpolar anchor at the receptor interface. Significance: Nonstandard aliphatic modification of residue B24 may enhance therapeutic properties of insulin analogs. Crystallographic studies of insulin bound to fragments of the insulin receptor have recently defined the topography of the primary hormone-receptor interface. Here, we have investigated the role of PheB24, an invariant aromatic anchor at this interface and site of a human mutation causing diabetes mellitus. An extensive set of B24 substitutions has been constructed and tested for effects on receptor binding. Although aromaticity has long been considered a key requirement at this position, MetB24 was found to confer essentially native affinity and bioactivity. Molecular modeling suggests that this linear side chain can serve as an alternative hydrophobic anchor at the hormone-receptor interface. These findings motivated further substitution of PheB24 by cyclohexanylalanine (Cha), which contains a nonplanar aliphatic ring. Contrary to expectations, [ChaB24]insulin likewise exhibited high activity. Furthermore, its resistance to fibrillation and the rapid rate of hexamer disassembly, properties of potential therapeutic advantage, were enhanced. The crystal structure of the ChaB24 analog, determined as an R6 zinc-stabilized hexamer at a resolution of 1.5 Å, closely resembles that of wild-type insulin. The nonplanar aliphatic ring exhibits two chair conformations with partial occupancies, each recapitulating the role of PheB24 at the dimer interface. Together, these studies have defined structural requirements of an anchor residue within the B24-binding pocket of the insulin receptor; similar molecular principles are likely to pertain to insulin-related growth factors. Our results highlight in particular the utility of nonaromatic side chains as probes of the B24 pocket and suggest that the nonstandard Cha side chain may have therapeutic utility.

Insulin Mimetic Peptide Disrupts the Primary Binding Site of the Insulin Receptor.

Sets of synthetic peptides that interact with the insulin receptor ectodomain have been discovered by phage display and reported in the literature. These peptides were grouped into three classes termed Site 1, Site 2, and Site 3 based on their mutual competition of binding to the receptor. Further refinement has yielded, in particular, a 36-residue Site 2-Site 1 fusion peptide, S519, that binds the insulin receptor with subnanomolar affinity and exhibits agonist activity in both lipogenesis and glucose uptake assays. Here, we report three-dimensional crystallographic detail of the interaction of the C-terminal, 16-residue Site 1 component (S519C16) of S519 with the first leucine-rich repeat domain (L1) of the insulin receptor. Our structure shows that S519C16 binds to the same site on the L1 surface as that occupied by a critical component of the primary binding site, namely the helical C-terminal segment of the insulin receptor α-chain (termed αCT). In particular, the two phenylalanine residues within the FYXWF motif of S519C16 are seen to engage the insulin receptor L1 domain surface in a fashion almost identical to the respective αCT residues Phe701 and Phe705. The structure provides a platform for the further development of peptidic and/or small molecule agents directed toward the insulin receptor and/or the type 1 insulin-like growth factor receptor.