Activation of the human insulin receptor by non-insulin-related peptides

The human insulin receptor signalling system plays a critical role in glucose homeostasis. Insulin binding brings about extensive conformational change in the receptor extracellular region that in turn effects trans-activation of the intracellular tyrosine kinase domains and downstream signalling. Of particular therapeutic interest is whether insulin receptor signalling can be replicated by molecules other than insulin. Here, we present single-particle cryoEM structures that show how a 33-mer polypeptide unrelated to insulin can cross-link two sites on the receptor surface and direct the receptor into a signalling-active conformation. The 33-mer polypeptide engages the receptor by two helical binding motifs that are each potentially mimicable by small molecules. The resultant conformation of the receptor is distinct from—but related to—those in extant three-dimensional structures of the insulin-complexed receptor. Our findings thus illuminate unexplored pathways for controlling the signalling of the insulin receptor as well as opportunities for development of insulin mimetics.

The regulation of plasma glucose levels is effected by insulin. Here, the authors reveal atomic detail of how peptides distinct from insulin bind to and activate the insulin receptor, with implications for design of small-molecule insulin mimetics.

How insulin-like growth factor I binds to a hybrid insulin receptor type 1 insulin-like growth factor receptor

Monomers of the insulin receptor and type 1 insulin-like growth factor receptor (IGF-1R) can combine stochastically to form heterodimeric hybrid receptors. These hybrid receptors display ligand binding and signaling properties that differ from those of the homodimeric receptors. Here, we describe the cryoelectron microscopy structure of such a hybrid receptor in complex with insulin-like growth factor I (IGF-I). The structure (ca. 3.7 Å resolution) displays a single IGF-I ligand, bound in a similar fashion to that seen for IGFs in complex with IGF-1R. The IGF-I ligand engages the first leucine-rich-repeat domain and cysteine-rich region of the IGF-1R monomer (rather than those of the insulin receptor monomer), consistent with the determinants for IGF binding residing in the IGF-1R cysteine-rich region. The structure broadens our understanding of this receptor family and assists in delineating the key structural motifs involved in binding their respective ligands.

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A cryo-EM structure of IGF-I bound to a hybrid IR/IGF-1R ectodomain is presented

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The structure is congruent to those of the single-liganded homodimeric receptors

How IGF-II Binds to the Human Type 1 Insulin-like Growth Factor Receptor

Human type 1 insulin-like growth factor receptor (IGF-1R) signals chiefly in response to the binding of insulin-like growth factor I. Relatively little is known about the role of insulin-like growth factor II signaling via IGF-1R, despite the affinity of insulin-like growth factor II for IGF-1R being within an order of magnitude of that of insulin-like growth factor I. Here, we describe the cryoelectron microscopy structure of insulin-like growth factor II bound to a leucine-zipper-stabilized IGF-1R ectodomain, determined in two conformations to a maximum average resolution of 3.2 Å. The two conformations differ in the relative separation of their respective points of membrane entry, and comparison with the structure of insulin-like growth factor I bound to IGF-1R reveals long-suspected differences in the way in which the critical C domain of the respective growth factors interact with IGF-1R.

The signalling conformation of the insulin receptor ectodomain

Understanding the structural biology of the insulin receptor and how it signals is of key importance in the development of insulin analogs to treat diabetes. We report here a cryo-electron microscopy structure of a single insulin bound to a physiologically relevant, high-affinity version of the receptor ectodomain, the latter generated through attachment of C-terminal leucine zipper elements to overcome the conformational flexibility associated with ectodomain truncation. The resolution of the cryo-electron microscopy maps is 3.2 Å in the insulin-binding region and 4.2 Å in the membrane-proximal region. The structure reveals how the membrane proximal domains of the receptor come together to effect signalling and how insulin's negative cooperativity of binding likely arises. Our structure further provides insight into the high affinity of certain super-mitogenic insulins. Together, these findings provide a new platform for insulin analog investigation and design.

How ligand binds to the type 1 insulin-like growth factor receptor

Human type 1 insulin-like growth factor receptor is a homodimeric receptor tyrosine kinase that signals into pathways directing normal cellular growth, differentiation and proliferation, with aberrant signalling implicated in cancer. Insulin-like growth factor binding is understood to relax conformational restraints within the homodimer, initiating transphosphorylation of the tyrosine kinase domains. However, no three-dimensional structures exist for the receptor ectodomain to inform atomic-level understanding of these events. Here, we present crystal structures of the ectodomain in apo form and in complex with insulin-like growth factor I, the latter obtained by crystal soaking. These structures not only provide a wealth of detail of the growth factor interaction with the receptor’s primary ligand-binding site but also indicate that ligand binding separates receptor domains by a mechanism of induced fit. Our findings are of importance to the design of agents targeting IGF-1R and its partner protein, the human insulin receptor.

The human type 1 insulin-like growth factor receptor (IGF-1R) is important for normal human growth and development. Here, the authors present the crystal structures of the IGF-1R ectodomain both in its apo form and in complex with its ligand insulin-like growth factor I and discuss the receptor activation mechanism.

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 Phe(701) and Phe(705) 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.

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.

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.

A thermodynamic study of ligand binding to the first three domains of the human insulin receptor: relationship between the receptor alpha-chain C-terminal peptide and the site 1 insulin mimetic peptides

The C-terminal segment of the insulin receptor (IR) alpha-chain plays a critical role in insulin binding. This 16-residue peptide together with the central beta-sheet of the receptor L1 domain forms one of the insulin binding surfaces of the IR monomer. Here we use isothermal titration calorimetry to assay directly the binding of the IR alphaCT peptide to an IR construct (IR485) consisting of the three N-terminal domains of the receptor monomer. Our measurements show further that the binding of the IR alphaCT peptide to IR485 competes with the binding of a prototypical "Site 1" insulin mimetic peptide to the same receptor fragment. The competitive nature of their binding appears to be reflected in a previously undetected sequence similarity between the IR alphaCT peptide and the Site 1 mimetic peptide. In contrast, a prototypical "Site 2" peptide has very limited affinity for IR485. Taken together, these results complement our recent observation that there is a possible structural relationship between these mimetic peptides and insulin itself. They also add support to the view that the segment of unexplained electron density lying on the surface of the central beta-sheet of the L1 domain in the IR ectodomain crystal structure arises from the IR alphaCT peptide. Finally, we show that mutation of the critical IR alphaCT peptide residue Phe714 to alanine does not affect the peptide's affinity for IR485 and conclude that the resultant loss of insulin binding with this mutation results from loss of interaction of the phenylalanine side chain with insulin.

Antibody responses to HPV6b E polyproteins and production of monoclonal antibodies

A range of fusion constructs (expressed in Escherichia coli) were produced that contained two or more HPV6b E proteins, producing a single continuous amino acid sequence corresponding to the sequences of the individual E proteins. The constructs also included a C-terminal hexahistidine tag fused in-frame to aid purification. The fusion proteins (polyproteins) were semipurified by Ni(++) metal affinity chromatography under denaturing conditions. Immunization of BALB/c mice with these polyproteins resulted in the production of specific E protein antibodies. The draining lymph nodes from these mice were used to produce monoclonal antibodies (MAbs). The specificity of the polyclonal and MAbs was confirmed by immunoblotting and by screening for reaction with a series of synthetic peptides of E proteins. HPV E polyproteins were found to be immunogenic and immunization with the polyproteins resulted in specific antibody responses to the component E proteins.

Overexpression, purification, and refolding of a Porphyromonas gingivalis cysteine protease from Escherichia coli

This paper describes the overexpression of the Rgp-1 (arginine) protease domain from Porphyromonas gingivalis. This protease and the related Kgp (lysine) protease, both of which display trypsin-like specificity, have been implicated as major virulence factors and may play a significant role in the etiology of periodontal disease. Both Rgp-1 and Kgp are initially translated as polyproteins, each containing a protease domain and multiple adhesin domains. The Rgp-1 protease domain was expressed in E. coli, purified, refolded, and assayed for activity. These expression studies demonstrated that prior to the formation of inclusion bodies in the E. coli cytoplasm, the protease was proteolytically active and could hydrolyze a specific synthetic substrate. When the Rgp-1 protease domain was purified from inclusion bodies and refolded, it was found to be autolytically active and displayed specific catalytic activity. This is the first report on the expression and purification of active Rgp-1 from E. coli. Polyclonal antisera raised against recombinant protein recognized the native form of the protease in the P. gingivalis strain W50, indicating that the recombinant protein contained some of the antigenic determinants of the native protease.