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.

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 leucinerich 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 ␣-chain (termed ␣CT). 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 ␣CT 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.In 2002, sets of synthetic peptides were identified that could compete for insulin binding to the human insulin receptor extracellular region and that had affinities in the high nanomolar to low micromolar range (1). Based on competition studies, the putative epitopes of these peptides were grouped into three non-overlapping sites, termed Sites 1, 2, and 3.3 Some Site 1 peptides were able to activate the receptor tyrosine kinase and act as agonists in an insulin-dependent fat cell assay, whereas Site 2 and Site 3 peptides were found to act as antagonists both in phosphorylation and fat cell assays. The highest affinity Site 1 peptides contained the motif FYXWF, whereas Site 2 peptides were characterized by either a short or long disulfide loop (1).Optimized versions of the Site 1 and Site 2 peptides were subsequently described (2), in particular those that were linked in tandem in various ways. Several of these linked peptides were found to function as either potent antagonists (e.g. peptide S661, a Site 1-Site 2 combination) or as potent agonists (e.g. peptide S597, a Site 2-Site 1 combination), depending on the order in which the individual Site 1 and Site 2 peptides were linked (2, 3). In earlier studies (4, 5), we provided isothermal titration calorimetry (ITC) 4 data that suggested how Site 1 peptides might bind to the insulin receptor. The insulin receptor (IR) itself is a disulfide-linked (␣␤) 2 homodimer; each ␣␤ monomer comprises, from its N terminus, two homologous leucine-rich repeat domains (L1 and L2) separated by a cysteine-rich region (CR) comprising eight disulfide-linked modules (6). These domains are followed by three fibronectin type III d...

SummaryWe have determined the structure of the core capsid of an unusual variant of hepatitis B virus, genotype G (HBV/G) at 14 Å resolution, using cryo-electron microscopy. The structure reveals surface features not present in the prototype HBV/A genotype. HBV/G is novel in that it has a unique 36- bp insertion downstream of the core gene start codon. This results in a twelve amino acid insertion at the N-terminal end of the core protein, and two stop codons in the precore region that prevent the expression of HBeAg. HBV/G replication in patients is associated with co-infection with another genotype of HBV, suggesting that HBV/G may have reduced replication efficiency in vivo. We localized the N-terminal insertion in HBV/G and show that it forms two additional masses on the core surface adjacent to each of the dimer-spikes and have modelled the structure of the additional residues within this density. We show that the position of the insertion would not interfere with translocation of nucleic acids through the pores to the core interior compartment. However, the insertion may partially obscure several residues on the core surface that are known to play a role in envelopment and secretion of virions, or that could affect structural rearrangements that may trigger envelopment after DNA second-strand synthesis.