Three-dimensional Structural Interactions of Insulin and Its Receptor

Abstract

The insulin receptor (IR) 1 belongs to the superfamily of transmembrane receptor tyrosine kinases (TKs) (reviewed in Ref. 1). In contrast to other family members that are monomeric in their structure, IR and its homologue, insulin-like growth factor I receptor (IGF-1R), are intrinsic disulfide-linked dimers of heterodimeric disulfide-linked proteins of the form (␣␤) 2 . The 135-kDa ␣ subunit of IR is extracellular, whereas the 95-kDa ␤ subunit contains an extracellular portion, a single transmembrane sequence, and an intracellular TK domain. Fig. 1a depicts the major structural features of the ␣␤ dimer (see review by Tavaré and Siddle (2)). Ligandspecific binding to the ␣ subunits activates the TK, initiating a signal cascade that results in numerous cellular responses. Our understanding of the mechanics of this signal transduction process has been hampered by the unavailability of an atomic structure of the whole IR protein. However, the quaternary structure of the isolated complex of biologically active IR and insulin was recently solved by three-dimensional reconstruction from low dose scanning transmission electron micrographs (STEM) (3) (Fig. 1, b1-b3). Atomic structures of subdomains of IR or of highly analogous proteins were fitted into the complex (e.g. Fig. 1, b4), creating the only available atomic structural model of IR. The model reveals structural details of the interaction of insulin with the receptor that lead to the activation of the intracellular TK (4). Here we review previous biochemical observations on IR binding of natural and modified insulins and of insulin-like growth factor 1 (IGF-1) against this atomic structural model, and in the light of recent structural data on the unbound receptor, we discuss the mechanics of a model of receptor activation arising from insulin binding. Three-dimensional STEMAlthough crystallography remains the technique of choice for atomic structure determination of large proteins, considerable advances in electron microscopy (EM) and in image processing have been made for three-dimensional structure determination of proteins that are refractory to crystallization. For a growing number of proteins, including the plant light-harvesting complex (5), a human water channel (6), and aquaporin-1 (7), three-dimensional structural information has been obtained at resolutions of several Angstrom units, revealing ␣-helices and other structural details.In STEM, image acquisition is digital, as a 3-Å small electron beam probe scans across the specimen at low temperature. Signal intensity is directly proportional to the molecular mass of each molecule. Moreover, STEM in dark field mode can readily visualize clusters of heavy atoms on specifically marked biological molecules (3,8). STEM images have been used to reconstruct three-dimensional structures of several proteins at resolutions of 12-20 Å, such as SRP54, the Klenow fragment of DNA polymerase I, and IR (3, 9 -12).The STEM IR reconstruction at 20 Å was docked with atomic structures of insulin and of subdomains of IR or hi...

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

“…18 Corresponding studies with detergentsolubilized IGF-1R and IGF-1R ectodomain have not been reported. Attempts to investigate the conformational changes in wtIR upon ligand binding using dark-field scanning transmission electron microscopy have also been made, 19 but the models derived have been treated with some skepticism 20 and, in particular, show no correlation with the crystal structure of IR ectodomain homodimer. 3 We have decided to address the question of conformational change in IR and IGF-1R by undertaking a small-angle X-ray scattering (SAXS) investigation of the receptor ectodomains in solution.…”

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

“…For the docked insulin, the total buried accessible surface area is 965 Å 2 for insulin and 960 Å 2 for the receptor. Combining this docking with the putative assignment of domains in the EM reconstruction of whole IR dimers (49), insulin would present a second hydrophobic face (site 2, centered on leucines A16 and B11) toward the hydrophobic patch of the dimer-related L1 domain. Insulin's site 2 is concave in the R state, and this cleft could be occupied, potentially, by the side chains of F88 and F89 of the other receptor.…”

Exploring the Implications of Vitamin B12 Conjugation to Insulin on Insulin Receptor Binding

“…[2][3][4][5] The former has been the focus of experimental and computational studies to determine the positions of…”