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...
