The insulin action sequence is initiated with specific binding to high-affinity receptors on the plasma membrane of target cells. The insulin receptor is a large transmembrane glycoprotein consisting of two α- and two β- subunits that form a heterotetramer, as depicted in Figure 12.1. The insulin receptor is synthesized from a single gene as a polypeptide pro receptor comprised of α- and β- subunit sequences in tandem. The preceptor is processed into separate α- and β- subunits which then assemble as a disulfide-linked holoenzyme of stoichiometry (αβ)2 . The 135 kDa α subunits, derived from the amino-terminal portion of the baroreceptor, reside entirely on the outside of the cell, tethered to the membrane via the 95 kDa β subunits which span the membrane. Insulin binds to the extracellular α subunit. Studies using chimeric receptors containing structural domains from the homologous insulin-like growth factor-1 (IGF-1) receptor have established that multiple extracellular regions including glycine-centered repeats in the α subunit establish the specificity for insulin binding ; however, a cysteine-rich domain has been shown to directly interact with the ligand. Insulin binding results in conformational changes that bring the α subunits closer together, and enables ATP to bind to the intracellular domain of the β subunit.
Tyrosine kinase activity
The β subunits possess tyrosine kinase activity [5,6]. The initial event following insulin-binding appears to be phosphorylation of one β subunit catalyzed by a specific domain in the other β subunit within the same heterotetramer, a reaction that has been termed “trans phosphorylation” or autophosphorylation . Autophosphorylation of the kinase renders it considerably more active, allowing the accelerated transfer of phosphate to other tyrosine sites within the same receptor as well as exogenous substrates . Ligand-dependent stimulation of the β-subunit tyrosine kinase activity is critical for promulgation of the insulin signal. There is an ATP-binding domain in the β subunit, and at least six tyrosine residues undergoing phosphorylation that have been shown in mutagenesis experiments to serve different roles in insulin signaling, as shown in Figure 12.1. These phospho tyrosines lie within three functional groups. Tyrosine phosphorylation sites at positions Y1158 , Y1162 , and Y1163 are essential for mediating an increase in subunit tyrosine kinase activity and signal transduction, and lie in the active loop of the kinase catalytic domain. When phosphorylated, a juxtamembrane tyrosine at position Y972 becomes part of a recognition motif and provides a docking site for several intracellular substrates. Y972 is required to assure sufficient stability of the receptor/substrate complex for substrate phosphorylation. Y972 also lies within a domain that mediates endocytosis of ligand-bound insulin receptors. Phosphorylation sites Y 1328 and Y 1334 near the COOH terminus are not essential for stimulation of glucose transport but may affect the sensitivity of Ras/MAP kinase pathway activation, and, so, are involved in the receptor’s mitogenic responses . Phosphorylation of serine and threonine residues within the insulin receptor also occurs following insulin binding, and, as described later, phosphorylation of these residues by intracellular serine kinases can diminish receptor tyrosine kinase activity; it represents a regulatory feedback mechanism  and can induce insulin resistance under pathophysiologic conditions.
Insulin-like growth factors
The insulin receptor is similar in structure to IGF-1 and other growth factor and cytokine receptors (all (αβ)2 stoichiometry), and IGF-II receptors ((αβ) stoichiometry), in having an extracellular ligand-binding domain that activates an intracellular tyrosine kinase domain [2,3,11]. The respective ligands, insulin and IGF-1, are also closely related hormones from an evolutionary perspective, but have widely different functions in regulating metabolism and growth, respectively. Nevertheless, it has been recognized for some time that these hormones can elicit similar effects to one another, if they are present at high concentrations. This suggested that they could bind to one another’s receptors, with differing affinities. In fact, both IGF-1 and IGF-II can bind to cell surface insulin receptors. The affinity of IGF-1 for the insulin receptor is 100- to 1000-fold less than that for insulin. Nevertheless, circulating IGF-I concentrations can be 100-fold higher than insulin, such that there is some potential for IGF-I binding and action through the insulin receptor. IGF-II binds insulin receptors during embryogenesis in rodents, and promotes fetal growth. This capability may be associated with expression of an alternatively spliced form of the insulin receptor with high affinity for IGF-II . Alternative splicing in exon 11 determines the insertion or deletion of 12 amino acids near the COOH terminus of the α subunit. Iso- form A lacking the 12 amino acids has high affinity for IGF-II, predominates during fetal development, and promotes growth as a consequence of IGF-II binding. Isoform B containing the 12 amino acids predominates postnatally and is activated mainly by insulin. Some evidence supports the contention that dysregulated expression towards the fetal pattern could occur in adult tissues and result in insulin resistance [12,13].
Because of the heterotetrameric nature of the insulin and IGF-1 receptors, it was natural to ask whether individual dimers could be formed from “hybrid” receptors, consisting of one α/β insulin receptor dimer and one α/β IGF-1 receptor dimer. Such hybrid receptors were indeed demonstrated to exist [14 – 16], and it appears likely that these hybrid receptors form stochastically, such that if half of the dimers present were insulin receptor dimers, and half were IGF-1 receptor dimers, half of the mature receptors present in the membrane would be hybrid insulin/IGF-1 receptors, with about one quarter each “pure” insulin or IGF-1 heterotetramers. This raises the question of what the main ligand is for such hybrid receptors, and what downstream effects would be triggered when the hybrid receptors are occupied. Most evidence would suggest that hybrid receptors bind IGF-1 with greater affinity than insulin and mediate IGF-1-like effects .
The implication of these findings is that the ratio of IGF-1 to insulin receptor dimer subunits could have an important influence on the number of insulin receptor heterotetramers that exist in the plasma membrane and can actually function to mediate insulin’s metabolic effects. Increased IGF-1 receptor expression can produce insulin resistance via increasing the proportion of hybrid receptors, essentially hijacking insulin receptor dimers to serve a function in IGF-1 signaling . For example, aldosterone can induce vascular insulin resistance by this mechanism , and IGF-1 receptor expression can modulate insulin sensitivity and nitric oxide availability in endothelium . Hybrid insulin/IGF-1 receptors may also play a role as growth factor receptors in myeloma cells  and endometrial cancer cells , as well as affecting the function of platelets . The role of hybrid insulin/IGF-1 receptors in regulating insulin action and disease processes is an understudied area.
Insulin receptor mutations
Identification of functional domains within the insulin receptor has been accomplished by studying site-directed mutations, knockout and transgenic mice, and rare native mutations of the insulin receptor in patients with severe forms of insulin resistance. Native insulin receptor gene mutations have been detected in all major structural and functional domains, and generally either diminish ligand binding and/or impair β-subunit tyrosine kinase activity . Five major categories of insulin receptor mutations have been delineated that cause severe defects in insulin binding. These include: (i) nonsense or frameshift mutations that lead to premature termination of translation and decreased receptor synthesis; (ii) mutations that impair intracellular transport and posttranslational processing of receptors such that fewer receptors reach the cell surface; (iii) mutations in the insulin-binding domain that affect ligand affinity; (iv) mutations that enhance ligand-mediated endocytosis and degradation of insulin receptors; and (v) mutations that cause defects in receptor kinase activity. These latter mutations usually consist of point mutations in one of three areas within the β subunit: the ATP binding site, the tyrosine kinase domain, or other areas in the intracellular domain. Phenotypically, patients with syndromes of severe insulin resistance can exhibit mild diabetes or glucose intolerance with marked hyperinsulinemia, acanthosis nigricans, hyperandrogenism, and abnormalities of growth and development with precocious pseudopuberty and cardiac enlargement. Related disease categories include type A severe insulin resistance, lipoatrophic diabetes, leprechaunism, and Rabson – Mendenhall syndrome [24,25].