Insulin receptor /3-subunit tyrosine kinase

The insulin receptor is composed of two heterodimers each heterodimer is composed of an a unit and a P unit. The a unit is extracellular and contains the insulin recognition and binding sites the p unit spans the cellular membrane and contains a tyrosine kinase. Although insulin can bind to a single ap dimer, it binds with higher affinity to the aPaP tetrameric complex. When insulin binds to an a unit, the tyrosine kinase associated with the corresponding p unit is stimulated. Following this, intracellular proteins such as IRS-1 and IRS-2 (IRS=insulin receptor substrate) are phosphorylated by the P subunit tyrosine kinase, and they in turn activate a network of phosphorylations within the receptor cell. 

The reduction in receptors may be a secondary consequence of hyper-glycaemia and hyperinsulinaemia and does not relate well to the impairment of insulin action. It therefore seems unlikely that a defect in the number or affinity of insulin receptors is a primary abnormality in NIDDM. However, it is possible that an abnormality of the portion of the insulin receptor projecting into the cell (/3-subunit) could contribute to insulin resistance. Tyrosine kinase activity of the /3-subunit appears to be intimately involved in mediating insulin action this tyrosine kinase activity is reduced in some animal models of diabetes and there is some evidence that the same is true in human NIDDM. 

The cellular insulin receptor (IR) is a tetrameric trans-membrane protein consisting of two a and two j8 subunits. In a first step, insulin docks to the a subunits at the extracellular surface, initiating autophosphorylation of tyrosines on the jS subunits at the intracellular surface (scenarios I and II in Figure 5.12). Subsequently, various endogenous substrates are phosphorylated. These substrates can be subsumed, in part, as insulin receptor substrates (IRS). In other words insulin stimulates the tyrosine kinase activity... 

Insulin Receptor. Figure 1 Structure and function of the insulin receptor. Binding of insulin to the a-subunits (yellow) leads to activation of the intracellular tyrosine kinase ((3-subunit) by autophosphorylation. The insulin receptor substrates (IRS) bind via a phospho-tyrosine binding domain to phosphorylated tyrosine residues in the juxtamembrane domain of the (3-subunit. The receptor tyrosine kinase then phosphorylates specific tyrosine motifs (YMxM) within the IRS. These tyrosine phosphorylated motifs serve as docking sites for some adaptor proteins with SRC homology 2 (SH2) domains like the regulatory subunit of PI 3-kinase. 

Concanavalin A is a plant lectin from the jack bean (Canavalia ensiformis) which binds with high affinity to mannose residues of glycoproteins. Concanavalin A is known to stimulate the tyrosine kinase activity of the INSR (3-subunit with consecutive activation of kinases downstream the insulin receptor (IRS, PI 3-kinase). It is believed that Concanavalin A stimulates the activation and autophosphorylation of the INSR kinase through aggregation of the receptor, although the precise mechanism of action is unclear. 

The catalytic pi 10 subunit has four isoforms, all of which contain a kinase domain and a Ras interaction site. In addition, the a, (3, and y isoforms possess an interaction site for the p85 subunit. The class I enzymes can be further subdivided class IA enzymes interact through their SH2 domains with phosphotyrosines present on either protein tyrosine kinases or to docking proteins such as insulin-receptor substrates (IRSs GAB-1) or linkers for activation of T cells (LATs in the case of T cells). 

The cytoplasmic domain of the P-subunit displays three distinct sub-domains (a) the juxtam-embrane domain , implicated in recognition/binding of intracellular substrate molecules (b) the tyrosine kinase domain, which (upon receptor activation) displays tyrosine kinase activity (c) the C-terminal domain, whose exact function is less clear, although site-directed mutagenesis studies implicate it promoting insulin s mitogenic effects. 

Figure 11.2 Structure of the insulin receptor (a). Binding of insulin promotes autophosphorylation of the (3-subunits, where each (3-subunit phosphorylates the other (3-subunit. Phosphate groups are attached to three specific tyrosine residues (tyrosines 1158, 1162 and 1163), as indicated in (b). Activation of the (3-subunit s tyrosine kinase activity in turn results in the phosphorylation of various intracellular (protein) substrates which trigger the mitogen-activated protein kinase and/or the phosphoinositide (PI-3) kinase pathway responsible for inducing insulin s mitogenic and metabolic effects. The underlying molecular events occurring in these pathways are complex (e.g. refer to Combettes-Souverain, M. and Issad, T. 1998. Molecular basis of insulin action. Diabetes and Metabolism, 24, 477-489)...

These molecules span the membrane with only one a-helix. The subunits of the dimeric receptor (red and blue) each consist of two polypeptides (a and P) bound by disulfide bonds. The a-chains together bind the insulin, while the p-chains contain the transmembrane helix and, at the C-terminus, domains with tyrosine kinase activity. In the activated state, the kinase domains phosphorylate themselves and also mediator proteins (receptor substrates) that set in motion cascades of further phosphorylations (see pp. 120 and 388). 

The insulin receptor (top) is a dimer with subunits that have activatable tyrosine kinase domains in the interior of the cell (see p. 224). Binding of the hormone increases the tyrosine kinase activity of the receptor, which then phosphorylates itself and other proteins (receptor substrates) at various tyrosine residues. Adaptor proteins, which conduct the signal further, bind to the phosphotyrosine residues. 

I Insulin binding activates I receptor tyrosine kinase activity in the intracellular domain of the 3 subunit of the insulin receptor. 

Insulin receptor The insulin receptor is synthesized as a single polypeptide that is glycosylated and cleaved into a and 3-sub-units, which are then assembled into a tetramer linked by disulfide bonds (Figure 23.7). A hydrophobic domain in each 3-subunit spans the plasma membrane. The extracellular a-subunit con tains the insulin binding site. The cytosolic domain of the 3-subunit is a tyrosine kinase, which is activated by insulin. 

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