Receptor Enzymes:-The Insulin Receptor Is a Tyrosine-Specific Protein Kinase

Insulin regulates both metabolism and gene expression: the insulin signal passes from the plasma membrane receptor to insulin-sensitive metabolic enzymes and to the nucleus, where it stimulates the transcription of specific genes. The active insulin receptor consists of two identical α chains protruding from the outer face of the plasma membrane and two transmembrane β subunits with their carboxyl termini protruding into the cytosol (Fig. 12–6, step 1). The α chains contain the insulin binding domain, and the intracellular domains of the β chains contain the protein kinase activity that transfers a phosphoryl group from ATP to the hydroxyl group of Tyr residues in specific target proteins. Signaling through the insulin receptor begins (step 1) when binding of insulin to the α chains activates the Tyr kinase activity of the β chains, and each αβ monomer phosphorylates three critical Tyr residues near the carboxyl terminus of the chain β of its partner in the dimer. This autophosphorylation opens up the active site so that the enzyme can phosphorylate Tyr residues of other target proteins (Fig. 12–7).

One of these target proteins (Fig. 12–6, step 2) is insulin receptor substrate-1 (IRS-1). Once phosphorylated on its Tyr residues, IRS-1 becomes the point of nucleation for a complex of proteins (step 3) that carry the message from the insulin receptor to end targets in the cytosol and nucleus, through a long series of inter mediate proteins. First, a P–Tyr residue in IRS-1 is bound by the SH2 domain of the protein Grb2. (SH2 is an abbreviation of Src homology 2; the sequences of SH2 domains are similar to a domain in another protein Tyr kinase, Src (pronounced sark).) A number of signaling proteins contain SH2 domains, all of which bind P–Tyr residues in a protein partner. Grb2 also contains a second protein-binding domain, SH3, that binds to regions rich in Pro residues. Grb2 binds to a proline-rich region of Sos, recruiting Sos to the growing receptor complex. When bound to Grb2, Sos catalyzes the re placement of bound GDP by GTP on Ras, one of a family of guanosine nucleotide–binding proteins (G proteins) that mediate a wide variety of signal transductions (Section 12.4). When GTP is bound, Ras can activate a protein kinase, Raf-1 (step 4), the first of three protein kinases—Raf-1, MEK, and ERK—that form a cascade in which each kinase activates the next by phosphorylation (step 5). The protein kinase ERK is activated by phosphorylation of both a Thr and a Tyr residue. When activated, it mediates some of the biological effects of insulin by entering the nucleus and phosphorylating proteins such as Elk1, which modulates the transcription of about 100 insulin-regulated genes (step 6).

The proteins Raf-1, MEK, and ERK are members of three larger families, for which several nomenclatures are employed. ERK is a member of the MAPK family (mitogen-activated protein kinases; mitogens are signals that act from outside the cell to induce mitosis and cell division). Soon after discovery of the first MAPK, that enzyme was found to be activated by another protein kinase, which came to be called MAP kinase (MEK belongs to this family); and when a third kinase that activated MAP kinase kinase was discovered, it was given the slightly ludicrous family name MAP kinase kinase kinase (Raf-1 is a member of this family; Fig. 12–6). Slightly less cumbersome are the acronyms for these three families, MAPK, MAPKK, and MAPKKK. Kinases in the MAPK and MAPKKK families are specific for Ser or Thr residues, but MAPKKs (here, MEK) phosphory late both a Ser and a Tyr residue in their substrate, a MAPK (here, ERK).

Biochemists now recognize the insulin pathway as but one instance of a more general theme in which hormone signals, via pathways similar to that shown in Fig ure 12–6, result in phosphorylation of target enzymes by protein kinases. The target of phosphorylation is often another protein kinase, which then phosphorylates a third protein kinase, and so on. The result is a cascade of reactions that amplifies the initial signal by many orders of magnitude (see Fig. 12–1b). Cascades such as that shown in Figure 12–6 are called MAPK cascades.

FIGURE 12–6 Regulation of gene expression by insulin. The insulin receptor consists of two chains on the outer face of the plasma mem brane and two chains that traverse the membrane and protrude from the cytoplasmic face. Binding of insulin to the chains triggers a con formational change that allows the autophosphorylation of Tyr residues in the carboxyl-terminal domain of the subunits. Autophosphorylation further activates the Tyr kinase domain, which then catalyzes phosphorylation of other target proteins. The signaling pathway by which insulin regulates the expression of specific genes consists of a cascade of protein kinases, each of which activates the next. The insulin receptor is a Tyr-specific kinase; the other kinases (all shown in blue) phosphorylate Ser or Thr residues. MEK is a dual-specificity kinase, which phosphorylates both a Thr and a Tyr residue in ERK (extracellular regulated kinase); MEK is mitogen-activated, ERK-activating ki nase; SRF is serum response factor. Abbreviations for other compo nents are explained in the text.

FIGURE 12–7 Activation of the insulin-receptor Tyr kinase by auto phosphorylation. (a) In the inactive form of the Tyr kinase domain (PDB ID 1IRK), the activation loop (blue) sits in the active site, and none of the critical Tyr residues (black and red ball-and-stick structures) are phosphorylated. This conformation is stabilized by hydro gen bonding between Tyr¹¹⁶² and Asp¹¹³². (b) When insulin binds to the α chains of insulin receptors, the Tyr kinase of each β subunit of the dimer phosphorylates three Tyr residues (Tyr¹¹⁵⁸, Tyr¹¹⁶², and Tyr¹¹⁶³) on the other β subunit (shown here; PDB ID 1IR3). (Phosphoryl groups are depicted here as an orange space-filling phospho rus atom and red ball-and-stick oxygen atoms.) The effect of introducing three highly charged P–Tyr residues is to force a 30 Å change in the position of the activation loop, away from the substrate-binding site, which becomes available to bind to and phosphorylate a target protein, shown here as a red arrow.

Grb2 is not the only protein that associates with phosphorylated IRS-1. The enzyme phosphoinositide 3 kinase (PI-3K) binds IRS-1 through the former’s SH2 domain (Fig. 12–8). Thus activated, PI-3K converts the membrane lipid phosphatidylinositol 4,5-bisphosphate (see Fig. 10–15), also called PIP2, to phosphatidylinositol 3,4,5-trisphosphate (PIP3). When bound to PIP3, protein kinase B (PKB) is phosphorylated and activated by yet another protein kinase, PDK1. The activated PKB then phosphorylates Ser or Thr residues on its target proteins, one of which is glycogen synthase kinase 3 (GSK3). In its active, nonphosphorylated form, GSK3 phosphorylates glycogen synthase, inactivating it and thereby contributing to the slowing of glycogen synthesis. (This mechanism is believed to be only part of the explanation for the effects of insulin on glycogen metabolism.) When phosphorylated by PKB, GSK3 is inactivated. By thus preventing inactivation of glycogen synthase in liver and muscle, the cascade of protein phosphorylations initiated by insulin stimulates glycogen synthesis (Fig. 12–8). In muscle, PKB triggers the movement of glucose transporters (GLUT4) from internal vesicles to the plasma membrane, stimulating glucose uptake from the blood (Fig. 12–8; see also Box 11–2). PKB also functions in several other signaling pathways, including that triggered by Δ⁹-tetrahydro cannabinol (THC), the active ingredient of marijuana

FIGURE 12–8 Activation of glycogen synthase by insulin. Transmission of the signal is mediated by PI-3 kinase (PI-3K) and protein kinase B (PKB).

and hashish. THC activates the CB1 receptor in the plasma membrane of neurons in the brain, triggering a signaling cascade that involves MAPKs. One consequence of CB1 activation is the stimulation of appetite, one of the well-established effects of marijuana use. The normal ligands for the CB1 receptor are endocannabinoids such as anandamide, which serve to protect the brain from the toxicity of excessive neuronal activity— as in an epileptic seizure, for example. Hashish has for centuries been used in the treatment of epilepsy.

As in all signaling pathways, there is a mechanism for terminating signaling through the PI 3K–PKB pathway. A PIP3-specific phosphatase (PTEN in humans) removes the phosphate at the 3position of PIP3 to produce PIP2, which no longer serves as a binding site for PKB, and the signaling chain is broken. In various types of advanced cancer, tumor cells often have a defect in the PTEN gene and thus have abnormally high levels of PIP3 and of PKB activity. The result seems to be a continuing signal for cell division and thus tu mor growth.

What spurred the evolution of such complicated regulatory machinery? This system allows one activated receptor to activate several IRS-1 molecules, amplifying the insulin signal, and it provides for the integration of signals from several receptors, each of which can phosphorylate IRS-1. Furthermore, because IRS-1 can activate any of several proteins that contain SH2 domains, a single receptor acting through IRS-1 can trigger two or more signaling pathways; insulin affects gene ex pression through the Grb2-Sos-Ras-MAPK pathway and glycogen metabolism through the PI-3K–PKB pathway.

The insulin receptor is the prototype for a number of receptor enzymes with a similar structure and receptor Tyr kinase activity. The receptors for epidermal growth factor and platelet-derived growth factor, for ex ample, have structural and sequence similarities to the insulin receptor, and both have a protein Tyr kinase activity that phosphorylates IRS-1. Many of these receptors dimerize after binding ligand; the insulin receptor is already a dimer before insulin binds. The binding of adaptor proteins such as Grb2 to P–Tyr residues is a common mechanism for promoting protein-protein in teractions, a subject to which we return in Section 12.5. In addition to the many receptors that act as protein Tyr kinases, a number of receptor like plasma membrane proteins have protein Tyr phosphatase activity. Based on the structures of these proteins, we can surmise that their ligands are components of the extracellular matrix or the surfaces of other cells. Although their signaling roles are not yet as well understood as those of the receptor Tyr kinases, they clearly have the potential to reverse the ac tions of signals that stimulate these kinases. A variation on the basic theme of receptor Tyr ki nases is seen in receptors that have no intrinsic protein kinase activity but, when occupied by their ligand, bind a soluble Tyr kinase. One example is the system that regulates the formation of erythrocytes in mammals. The cytokine (developmental signal) for this system is erythropoietin (EPO), a 165 amino acid protein produced in the kidneys. When EPO binds to its plasma membrane receptor (Fig. 12–9), the receptor dimerizes and can now bind the soluble protein kinase JAK (Janus kinase). This binding activates JAK, which phosphory lates several Tyr residues in the cytoplasmic domain of the EPO receptor. A family of transcription factors, collectively called STATs (signal transducers and activators of transcription), are also targets of the JAK kinase activity. An SH2 domain in STAT5 binds P–Tyr residues in the EPO receptor, positioning it for this phosphorylation by JAK. When STAT5 is phosphorylated in response to EPO, it forms dimers, exposing a signal for its transport into the nucleus. There, STAT5 causes the ex pression (transcription) of specific genes essential for erythrocyte maturation. This JAK-STAT system operates in a number of other signaling pathways, including that for the hormone leptin, described in detail in Chapter 23 (see Fig. 23–34). Activated JAK can also trigger, through Grb2, the MAPK cascade (Fig. 12–6), which leads to altered expression of specific genes. Src is another soluble protein Tyr kinase that associates with certain receptors when they bind their lig ands. Src was the first protein found to have the characteristic P–Tyr-binding domain that was subsequently named the Src homology (SH2) domain. Yet another example of a receptor’s association with a soluble protein kinase is the Toll-like receptor (TLR4) system through which mammals detect the bacterial lipopolysaccharide (LPS), a potent toxin. We return to the Toll-like receptor system in Section 12.6, in the context of the evolution of signaling proteins.

FIGURE 12–9 The JAK-STAT transduction mechanism for the erythropoietin receptor. Binding of erythropoietin (EPO) causes dimerization of the EPO receptor, which allows the soluble Tyr kinase JAK to bind to the internal domain of the receptor and phosphorylate it on several Tyr residues. The STAT protein STAT5 contains an SH2 domain and binds to the P–Tyr residues on the receptor, bringing the receptor into proximity with JAK. Phosphorylation of STAT5 by JAK allows two STAT molecules to dimerize, each binding the other’s P–Tyr residue. Dimerization of STAT5 exposes a nuclear localization sequence (NLS) that targets STAT5 for transport into the nucleus. In the nucleus, STAT causes the expression of genes controlled by EPO. A second signaling pathway is also triggered by autophosphorylation of JAK that is associated with EPO binding to its receptor. The adaptor protein Grb2 binds P–Tyr in JAK and triggers the MAPK cascade, as in the insulin system.

اخر الاخبار

اخبار العتبة العباسية المقدسة

بمشاركة طالبات دورة (ينابيع الرحمة).. شعبة السيدة فاطمة بنت أسد تحيي ذكرى زواج النورين (عليهما السلام)

قسم شؤون المعارف يوضح مراحل الإخراج الطباعي لإصدارات مركز تراث الجنوب

دار علوم نهج البلاغة تناقش رسائل الإمام علي (عليه السلام) ضمن المحاضرة الرابعة عشرة في ذي قار

قسم الشؤون الفكرية وجمعية العميد يدعوان المهتمّين لحضور فعّاليات مسابقة العميد البحثية الأولى

المزيد

الآخبار الصحية

طريقة مبتكرة للكشف المبكر عن أمراض القلب والشرايين

منظمة الصحة تحذر من خطر إدمان الشباب لأكياس النيكوتين

خبراء يحذرون من عادة شائعة تهدد صحة الملايين

خبير يكشف 5 حقائق طبية صادمة وغير معروفة

المزيد

الآخبار التكنلوجية

الأولى في العالم.. الصين تجري تجربة رائدة في الفضاء باستخدام "جنين اصطناعي"

دراسة حديثة تفند ربط التستوستيرون بالمخاطرة لدى الرجال

ميتا تضيف وضع "المحادثة المخفية" إلى تطبيق "واتساب"

فوبيا المرتفعات ليست من الدماغ.. دراسة تكشف السبب

المزيد

مواضيع ذات صلة

Solute Transport across Membranes:-The β-Adrenergic Receptor Is Desensitized by Phosphorylation

G Protein–Coupled Receptors and Second Messengers:-The -Adrenergic Receptor System Acts through the Second Messenger cAMP

Receptor Enzymes:- Receptor Guanylyl Cyclases Generate the Second Messenger cGMP

Gated Ion Channels:- Neurons Have Receptor Channels That Respond to Different Neurotransmitters

Gated Ion Channels:- Voltage-Gated Ion Channels Produce Neuronal Action Potentials

Gated Ion Channels:- Ion Channels Underlie Electrical Signaling in Excitable Cells

Solute Transport across Membranes:- Defective Ion Channels Can Have Adverse Physiological Consequences

Solute Transport across Membranes:- The Acetylcholine Receptor Is a Ligand-Gated Ion Channel

Solute Transport across Membranes:- The Neuronal Na+ Channel Is a Voltage-Gated Ion Channel

Solute Transport across Membranes: -The Structure of a K+ Channel Reveals the Basis for Its Specificity

Solute Transport across Membranes:- Ion-Channel Function Is Measured Electrically

Solute Transport across Membranes:- Ion-Selective Channels Allow Rapid Movement of Ions across Membranes