Chapter 16, see p330 in text book Cell-to-Cell Signaling: Hormones and Receptors
signaling molecules receptors signal transduction
Overview of Extracellular Signaling (1) synthesis and (2) release of the signaling molecule; (3) transport of the signal to the target cell; (4) detection of the signal by a specific receptor protein; (5) a change in cellular metabolism, function, or development triggered by the receptor-signal complex; (6) removal of the signal, which often terminates the cellular response.
1.1 Signal molecules: • Signaling Molecules Operate over Various Distances in Animals • endocrine, 2) paracrine, or autocrine • 3) Synaptic signal
In endocrine signaling, signaling molecules, called hormones, act on target cells distant from their site of synthesis by cells of endocrine organs.
Receptor Proteins Exhibit Ligand-Binding and Effector Specificity Hormones Can Be Classified Based on Their Solubility and Receptor Location Most hormones: (1) small lipophilic molecules that diffuse across the plasma membrane and interact with intracellular receptors; and (2) hydrophilic or (3) lipophilic molecules
Lipophilic Hormones with Intracellular Receptors steroids (cortisol, progesterone, estradiol, and testosterone), thyroxine, and retinoic acid
Water-Soluble Hormones with Cell-Surface Receptors (1)peptide hormones, such as insulin, growth factors, and glucagon, and (2) small charged molecules, such as epinephrine and histamine Lipophilic Hormones with Cell-Surface Receptors
Cell-Surface Receptors Belong to Four Major Classes G protein coupled receptors Ion-channel receptors Tyrosine kinase linked receptors Receptors with intrinsic enzymatic activity
Effects of Many Hormones Are Mediated by Second Messengers second messengers,intracellular signaling molecules, cAMP; cGMP; 1,2-diacylglycerol (DAG); IP3; various inositol phospholipids (phosphoinositides); and Ca2+. Other Conserved Proteins Function in Signal Transduction GTPase Switch Proteins: there are two classes of GTPase switch proteins, and monomeric Ras and Ras-like proteins. Protein Kinases Adapter Proteins
Common Signaling Pathways Are Initiated by Different Receptors in a Class
The Synthesis, Release, and Degradation of Hormones Are Regulated Peptide Hormones and Catecholamines Steroid Hormones, Thyroxine, and Retinoic Acid Feedback Control of Hormone Levels
SUMMARYsignaling molecules, membrane-anchored and secreted proteins, lipophilic and hydrophilic molecules, and gases. Binding of extracellular signaling molecules to cell-surface receptors trigger intracellular pathways that modulate cellular metabolism, function, or development.The second messengers, such as Ca2+, cAMP, and IP3 ;Conserved proteins in signal-transduction pathways include GTPase switch proteins, protein kinases, and adapter proteins. Extracellular signals are often integrated into complex regulatory networks in which the synthesis, release, and degradation of hormones are precisely regulated.
Identification and Purification of Cell-Surface Receptors Binding of a hormone to a receptor involves of weak interactions ionic and van der Waals bonds and hydrophobic interactions.
Hormone Receptors Are Detected by Binding Assays The KD, the hormone concentration at which the receptor is half-saturated, also can be calculated from the specific binding curve.KD Values for Cell-Surface Hormone Receptors Approximate the Concentrations of Circulating Hormones
Affinity Techniques Permit Purification of Receptor Proteins Many Receptors Can Be Cloned without Prior Purification
SUMMARYReceptors bind to ligands. This specificity is determined by interactions between ligand determinants and specific amino acids in the receptor.Receptors can be purified directly using ligands as affinity reagents. In some cases, the genes encoding receptors for specific ligands can be isolated from cDNA libraries transfected into cultured cells. Cells expressing the receptor are detected using labeled ligand as a probe.
G Protein Coupled Receptors and Their Effectors cell-surface receptors --- trimeric signal---transducing G protein---effector enzyme --- an intracellular second messenger All G protein coupled receptors (GPCRs) contain seven membrane-spanning regions with N-terminal segment on the exoplasmic face and C-terminal segment on the cytosolic face of the plasma membrane.
Hormone—receptor—G protein—enzyme—the second message—kinase—enzyme or functional protein—biological effect
Binding of Epinephrine to Adrenergic Receptors Induces Tissue-Specific Responses Stimulation of b-Adrenergic Receptors Leads to a Rise in cAMP Critical Features of Catecholamines and Their Receptors Have Been Identified
Trimeric Gs Protein Links b-Adrenergic Receptors and Adenylyl Cyclase b-adrenergic receptors, which are coupled to Gs, or stimulatory G protein b-adrenergic receptors is an elevation in the intracellular level of cAMP.
Cycling of Gs between Active and Inactive Forms The G proteins and other GPCRs contain three subunits designated a, b, and g. GTPase switch proteins alternate between an "on" state with bound GTP and an "off" state. Binding of a hormone or agonist to the receptor changes its conformation, causing it to bind to the trimeric Gs protein in such a way that GDP is displaced from Gsaand GTP is bound. The Gsa·GTP complex, which dissociates from the Gbgcomplex, then binds to and activates adenylyl cyclase.
Amplification of Hormone Signal Termination of Cellular Response Some Bacterial Toxins Irreversibly Modify G Proteins Adenylyl Cyclase Is Stimulated and Inhibited by Different Receptor-Ligand Complexes
GTP-Induced Changes in GsaFavor Its Dissociation from Gbgand Association with Adenylyl Cyclase Giaand GsaInteract with Different Regions of Adenylyl Cyclase Degradation of cAMP Also Is Regulated
SUMMARYMany cell-surface receptors, seven transmembrane domains - trimeric G proteins. All G proteins contain three subunits: a, b, and g. The intrinsic GTPase activity of Gainactivates Ga· GTP by catalyzing GTP hydrolysis: Pi is released and the resulting Ga· GDP then dissociates from its effector and reassociates with Gbg.Adenylyl cyclase, which catalyzes the formation of cAMP from ATP, is the best-characterized effector regulated by trimeric G proteins. All adenylyl cyclase isoforms are stimulated by Gsa, but only specific isoforms are inhibited by Giaand Gbg. Gsa, Gia, and Gbginteract with different regions of the catalytic domain of adenylyl cyclase.
Receptor Tyrosine Kinases and Ras receptor tyrosine kinases (RTKs) , The ligands for RTKs are soluble or membrane-bound peptide/protein hormones including nerve growth factor (NGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), and insulin.
All RTKs comprise an extracellular domain containing a ligand-binding site, a single hydrophobic transmembrane a helix, and a cytosolic domain that includes a region with protein-tyrosine kinase activity. Binding of ligand causes most RTKs to dimerize; the protein kinase of each receptor monomer then phosphorylates a distinct set of tyrosine residues in the cytosolic domain of its dimer partner, a process termed autophosphorylation. The phosphotyrosine residues in activated RTKs interact with adapter proteins containing SH2 or PTB domains.
Ras and GaSubunits Belong to the GTPase Superfamily of Intracellular Switch Proteins Activation of both Ras and Gais triggered by hormone binding to an appropriate cell-surface receptor. Ras activation is accelerated by a protein called guanine nucleotideexchange factor (GEF), which binds to the Ras · GDP complex, causing dissociation of the bound GDP.
Hydrolysis of the bound GTP deactivates both Ras and Ga. The average lifetime of a GTP bound to Ras is about 1 minute, which is much longer than the lifetime of Ga· GTP. The reason for this difference is that the deactivation of Ras, unlike the deactivation of Ga, requires the assistance of another protein: a GTPase-activating protein (GAP), which binds to Ras · GTP and accelerates its intrinsic GTPase activity by a hundredfold.
An Adapter Protein and GEF Link Most Activated RTKs to Ras SH2 Domain in GRB2 Adapter Protein Binds to a Specific Phosphotyrosine in an Activated RTK Sos, a Guanine Nucleotide Exchange Factor, Binds to the SH3 Domains in GRB2
SUMMARYReceptor tyrosine kinases (RTKs), bind to peptide hormones, may exist as dimers or dimerize during binding to ligands.Ligand binding leads to activation of the kinase activity of the receptor and autophosphorylation of tyrosine residues. Ras is an intracellular GTPase switch protein that acts downstream from most RTKs. Like Gsa, Ras cycles between an inactive GDP-bound form and active GTP-bound form. Unlike GPCRs, which interact directly with an associated G protein, RTKs are linked indirectly to Ras via two proteins, GRB2 and Sos.The SH2 domain in GRB2, an adapter protein, binds to specific phosphotyrosines in activated RTKs. Normally, Ras activation and the subsequent cellular response is induced by ligand binding to an RTK. However, in cells that contain a constitutively active Ras, the cellular response occurs in the absence of ligand binding.
MAP Kinase Pathways Activated Ras then induces a kinase cascade that culminates in activation of MAP kinase. This serine/threonine kinase, which can translocate into the nucleus, phosphorylates many different proteins including transcription factors that regulate expression of important cell-cycle and differentiation-specific proteins. Signals Pass from Activated Ras to a Cascade of Protein Kinases Ksr May Function as a Scaffold for the MAP Kinase Cascade Linked to Ras
Phosphorylation of a Tyrosine and a Threonine Activates MAP Kinase Various Types of Receptors Transmit Signals to MAP Kinase Multiple MAP Kinase Pathways Are Found in Eukaryotic Cells
SUMMARYActivated Ras promotes formation of signaling complexes at the membrane containing three sequentially acting protein kinases and a scaffold protein Ksr. Raf is recruited to the membrane by binding to Ras · GTP and then activated. It then phosphorylates MEK, a dual specificity kinase that phosphorylates MAP kinase. Phosphorylated MAP kinase dimerizes and translocates to the nucleus where it regulates gene expression.RTKs, GPCRs, and other receptor classes can activate MAP kinase pathways. In MAP kinase pathways containing common components, the activity of shared components is restricted to only a subset of MAP kinases by their assembly into large pathway-specific signaling complexes.Some MAP kinases have kinase-independent functions that can restrict signals to only a subset of MAP kinases.
The pathway of signal transduction, • Second Messengers • cAMP and Other Second Messengers Activate Specific Protein Kinases • cAMP-dependent protein kinases (cAPKs) cAPKs Activated by Epinephrine Stimulation Regulate Glycogen Metabolism Kinase Cascades Permit Multienzyme Regulation and Amplify Hormone Signals