Chapt. 15 Cell Signaling

Chapt. 15 Cell Signaling

Student Learning Outcomes:Transmembrane proteins communicate signals

from outside the cell to the inside

1*. Explain various signaling molecules and their receptors: how cells receive information from outside.

2*. Explain several ways signals to receptors are transmitted through the cell (second messenger pathways)

3. Describe examples of signal transduction

4. Describe involvement of cytoskeleton in signal transduction.

Fig 15.1 Modes of cell-cell signaling

1. Signaling molecules and receptors:

Modes of cell signaling include:• Direct cell-cell interaction (ex. Integrins, cadherins) • Secreted molecules:

– Endocrine – distant• Estrogen hormone

– Paracrine – local• neurotransmitter

– Autocrine – self• T cell response• Cancer cells

Fig. 15.1

Signaling Molecules and Their Receptors

Signal molecules bind receptors:

• Intracellular receptors • bind small hydrophobic signaling molecules that

cross plasma membrane• Ex. nuclear receptors (steroid hormone)

• Cell surface receptors• bind hydrophilic molecules, peptides, growth

factors that don’t cross plasma membrane• Ex. insulin receptor, epidermal growth factor,

neurotransmitters

filopdia

Steroid hormones, thyroid hormone, vitamin D3, retinoic acid

Nuclear Receptor superfamily:

Intracellular receptors bind hydrophobic hormones• steroid hormones • thyroid hormone,

• vitamin D3,

• retinoic acid• cortisolReceptors are transcription factors,active after binding hormone

Fig. 15.2

Histone tails can be marked with acetylation, methylation, phosphorylation

Modifications of histone tails can alter gene regulation – bind coactivators, corepressors

Figs. 7.34, 35Acetylation of lysines (HAT)Deaceylation (HDAC)

Fig 15.3 Glucocorticoid action

Some nuclear receptors inactive without hormone:• Glucocorticoid receptor (GR) is bound to Hsp90

chaperone in absence of hormone.• Glucocorticoid binding displaces Hsp90 →• GR binds specific DNA sequences, activates transcription• Note coactivator HAT(histone acetyl transferase)

Fig. 15.3

Fig 15.4 Gene regulation by thyroid hormone receptor

Some nuclear receptors alter activity after hormone• Absence of hormone, thyroid hormone receptor (TR)• Binds DNA and corepressor complex (HDAC) (histone deacetylase)

• represses transcription

• Hormone binding →

TR binds coactivator (HAT)• activates transcription

(see Fig. 7.34)Fig. 15.4

Fig 15.5 Synthesis of nitric oxide

Nitric oxide (NO) is a paracrine signaling molecule in nervous, immune, circulatory systems

• Alters activity of enzymes (guanylyl cyclase makes cGMP)• Synthesized from Arg by nitric oxide synthase (NOS)• Only local effects, extremely unstable, t1/2 of seconds• Ex. Dilation of blood vessels (after neurotransmitter signals)

CO is also paracrine

Fig. 15.5

Fig 15.6 Structure of some neurotransmitters

Neurotransmitters carry signals between neurons, from neurons to other target cells

• Small hydrophilic • released after arrival of action potential at end of neuron. • Diffuse across cleft• Bind receptors on target cell surface:ligand-gated ion channelscell-surface receptors

– Coupled to G proteins

Fig. 15.6


Peptide signaling molecules: Table 1diverse sizes (5 – 230 amino acids) and roles

• Peptide hormones • Insulin, glucagon, pituitary gland hormones (growth

hormone, follicle-stimulating hormone, prolactin)

• Neuropeptides & neurohoromone • Enkephalins and endorphins act as neurotransmitters at

synapses, oxytocin stimulates smooth muscle contraction

• Polypeptide growth factors • Nerve growth factor (NGF)• Epidermal growth factor (EGF) stimulates cell proliferation


Eicosanoids are lipid signaling molecules:• Prostaglandins and leukotrienes• Autocrine or paracrine pathways• Inflammation, smooth-muscle contraction, platelet aggregation• Synthesized from arachidonic acid (from phospholipid by PLA2)• Aspirin (NSAID) targets cyclooxygenase (COX) (1st step in Prostaglandin syn)

Fig. 15.8

COX


Plant hormones: small molecules regulate• Gibberellins— stem elongation• Auxins— cell elongation• Ethylene— fruit ripening• Cytokinins— cell division• Abscisic acid— onset of dormancy

Fig. 15.9

Functions of Cell Surface Receptors

2** Functions of cell-surface receptors:Most ligands for cell-cell signaling bind surface receptors on targets• Ligand does not enter cell

Binding initiates:• chain of intracellular reactions

(amplify signal) • Alter activity of enzymes• Affect ion channels• Often change in gene expression* 2 Main types: G protein-coupled, Receptor tyr kinase (RTK)

Fig. 8.40 ex. signal cascade

G-coupled Surface Receptors

Largest family of cell surface receptors.

•Signals transmitted via guanine nucleotide-binding proteins (G proteins) in cytoplasm•Receptors have 7 transmembrane α helices•G proteins discovered during studies of cyclic AMP (cAMP), second messenger that mediates cellular responses to many hormones

2a. G protein-coupled receptors:

Fig. 15.11 G-protein coupled receptor

Fig 15.12 Hormonal activation of adenylyl cyclase

Binding of ligands induces conformational change:•Cytosolic domain activates G protein on inner face of plasma membrane.•Activated G protein subunit dissociates from receptor•Carries signal to intracellular targetEx. Heterotrimeric G protein intermediary in activation of adenylyl cyclase, which synthesizes cAMP

*Fig. 15.12 G-protein coupled receptor

Fig 15.13 Regulation of G proteins

G proteins have 3 subunits: α, β, and γ.• Called heterotrimeric G proteins to distinguish from small

guanine nucleotide-binding proteins, Ras (Rab, Ran)

subunit is regulator: Hormone binding stimulates GTP to bind (exchange GDP); dissociate, (interact different targets)GTP hydrolysis terminates

(details later)

Fig. 15.13 G-protein coupled receptor (ex. binds adenylyl cyclase)

Functions of G-protein coupled Receptors

Different G proteins connect receptors to distinct targets: Humans have 21 , 6 , 12

Ex. enzyme regulation: epinephrine G protein is Gs,: subunit stimulates adenylyl cyclase

G proteins can also regulate ion channels:Ex. Heart muscle cells have different acetylcholine

receptor (G protein-coupled) than nerve and skeletal muscle cells (ligand-gated ion channel (Fig. 13.25):

• α subunit of this G protein (Gi) inhibits adenylyl cyclase. The Gi βγ subunits open K+ channels in plasma membrane, which slows heart muscle contraction.

Key Experiment 15.1 G Protein-Coupled Receptors and Odor Detection: The odorant receptor protein family

Largest family of G protein-coupled receptors is responsible for odor detection. • Odorant receptors on surface of olfactory neuronsencoded by multigene family (400 humans, 1000 in dogs, rats).• Odor binds receptor on surface of olfactory neurons; stimulates adenylyl cyclase, increased

cAMP opens Na+ channel and nerve impulse

Buck, Axel 1991 cloned receptors:White conserved aaBlack variable


2b. Receptor protein-tyrosine kinases (RTK):Receptors directly link to intracellular enzyme:• Largest family phosphorylates substrates on tyrosine residues • Receptors for most polypeptide growth factors: EGF, NGF, PDGF, insulin, and others

Ligands binding outside:• activate cytosolic kinase domains• phosphorylation of receptors and binding intracellular targets propagates signal

Fig. 15.14 RTK receptors Conserved structure

Fig 15.15 Dimerization and autophosphorylation of RTK receptor

• Ligand-binding induces receptor dimerization.• Receptor autophosphorylation - 2 polypeptide

chains cross-phosphorylate one another

Fig. 15.15 RTK receptors dimerize, autophosphorylate

Fig 15.16 Downstream signaling molecules bind RTK receptors

Autophosphorylation has two roles:• Tyr PO4 in catalytic domain increases kinase activity• Tyr PO4 outside catalytic domain → binding sites for

proteins that transmit downstream signals– Common motif is the SH2 domain on downstream molecule

Fig. 15.16 RTK receptors bind downstream signal proteins

Complex between an SH2 domain and phosphotyrosine peptide

Fig. 15.17: SH2 domain: p-tyr: Blue, 3 aa that bind p-tyr peptide(red = P; yellow backbone); Purple is groove

Downstream signaling molecules have domains that bind to specific phosphotyrosine-containing peptides

• SH2 domains were the first characterized (~100aa):

• initially recognized in nonreceptor protein-tyrosine kinases related to Src, (oncogenic protein of Rous sarcoma virus)

Other proteins bind via PTB domains (phosphotyrosine-binding).

Fig 15.18 Signaling from cytokine receptors

2c. Cytokine receptor superfamily:• Receptors for cytokines (interleukin-2, erythropoietin)

and some polypeptide hormones (growth hormone)

• Structure similar to receptor protein-tyrosine kinases, but no catalytic activity cytosolic domains

• Work with nonreceptor protein-tyrosine kinases; • Phosphorylated receptor binds downstream molecules via

SH2 domains.

Fig. 15.18


• Nonreceptor Kinases associated with cytokine receptors belong to Janus kinase (or JAK) family.

• Members of JAK family appear to be universally required for signaling from cytokine receptors

Fig. 15.40 part;More later


Other nonreceptor protein-tyrosine kinases belong to Src family, signal downstream of:

• cytokine receptors, • receptor protein-tyrosine kinases, • antigen receptors on B and T lymphocytes,• integrins at sites of cell attachment to ECM (matrix)

Src protein:Oncogene of RSV virus


2d. Receptors linked to other enzymatic activities:Protein-tyrosine phosphatases:• remove phosphate groups from phospho-tyr, • counterbalance effects of protein-tyrosine kinases

Protein-serine/threonine kinasesEx. Receptors for transforming growth factor β (TGF- β)

polypeptide - growth factor controls cell proliferation

Receptor guanylyl cyclases - cytosolic domain catalyzes formation of cyclic GMP, another second messenger

Some receptors have associated protease:Ex. Tumor Necrosis factor (TNF) binding receptor induces

cell death (apoptosis; Chapt. 17); downstream proteases.

Pathways of Intracellular Signal Transduction

15.3 Intracellular signal transduction:• Chain of reactions transmits signals from cell

surface to intracellular targets.• Targets often include transcription factors that

regulate gene expression• Different mechanisms:

• cAMP and protein phosphoryation (PKA)• cGMP• Phospholipids and Ca2+

• DAG and PKC, IP3 and Ca2+, PIP3/AKT

• Ras, Raf, MAP kinase• JAK/STAT; TGF/Smad

Pathways of Intracellular Signal Transduction - cAMP

**3a. cAMP/ PKA path

Intracellular signaling studied for hormone epinephrine, (breakdown of glycogen to glucose)

In 1958 Sutherland discovered action of epinephrine was mediated by increase in cyclic AMP (cAMP), leading to concept of cAMP as a second messenger. (Fig. 8.40)

Sequential cascade of activations, amplification from phosphorylation:

1 hormone to receptor → many Gs → many adenylyl cyclases and cAMP

Fig. 8.40Signaling pathway

Fig 15.19 Synthesis and degradation of cAMP

• cAMP is formed from ATP by adenylyl cyclase; degraded to AMP by cAMP phosphodiesterase.

• Epinephrine receptor is coupled to adenylyl cyclase via a G protein that stimulates enzymatic activity, increasing concentration of cAMP.

Fig. 15.19cAMP 5’-3’

Fig 15.20 Regulation of protein kinase A

Effects of cAMP in animal cells are mediated by cAMP-dependent protein kinase, or protein kinase A (PKA)

• Inactive form has 2 regulatory, 2 catalytic subunits

• cAMP binds to regulatory subunits, which dissociate

• Free catalytic subunits phosphorylate serine on target proteins

Fig. 15.20; Protein kinase A activated by cAMP binding regulatory subunitAlso Fig. 8.42

Fig 15.21 Regulation of glycogen metabolism by PKA

Increased cAMP affects enzyme activityEx. PKA stimulates breakdown of glycogen:PKA phosphorylates 2 enzymes:• Phosphorylase kinase activated, → activates glycogen phosphorylase:

stimulates glycogen breakdown• Glycogen synthase is inactivated: blocks new synthesis Signal amplification:• 1 hormone to receptor many Gs → many adenylyl cyclases, lots of cAMP• PKA adds PO4 to multiple enzymes

Fig. 15.21 example PKA cascade

Fig 15.22 Cyclic AMP-inducible gene expression

Increased cAMP can also activate transcription of genes:

• Through regulatory sequence: cAMP response element (CRE)• Free catalytic subunit of PKA goes to

nucleus, phosphorylates transcription factor CREB (CRE-binding protein)

• CREB binds DNA (and coactivators)• Expression of cAMP-inducible genes.

Fig. 15.22CREB

Regulation of phosphorylation by protein kinase A, protein phosphatase 1

Protein phosphorylation is rapidly reversed by protein phosphatases,

• Terminates responses initiated by (signal) receptor activation of protein kinases.

Fig. 15.23


cAMP can also directly regulate ion channels:(no need for PKA)

• cAMP is the second messenger in sensing smells:• Odorant receptors are G protein-coupled; stimulate adenylyl

cyclase, leading to an increase in cAMP.

• cAMP opens Na+ channels in plasma membrane, leading to initiation of nerve impulse.

Fig. 13.26


3b. Cyclic GMP (cGMP) • Important second messenger in animal cells.• cGMP formed from GTP by guanylyl cyclase, degraded to GMP by phosphodiesterase.• cGMP effects often mediated by activation of cGMP-dependent kinases • cGMP regulates ion channels, phosphodiesterases• ex. cGMP mediates biological responses such as

blood vessel dilation (after NO)• ex. In vertebrate eye, cGMP is second messenger

that converts visual signals to nerve impulses.

Fig 15.24 Role of cGMP in photoreception

Rhodopsin - Photoreceptor in rod cells of retina is a G protein-coupled receptor

• Rhodopsin is activated when retinal absorbs light

• Rhodopsin activates G protein transducin; α subunit stimulates cGMP phosphodiesterase, → decreased cGMP.

• Change in cGMP levels translates to nerve impulses by direct effect of cGMP on ion channels.

Fig. 15.24 cGMP, photoreceptor


*3c Membrane Phospholipid, Ca2+, DAG, IP32 major paths use second messengers derived from

phosphatidylinositol 4,5-bisphosphate (PIP2).

• Hydrolysis of PIP2 by phospholipase C (PLC) produces 2 second messengers: diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3).

DAG → PKCIP3 → Ca2+

Fig. 15.25 DAG, IP3

Activation of phospholipase C by protein-tyrosine kinases

Two forms of phospholipase C:• PLC-β stimulated by G proteins• PLC-γ has SH2 domains, binds to receptor protein-tyrosine

kinases (RTK)Tyr phosphorylation increases PLC- γ activity, stimulates hydrolysis of PIP2 to DAG and IP3

DAG stays in plasma membrane:• activates ser/thr kinases of protein kinase C (PKC) family (growth, differentiation)

Fig. 15.26 RTK signals through PLC to form DAG and IP3

Fig 15.27 Ca2+ mobilization by IP3

IP3 is small polar molecule released to cytosol: signals release of Ca2+ from ER

• Cytosol concentration of Ca2+ is maintained at extremely low level by Ca2+ pumps

Fig. 13.32; [Ca2+ ~0.01 M]• IP3 stimulates release of Ca2+ from ER by binding to receptors - ligand-gated Ca2+ channels

• Increased Ca2+ affects activity of several proteins: protein kinases protein phosphatases (some PKC require DAG and Ca2+)

Fig. 15.27 IP3

Fig 15.28 Function of calmodulin

Increased Ca2+ affects activity of proteins: including protein kinases and phosphatases•Calmodulin activated when Ca2+ concentration increases to about 0.5 M

Ca2+/calmodulin binds target proteins:•Myosin light-chain kinase (Fig. 12.31)•CaM kinase family phosphorylates:

– metabolic enzymes– ion channels– regulate synthesis and release of

neurotransmitters– transcription factors (CREB)

Intersection of cAMP, Ca2+ signaling paths

Fig. 15.28

Regulation of intracellular Ca2+ in electrically excitable cells

Ca2+ also increased by uptake of extracellular Ca2+ by regulated channels in plasma membrane.

• Electrically excitable cells (nerve and muscle) open voltage-gated Ca2+ channels by membrane depolarization

• Increase in intracellular Ca2+ signals further release of Ca2+ from ER by opening Ca2+ channels (ryanodine receptors) in ER• Increased Ca2+ in neurons signals release

of neurotransmitter• Increased Ca2+ in muscles → contractions

Ca2+ is versatile second messengerFig. 15.29


c. [PI 3-kinase, AKT, mTOR path]• PIP2 also another signaling pathway (survival).• PIP2 phosphorylated by PI 3-kinase phosphatidylinositide (PI) 3-kinase• yields second messenger (PIP3) phosphatidylinositol 3,4,5-trisphosphate• PIP3 targets protein ser/thr kinase Akt, also

binds protein kinase PDK1• Activation of Akt requires protein kinase

mTOR ; growth factors stimulate• AKT phosphorylates target proteins,

transcription factors

Fig. 15.30, 31


*3d. MAP kinase pathway (Ras, cancer)• cascade of protein kinases, highly conserved

• MAP kinases (mitogen-activated protein kinases) are protein ser/thr kinases

• MAP kinases initially characterized in mammalian cells belong to ERK (extracellular signal-regulated kinase) family.

• ERK signals cell proliferation,• responds to signals from many paths

Fig 15.34 Activation of ERK MAP kinases

Activation of Ras → activation of Raf protein ser/thr kinase, → phosphorylates, activates protein kinase MEK (for MAP kinase/ERK kinase → ERK gets PO4 on thr and tyr residues

ERK pathway: • mediated by upstream protein kinases• coupled to growth factor receptors by Ras GTP-binding

Fig. 15.34

Fig 15.35 Regulation of Ras proteins

Ras proteins - small guanine nucleotide-binding proteins (function like α subunits of G proteins)

• Activated by guanine nucleotide exchange factors (GEF) that stimulate exchange of GTP for bound GDP.

• Ras-GTP activity is terminated by GTP hydrolysis, (stimulated by interaction of Ras-GTP with GTPase-activating proteins)

Note: Ras bound in membrane by prenyl group (Fig. 13.11)

relatives Rab (vesicles) Ran (nucleus)

Fig. 15.35

Molecular Medicine 15.1 Cancer: Signal Transduction and ras Oncogenes: A human colon polyp (an early stage of colon cancer)

Mutations of ras genes in human cancers: inhibit GTP hydrolysis by Ras proteins.

• Mutated Ras proteins continuously in active GTP-bound form, driving proliferation of cancer cells in absence of growth factor

Ras mutated in 25% all cancers: 25% of lung cancers 50% of colon cancers 90% lung cancers

Ras is proto-oncogeneMutated Ras is oncogene

Colon polyp (early stage of cancer

Ras activation downstream of receptor protein-tyrosine kinases

*Receptor protein-tyrosine kinases activate Ras• Autophosphorylation of RTK → binds to the Ras GEF

factor SOS via the SH2-mediated binding of Grb2• SOS activates membrane-bound RAS by GTP• Ras-GTP binds Raf ser/thr protein kinase. • Raf initiates kinase cascade to activate ERK

Fig. 15.36

Mutated Ras proteins in active GTP-bound form stimulate pathway in absence of growth factor

Fig 15.34 Activation of ERK MAP kinases

Activation of Ras → activation of Raf protein ser/thr kinase, → phosphorylates, activates protein kinase MEK (for MAP kinase/ERK kinase → ERK gets PO4 on thr and tyr residues

ERK pathway: • mediated by upstream protein kinases• coupled to growth factor receptors by Ras GTP-binding

Fig. 15.34

Fig 15.37 Induction of immediate-early genes by ERK

Activated ERK phosphorylates transcription factors:

• Primary response to growth factor stimulation is rapid transcriptional induction of immediate-early genes

• Mediated by regulatory sequence called the serum response element (SRE), bound by transcription factors including serum response factor (SRF) and Elk-1

• Many of the Immediate-early genes encode transcription factors: (Secondary response genes)

Fig. 15.37

Pathways of MAP kinase activation in mammalian cells

Multiple MAP kinase pathways• In mammalian cells, 3 groups of MAP kinases:

ERK family, JNK and p38 MAP kinases• Yeast 5 groupsSpecificity of signaling from physical association (scaffold)

Fig. 15.38, 39 mammal MAP kinases

Fig 15.40 The JAK/STAT pathway

3e. JAK/STAT pathway:

Direct connection from growth factor receptors to transcription factors:

• Key elements are STAT proteins (signal transducers and activators of transcription): Transcription factors contain SH2 domains that bind tyr-PO4

• STAT binds phosphorylated receptor (cytokine receptor PO4 by JAK nonreceptor tyr kinase; or RTK receptor)

• STAT activates transcriptionFig. 15.40

Fig 15.41 Signaling from TGF-β receptors

TGF-β family receptors for growth factors are protein-ser/thr kinases:•Directly phosphorylate Smadfamily of transcription factors•Receptors have two polypeptides, which associate following ligand binding, phosphorylate Smad protein

•30 members of TGF-•8 different SMADs•Different responses in cells

Fig. 15.41


The NF-κB signaling pathway also targets specific family of transcription factors

Ex. downstream of receptor for tumor necrosis factor (cytokine induces inflammation and cell death)

Activation of receptors activatesIkB kinase, phosphorylates IκB, frees NF-κB to bind importin,move to nucleus, induce expression of target genes(see also Fig. 9.13)

Fig. 15.42


[3f. Hedgehog, Wnt and Notch pathways] • Connected signaling systems play key roles in determining cell fate during

embryonic development, regulating proliferation of stem cells in adult tissues.• Model organisms Drosophila

• Hedgehog and Wnt act by preventing Ub-degradation of transcription factors in cytoplasm

• Notch involves direct cell-cell interactions, proteolysis and movement of fragment to nucleus

Signal Transduction and the Cytoskeleton

15.4 signal transduction and cytoskeleton:

• Functions of most cells are directly affected by cell adhesion and organization of cytoskeleton.

• Receptors responsible for cell adhesion initiate intracellular signaling pathways, regulate other aspects of cell behavior, including gene expression

• Growth factors often induce cytoskeletal alterations, result in cell movement, changes in shape.

• Components of cytoskeleton act as both receptors and targets in cell signaling pathways.


4a. Cytoskeleton – Integrin signaling • Integrins- receptors attach cells to extracellular matrix • integrins interact with cytoskeleton (Figs. 12.16, 12.39)

• receptors activate intracellular signaling path

Ex. binding of integrins to extracellular matrix leads to activation of FAK ( focal adhesion kinase), nonreceptor protein-tyr kinase.

Autophosphorylation of FAK followed by Src binding and more phosphorylation of FAK Fig. 15.46

Fig 15.46 Integrin signaling (Part 2- 3)

Phosphorylated FAK provides binding sites for several signaling molecules, including Src, Grb2-Sos complex, (→ activation of Ras), PI 3-kinase, and phospholipase Cγ.

Fig. 15.46• Integrins also can interact with RTK like EGFR,

stimulate signaling by growth factors


Actin cytoskeleton • Signaling from integrins and growth factor receptors

regulates dynamic behavior of actin cytoskeleton.

• Members of Rho subfamily of small GTP-binding proteins (including Rho, Rac, and Cdc42) have critical roles

Fig. 15.47 Microinjection of fibroblasts with Rho proteins remodeled cytoskeleton: •cell surface protrusions •formation of focal adhesions, stress fibers.

Stimulation of actin polymerization by Rho family proteins

Rho family members are activated by integrin signaling and by growth factor receptors.

• Multiple target proteins do cytoskeletal changes. • Rho family proteins promote actin polymerization.• Rho proteins cause stress fibers

Fig. 15.48,49

ROCK is ser/thr kinase

Signaling Networks

5. Signaling networks: • Signaling pathways not work in isolation• Intracellular signal transduction is integrated network

of connected pathways• Ex. negative feedback loop is the NF-κB pathway• Ex. Crosstalk at junctions between paths

• between Ca2+ and cAMP signaling, • between the cAMP and ERK pathways, • between integrin signaling and receptor protein-

tyrosine kinases.

• Computational modeling of signaling networks is major challenge in cell biology.

Signaling Networks

• NF-κB activated after phosphorylation, degradation of IκB

• one gene activated by NF-κB encodes IκB, → feedback loop inhibits NF-κB activity.

• Limits activation

Ex. Negative feedback loop: NF-κB pathway

Fig. 15.50

Signaling Networks

Ex. crosstalk: G protein-coupled receptors and ERK signaling by β-arrestin.

• Activity of receptors turned off as result of phosphorylation by GRKs (G protein-coupled receptor kinases), association of β-arrestin with phosphorylated receptor.

• β-arrestin also scaffold protein for Raf, MEK, and ERK

• Links G protein-coupled receptors to ERK path

Review

Review:

Lot of terms, proteins and protein complexes:• Draw the two main G-coupled receptor paths,• Draw the two main Receptor tyrosine kinase paths

• New proteins: – Ras, Rho, NFk, heterotrimeric G proteins, CREB

• New Enzymes: – PKA, PKC, PLC

• New molecules: – DAG, IP3, PIP3, PIP2, cAMP, cGMP, Ca2+ ,

Review

Review questions

2. How does signaling by hydrophobic molecules like steroid hormones differ from signaling by peptide hormones?

4. Hormones that activate a receptor coupled to Gs stimulate the proliferation of thyroid cells. How would inhibitors of cAMP phosphodiesterase affect the proliferation of these cells?

7. You have generated a truncated version of the EGF receptor that lacks the tyrosine kinase domain. Expression of this truncated receptor inhibits the response of cells to EGF. Why?

Documents

Chapt. 15 Cell Signaling