Signaling Processes and Figures

Question 1

Figure 15–1 A simple intracellular signaling pathway activated by an extracellular signal molecule.

Answer 1

The signal molecule usually binds to a receptor protein that is embedded in the plasma membrane of the target cell and activates one or more intracellular signaling pathways mediated by a series of signaling proteins. Finally, one or more of the intracellular signaling proteins alters the activity of effector proteins and thereby the behavior of the cell.

Question 2

Figure 15–3 The binding of extracellular signal molecules to either cell-surface or intracellular receptors.

Answer 2

In most cases, the receptors are transmembrane proteins on the target cell surface. When these proteins bind an extracellular signal molecule (a ligand), they become activated and generate various intracellular signals that alter the behavior of the cell. In other cases, the receptor proteins are inside the target cell, and the signal molecule has to enter the cell to bind to them: this requires that the signal molecule be sufficiently small and hydrophobic to diffuse across the target cell’s plasma membrane

Question 3

Figure 15–4 Four forms of intercellular signaling.

Answer 3

(A) Contact-dependent signaling requires cells to be in direct membrane–membrane contact.
(B) Paracrine signaling depends on signals that are released into the extracellular space and act locally on neighboring cells. (C) Synaptic signaling is performed by neurons that transmit signals electrically along their axons and release neurotransmitters at synapses, which are often located far away from the neuronal cell body. (D) Endocrine signaling depends on endocrine cells, which secrete hormones into the bloodstream for distribution throughout the body. Many of the same types of signaling molecules are used in paracrine, synaptic, and endocrine signaling; the crucial differences lie in the speed and selectivity with which the signals are delivered to their targets.

Question 4

Cellular Response (Fig 15-6)

Answer 4

Certain types of signaled responses, such as increased cell growth and division, involve changes in gene expression and the synthesis of new proteins; they therefore occur slowly, often starting after an hour or more. Other responses—such as changes in cell movement, secretion, or metabolism—need not involve changes in gene transcription and therefore occur much more quickly, often starting in seconds or minutes; they may involve the rapid phosphorylation of effector proteins in the cytoplasm, for example. Synaptic responses mediated by changes in membrane potential can occur in milliseconds (not shown).

Question 5

Structure of IP3

Answer 5

Question 6

Structure of cGMP

Answer 6

Activates protein kinase G (PKG) and opens cation channels in rod cells

Question 7

Structure of DAG

Answer 7

Question 8

Structure of cAMP

Answer 8

Question 9

Switch proteins

Answer 9

Question 10

Structure of GPCRs

Answer 10

Question 11

FIGURE 13-11 Operational model for ligand-induced activation of effector proteins associated with G protein– coupled receptors.

Answer 11

1. Binding of hormone induces a conformational change in receptor
2. Activated receptor binds to Gα subunit
3. Binding induces conformational change in Gα; bound GDP dissociates and is replaced by GTP; Gα dissociates from Gβγ
4. Hormone dissociates from receptor; Gα binds to effector, activating it

5.

Question 12

functional expression assay

Answer 12

Question 13

structure of adenylyl cyclase

Answer 13

Question 14

▲ FIGURE 13-15 Hormone-induced activation and inhibition of adenylyl cyclase in adipose cells.

Answer 14

Question 15

FIGURE 13-20 Localization of protein kinase A (PKA)
to the nuclear membrane in heart muscle.

Answer 15

This A kinase– associated protein mAKAP anchors both PKA and cAMP phosphodiesterase (PDE) to the nuclear membrane, maintaining them in a negative feedback loop that provides close local control of the cAMP level. Step 1 : The basal level of PDE activity in
the absence of hormone (resting state) keeps cAMP levels below those necessary for PKA activation. Step 2: Activation of

Question 16

FIGURE 13-21 Operational model of muscarinic acetylcholine receptor in the heart muscle plasma membrane.

Answer 16

Question 17

FIGURE 13-23 The light-triggered step in vision.

Answer 17

The light-absorbing pigment 11-cis-retinal is covalently bound to the amino group of a lysine residue in opsin, the protein portion of rhodopsin. Absorption of light causes rapid photoisomerization of the cis-retinal to the all-trans isomer, forming the unstable intermediate meta-rhodopsin II, or activated opsin, which activates Gt proteins. Within seconds all-trans-retinal dissociates from opsin and is converted by an enzyme back to the cis isomer, which then rebinds to another opsin molecule.

Question 18

FIGURE 13-24 Operational model for rhodopsin-induced closing of
cation channels in rod cells.

Answer 18

In dark-adapted rod cells, a high level of cGMP keeps nucleotide-gated nonselective cation channels open. Light absorption generates activated opsin, O* (step 1 ), which binds inactive GDP-bound Gt protein and mediates replacement of GDP with GTP (step 2 ). The free Gt

Question 19

FIGURE 13-25 Structural models of rhodopsin and its associated Gt protein.

Question 20

Role of opsin phosphorylation in adaptation of rod cells to changes in ambient light levels.

Answer 20

Light-activated opsin (O*), but not dark-adapted rhodopsin, is a substrate for rhodopsin kinase. The extent of opsin phosphorylation is directly proportional to the amount of time each opsin molecule spends in the light- activated form and thus to the average ambient light level over the previous few minutes. The ability of O* to activate
Gt

Question 21

Synthesis of DAG and IP3 from membrane-bound phosphatidylinositol (PI).

Answer 21

Each membrane- bound PI kinase places a phosphate (yellow circles) on a specific hydroxyl group on the inositol ring, producing the phosphoinositides PIP and PIP2. Cleavage of PIP2 by phospholipase C (PLC) yields the two important second messengers DAG and IP3.

Question 22

IP3/DAG pathway and the elevation of cytosolic Ca2

Answer 22

This pathway can be triggered by ligand binding to certain G protein–coupled receptors and several other receptor types, leading to activation of phospholipase C. Cleavage of PIP2 by phospholipase C yields IP3 and DAG (step 1 ). After diffusing through the cytosol, IP3 interacts with and opens Ca2

Question 23

Regulation of contractility of arterial smooth muscle by nitric oxide (NO) and cGMP.

Answer 23

Nitric oxide is synthesized in endothelial cells in response to acetylcholine and the subsequent elevation in cytosolic Ca2

Question 24

Activation of the Tubby transcription factor following ligand binding to receptors coupled to Go or Gq.

Answer 24

In resting cells, Tubby is bound tightly to PIP2 in the plasma membrane. Receptor stimulation (not shown) leads to activation of phospholipase C, hydrolysis of PIP2, and release of Tubby into
Tubby
Transcriptional activation
domain
DNA binding domain
Cytosol the cytosol ( 1 ). Directed by two functional nuclear localization sequences (NLS) in its N-terminal domain, Tubby translocates into the nucleus ( 2 ) and activates transcription of target genes ( 3 ). It is not known whether IP3 remains bound to Tubby.

Question 25

Activation of gene expression following ligand binding to Gs protein–coupled receptors.

Answer 25

Receptor stimulation ( 1 ) leads to activation of PKA ( 2 ). Catalytic subunits of PKA translocate to the nucleus ( 3 ) and there phosphorylate and activate the transcription factor CREB ( 4 ). Phosphorylated CREB associates with the co-activator CBP/P300 ( 5 ) to stimulate various target genes controlled by the CRE regulatory element. See the text for details.

Question 26

GPCRs

Answer 26

Linked to a trimeric G protein that controls the activity of an effector protein (here adenylyl cyclase) Activate cytosolic or nuclear transcription factors via several pathways (here one involving protein kinase A)

Question 27

Schematic model of Ski-mediated down-regulation of the response to TGF

Answer 27

Ski binds to Smad4 in Smad3/Smad4 or Smad2/Smad4 (not shown) signaling complexes and may partially disrupt interactions between the Smad proteins. Ski also recruits a protein termed N-CoR that binds directly to mSin3A, which in turn interacts with histone deacetylase (HDAC), an enzyme that promotes histone deacetylation (Chapter 11). As a result, transcription activation induced by TGF

Question 28

General structure and ligand-induced activation of receptor tyrosine kinases (RTKs) and cytokine receptors.

Answer 28

The cytosolic domain of RTKs contains a protein tyrosine kinase catalytic site, whereas the cytosolic domain of cytokine receptors associates with a separate JAK kinase (step 1 ). In both types of receptor, ligand binding causes a conformational change that promotes formation of a functional dimeric receptor, bringing together two intrinsic or associated  kinases, which then phosphorylate each other on a tyrosine residue in the activation lip (step 2 ). Phosphorylation causes the lip to move out of the kinase catalytic site, thus allowing ATP or a protein substrate to bind. The activated kinase then phosphorylates other tyrosine residues in the receptor’s cytosolic domain (step 3 ). The resulting phosphotyrosines function as docking sites for various signal-transduction proteins (see Figure 14-6).

Question 29

JAK-STAT signaling pathway.

Answer 29

Following ligand binding to a cytokine receptor and activation of an associated JAK kinase, JAK phosphorylates several tyrosine residues on the receptor’s cytosolic domain (see Figure 14-5, bottom). After an inactive monomeric STAT transcription factor binds to a phosphotyrosine in the receptor, it is phosphorylated by active JAK. Phosphorylated STATs spontaneously dissociate from the receptor and spontaneously dimerize. Because the STAT homodimer has two phosphotyrosine–SH2 domain interactions, whereas the receptor-STAT complex is stabilized by only one such interaction, phosphorylated STATs tend not to rebind to the receptor. The STAT dimer, which has two exposed nuclear-localization signals (NLS), moves into the nucleus, where it can bind to promoter sequences and activate transcription of target genes.

Question 30

wo mechanisms for terminating signal transduction from the erythropoietin receptor (EpoR).

Answer 30

(a) SHP1, a protein tyrosine phosphatase, is present in an inactive form in unstimulated cells. Binding of an SH2 domain in SHP1 to a particular phosphotyrosine in the activated receptor unmasks its phosphatase catalytic site and positions it near the phosphorylated tyrosine in the lip region of JAK2. Removal of the phosphate from this tyrosine inactivates the JAK kinase. (b) SOCS proteins, whose expression is induced in erythropoietin-stimulated erythroid cells, inhibit or permanently terminate signaling over longer time periods. Binding of SOCS to phosphotyrosine residues on the EpoR or JAK2 blocks binding of other signaling proteins (left). The SOCS box can also target proteins such as JAK2 for degradation by the ubiquitin- proteasome pathway (right). Similar mechanisms regulate signaling from other cytokine receptors.

Question 31

Activation of Ras following ligand binding to receptor tyrosine kinases (RTKs).

Answer 31

The receptors for epidermal growth factor (EGF) and many other growth factors are RTKs. The cytosolic adapter protein GRB2 binds to a specific phosphotyrosine on an activated, ligand-bound receptor and to the cytosolic Sos protein, bringing it near its substrate, the inactive Ras

Question 32

Kinase cascade that transmits signals downstream from activated Ras protein to MAP kinase

Answer 32

In unstimulated cells, most Ras is in the inactive form with bound GDP; binding of a ligand to its RTK or cytokine receptor leads to formation of the active Ras

Question 33

Induction of gene transcription by activated MAP kinase

Answer 33

In the cytosol, MAP kinase phosphorylates and activates the kinase p90RSK, which then moves into the nucleus and phosphorylates the SRF transcription factor. After translocating into the nucleus, MAP kinase directly phosphorylates the transcription factor TCF. Together, these phosphorylation events stimulate transcription of genes (e.g., c-fos) that contain an SRE sequence in their promoter. See the text for details.