Cell Signaling - Chapter 16

signal transduction

the process whereby one type of signal is converted into another

true

true or false: signals can act over a long or short range

hormones

______ produced in endocrine glands are secreted into the bloodstream and are distributed widely throughout the body

paracrine

______ signals are released by cells into the extracellular fluid in their neighborhood and act locally via diffusion

neuronal, neurotransmitters

_______ signals are transmitted electrically along a nerve cell axon, when this electrical signal reaches the nerve terminal, it causes the release of _________ onto adjacent target cells

contact-dependent

in _________ signaling, a cell-surface-bound signal molecule binds to a receptor protein on an adjacent cell

• both signal molecules and receptors embedded in membrane

cell-surface, intracellular

extracellular signal molecules bind either to _________ receptors or to _________ receptors

large, hydrophilic, intracellular

most extracellular signal molecules are ___ and ______ and are therefore unable to cross the plasma membrane directly; instead they bind to cell-surface receptors, which in turn generate one or more _________ signaling molecules in the target cell

small, hydrophobic, intracellular

some _____, _____, extracellular signal molecules pass through the target cell’s plasma membrane and bind to _______ receptors - in the cytosol or in the nucleus - that then regulate gene transcription or other functions

• ex: steroid hormone

different

the same signal molecule can induce _____ responses in different target cells

• 1 signal can trigger different cellular responses with different intracellular pathways

true

true or false: acetylcholine can act on receptors expressed on a wide variety of different cells

effector proteins, transduce

how a cell responds to a particular signal depends on activated intracellular proteins called _______ _____, which _____ the signal in the cytosol and/or nucleus

• these can differ between cell types although receptors are the same

• information conveyed by signal depends on how the target cell receives and decodes the signal

multiple, apoptosis

cells may require _____ signals to survive, additional signals to grow and divide, and still other signals to differentiate

• cell can change its receptors

• if deprived of the necessary survival signals, most cells undergo a form of cell suicide called _____

differs

a combination of signals can evoke a response that ____ from the sum of the effects a single signal might normally trigger

gene expression, proteins, slowly

certain types of cell responses, such as cell differentiation or increased cell growth and division, involve changes in _____ _______ and the synthesis of new ______

• these responses occur relatively ______

don’t, quickly

other responses, such as changes in cell movement, secretion, or metabolism, ____ necessarily involve changes in gene expression

• these responses occur more ____

intracellular, behavior

many extracellular signals activate _______ signaling pathways to change the _____ of the target cell

• these can be proteins, peptides, or small hydrophilic molecules

series

a cell-surface receptor protein activates one or more intracellular signaling pathways, each mediated by a _____ of intracellular signaling molecules

• can be proteins or small messenger molecules that carry on signal transduction

effector proteins, different

signaling molecules eventually interact with specific ______ ______, altering them to change the behavior of the cell in various ways

• same signaling molecules reach different effector proteins and evoke _______ target-cell responses

relay, amplify, integrate, distribute, feedback

intracellular signaling pathways perform many different functions:

1. _____ the signal and help it spread through the cell

2. ____ the signal and make it stronger, so fewer signaling molecules can have stronger effects

3. ______ signals from multiple pathways

4. ______ to more than one effector protein to evoke complex responses

5. ______ regulation of upstream signaling components (downstream molecules exert effect on upstream molecules)

positive feedback

a downstream component of the pathway enhances the response of the initial signal (explosive responses)

• intensity signal pathway

negative feedback

downstream component of the pathway diminishes response of an upstream signal (oscillatory responses)

• weaken pathway

• maintains balance

molecular switches

many intracellular signaling proteins act as ______ ______

• extracellular signals cause proteins to toggle between active and inactive states

true

true or false: extracellular signals often result in the phosphorylation of intracellular effector proteins, usually activates protein but sometimes can cause inactivation

protein kinases

phosphates are added by _____ _____, one of the most commonly activated effector proteins

• serine/threonine kinases

• tyrosine kinases

protein phosphatases

phosphates are removed by ____ ________, which commonly deactivates the effector protein following removal of the signal

• turn off signal

GTP-binding proteins

another major class of proteins involved in switching intracellular signaling pathways “on” and off” are __________

• typically located close to cell membrane

• proteins become active upon binding GTP nucleotide

• become inactive by hydrolyzing GTP —> GDP

guanine nucleotide exchange factors (GEFs)

____ activate monomeric GTPase proteins by promoting exchange of GDP for GTP

GTPase activating proteins (GAPs)

______ turn off monomeric GTPase proteins by stimulating the hydrolysis of GTP to GDP

ion channel coupled receptor

class of cell-surface receptor: an __________________ opens in response to binding an extracellular signal molecule

• common in heart, neurons

• changes the membrane potential of the cell and can produce an electrical current

• rapid transmission of signals across synapses in nervous system

• transduce chemical signals (neurotransmitters) into electrical signals

• ions flow in or out of cells, driven by electrochemical gradients

G protein coupled receptors

Class of cell-surface receptors that activates membrane bound trimeric GTP-binding proteins (G proteins)

• the proteins then activate intracellular enzymes or also ion channels in the plasma membrane to stimulate an intracellular signaling cascade

enzyme coupled receptors

class of cell-surface receptors: act like enzymes or associate with enzymes

• upon stimulation by a ligand, the enzymes can activate a variety of intracellular signaling pathways

largest, polypeptide, 7, tail

GPCRs are the _______ family of cell-surface receptors

• activated by a wide variety of different ligands and in turn activate a variety of intracellular signaling molecules

• 1/3 of all drugs prescribed target these receptors

• all with a similar structure: a single ______ with _ transmembrane domains and an intracellular ____ (c-terminal)

• ancient receptors, prokaryotes with structurally similar transmembrane proteins

alpha, conformation, dissociate, bound

an activated GPCR activates G proteins by causing the ___ subunit to exchange GDP for GTP

• alpha and gamma subunits of G protein have covalently attached lipid molecules

• in unstimulated state, both receptor and G protein inactive

• binding of extracellular signal molecule to receptor changes _______ of receptor, which alters conformation of bound G protein

• alteration of alpha subunit of G proteins allows it to exchange GDP for GTP, triggering conformational change that activates both alpha subunit and beta-gamma complex, which ______ to interact with their target proteins in plasma membrane

• receptor stays active as long as external signal molecule is ____ to it, and therefore can activate many molecules of G protein

hydrolyzing, inactivates, inactive

The G protein alpha subunit switches itself off by _______ its bound GTP to GDP

• when an activated alpha subunit interacts with its target protein, it activates that target protein for as long as two remain in contact

• alpha subunit then hydrolyzes bound GTP to GDP within seconds of G protein activation

• hydrolysis of GTP _____ the alpha subunit, which dissociates from its target protein, and alpha subunit can now reassociate with a beta-gamma complex to reform an _____ G protein

• G protein now ready to couple with another activated receptor

• both activated alpha subunit and activated beta-gamma complex can interact with target proteins in plasma membrane

bacteria

_______ can exploit G-protein coupled receptor signaling and cause disease

• different effects but causes same outcome of prolonged activation

Cholera toxin

infects the human intestine

• modifies the alpha subunit of the Gs protein

• GTP does not get hydrolyzed to GDP

• prolonged activation of alpha subunit, prevents inactivation of G proteins

• stimulation of effector proteins (adenylyl cyclase)

• impairs Cl^- trafficking, causing excess water to flow into gut leading to diarrhea

Pertussis Toxin

infects the lung

• modifies the alpha subunit of the Gi protein

• blocks exchange of GDP for GTP

• inactivating Gi protein, kept in inactive state

• Gi normally inhibits adenylyl cyclase

• undergoes excessive, prolonged activation

opening, harder, slows

Gi protein directly couples receptor activation to the _____ of K⁺ channels in the plasma membrane of heart pacemaker cells

• binding of the neurotransmitter acetylcholine (released by nerves) to its GPCR on the heart cells results in the activation of the G protein (Gi)

• activated beta-gamma complex directly opens K+ channel in plasma membrane, increasing its permeability to K⁺ and thereby making it _____ to electrically activate membrane, which ____ the heart rate

• inactivation of the alpha subunit by hydrolysis of its bound GTP returns G protein to its inactive state, allowing K⁺ channel to close

increase

enzymes activated by G proteins (adenylyl cyclase, phospholipase C) _____ the concentrations of small intracellular signaling molecules

second messenger molecules, amplified, slower, complex

because each activated enzyme generates many ___ ____ _______ (cyclic AMP, diacylglycerol), the signal is greatly _______ at this step in the pathway

• signal relayed onward by these molecules which bind to specific signaling proteins in the cell and influence their activity

• the effects of this process are ____ and more ___ compared to those that occur with ion channels

adenylyl cyclase

many GPCRs affect the activity of _____ (enzyme)

cyclic AMP

______ (2nd messenger) is synthesized by adenylyl cyclase and degraded by cyclic AMP phosphodiesterase

ATP, 2, sugar

cAMP is formed from ____ by a cyclization reaction that removes _ phosphate groups from ATP and joins the “free” end of the remaining phosphate group to the ___ part of the AMP molecule

AMP

degradation by cyclic AMP phosphodiesterase breaks this new bond, forming ____

• caffeine inhibits AMP phosphodiesterase and thus helps maintain high concentrations of cyclic AMP

rapidly, serotonin, eliminate

• concentration of cyclic AMP rises _____ in response to extracellular signal

• binding of the neurotransmitter _____ to a GPCR boosts the synthesis of cyclic AMP

• more NT receptors = more concentration

• cyclic AMP phosphodiesterase is always active, so it can _______ cyclic AMP quickly. Thus, the levels of cAMP are constantly fluctuating throughout the cell

increase, glycogen, fat

the extracellular signal molecule epinephrine (fight or flight hormone)

• in heart: causes ______ in heart rate and force of contraction

• in skeletal muscle: ______ breakdown

• in fat: __ breakdown

PKA, inactivated

cyclic AMP exerts most of its effects via ___, aka cyclic-AMP-dependent protein kinase

• ^ is normally kept in an ________ state by an associated protein complex that is inhibitory and suppresses it

• cyclic AMP binds to regulatory complex, releasing it so it is now active

phosphorylate, modulate

After becoming active, PKA is then free to ______ and ____ the activity of intracellular signaling proteins

• type of response depends on which intracellular proteins are present in the cell

G protein, adenylyl cyclase, cAMP, PKA, phosphorylase kinase, glycogen phosphorylase

epinephrine stimulates glycogen breakdown in skeleton muscle cells

• GPCR binding activates ________, which activates ______ ______ to boost _____ production

• the latter activates ___, which phosphorylates and activates an enzyme called _______ ____

• the latter then activates _______ __________, the enzyme that breaks down glycogen into glucose (fuel for energy production)

• fairly rapid process

gene transcription, nucleus, transcription regulators

a rise in intracellular cyclic AMP can also activate ___ _____

• intracellular cAMP activates PKA, allowing it to enter the ______ and phosphorylate specific ______ _____

• once phosphorylated, these proteins stimulate transcription of target genes

• less rapid process (depends on gene transcription)

phospholipase C

GPCRs that mediate their effects via Gq protein activate membrane bound _____________

lipid, inositol phospholipid, DAG, IP3

Phospholipase C cleaves a _____ in the membrane called _____ ______, generating 2 second messenger molecules: ____ and ____

diacylglycerol (DAG), protein kinase C (PKC)

____________: this is generated by phospholipase C, and upon activation, this remains embedded in plasma membrane, where it can recruit and activate __________, which can then phosphorylate and modulate the activity of downstream intracellular signaling proteins

Inositol 1,4,5 triphosphate (IP3), calcium, endoplasmic reticulum, cytosolic

___________: this is generated by phospholipase C, binds and opens _____ channels in the _____ _______, causing a sharp rise in _____ calcium levels

amylase, contraction

acetylcholine can trigger different cellular responses in different target tissue from the same signal molecule

• in pancreas: response is secretion of ______ (a digestive enzyme)

• in skeletal muscle: response is _____

endoplasmic reticulum, zona pellucida

when a sperm fertilizes an egg, cytosolic calcium is released from the _______ ______ of the egg, which alters surface membrane receptors preventing other sperm from fertilizing the egg, and initiates development

• hardens outer membrane of egg (____ _______)

intracellular, extracellular, Ca+2, released

calcium’s role as intracellular messenger is diverse and widespread

• _________ calcium levels are low compared to _________ concentrations

• cytosolic calcium levels are controlled by _______ channels on the cell membrane, and many GPCR and other signaling pathways regulate opening/closing

• calcium is also stored and ________ by the endoplasmic reticulum

strictly, ATP, rush into

Calcium gradients are also ______ controlled by membrane-embedded pumps that use ___ to actively transport Calcium out of the cell or into ER storage sites

• when channels are opened, the electrochemical gradient causes Calcium to ___ ___ the cell from the extracellular environment or from the ER

• Calcium can signal by binding and activating Calcium binding proteins

calmodulin, CaM kinase (Ca+2/Calmodulin-dependent protein kinase)

_______ is a cytosolic calcium-binding protein widely expressed in Eukaryotes

• upon binding calcium, it undergoes a conformational change (2 globular domains connected by alpha helix, can now bind with 2 Ca+2 ions)

• it can now alter the activities of its target proteins

• an important target protein is ____ _______

phosphorylate, synapses, learning, memory

once CaM kinase is activated, it can ______ target proteins further down the pathway

• in neurons, it is abundant in ______ and is thought to regulate _______ and ______ via LTP

• repeat stimulation —> strengthening circuit —> better memory

smooth muscle, dilation

some second messengers are produced in the cell but can pass through the cell membrane to influence the activity of neighboring cells

• example is Nitric oxide, which can trigger ______ _____ relaxation and blood vessel _______

endothelial, Ca+2, nitric oxide synthase, nitric oxide, guanylyl cyclase, GMP, GTP

acetylcholine binds to a GPCR on the surface of the ________ cells and activates Gq to trigger ____ release

• the latter activates ____ ____ ______, stimulating the production of _____ ____, which can diffuse out of the cells and into adjacent smooth muscle cells

• there, it regulates the activity of specific proteins such as _______ ____

• the latter catalyzes the production of cyclic ___ from ____

• drugs such as Viagra prevent cyclic GMP degradation, prolonging NO signal

retina, light, electrical energy

one of the fastest responses mediated by GPCRs is that of light activating rod photoreceptors in the ____

• photoreceptors convert _____ (photons) into ______ ____

rhodopsin, transducin, close, neurotransmitter

light is sensed by _______, which is a GPCR located in the outer segment of the photoreceptor

• it will activate a G protein called ______

• the latter causes cation channels to ___ in the plasma membrane, causing a change in voltage across the membrane

• this affects _______ release and causing signals to be sent to the brain

amplify, perceptible, adaptation, negative feedback

the light-induced signaling case in rod photoreceptor cells greatly ____ the light signal

• in the case of vision, even very low light conditions will lead to a _______ signal due to amplification

• when the external signal is intense or plentiful, amplification can be dampened through a process known as ________, which relies on _____ ______ to allow cells to fine-tune their responses when extracellular signals are fluctuating

• rapid, sensitive, adaptive process

one photon, 500, 500, 10⁵, 250, 10⁶, 10⁷, 1

Pathway:

1. light

2. one rhodopsin molecule (receptor) absorbs ____ _____

3. _____ G protein (transducin) molecules are activated (amplified)

4. ____ cyclic GMP phosphodiesterase molecules are activated

5. ____ cyclic GMP molecules are hydrolyzed

6. ____ cation channels in plasma membrane close

7. _____ to ____ Na+ ions per second are prevented from entering the cell for a period of about 1 second

8. membrane potential is altered by _ mV

9. signal relayed to brain

ligand, extracellular, regulate, slow, gene transcription, cytoskeleton, rapid

enzyme-coupled receptors: basics

• transmembrane proteins that have _____-binding domains on the _______ surface

• do NOT associate with G proteins (intracellular domains)

• the receptor’s intracellular domains can act as an enzyme or can form a complex with another protein that acts as enzyme

• these receptors commonly ______ the growth, proliferation, differentiation, and survival of cells

• responses are often ___ and involve multiple steps leading to ___ ________

• however, some are directly linked to the ______ and can cause ___ changes in cell shape and movement

Receptor Tyrosine Kinases (RTKs), phosphorylates, dimers, tails

• __________ comprise the largest class of enzyme-coupled receptors

• cytoplasmic domain functions as a tyrosine kinase, which ________ tyrosine residues on intracellular signaling proteins

• ligand-binding causes RTKs to form _____ at the cell membrane, bringing together the two intracellular ____ and activating their kinase domains

  • each receptor tail phosphorylates the other on specific tyrosine residues

docking, phosphorylated, scaffolds

• phosphorylated tails of RTKs serve as ______ sites for intracellular signaling proteins

  • some become ________ and activated

  • others function as ________ to couple the receptor to other signaling proteins (recruiting)

signaling complexes, interaction domain

for RTKs, the fundamental action is to build _____ ____

• intracellular signaling proteins have a specialized _____ ____ that specifically recognizes phosphorylated residues on the intracellular tails of RTKs

tyrosine phosphatases, endocytosis, degradation

RTKs: as long as the signaling complex remains assembled, the receptors can activate multiple signaling pathways

• response are terminated by ______ _____, which remove phosphates from the intracellular tails (inactivation)

• responses can also be terminated by __________, which internalizes the receptor to cytosol and targets it for ______ (receptor proteins gone —> shuts down signal)

monomeric GTPase, lipid

Ras is a key member of the cytosolic signaling complex

• it is activated by most RTK receptors and can stimulate several downstream signaling pathways

• it is a ______ _____, a small GTP binding protein that associates with the plasma membrane via its ____ tail

• it resembles the alpha subunit of a G protein and similarly functions as molecular switch

GTP, GDP

Ras is active when it is bound to ____ and inactive when bound to ___

Ras-GEF (guanine exchange factor)

_____ causes Ras to switch GDP for GTP, thereby activating the protein

Ras-GAP (GTPase activating protein)

switches Ras “off” by promoting GTP hydrolysis

phosphorylation

Ras can initiate a ______ cascade that involves a series of serine/threonine kinases

• ex: MAP-kinase signaling module (3 kinase signaling module)

• activated MAP-kinase phosphorylates a variety of downstream signaling proteins

inactivate, activation

Ras mutations that _____ the GTPase activity of Ras cause excessive _______

• this can result in uncontrolled cell proliferation and cancer

PI 3-kinase (phosphoinositide 3-kinase), inositol phospholipids, cytosol

RTK activation of ________ is important for cell growth and survival

• the latter phosphorylates _____ _______ in the cell membrane

• these phosphorylated lipids act as docking sites for intracellular signaling proteins, which relocate from ____ to the cell membrane and can activate one another

protein kinase, survival

Akt is a ____ _____ that is phosphorylated and activated by protein kinase 1/2

• activated Akt is released from the cell membrane into cytosol and can phosphorylate downstream signaling proteins

• activated Akt promotes cell _______

• Akt phosphorylates downstream targets which can cause them to become inactivated

cell death, Bcl2

Akt phosphorylates Bad

• in the active state, Bad promotes apoptosis and ____ ____ by binding and inhibiting the activity of ____ (suppresses apoptosis)

• phosphorylation by Akt inactivates Bad and shuts down apoptotic machinery to release it, to now promote cell survival

Tor (target of rapamycin)

Akt stimulates cells to grow in size by activating the serine/threonine kinase ___

• Akt phosphorylates and inhibits a protein that normally keeps it inactivated

protein synthesis, protein degradation

Tor causes cells to grow by enhancing ______ ____ and inhibiting _____ ______

rapamycin

______ is a well-known cancer drug that works to inactivate Tor, inhibiting cell growth and survival

PI 3-kinase, Akt, Tor

Pathway:

1. growth factor attached to RTK

2. activated RTK by phosphorylation

3. activates ________

4. activates ___

5. downstream signaling molecules

6. activates ___

7. inhibition of protein degradation, stimulation of protein synthesis

8. effect: cell growth

cross talk, protein kinases

each pathway differs from the others, yet they use common components and ___ ___ in order to transmit their signals

• because all eventually activate ____ ____, each is capable of regulating practically any process in the cell

Co-immunoprecipitation

used to detect protein-protein interactions

• antibody against target protein (A) of interest is bound to a column, and cell lysates are poured over the column

• target protein A will bind to its antibody on column

• target protein is then removed from column, along with its protein binding partners

• mass spectrometry

targeted mutagenesis, phenylalanine

in some cases, protein-protein interactions are known, but the specific interaction domains have yet to be identified

• ______ ____ can be used to disrupt potential protein binding domains to determine which are required for interactions

• in the example shown here, in order to determine which tyrosine residues on RTKs bind to signaling proteins, each is selectively mutated to a _______, which is not phosphorylated by RTK —> determines effects on protein binding and cell responses to signal

dominant interfering effect, activation, inactivation

• other methods to untangle cell signaling pathways may involve overexpressing individual components of a signaling pathway in cells

• over-expressing mutated copies of the gene may cause a ______ ____ ___ on the normal pathway’s functions, which can then be deduced

• can over-express mutated versions that cause both excessive _____ and/or ______

Ras acts downstream of Protein X

cell with mutant protein X, normal protein Y

1. introduce signal molecule —> no signal

2. introduce overactive Ras —> signaling is restored

conclusion?

Protein Y acts downstream of Ras

cell with mutant protein Y, normal protein X

1. introduce signal molecule —> no signaling

2. introduce overactive Ras —> no signaling

Conclusion?

transcriptional regulator, cleaved, nucleus

Notch (delta receptor) is a transmembrane signaling receptor that is critical for embryonic and adult functions

• it can also act as a _______ ____

• upon activation by Delta, a transmembrane signaling protein on neighboring cells, the Notch receptor is ____

• the cleaved cytoplasmic tail can translocate to the ____ and activate Notch-responsive genes

endothelial, neurons, non-neuronal, lateral inhibition, contact-dependent

Notch signaling controls nerve-cell production in the fruit fly Drosophila

• the fly nervous system originates from a sheet of ______ cells

• isolated cells in this sheet begin to specialize as _____, while their neighbors remain _______ (become glial cells) and maintain the structure of the sheet as instructed

• the signals that control this process are transmitted via direct cell-cell contacts: each future neuron delivers an inhibitory signal to the cells next to it, deterring them from specializing as neurons too: a process called _______ _______

• This is a form of ______________ signaling

nuclear receptors, cytosol, nucleus, gene transcription

steroid hormones (can pass lipid bilayer) are small, hydrophobic molecules that bind to intracellular receptors that act as transcription regulators

• these receptors, called ______ _______, can be located in either the ____ or the _____

• regardless of where they are located, these receptors regulate ___ ______ in the nucleus (activate or repress)

inactive, different

• in unstimulated cells, nuclear receptors exist in ______ form

• upon binding a hormone, the receptor undergoes conformational change and becomes activated

• each hormone binds to a different nuclear receptor that recognizes specific regulatory DNA sequences

• hormones can stimulate _____ sets of genes in different cell types

cortisol

____ is produced by adrenal glands in response to stress

• it happens to bind its receptor protein in the cytosol, which then translocates to the nucleus

testosterone

______ is another steroid hormone that is critical for sexual development

• loss of nuclear receptors for this cause people to develop as female even with XY chromosomes

enzyme coupled

• _______ _____ receptors are very common in plants

• many encode receptor serine/threonine kinases

• important for growth, development, disease resistance

True

true or false: plants do NOT use receptor tyrosine kinases, steroid-hormone receptors, and few GPCRs

ethylene, relieving inhibition, destruction

_______ is a gaseous hormone that regulates seed germination and ripening

• the pathway turns ON genes by _____ _______

• in absence of this, the receptor directly activates associated protein kinase, which promotes _______ of the transcription regulator that switches on hormone-responsive genes