Chapter 16 - Cell Signaling Flashcards
(42 cards)
When cells respond to an extracellular signal, they most often convert the information carried by this molecule from one form to another. What is this process called?
- signal transduction
- signal detection
- signal integration
- signal amplification
- signal production
Signal transduction
(Signal transduction begins when the receptor on a target cell receives an incoming extracellular signal and then produces intracellular signaling molecules that alter cell behavior.)
Many of the extracellular signal molecules that regulate inflammation are released locally at the site of infection. What form of cell–cell signaling is being used?
- paracrine
- endocrine
- contact-dependent
- neuronal
Paracrine
(In paracrine signaling, the extracellular signaling molecule acts as a local mediator on cells located nearby the signaling cell that produced it.)
Which statement about cell signaling is correct?
- All extracellular signal molecules act by binding to receptors on the cell surface.
- All cell types are able to respond to the same set of signal molecules.
- Each receptor triggers one particular type of cell behavior, for example, activating gene expression.
- Each receptor is generally activated by only one type of signal molecule.
- Each type of extracellular signal molecule induces the same response in all target cells.
Each receptor is generally activated by only one type of signal molecule.
(Small molecules like acetylcholine, for example, usually bind to a pocket formed by amino acids from several transmembrane segments of the receptor. These amino acids form noncovalent interactions with the signal molecule, allowing the receptor to recognize and bind specifically to its signal.)
Which statement about positive feedback regulation is accurate?
- In positive feedback, a downstream component acts to inhibit an earlier component in the pathway to diminish the response to the initial signal.
- Positive feedback can generate responses that oscillate on and off as the activities or concentrations of the participating components rise and fall.
- Positive feedback is rare in biological systems.
- Positive feedback regulation can generate an abrupt, all-or-none response in which the cell moves from ignoring a signal to responding to it very strongly.
- In positive feedback, a molecular switch enhances the response to a signal by activating a component that lies downstream in the pathway.
Positive feedback regulation can generate an abrupt, all-or-none response in which the cell moves from ignoring a signal to responding to it very strongly.
(In some cases, such a response can be self-sustaining and will persist even after the signal is no longer present.)
What is true of the GTP-binding proteins that act as molecular switches inside cells?
- They turn themselves on by hydrolyzing GTP to form GDP.
- They are active when GTP is bound.
- They are active when GDP is bound.
- They turn themselves on by phosphorylating GDP to form GTP.
- They are active only in their trimeric forms.
They are active when GTP is bound.
(Once activated by GTP binding, many of these proteins have intrinsic GTP-hydrolyzing (GTPase) activity, and they shut themselves off by hydrolyzing their bound GTP to GDP.)
Which is true of the GTP-binding proteins that participate in intracellular signaling?
- Only trimeric GTP-binding proteins relay messages from G-protein-coupled receptors.
- Only trimeric GTP-binding proteins interact with guanine nucleotide exchange factors (GEFs).
- Only monomeric GTP-binding proteins relay messages from G-protein-coupled receptors.
- Only trimeric GTP-binding proteins participate in intracellular cell signaling.
- G-protein-coupled receptors interact with all types of GTP-binding proteins.
Only trimeric GTP-binding proteins relay messages from G-protein-coupled receptors.
(There are several varieties of G proteins. Each is specific for a particular set of receptors and for a particular set of target enzymes or ion channels in the plasma membrane.)
What is true of ion-channel-coupled receptors?
- They transduce signals in a simple and direct manner.
- They are voltage-gated.
- They transduce signals via elaborate intracellular signaling pathways.
- They are found only in nerve cells.
- They integrate multiple signals at the same time.
They transduce signals in a simple and direct manner.
(Ion-channel-coupled receptors do not activate the sort of elaborate intracellular signaling pathways used by G-protein-coupled receptors or enzyme-coupled receptors.)
Why does nitric oxide (NO) act as a paracrine signal that affects only neighboring cells?
- It is rapidly converted to nitrates and nitrites in the target cell.
- Cell-surface NO receptors are expressed only on cells in close proximity to NO signaling cells.
- It is rapidly washed away by the bloodstream.
- It is rapidly converted to nitrates and nitrites in the extracellular fluid.
- It diffuses rapidly into the atmosphere.
It is rapidly converted to nitrates and nitrites in the extracellular fluid.
(Reaction with oxygen and water in the extracellular environment converts NO into nitrates and nitrites within seconds of its release by the cells that produce it.)
Signaling via a GPCR ceases when which condition occurs?
- The G protein associates with an activated GPCR.
- The G protein dissociates from the activated GPCR.
- The α subunit hydrolyzes its bound GTP.
- The α subunit exchanges GDP for GTP.
- The α subunit dissociates from the βγ complex.
The α subunit hydrolyzes its bound GTP.
(GTP hydrolysis and inactivation usually occur within seconds after the G protein has been activated. The inactive G protein is then ready to be reactivated by another activated receptor.)
Which response to GPCR stimulation would be most rapid?
- activation of a G protein that inhibits adenylyl cyclase
- change in the activation of a set of target genes
- activation of a G protein that activates adenylyl cyclase
- activation of a G protein that activates phospholipase C
- activation of a G protein that regulates the opening of an ion channel
activation of a G protein that regulates the opening of an ion channel
(When G proteins interact with ion channels, they cause an immediate change in the state and behavior of the cell.)
When activated phospholipase C cleaves an inositol phospholipid, what happens to the small signaling molecules the enzyme produces?
- Both inositol 1,4,5-trisphosphate (IP3) and Ca2+ are released into the cytosol.
- Inositol 1,4,5-trisphosphate (IP3) is released into the cytosol, while diacylglycerol is retained in the membrane.
- Inositol 1,4,5-trisphosphate (IP3) remains in the membrane, while diacylglycerol is released into the cytosol.
- Both inositol 1,4,5-trisphosphate (IP3) and diacylglycerol are retained in the membrane.
- Both inositol 1,4,5-trisphosphate (IP3) and diacylglycerol are released into the cytosol.
Inositol 1,4,5-trisphosphate (IP3) is released into the cytosol, while diacylglycerol is retained in the membrane.
(IP3 binds to and opens Ca2+ channels that are embedded in the endoplasmic reticulum (ER) membrane; diacylglycerol remains in the membrane, where it helps to recruit and activate protein kinase C (PKC).)
What are small intracellular signaling molecules often called?
- first messengers
- transmitter-gated ion channels
- second messengers
- intracellular signaling proteins
- extracellular signaling molecules
Second messengers
(The “first messengers” are the extracellular signals that activate the enzymes that produce the second messengers. These second messengers rapidly diffuse from their site of synthesis and thereby amplify and spread the intracellular signal.)
Because cyclic AMP diffuses quickly through the cytosol and a cyclic AMP phosphodiesterase rapidly converts cyclic AMP to AMP, what is the consequence?
- The cytosolic concentration of cyclic AMP can change rapidly.
- The responses triggered by an increase in cyclic AMP are slow.
- The responses triggered by an increase in cyclic AMP are rapid.
- Cells must sequester cyclic AMP.
- Cyclic AMP is not often used to mediate cell responses.
The cytosolic concentration of cyclic AMP can change rapidly.
(The rapid synthesis, diffusion, and elimination of cAMP allows the concentration of this second messenger to rise or fall tenfold in a matter of seconds.)
What do the phosphorylated tyrosines on activated RTKs do?
- They serve as binding sites for a variety of intracellular signaling proteins.
- They promote receptor internalization.
- They serve as binding sites for G proteins.
- They help the receptor dimerize.
- They activate the enzymatic activity of the RTKs.
They serve as binding sites for a variety of intracellular signaling proteins.
(Some of these proteins become phosphorylated and activated on binding to the receptors, and they then propagate the signal; others function solely as scaffolds, which couple the receptors to other signaling proteins, thereby helping to build the active signaling complex.)
Ras is activated by a Ras-activating protein that does what?
- causes Ras to hydrolyze its bound GTP
- causes Ras to exchange GDP for GTP
- causes Ras to interact with a phosphorylated RTK
- dephosphorylates Ras
- phosphorylates Ras
causes Ras to exchange GDP for GTP
(An adaptor protein, docked on a particular phosphotyrosine on the activated receptor, recruits the Ras guanine nucleotide exchange factor (Ras‑GEF), which stimulates Ras to exchange its bound GDP for GTP.)
RTKs can activate the enzyme phosphoinositide 3-kinase, which phosphorylates inositol phospholipids. These phospholipids then do what?
- activate Ras
- activate G proteins
- serve as docking sites that recruit specific intracellular signaling proteins to the plasma membrane
- serve as phosphate donors in phosphorylation reactions
- activate Ca2+ channels in the plasma membrane, promoting an influx of Ca2+ into the cytosol
Serve as docking sites that recruit specific intracellular signaling proteins to the plasma membrane
(One of the most important of these recruited proteins is a kinase called Akt.)
PI 3-kinase acts by phosphorylating what molecule(s)?
- serines or threonines on protein kinases that phosphorylate Akt
- tyrosines on protein kinases that phosphorylate Akt
- tyrosines of Akt
- inositol phospholipids
- serines or threonines of Akt
Inositol phospholipids
(These phosphorylated lipids serve as docking sites for other intracellular signaling proteins, which then activate one another.)
Ethylene is a hormone that promotes the ripening of fruit. What happens in the absence of ethylene?
- The activated ethylene receptor promotes the degradation of a transcription regulator and the ethylene-responsive genes remain turned off.
- The ethylene receptor is internalized and degraded, shutting down the expression of the ethylene-responsive genes.
- The ethylene receptor is inactive and the transcription regulator needed to turn on the ethylene-responsive genes is degraded.
- The ethylene receptor is inactive and a transcription regulator shuts off the ethylene-responsive genes.
- The activated ethylene receptor switches on a transcription regulator that inhibits the expression of ethylene-responsive genes.
The activated ethylene receptor promotes the degradation of a transcription regulator and the ethylene-responsive genes remain turned off.
(In the absence of ethylene, the receptor directly activates an associated protein kinase, which then indirectly promotes the destruction of the transcription regulator that switches on ethylene-responsive genes. As a result, the genes remain turned off.)
Ras can exist in two different conformations or states, inactive and active. Which of the following correctly describe(s) the stably active state of Ras?
Choose one or more:
- Ras is bound to GTP.
- Ras is bound to GDP.
- Ras is bound to Ras-GAP.
- Switch 1 and switch 2 regions are in an active conformation.
- Ras is bound to GTP.
- Switch 1 and switch 2 regions are in an active conformation.
(Binding to GTP changes the switch 1 and switch 2 regions to the active conformation, leading to activation of the Ras protein.)
Specific amino acids, including Arg 789 in Ras-GAP and amino acids Gln 61 and Thr 35 in Ras, all play an important role in Ras function. Why are these amino acids important?
- They help Ras bind GDP tightly.
- They help Ras bind GTP tightly.
- They help facilitate GTP hydrolysis by Ras.
- They help downstream signaling by binding to additional signaling proteins.
They help facilitate GTP hydrolysis by Ras.
The amino acids in Ras-GAP and Ras hydrolyze the bound GTP, inactivating Ras.
Which of these represents a mechanism used to terminate a signal transmitted by activated RTKs?
- SH2 domains are internalized and digested in lysosomes.
- The RTKs are internalized and digested in lysosomes.
- SH2 domains remove the phosphates from RTKs and other intracellular signaling proteins.
- Tyrosine kinases remove the phosphates from RTKs and other intracellular signaling proteins.
- The RTK is inactivated by phospholipase C.
The RTKs are internalized and digested in lysosomes.
(Activated receptors can also be switched off in a more readily reversible manner, by removal of their activating phosphates by a tyrosine phosphorylase.)
What makes it possible for a combination of signal molecules to evoke a response that differs from the sum of the effects that each signal could trigger on its own?
- the ability of different receptors to activate different intracellular relay systems
- the ability of the same signal molecule to bind to different receptors
- the ability of different signal molecules to bind to the same receptor
- the ability of different intracellular relay systems to interact
- the tendency of different cells to display different receptors
The ability of different intracellular relay systems to interact
(A cell can integrate and tailor its response to a combination of signals in part because the intracellular relay systems activated by the different signals interact. Thus, the presence of one signal will often modify the effects of another.)
Which of the following signaling pathways would be likely to trigger the most rapid cell response?
- Adrenaline binds to a GPCR to activate a cyclic AMP signaling pathway that triggers glycogen breakdown.
- Acetylcholine binds to anion-channel-coupled receptor that allows Na+ to flow down its electrochemical gradient, triggering contraction of a skeletal muscle cell.
- Adrenaline binds to a GPCR to activate a cyclic AMP signaling pathway that triggers the synthesis of hormones in endocrine cells.
- Platelet-derived growth factor binds to a receptor tyrosine kinase to activate a signaling pathway that stimulates cell proliferation at the site of a wound.
- Nerve growth factor binds to a receptor tyrosine kinase to activate a signaling pathway that enhances the transcription of Bcl2, a protein that suppresses cell death.
Acetylcholine binds to anion-channel-coupled receptor that allows Na+ to flow down its electrochemical gradient, triggering contraction of a skeletal muscle cell.
(Ion-channel-coupled receptors are the simplest and most direct cell-surface receptors. Activation of an ion-channel-coupled receptor can trigger a response in milliseconds—thousands of a second.)
In skeletal muscle cells, epinephrine (adrenaline) stimulates the breakdown of glycogen by activating a GPCR. In the signaling pathway shown here, which step does not amplify the original signal?
- activation of glycogen phosphorylase by activated phosphorylase kinase
- activation of adenylyl cyclase by an activated G protein
- activation of phosphorylase kinase by activated PKA
- activation of a G protein by an activated GPCR
- production of cyclic AMP by activated adenylyl cyclase
activation of adenylyl cyclase by an activated G protein
(Adenylyl cyclase is activated by the binding of an activated α subunit of the G protein Gs. When the α subunit hydrolyzes its bound GTP, it will dissociate from adenylyl cyclase, rendering the enzyme inactive once again.)
