Cell Signaling (Lectures 3 and 4)

Question 1

What are the 4 types of signaling receptors?

Answer 1

1. ) ligand-activated ion channels
2. ) G protein coupled receptors
3. ) ligand-activated transcription factors
4. ) tyrosine kinase receptors

Question 2

About 40% of drugs target what type of receptor?

Answer 2

G protein coupled receptor

Question 3

When can ligand-activated ion channels be found?

Answer 3

1. ) plasma membrane

2. ) organells

Question 4

Describe plasma membrane ligand-activatd ion channels

Answer 4

• signaling in excitable cells
• multi-subunit proteins
• produce graded potential (effects are local)

Question 5

Describe depolarization vs hyperpolarization in ligand-activated ion channels

Answer 5

depolarization: lets in Na+ (usually not particularly selective)
- acetylcholine, serotonin, glutamate
hyperpolarization: lets in Cl-
- glycine, GABA (gamma amino butyric acid)

Question 6

Activating and inhibiting ligand activated plasma membrane ion channels is the basis of?

Answer 6

synaptic transmission of nerve impulses

note: plenty of drugs target here

Question 7

What are some ways drugs affect the synaptic transmission of nerve impulses?

Answer 7

1. ) mimicking or blocking the action of neurotransmitters
2. ) blocking the ion channel
3. ) acting on transmitter re-uptake
4. ) acting on transmitter metabolism
   note: this was in the section of ligand-activated ion channels in the plasma membrane

Question 8

What is an example of ligand activated ion channels in an organelle?

Answer 8

IP3 mediated Ca+ release from the ER

note: this is the receptor mediated Ca2+ release from the ER internal source. IP3 sensitive Ca2+ chanel. (IP3 is the ligand)

Question 9

Most signaling by Ca++ is mediated by?

Answer 9

Ca++/Calmodulin

Question 10

Describe Ca2+/Calmodulin signaling

Answer 10

IP3 –> Ca2+

[Ca2+] in the cytoplasm of a cell greatly increases after IP3 mediated release

1. ) The increase in [Ca2+] increases the binding of Ca2+ to calmodulin, altering the structure of the Ca/calmodulin complex
2. ) The Ca2+/calmodulin complex is able to interact and activate numerous effector proteins

Question 11

What are 2 examples of proteins activated by Ca2+/calmodulin

Answer 11

1. ) calcium calmodulin dependent protein kinase
   
   • endothelial
   • many substrates for this kinase
2. ) nitric oxide synthase
   
   • neuronal
   • leads to synthesis of cGMP

Question 12

How do agonists activate G-protein coupled receptors (GPCRs)?

Answer 12

Agonists bind to the GPCR’s, alter the proteins’s conformation, leading to the activation of a trimeric G protein

Question 13

Describe the G protein cycle

Answer 13

1. ) Ligand bound GPCR stimulates GDP-GTP exchange
2. ) The GPCR complex separates into two pieces
   - G alpha and G beta G gamma
3. ) G alpha has intrinsic GTPase activity. Eventually the GTP is hydrolyzed to GDP and Pi. RCS proteins accelerate this process
4. ) After GTP hydrolysis the complex reassembles

important note: Both G alpha and G beta G gamma participate in cell signaling events

Question 14

What are the components of the GPCR complex and what do they do?

Answer 14

major component: G alpha
minor component: G beta G gamma

both components bind to and activate other proteins in the cell

Question 15

What are the major effector of G alpha’s?

Answer 15

1. ) Adenylate cyclase (10 isoforms)
2. ) Phospholipase A2

note: major does not mean only.
also : he lists phospholipase C as a minor effector and phospholipase Cbeta as a major effector later

Question 16

Describe the mechanism of Adenylate cyclase

Answer 16

1.) along with ATP adenylate cyclase it forms cAMP and PPi
2.) cAMP acts on (activates) protein kinase A (PKA)
3.) PKA driven phosphorylation of a large number of proteins produces a large number of biological effects
-PKA alters the activity of target proteins (literally
hundreds of targets)
- phosphorylation of transcription factors by PKA and
other protein kinases can lead to gene expression in
cells
-the whole process is pretty complicated

note: his notes say to know this

Question 17

Where are signaling molecules often embedded and how to GPCRs liberate them?

Answer 17

-they’re embedded within the phospholipids in the
membrane
-GPCRs liberate them by activating phospholipases

Question 18

Why do we care about arachidonic acid?

Answer 18

its the source of eicosinoid signaling molecules

Question 19

What is the substrate for phospholipase C?

Answer 19

• PIP2 (phosphatidyllinositol-4,5 bis phosphate)

- it is a phospholipid (hence phosphatidyl)

Question 20

Cleavage of PIP2 by phospholipase C (PLC beta) yields?

Answer 20

• DAG
• IP3

note: they are important signaling molecules. Remember IP3 increases [Ca2+] in the cytoplasm

Question 21

What does DAG activate?

Answer 21

-PKC

note : PKC is Ca2+ dependent. DAG allows PKC to bind Ca2+ tighter so the enzyme is able to function at lower Ca2+ concentrations

Question 22

What critical role does G beta G gamma play?

Answer 22

it helps turn of the GPCR

Question 23

The trimeric G protein turns itself off but the agonist GPCR complex is free to activate other trimeric G proteins that it comes in contact with. Does this signal last for as long as the agonist is present?

Answer 23

No. The receptor gets turned off by desensitization.

Question 24

Describe the process of desensitization and endocytosis of GPCRs.

Answer 24

1.) The agonist GPCR complex is phosphorylated by a GRK    
      (recruited by G beta G gamma)
2.) B-arrestin binds 
3.) A clathrin-coated pit forms
4.) Endocytosis

Question 25

What are the possible fates of an endocytosed receptor?

Answer 25

1. ) recycling | 2. ) degradation

Question 26

Why should you care about receptor desensitization?

Answer 26

Sometimes drugs are administered chronically. This can lead to a loss of responsiveness to the drug and results that would seem paradoxical if one failed to consider desensitization.

Question 27

Describe signaling by receptor tyrosine kinases (RYK).

Answer 27

1.) receptors contain either and intrinsic or extrinsic tyrosine kinase enzyme 2.) binding of a ligand leads to dimerization of the receptor - some form homodimers, heterodimers or both 3.) dimerization leads to transphosphorylation of receptors 4.) tyrosine phosphorylation leads to docking of signaling molecules 5.) the positioning of proteins near the membrane leads to different cascades of signaling

Question 28

When protein kinases (PKA, PKA, MAP kinase) phosphorylate transcription factors this can lead to ?

Answer 28

Changes in gene expression in cells note: this is just the basic concept. Something is phosphorylated , it activates transcription factors (usually via phosphorylation), the transcription factors bind to target genes and the gene expression changes.

Question 29

Activation of Akt kinase is an example of what and why is Akt important?

Answer 29

- a signaling cascade - it pushes cells towards growth and proliferation. It inhibits apoptosis note: RYK recruits P13K to form PIP3. PIP3 recruits PDK. PDK phosphorylates Akt

Question 30

Describe JAK-STAT signaling

Answer 30

1.) binding of ligand (cytokine) leads to phosphorylation of receptor by JAK kinase 2.) one of six STAT proteins bind to the phosphorylated receptor. 3.) JAK kinase then phosphorylates the STAT protein 4.) the STAT protein dimerizes 5.) the STAT protein is translocated to the nucleus where transcriptional activation of panels of genes occurs

Question 31

What is the significance of JAK-STAT signaling?

Answer 31

-Different cytokines recruit different STAT proteins. - Different STATs have different target sites which leads to the transcription of different genes

Question 32

Cross talk amongst different signaling pathways creates?

Answer 32

Complex signaling possibilities

Question 33

Nuclear hormone receptors are receptors for?

Answer 33

steroids, hormones and other molecules (vitamin D, retinoic acid, etc)

Question 34

Nuclear hormone receptors are classified as either?

Answer 34

class I or class II

Question 35

How are class I and class II nuclear receptors different?

Answer 35

Class I receptors -ligand binding causes the receptor complex to bind to target DNA leading to transcription of certain genes Class II receptors -the receptor is already bound to the target DNA and binds co-repressors in the absence of ligand (represses transcription) -binding of the ligand causes the receptor complex to bind co-activators which leads to transcription