Chapter 9

Question 1

What are the four basic steps to cell signalling?

Answer 1

1. Signalling molecules are released (ligands)
2. Recognition of the signalling molecule by target cell (receptors)
3. Signal transduction- conversion of the extracellular signal into intracellular instructions (also called signalling cascade)
4. Final impact on target cell and impact on organism as a whole
   Slide 4

Question 2

What are the two modes of communication in cell signalling?

Answer 2

1. Direct interaction of a cell with its neighbour
2. Action of a diffusible signalling molecules over a distance

Cell communication and signal transduction increases in complexity with multicellular organisms

Question 3

What are the 4 modes of cell signalling?

Answer 3

1. cell-cell: embryo development
2. Paracrine: neurotransmitters across a synapse
3. Endocrine: hormones such as glucocorticoids, thyroid hormone
4. Autocrine: T-lymphocytes proliferate with a growth factor

Question 4

What are the 5 classes of ligands?

Answer 4

1. Steroid hormones- hydrophobic derived from lipid cholesterol so it diffuses across membrane
2. Eicosanoids- hydrophobic synthesized from lipids and are rapidly broken down
3. Neurotransmitters- hydrophilic do not cross membrane
4. Peptide hormones and polypeptide growth factors- largest and most variable hydrophilic and don’t cross membrane
5. Simple gases- passive diffusion across membrane, bind to enzymes without receptors

Question 5

What are the 2 classes of receptors?

Answer 5

1. Intracellular receptors- nuclear receptor superfamily which primarily function as transcription factors
2. Cell surface receptors- G protein coupled receptors, receptor protein tyrosine kinases, cytokine receptor superfamily

Question 6

What are the characteristics of the nuclear receptor superfamily? (4 steps)

Answer 6

• molecules that bind these receptors include steroid hormones and thyroid hormones (small and hydrophobic)
• receptors and intracellular proteins (not associated with membrane)
• these receptors + ligand= transcription factors
• receptors contain both a ligand binding domain and a DNA binding domain

Question 7

What is glucocorticoid action?

Answer 7

Inactive when bound to chaperone
Glucocorticoid, active when bound to ligand
receptor+ligand+HAT coactivator= active gene transcription

2 active receptors form a dimer, dimer associates with HAT coactivator, hormone complex bonds to a specific DNA binding site and activated gene transcription
Slide 16

Question 8

How are genes regulated by the thyroid hormone receptor?

Answer 8

Thyroid hormone receptor binds to DNA with or without ligand, without the ligand the receptors binds the corepressor HDAC to repress gene transcription
When hormone is present, it binds receptor changing it conformation to dissociate from HDAC and associate with the coactivator HAT thereby allowing gene transcription
Slide 17

Question 9

What are the characteristics of G protein coupled receptors? (5 of them)

Answer 9

• largest family of cell surface receptors
• signals are transmitted to intracellular targets by an intermediary G protein (alpha beta gamma)
• are transmembrane proteins with multiple transmembrane domains
• extracellular receptor domain binds Logan which causes conformation change which allows cytosolic domain to activate G protein
• alpha subunit of G protein dissociates from beta and game subunits and carries signal to intracellular target
  Slide 21

Question 10

What are the characteristics of tyrosine kinase receptors? (4 of them)

Answer 10

• are cell surface receptors linked to intracellular enzymes (have 1 transmembrane domain)
• receptors dimerize when bound to ligand
• activate the receptors by phosphorylating tyrosine residues in receptor and target substrates
• phosphorylated receptors can associate downstream target which initiate signalling cascade
  Slide 24

Question 11

How are nonreceptor tyrosine kinases activated?

Answer 11

They are associated with receptors that contain no catalytic activity
Ligand binding indices dimerizatipn and activate tyrosine kinases to autophosphorylate themselves as well as the receptor, the. The receptor can associate with downstream signalling molecules beginning a signalling cascade
Slide 25

Question 12

What is intracellular signal transduction?

Answer 12

Chain of reactions that transmits chemical signals from the cell surface to their intracellular targets
Also called signalling cascade
Transcription factors are often the final targets of the signal cascade

Question 13

What is the Cyclic AMP pathway?

Answer 13

Cyclic AMP is a second messenger which is a compound that is modified as a result of ligand receptor interaction (relays message from receptor to target is signal transduction)
Cyclic AMP is important second messenger associated with G protein receptors
cAMP is important for response of cells to a variety of hormones

Question 14

What are the two second messengers of cyclic amp? (Adenylyl Cyclase and cAMP phosphodiesterase)

Answer 14

Adenylyl cyclase- enzyme that catalyzes formation of cyclic AMP from ATP (activated G protein alpha subunit stimulates the activity of this)
cAMP phosphodiesterase- enzyme that degrades cyclic AMP
Pictures of them on slide 32 and 33

Question 15

How is signal transduction initiated through PKA? (Start with second messenger cAMP)

Answer 15

cAMP binds to a cAMP dependent protein kinase called protein kinase A (PKA)
This causes dissociation of PKA regulatory subunits
PKA phosphorylates downstream target proteins
Slide 35

Question 16

What is the transcription factor CREB?

How is it activated?

Answer 16

Recruits co activators and initiated transcription of genes at CRE binding sites
PKA activates transcription factor CREB by phosphorylation
Slide 36

Question 17

How does protein kinase A (PKA) regulate glycogen metabolism?
(Activates and inhibits something)

Answer 17

PKA phosphorylates two key downstream target enzymes

• activates phosphorylase kinase
• inhibits glycogen synthase

cAMP signalling pathways can effect both transcription factors and metabolic enzymes

Question 18

How is protein phosphorylation regulated by protein kinase A (PKA) and protein phosphatase 1 (PP1)

Answer 18

Protein kinases (PKA) do not function in isolation within the cell
So protein phosphatase 1 (PP1) activity counterbalances PKA activity to fine tune this signalling mechanism
Slide 38

Question 19

What are MAP kinase pathways?
What does MAP stand for?
What is the stereotypical pathway?

Answer 19

Cell surface receptors linked to enzymes to produce intracellular signals
MAP- mitogen-activated-protein
Can be associated with receptor and nonreceptor tyrosine kinases

Stereotypical pathway- Ras-Raf-MEK-ERK
Ex: Raf phosphorlyates MEK
MEK phosphorylates ERK
Slide 42

Question 20

What are Ras proteins?

how are they activated and terminated?

Answer 20

Ras (Rat Sarcoma)
Integral membrane lipoprotein
One of first oncogenes found in human cancers
Activated by GEF-guanine exchange factor
Terminated by GAP- guanine activations protein

Question 21

What are Raf and MEK in MAP kinase pathways?

Answer 21

RAF- (rapidly accelerating fibrosarcoma) is a protein serine/threonine kinase that is activated by Ras and activates ERK
MEK- (map kinase/ERK Kinase) is a 2 specificity protein kinase that activates members of the ERK family

Question 22

What is Notch signalling?

Answer 22

Example of cell-cell mode of signalling
Notch receptor on plasma membrane receivers signal from delta ligand on surface of another adjacent cell
Ligand receptor binding activates γ-secretase enzyme to cleave intracellular domain of notch
This notch intracellular domain translocates to the nucleus to bind and activate transcription factors
Slide 51

Question 23

What is the Wnt pathway?

Answer 23

Wnt proteins are family of secreted growth factors in cell fate pathways
Bind to receptor frizzled and LRP
interaction of Wnt and frizzled and LRP recruits protein dishevelled which disrupts construction complex of proteins, this prevents phosphorylation of transcription factor β-catenin, this then translocates to nucleus to activate gene transcription
Slides 53-54

Question 24

What are feedback loops?

Answer 24

Regulate the activities of individual pathways and control the extent and duration of signalling activity
They are similar in principle to feedback regulation of metabolic pathways in that they control activity of signalling pathways

Question 25

What are signalling networks?

What is crosstalk?

Answer 25

Signalling pathways do not operate in isolation, there is frequent crosstalk between different pathways so intracellular signal transduction ultimately needs to be understood as an integrated network of connected pathways
Crosstalk is the interaction of one signalling pathway with another

Question 26

Look at the 5 elements of signalling networks on slide 61 (negative feedback, positive feedback, feed forward relay, stimulatory crosstalk, inhibitory crosstalk)

Answer 26

Okay

Question 27

How can an intracellular signal received by a receptor be amplified?

Answer 27

• a single receptor can activate multiple G proteins which stimulates Adenylyl cyclase to make cAMP which activates protein kinase A which then can phosphorylate multiple targets