Slide set 10

Question 1

not all cells in a population behave identically!

Answer 1

when looking at a cellular response, it is the # of cells that are responding, not a gradual response of all cells

WHY: each cell has a different concentration of various proteins

Question 2

after signal is removed, what happens to the response rate?

Answer 2

response decreases at a rate that depends (in part) on the turnover rate of intracellular intermediates

red = rapid turnover (largest fluctuation in amount of molecule over time

Question 3

signal pathways impact response to a gradually increasing signal

Answer 3

you may say smoothly graded or switchlike responses

hyperbolic: response increases gradually as concentration of extracellular signal molecule increases (reaches a plateau)
sigmoidal: signaling system reduces response at low signal conc and produces a steepr response at an intermediation conc

all-or-none: cell quickly switches between low and high response

Question 4

response varies with number of regulators that must bind simultaneously to target protein

Answer 4

activation curves for an allosteric protein as a function of effector molecule concentration

Question 5

positive and negative feedback loops

Answer 5

a stimulus activates protein A, which in turn activates protein B which then acts to either increase or decrease the activity of A

Question 6

positive feedback loop example

Answer 6

• response can remain on even after initial signal is removed

1. activated E kinase acts back to promote its own phosphorylation and activator
2. basal activity of the I phosphatase dephosphorylates activated E at a steady, low rate
   
   1. even after stimulus is removed, stimulation by S kinase keeps system ON

Question 7

negative feedback loop

Answer 7

• response pattern depends on delay in negative feedback
  1. activated E kinase phosphorylates and activates the I phosphatase, increasing the rate at which the phosphatase dephosphorylates and inactivates the phosphorylated E kinase

Question 8

desensitization

Answer 8

• allows cells to detect a change in the signal concentration
  
  • this allows cells to respond over a wide range of signal concentration
• cells aren’t responding to an absolute amount of signal, but changes in signal
• cells adapt to the signal

Question 9

Target cell desensitization example!

Answer 9

*these mechanisms operate at level of the receptor and often involve phosphorylation or ubiquitylation of the receptor proteins

1. receptor sequestration
2. receptor down-regulation
3. receptor inactivation
4. inactivation of signaling protein
5. production of inhibitory protein

Question 10

G-protein-coupled receptor pathway

Answer 10

1. signal molecular binds the receptor
2. signal is relayed across plasma membrane to activate a G protein
3. G protein activates downstream effectors

Question 11

G-protein-coupled receptor facts

Answer 11

• GPCR does NOT have enzymatic activity itself :(
• GPCR are multipass transmembrane receptors
  
  • 7 membrane-spanning domains
  • binding of signal changes the way the helices interact with each other

Question 12

Hetero-trimeric G protein

Answer 12

• has 3 components: alpha, beta, gamma that exist as a complex
• alpha subunit binds GTP/GDP
• associates with the membrane
  
  • alpha and gamma subunits have covalent attachments to lipids

Question 13

Activation of a G protein by an activated GPCR

Answer 13

1. signal binds to receptor
2. receptor binds to G protein
3. this causes GDP to dissociate and then GTP can bind
4. G protein is now activated, so alpha subunit is unbound from other subunits
5. activated G protein subunits can now activate downstream effector proteins

Question 14

adenylyl cyclase

Answer 14

converts ATP to cAMP

Question 15

activating adenylyl cyclase

Answer 15

active G protein alpha (G_s) often activates adenylyl cyclase

• adenylyl cyclase = ATP to cAMP
• other signals can activate a G protein that inhibits adenylyl cyclase (G_i)
• Balance between stimulation (through G_s) and inhibition (through G_i) of adenylyl cyclase that dictates how much cAMP this is in the cell

Question 16

G_s

Answer 16

stimulates enzyme (adenylyl cyclase)

more cAMP

Question 17

G_i

Answer 17

inhibits enzyme (adenylyl cyclase)

less cAMP

Question 18

cholera toxin

Answer 18

modifies G_s

adds an ADP-ribose to G_s

In this form, G_s can’t hydrolyze GTP to GDP

Question 19

Pertussis toxin

Answer 19

modifies Gi (pertussis = G_{<strong>i</strong>})

adds an ADP-ribose to G_i

In this form, G_i can’t interact with its receptor

Question 20

cAMP is a common second messenger

Answer 20

Question 21

cAMP activates….

Answer 21

cAMP activates Protein Kinase A (PKA)

Question 22

cAMP activation of PKA

Answer 22

1. PKA phosphorylates serine/threonine residues
2. in its inactive form, PKA is 4 subunits:
   
   1. 2 inactive catalytic subunits
   2. 2 regulatory subunits (each has 2 cAMP binding sites)
3. When each regulatory subunit binds to 2 cAMP, the active catalytic subunits dissociate (due to a conformational change)

Question 23

4 cAMPs lead to activation of PKA catalytic subunits

what type of response (think graph)?

Answer 23

sigmoidal!!

Question 24

example of PKA activity and regulation

Answer 24

cardiac muscle!!

• PKA activity is kept low by phosphodiesterase
• cAMP levels rise in response to a signal, which activates PKA
• PKA activates phosphodiesterase to lower cAMP concentration

Question 25

PKA in the nucleus

Answer 25

in some cell types, active PKA can enter the nucleus and turn on expression of specific genes

Question 26

rise in intracellular cyclic AMP conc = ?

Answer 26

altered gene transcription! 1. extracellular signal molec binds to its GPCR and activates adenylyl cyclase via Gs (increases cAMP conc in cytosol) 2. this rise activates PKA, released catalytic subunits of PKA can enter the nucleus 3. in nucleus, PKA catalytic subunits phosphorylate transcription regulatory protein CREB 4. once phosphorylated, CREB recruits the coactivator CBP, which stimulates gene transcription