Slide set 10 Flashcards
not all cells in a population behave identically!
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
after signal is removed, what happens to the response rate?
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
signal pathways impact response to a gradually increasing signal
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
response varies with number of regulators that must bind simultaneously to target protein
activation curves for an allosteric protein as a function of effector molecule concentration
positive and negative feedback loops
a stimulus activates protein A, which in turn activates protein B which then acts to either increase or decrease the activity of A
positive feedback loop example
- response can remain on even after initial signal is removed
- activated E kinase acts back to promote its own phosphorylation and activator
- basal activity of the I phosphatase dephosphorylates activated E at a steady, low rate
- even after stimulus is removed, stimulation by S kinase keeps system ON
negative feedback loop
- 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
desensitization
- 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
Target cell desensitization example!
*these mechanisms operate at level of the receptor and often involve phosphorylation or ubiquitylation of the receptor proteins
- receptor sequestration
- receptor down-regulation
- receptor inactivation
- inactivation of signaling protein
- production of inhibitory protein
G-protein-coupled receptor pathway
- signal molecular binds the receptor
- signal is relayed across plasma membrane to activate a G protein
- G protein activates downstream effectors
G-protein-coupled receptor facts
- 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
Hetero-trimeric G protein
- 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
Activation of a G protein by an activated GPCR
- signal binds to receptor
- receptor binds to G protein
- this causes GDP to dissociate and then GTP can bind
- G protein is now activated, so alpha subunit is unbound from other subunits
- activated G protein subunits can now activate downstream effector proteins
adenylyl cyclase
converts ATP to cAMP
activating adenylyl cyclase
active G protein alpha (Gs) often activates adenylyl cyclase
- adenylyl cyclase = ATP to cAMP
- other signals can activate a G protein that inhibits adenylyl cyclase (Gi)
- Balance between stimulation (through Gs) and inhibition (through Gi) of adenylyl cyclase that dictates how much cAMP this is in the cell
cAMP is a common second messenger
cAMP activation of PKA
- PKA phosphorylates serine/threonine residues
- in its inactive form, PKA is 4 subunits:
- 2 inactive catalytic subunits
- 2 regulatory subunits (each has 2 cAMP binding sites)
- When each regulatory subunit binds to 2 cAMP, the active catalytic subunits dissociate (due to a conformational change)
4 cAMPs lead to activation of PKA catalytic subunits
what type of response (think graph)?
sigmoidal!!
rise in intracellular cyclic AMP conc = ?
altered gene transcription!
- extracellular signal molec binds to its GPCR and activates adenylyl cyclase via Gs (increases cAMP conc in cytosol)
- this rise activates PKA, released catalytic subunits of PKA can enter the nucleus
- in nucleus, PKA catalytic subunits phosphorylate transcription regulatory protein CREB
- once phosphorylated, CREB recruits the coactivator CBP, which stimulates gene transcription
