Chapter 15: Cell Signaling

a ligand receptor binding event is

highly specific

• the binding event changes the shape of the receptor protein

3 types of pathway effector proteins

• altered metabolism through a metabolic enzyme

• altered gene expression through a transcription regulatory protein

  • creating a TF to bind to an enhancer/repressor

• altered cell shape or movement through a cytoskeletal protein

4 types of intercellular signaling

• contact-dependent

• paracrine

• synaptic

• endocrine

contact dependent pathways

use a membrane-bound signal molecule, which binds to a transmembrane receptor on another cell

• cells must be touching

• used in immune cells to be showed something foreign

paracrine pathways

• local signaling for short distances

• signals go to transmembrane receptors

• ex: in blood clot formation of nearby cells by the cut

synaptic pathways

• long distance signaling in neurons/ nervous tissue

• the axon can be any length (longest is from tailbone to big toe)

• neurotransmitters are released from synapse and go to the target cell

endocrine pathways

• signaling over long distances through the bloodstream

• hormones are released by endocrine glands and bind to receptors on distant target cells

• best way to reach the most cells the fastest

• if any other cell in the bloodstream has that receptor, it will get the signal

ion-channel coupled receptor

• like a ligand gated channel

• when solute (non-ion usually) binds to the binding sites, ions can flow through the receptor, which is now open

3 types of receptors

• ion channel coupled receptors

• g-protein coupled receptors (GPCR)

• enzyme coupled receptors

how np signal molecules get in to the cell

• attach to carrier protein (which is polar) to get through the polar ECM

• need an intracellular receptor protein to get into the nucleus

Phosphorylation can turn pathways

on or off

GTP binding turns pathways

ON

• mediated by GEF and GAP

Factors in cell signaling

• response timing

• sensitivity (how much signal is needed)

• dynamic range (over what range of [signal] do you get activation)

• persistence (how long response lasts)

• signal processing (on/off/oscillatory/cyclical)

• integration (need multiple signals/inputs?)

• coordination (multiple responses to one signal)

Cells require signals just for

baseline survival

Acetylcholine

causes many different responses in diff cells

Heart pacemaker cell: causes decr firing of heart rate

Salivary gland cell: causes saliva to be released

Skeletal muscle cell: causes muscle contraction

arrow pushing pathways

arrow shows activation

perpendicular sing shows inhibition

• double negatives show activation

different ways to show a pathway

Scaffold proteins

collect/ hold things needed for a pathway so that it can occur readily as the proteins are already recruited

assembly of a signaling complex on an activated receptor

• when things assemble on the activated receptor of a given pathway

• ex: phosphates on a receptor tail so that proteins can bind and be activated to help the pathway take place

SH2 domains

when there is a binding site for a specific AA and another for something phosphorylated (usually a phsophotyrosine) that helps time the reaction

• bind tyrosine-phosphorylated sequences in specific protein targets

membrane as a binding site

• pips assemble on the inside of a membrane and phosphates on these pips help activate proteins that bind to the pips

• occurs when an activated signal molecule phosphorylates the pips even more, causing hyperphsophorylation

speed of response of signaling pathways depends on

• whether or not a protein had to be transcribed or translated

• going in and out of nucleus takes more time than something that already has all of its proteins ready

• having to make proteins can cause a process to take up to hours long while ones already with proteins take a few seconds usually

turnover

• like the half-life of proteins

• the tome a protein is stable before it has been used too much and needs to be degraded

3 forms of responses in pathways

• All or none: response is either 100% on or completely off. requires a certain threshold to turn on

• Hyperbolic: response is on immediately but requires a bit of signal to start. As the amount of signal incr, the response incr

• Sigmoidal: S-shaped curve. Requires a certain threshold to start to respond, starts to respond more and more before it reaches a max

positive feedback control

• cyclic loops when a product incr the production of itself

• there is still a response after the signal is removed since activated kinases can activate other inactive ones for a while after the signal is turned off

positive feedback vs no feedback

• with no feedback, the response lasts as long as the signal is present

negative feedback

a product of a pathway inhibits something upstream from it

negative feedback control

• when the activated kinase activates the inhibitor

• short delay: high graph when the signal is received, then the product immediately activates the inhibitor, so graph is dampened

• long delay: signal yields a 100% response, activated kinase activates inhibitor to stop it then another signal immediately goes to activate the response again

  • takes awhile to be inactivated then is immediately reactivated

ex of long delay negative feedback control

when a cell needs to get rid of toxins produced in an immune response to gradually stop the response

Is this graph All or none or something else?

• it is S shaped, and if that was all the information given, it would be sigmoidal

• if information to the right was given of how different cells responded you can see if it was synchronized or asynchronized

• synchronized: all cells start responding more and more

• asynchronized: more and more different cells start to respond (shows all or none)

desensitization of cells to signals/ adaption

• when cells get used to a signal and start to ignor ethem if they are used to a certain conc of signal and now need higher (like building up drug tolerance)

Ways for cells to NOT respond to a signal

• receptor sequestration (separates and eats the protein)

• receptor down-regulation (using ESCRT pathway with intraluminal vesicle leading to lysosomal degradation)

• receptor inactivation via self inhibition

• inactivation of signaling protein by something other than the receptor

• production of an inhibitory protein (signals for an inhibitory protein to be synthesized)

What sense use GPCRs?

• sight

• small

• taste

GPCR mechanism

• are 7 pass transmembrane proteins

• can have one or more subunit

• alpha and gamma have lipid tails in the membrane

• alpha subunit binds to GTP/GDP and to Beta and Gamma subunits (sums to one trimeric complex)

• the signal molecule binds to the receptor, which becomes a GEF and phosphorylates the G-protein, activating it

• Now the activated G-protein binds to beta and gamma (with GTP bound to alpha)

• Effector (brown thing) binds to alpha and alpha drops beta and gamma, which are now activated

GPCR

• indent at top is the ligand/signal binding site

• 7 transmembrane regions

inactive form of G-protein

• has GDP bound to alpha

• once GTP binds again, alpha kicks off beta and gamma

Which subunits of a GPCR has lipid tails

alpha and gamma

• so they are stuck in the membrane

the beta subunit of a GPCR binds best to

gamma

• then alpha

an activated GPCR acts as a

GEF

• phosphorylates the alpha subunit g-protein to GTP so beta and gamma can be activated and break off

second messengers

• amplify signals

• usually ions, a single amino acid, or a single nucelotide

• small non-protein molecules that are specific

• ex: cAMP: cyclical AMP

Production of cAMP

1) ATP (from RNA) is converted to cAMP via adenylyl cyclase, which takes two phosphates off of the ATP

2) this cAMP is converted to 5’-AMP via cyclic AMP phosphodiesterase

• the 5’-AMP is a positive feedback thing for glycolysis and a building block of ATP

cAMP

is a nucleotide derivative of ATP

signals can alter cAMP levels

Gs: stimulatory G-protein

Gi: inhibitory G-protein

• can cause very horrific affects in the body

• ex: Cholera and Pertussis toxin

How cAMp activates protein kinase A (PKA)

• cAMP is a second messenger and puts two cyclic AMPs in each of the two catalytic subunits of the PKA

• this activates the catalytic subunit

• the inactive PKA has 4 total subunits: 2 regulatory and 2 catalytic

cAMP altering gene transcription mechasnism

1) signal molecule binds to the GPCR receptor and becomes activated into GEF

2) The alpha subunit of the stimulatory GPCR (Gs) is now activated by GTP from the GEF

3) adenyl cyclase is activated, causing ATP to breakdown into cAMP

4) the cAMP binds to the regulatory subunits of the previously inactive PKA, causing it to break away and be activated

5) the activated PKA goes into nucleus (importin, NLS)

6) once in nucleus. the activated PKA activates CREB

7) CREB is now phosphorylated and CBP (CREB binding protein) binds to it at the enhancer at the CRE (cAMP response element) on the enhancer

8)This causes transcription of all genes with the CRE enhancer that is also in euchromatin