Cell Signaling GPCRs Flashcards
Signal Transduction steps
- Extracellular signal binds to receptor (on cell surface) - Reception
- Cascade of intracellular reactions that triggers the cell’s response tot he original signal - Transduction
- Intracellular Signaling proteins alter the activity of an effector protein(s) - Response
Signal Transduction of Insulin (signaling cascade)
Activation of effector -> binding of glucose transporters to surface
Second Messengers
Chemicals (Ions, water soluble, lipid soluble, etc.) that are generated in large amounts (in short time) in response to receptor activation and diffuse away from their source, spreading signal to other parts of the cell.
They are NOT intracellular proteins
Examples of second messengers
Water soluble
IP3 (Inositol 1,4,5 phosphate), cAMP (cyclic AMP), and Ca2+
Lipid soluble
DAG (Diacylglycerol)
Intercellular signaling
between cells
1. Contact dependent
2. Paracrine
3. Synaptic
4. Endocrine
Contact dependent signaling
ex. Immune cells - ingulf and digest foreign antigen - requires close contact
Fast response
Paracrine Signaling
More localized
ex. secretion of growth factors - bind to specific receptor on neighboring cells - fast, short lived response
Autocrine cells - cancer - secrete growth factors that act on themselves
Synaptic Signaling
AP, propagate
NT release in synapse, because small area, fast response
Endocrine Signaling
Travel long distance via blood circulation, bind to target with receptor
slower - traveling through blood
Types of receptors extracellular signals can bind to
Cell surface receptors
Intracellular receptors
Cell surface receptors
Bind to hydrophilic signals (Growth factors, peptide hormones, neurotransmitters)
Intracellular receptors
Hydrophobic signals (Steroids) diffuse across plasma membrane and bind to receptor proteins inside the target cell
How does a cell integrate all of the signaling information in order to make a decision
An individual cell - controls receptors at the surface and intracellular pathways - regulates signals and response - specificity
Is many/combo of signals - not just one signal - integrates them
How does a signaling molecule have different effects on different target cells
Small number of types of signals (A) and receptors - Different response (B/C) - different intracellular pathway activated (D-also same signal can bind to different receptors)
Binding of ligand to R promotes signaling of predetermined state
GPCRs are the largest family of ____
_____ % of known drugs work through them.
They are ____ proteins with ___ helices
cell surface receptors (>800 in humans)
Almost 50%
multipass TM, GTPases
alfa helices - more flexible conformation changes/sliding - receptor activation, Has a binding pocket
G Protein subunits
have 3 subunits (Alfa, betta, gamma)
Are bound/associated with membrane
Inactive G Protein
alfa-GDP - inactive
alfa subunit is a GTPase which becomes inactive when it hydrolyzes GTP to GDP
Signaling steps through GPCRs
- Extracellular signal binds to GPCR
- Receptor undergoes conformational change, allowing it to bind to the G protein
- Receptor acts like a GEF, inducing the alfa subunit to release GDP, allowing GTP to take its place
- Conformational change in alfa subunit disassociates from receptor, resulting in activated alfa subunit and activated betta gamma subunit - both lead to other changes/response
Alfa subunit is inactivated with it
hydrolyzes GTP to GDP
This is greatly enhanced by binding of specific regulators of G protein signaling (RGS) to alfa subunit
Turn off signal
RGS
similar to GAP
enhances ability of alfa subunit to hydrolyze its own GTP
cAMP
Timing -
Synthesized from -
Destroyed by -
An extracellular signal can increases cAMP concentrations more than 20 fold in seconds (a lot in a short period)
Is synthesized from ATP by adenylyl cyclase (adenylate cyclase) and destroyed by cAMP phosphodiesterase
Both adenylyl cyclase and phosphodiesterase balance/regulate each other
How do Extracellular signals influence concentrations of cAMP
Activates GPCR that are coupled to stimulatory G protein (Gs) - binds to and activates adenylyl cyclase - increases cAMP
different GPCR (other signals) reduces cAMP - activating a inhibitory G protein (Gi)
How does cholera toxin affect cAMP production?
Cholera toxin is an enzyme that catalyzes the transfer of ADP ribose from intracellular NAD+ to the alfa subunit of Gs (Gsα is covalently modified (ADP ribosylation)), it can no longer hydrolyze its bound GTP, causing it to remain in an active state, stimulating AC indefinitely. Prolonged elevation in cAMP causes an efflux of Cl- and water into the gut, causing severe diarrhea.
Most animal cells, cAMP exerts its effects mainly by
activating PKA
PKA activation
inactive state, PKA consists of a complex of two catalytic subunits and two regulatory subunits.
binding of cAMP to the regulatory subunits alters their conformation, causing them to dissociate from the complex.
released catalytic subunits are thereby activated to phosphorylate specific target proteins
G Proteins Signaling by Phospholipids
G protein activates the plasma-membrane-bound enzyme phospholipase C-β (PLCβ) (by activated alfa betta and gamma subunits) which acts on/cleaves [PI(4,5)P2] into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol.
IP3 - water soluble - leaves PM and diffuses through cytosol to ER, binds to IP3 R - conf. change - exposes a high Ca2+ affinity binding site- cytosolic Ca2+ binds - these two (IP3 and Ca2+) causes the opening of the Ca2+ channel. Ca2+ stored in ER is released and binds to other IP3 receptors, causing more to open.
Concentration of cytosolic Ca2+ rises. this alters the PKC so that it translocates from the cytosol to the cytoplasmic face of the plasma membrane.
Diacylglycerol (remains embedded in PM) and Ca2+ and and neg charged membrane phospholipid phosphatidylserine (attracts PKC) - activates PKC
PKC then phosphorylates target proteins that vary depending on the cell type.
What is the term for the protein that is an α-subunit-specific GTPase-activating protein (GAP)?
Regulator of G protein signaling (RGS)
PKA is turned off when
cAMP is converted to AMP by cAMP phosphodiesterase.
