Cell Signaling/communication Flashcards
What microbe “communicates” to find a mate?
Yeast! They sense presence of other cells when looking for a mate
Is it one signal-> one action?
No! Cells are constantly undergoing a multitude of signals and reactions
What is signal transduction?
The whole pathway of taking a signal from outside the cell to make an internal change
What are the 4 “players” in signal transduction?
1: First Messengers/Ligands (bind to receptor- can be hydrophobic or hydrophilic)
2: Signal Transducers/receptor protein (transduces signal from outside to inside the cell)
3: Secondary Messengers (passes message to our effector proteins)
4: Effector Proteins (actually “does the work”)
What are three examples of effector proteins and what they could cause?
1: Metabolic enzyme (altered metabolism)
2: transcription regulatory protein (altered gene expression)
3: cytoskeletal protein (altered cell shape or movement)
Why would a cell signal not be able to be transduced?
If the cell doesn’t have a receptor for the signal molecule!
Where are receptors for hydrophilic signals found?
The cell surface ! (They will not cross the PM)
Ex: insulin receptors
Where are receptors for hydrophobic signals found?
Inside the cell, often inside the nucleus! (Signal will often diffuse across the plasma membrane)
Ex: testosterone and progesterone receptors
What are the 3 types of short-distance signaling?
1: Autocrine signaling: self signaling!
2: Contact dependent: two cells must be touching to be membrane-transduced from surface-surface
3: Paracrine Signaling: within the general vicinity of that cell (diffusion is over short distance)
What are the two types of long-distance signaling?
1: Synaptic (between neurons)
2: Endocrine (uses the circulatory system to go anywhere in the body, ex: hormones)
What’s an example of a signal that acts over a fast period of time?
-seconds to minutes
-altered protein function
-ex: neutrophil chasing bacteria
What’s an example of a signal that acts over a slow period of time?
-minutes to hours
-relies on altering gene expression and protein synthesis
Can both slow and fast pathways be activated at the same time?
Yes! This is most common that both occur same time
What are gap junctions?
A type of signaling, when cells are physically attached to each other and a junction allows direct contact between cytoplasm of two different cells
-allows exchange of ions and small molecules (both good and bad)
Why can one signal molecule have different effects on different cells?
The signal’s receptors activate different effector proteins! These do different pathways throughout the cell
What is the relationship between second messengers and phosphorylation?
-many second messengers activity are controlled by phosphorylation
What is Kinase versus Phosphatase?
Kinase: uses ATP to add phosphate to protein, turns it “on”’!
Phosphatase: removes phosphate, turns protein “off”!
What are the two ways that second messenger’s activity can be controlled?
1: kinase and phosphatase
2: GTP binding proteins (GAPs and GEFs)
Are all second messengers proteins?
No!
What are two examples of non-protein second messengers?
Calcium and Cyclic Adenosine MonoPhosphate (Cyclic AMP)
What are the three types of cell surface receptors?
1: ion channel- coupled receptors (ex: glucose symport)
2: G-protein-coupled receptors
3: enzyme-coupled receptors (RTKs)
What are Receptor Tyrosine Kinases? (RTKs)
-Most common type of enzyme, coupled receptors
-Usually a single pass Transmembrane protein
-kinase adds phosphate to the TYROSINE KINASE DOMAIN on inside of membrane (to send signal)
How are RTKs activated?
Dimerization!
How does dimerization of RTKs occur?
-RTKs need to be in pairs to work
-when each subunits finds a signal protein, it phosphorylates the other subunit on a TYROSINE residue
-this is called TRANS-AUTOPHOSPHORYLATION and activated the receptor
What is trans-autophosphorylation?
When two RTKs swap phosphate groups with each other on their tyrosine amino acids
What do most RTKs activate?
Ras!
What is Ras?
A small GTPase anchored to the PM (GTP and GDP forms when activated/not)
-activation of RTK results in activation of Ras
-Ras activates the MAPK Cascade
What is the MAPK Cascade?
A “Phosphorylation Cascade” where is phosphorylation is passed down
-MAP Kinase Cascade carries signal into the nucleus to change the expression of genes!
What are the three “dominos” in the MAP Kinase Cascade?
1: MAP kinase kinase kinase (Raf) (activated by Ras)
2: MAP kinase kinase (Mek)
3: MAP kinase (Erk)
Why do we need the MAPK Cascade?
Because Ras activation is only short-lived, we need the MAP kinase cascade to carry the signal to the nucleus!
What two things does the MAPK Cascade cause?
1: Changes in protein activity
2: Changes in gene expression
What happens when Ras is over-activated?
Human tumors and cancers form
-would need to target “downstream” of Ras to treat, because if overactive then regardless of inhibitors it would keep going
What are G-protein coupled receptors? (GCPR)
-composed of 7 transmembrane helices
-always associated with a G-protein on the cytosolic face of the membrane
-N-term is always outside the cell (the receptor), C-term is always inside the cytosol
What are G proteins?
They interact with C-term of GPCR
-trimeric: made up of three parts
-“alpha” and “Y” are lipid linked to plasma membrane, “beta” is free to move around
-the “alpha” subunit is a GTPase (GDP and GTP forms)
-A is bound to GTP/GDP and GPCR, B and Y separate when GDP-> GTP
How does G protein Activation occur?
1: Signal molecule binding activates the GPCR
2: GPCR becomes a GEF and turns A-GDP to A-GTP
3: A-GTP dissociates from B and Y (but stays stuck to the membrane)
4: A-GTP binds to its target/effector and activates it (binding to the target causes A-GTP to inactive itself back to A-GDP)
5: Regulator of G-protein Signaling (RGS) is a GAP and makes sure all the As are turned back to A-GDP
What are five ways that GPCR can prevent adaptation/desensitization of a pathway?
1: Receptor Sequestration: (signal pulled from outside the cell to endosome that acts like momentary jail cell)
2: Receptor Down-Regulation: (we degrade receptor + signal to stop activation of pathway)
3: Receptor Inactivation: (we inhibit the receptor of the signal)
4: Inactivation of Signaling Protein: (we inhibit intermediate)
5: Production of Inhibitory Protein: (we made an inhibitor to physically block the pathway)
What does Arrestin do?
causes GPCR Desensitization!
-inhibits GPCR from interacting with G protein
-is an adaptor for Clathrin, and induced receptor-medicated endocytosis
-ONLY BINDS TO PHOSPHORYLATED GPCR, not normal functioning ones
What does GPCR kinase do? (GRK)
Phosphorylated active GPCR on multiple sites, stops them from being able to send signal and tags them for endocytosis via Arrestin
What do Second messengers do?
They carry the activated signal away from the membrane, and can serve to amplify the cellular response!
What are the two main second messengers of GPCRs? (GPCRs go to activate both of these) and which pathway/effector protein does it signal?
1: Cyclic AMP (cAMP) signals for PKA
2: Ca2+ signals for PKC
How do we make Cyclic AMP? (cAMP)
-it is derived from ATP!
- we use ADENYLYL CYCLASE to turn from ATP to cAMP
-we use CYCLIC AMP PHOSPHODIESTERASE to turn cAMP to ATP
How is cyclic AMP activated by G-protein? What does it then do?
1: A-GTP activates adenylyl cyclase, which is a surface protein that turns ATP to cAMP
2: cAMP then diffuses throughout the cell and activates Protein Kinase A (PKA)
3: activated PKA then enters the nucleus and regulates the transcription of genes
What is an example of a cellular response that is regulated by cAMP?
Extracellular signal molecule: Adrenaline
Target Tissue: skeletal muscle
Major response: glycogen breakdown for immediate energy
What are some ways that Cytosolic Ca2+ is usually kept low?
1: Calcium is sequestered outside the cell
2:Ca2+ is sequestered inside organelles
Why use Ca2+ instead of Na+ for cell signaling?
Because there’s not as much everywhere, so small changes are noticed! (Such as sand on beach example)
What happens in the GPCR pathway when Ca2+ is the second messenger? (Practice drawing out this pathway flowchart)
1: G-protein is activated, which then causes B and Y subunits to activate PHOSPHOLIPASE C-B in the PM
2: PHOSPHOLIPASE C-B then converts PI(4,5)P2 into IP3 and diacylglycerol (DAG)
3: IP3 and DAG then work together to get PKC activated and attached to the PM. (DAG = is PM bound and activates PKC once it comes), (IP3= goes to ER calcium release channels and releases Ca2+ into cytosol, signaling for PKA to go to PM)
4: PKC then does a bunch of cool shit
What is PKC?
Protein Kinase C (PKC)
-bound to PM when activated by Ca2+ and DAG
-does cool shit
Positive feedback versus negative feedback?
Positive feedback: self promoting, continues to activates itself (ex: oxytocin in childbirth)
Negative feedback: inhibitory, pathway inhibits pathway upstream (ex: TH4 thyroid)
What is Heparin?
-anticoagulant/ blood thinner
-reduces inflammation
-regulates vascular smooth muscle cell proliferation
-causes Map Kinase Phosphatase, which steals phosphates from ERK to turn it off! As a result: reduced inflammation!
What is Angiotensin?
Contraction signal that transiently increases blood pressure
(Causes contracted blood vessel/vasoconstricts)
What does cGMP do?
Vasodilates our blood vessels! Lets them relax more
-increases in cGMP are caused by NO (nitric oxide) and atrial natriuretic factor
What can cause increased cGMP?
-NO (Nitric Oxide)
-atrial natriuretic factor
-nitroglycerin
What are the 6 steps to relaxing the blood vessels?
1: Acetylcholine released, activates NO synthase (NOS)
2: NOS turns arginine into NO (nitrous oxide)
3: NO is nonpolar gas, so diffuses out of endothelial cell into outer smooth muscle cell lining
4: NO binds to guanylyl cyclase to turn it on
5: Guanylyl Cyclase now makes cyclic GMP (cGMP) using GTP
6:cGMP vasodilates and causes relaxation of outer smooth muscle cells of blood vessel
What does PDE-5 do? (Phosphodiesterases)
-converts cGMP to GMP
-ergo causes tightening of smooth muscle cells!
What does Sildenafil citrate do? (Viagra)
-was first made to treat high BP in kids
-blocks PDE-5, so prevent GMP from being made and ergo prevents constriction of outer smooth muscle cells
-causes vasodilation and increased blood flow
What is the point of transautophosphorylation?
It happens right after dimerization of the two RTKs and GENERATES BINDING SITES FOR SIGNALING PROTEINS
What is the order of events that happens for RTKs?
Ligand binds- receptor dimerization- transautophosphorylation- recruitment of other proteins to pass message along downstream
What effector protein is used in response to adrenaline?
PKA!
After ATP/GTP have been converted to cGMP/cAMP, what do they convert to bc of the phosphodiesterases?
AMP/GMP!
-they do not got back to ATP/GTP
