Cell signaling Flashcards
Signal molecules activate receptors
A variety of signal molecules initiate signal transduction:
Proteins, small peptides, amino acids, nucleotides, steroids, fatty acids, dissolved gases (NO, CO)
Cells respond to a signal molecule by means of a receptor
Signal molecules act over short or long distances
signals can act slowly or rapidly to change the behavior of the cell
Different types of cells usually respond differently to the same signal
Intracellular receptor mediated signaling
nitric oxide stimulates guanylyl cyclase:
Ach activates the endothelial cell to convert arginine to NO
NO rapidly diffuses across the membrane where it causes gyanylyl cyclase to convert GTP to cyclic GMP and that causes relaxation of smooth muscle
Certain hormones activate nuclear receptors:
hydrophobic hormones that require carrier proteins will activate nuclear receptors to bind to DNA-binding domains (most times its by a ligand binding to the DNA binding domain causing inhibitory proteins to fall off and the DNA binding domain can then attach to the receptor binding element)
Nuclear receptors mediate the biphasic regulation of transcription (hormone induces synthesis of primary response proteins, that in turn inhibit the primary response genes and the primary response protein turns on secondary response genes)
Cell surface receptor mediated signaling classes
- ion channel coupled receptors (involved in rapid synaptic signaling)
- G-protein coupled receptors (regulate plasma membrane bound enzymes or ion channels through trimeric GTP binding protein called G protein)
- Enzyme coupled receptors (either function as enzymes themselves or associate directly with the enzymes they activate)
From Cell surface receptors to intracellular protein network
- Extracellular signal molecules (1st messengers) stimulate the receptors
- Scaffold proteins bring together one or more signaling proteins together in close proximity
- The second messengers amplify the signals (Transduce and amlify)
- Coincidence detectors integrate different signals (and then spread out another signal)
- anchors determine a particular structure of a signaling complex in the cell (will anchor one or more signaling proteins in a cascade pathway to a particular structure in the cell)
- Modulators determine signaling intensity
Molecular switching
(phosphorylation vs. GTP binding)
Signaling by phosphorylation:
Signal stimulates protein kinase to hydrolyze ATP->ADP and adds the P to turn ON the enzyme, then when the signal leaves protein phosphatases take off the P and turn OFF the enzyme
Signaling by GTP binding:
Signal causes the GDP to come off an enzyme and GTP to come on causeing the enzyme to be turned ON, then GTP hydrolysis removes the P to turn OFF the enzyme
Trimeric vs monomeric GTP binding proteins:
two types of GTP binding proteins:
G proteins: trimeric
small GTPases: monomeric
Guanine nucleotide exchange factors (GEFs) promote the release of bound GDP in exchange of GTP to activate small GTPases. The GPCR serves as the GEF for G proteins
GTPase activating proteins (GAPs) increase the rate of GTP hydrolysis of small GTPases. GAPs that regulate G protiens are called regulators of G protein signaling RGSs
signaling complex formation
Scaffold proteins regulate the specificity by putting required signaling proteins in close proximity
docking site on receptor: receptor tyrosine kinase activation generates docking sites for the formation of transient signaling complexes in response to an extracellular signal)
docking site on lipids: receptor activation–> hyperphosphorylates phosphoinositides in the adjacent plasma membrane to recruit intracellular signaling proteins
Modular interaction domains:
PTB (phosphostyrosine-binding) and SH2 domains bind to phosphorylated tyrosines (on receptors)
SR3 domains bind to proline rich peptides
PH (pleckstrin homology) domains bind to charged heads of phosphoinositides)
Feedback regulation and desensitization/adaptation
positive feedback loop sustains signaling even after signal strength drops. (can contribute to making permanent decision for development)
negative feedback (counteracts the effects of the of a stimulus and therby abbreviates and limits the level of the response
Desensitization/adaptation: receptor sequestoration, receptor down regulation, receptor inactivation, inactivation of signaling protein, production of inhibitory protein (prolonged exposure to a stimulus decreases the cells response to the level of stimulus)
G protein coupled receptors signaling
All eukaryotes use GPCR (the largest family of cell surface receptors)
GPCRs mediate most responses to signals from the external world as well as other signals from cells, can be pretty much stimulated by anything
All GPCRs share a structural similarity (a single peptide that threads back and forth across the lipid bilayer seven times)
GPCRs activate various intracellular signaling pathways including some that are also activated by enzyme coupled receptors
G proteins are the molecular switches for GPCRs:
Examples of G proteins (Gs- stimulates adenylyl cyclase, Gi inhibits adenyl cyclase, Gq- activates phospholipase C)
cAMP role in GPCR signaling
cAMP mediates GPCR signaling via protein kinase A
cyclic AMP (cAMP) is a second messenger. Normal concentration of cAMP is low, and when its activated adenylyl cyclase increases cAMP twenty fold in second
cAMP phosphodiesterases hydrolyze cAMP to 5 AMP
GPCR activates adenylyl cyclase thru Gs (g protein)
caMP binds to the regulatory subunits of protein kinase A and allows the catalytic subunits to dissociate, activated protein kinase a mediates short term responses in the cytosol, protein kinase A also mediates long term responses which require gene transcription via cAMP response elements (CRE)
Cholera: adds an ADP (from NAD) to the alpha subunit of Gs, inhibiting GTP hydrolysis, sustains active conformation of Gs -> continued activation of adenylyl cyclase-> increased levels of cAMP-> large efflux of CL and water into gut
Pertussis toxin (whooping cough) catalyzes ADP ribosylation of the alpha subunit of Gi, maintaining the G protein in the GDP bound (OFF) form
GPCR/ IP3/ Ca/ and protein kinase C
IP3 mediates GPCR signaling via Ca and protein kinase C
PIP2-> diacylglycerol and IP3 via phospholipase C
diacylglycerol activates protein kinase C
IP3 releases Ca from the endoplasmic reticulum
Phospholipase C is activated from the GPCR and G protein (Gq)
The Ca thats release activates protein kinase C
Desensitization of GPCR
The activated G protein coupled receptor stimulates GRK (GPCR kinase) to phosphorylate the GPCR at multipple sites which then allows arrestin protein to bind and inhibit GPCR
3 modes of desensitization:
Receptor inactivation
receptor sequester (GPCR is internalized
receptor down regulation: GPCR is destroyed in lysosme after internalization
Enzyme coupled receptors classes
6 classes (five used in humans)
- receptor tyrosine kinases: directly phosphorylate specific tyrosines on themselves and on a small set of intracellular signaling proteins
- tyrosine kinase associated receptors: have no intrinsic tyrosine kinase activity but directly recruit cytoplasmic kinases to relay the signals
- receptor Ser/ Thr kinases: directly phosphorylate specific Ser/thr on themseslve ans the proteins they associate with
- receptor guanylyl cyclase directly catalyze the production of cGMP
- receptor like tyrosine phosphatases: remove phosphates from tyrosines of specific intracellular signaling proteins. These are receptor like because their ligands are not known
Receptor tyrosine kinases
there are 60 genes that encode human receptor tyrosine kinases
RTK require receptor dimerization and transautophosphorylation (exception is insulin and IGF 1 receptors that use IRS 1 (bc they are already dimers) and whe activated move close together
RTKs serve as the docking sites for signaling proteins
Transautophosphorylation facilitates RTK signaling in 2 ways:
- increases kinase activity of the RTK
- creates high affinity dockin sites for SH2 and PTB domains
The small GTPase (g protein monomeric) RAS is the key mediator of RTK (also RHO) , they spread the signal along several distinct downstream signaling pathways and act as a signaling hub
Ras and human disease (Ras is mutated in tumors) and targeting the Ras/ Mek/ erk/ MAPK signaling for cancer treatments
small molecules inhibit kinases (ATP analog and peptides
The PI3K-AKT pathway
phosphatidyl inositol (PI) undergoes phosphorylation to make the lipid phosphoinositides
PI3K phosphorylates inositol phospholipids PI3k class 1a is activated by RTK and class 1 b is activated by GPCRs
PIP3 serves as the lipid docking site for Pelkrin homology (PH) domains, this is major pathway activated by insulin and plays a major role in promoting cell survival and growth in response to other signals
Jak stat pathway
Tyr kinase associated receptors:
A cytokine dimerizes a tyrosine kinase, transautophosphorylation of the jaks, causing auto phosphorylation of the the tyrosine kinase monomer of the corresponidng jak from the jak, creates an SH2 domain for stat to bind to. Jaks phosphorylate the stats and releases STATS dimerize and bind to nuclear response elements and transcription initiates
