Signal Transduction Flashcards
Why is Signal Transduction important?
In a multicellular organism, cells need to respond to signals in order to communicate with each other
How do cells receive and respond to signals?
Cells receive signals by interacting with cellular signaling molecules which transduce the signal and induce a response to the signal
Signaling by secreted molecules (4 types)
- Paracrine signaling: Local mediators are secreted by the signaling cell and received by surrounding target cells
- Autocrine signaling: mediators are secreted and received by the same cell
- Synaptic signaling: Neurotransmitters are secreted by the pre-synaptic cleft and are received by the post synaptic cell
- Endocrine signaling: Hormones are secreted into the blood stream
Signaling by plasma membrane bound molecules
Signaling molecule is a transmembrane protein
Target cell in direct contact with signaling cell
Two major unique characteristics of intracellular steroid receptors
- These receptors are intracellular, and interact with their ligand in either the cytoplasm or nucleus
- These receptors are ligand-activated transcription factors
Carrier protein for cortisol
Corticosteroid-binding globulin (CBG)
Three parts of a steroid receptor
1) Hormone binding site
2) DNA binding domain
3) Transcription-activating domain
In the basal state, GR receptors are found in the _______
Thyroid and retinoid receptors are found in the _______
Ctyoplasm; Nucleus
GRE (Glucocorticoid-response element)
glucocorticoid response element - A DNA sequence that binds steroid receptors
GR (Glucocorticoid receptor) in basal state
Exists as an inactive complex in the cytosol associated with HSP90 and other cytosolic (inhibitory) proteins
Membrane Receptors
Ion Channel-Linked Receptor
G-Protein Coupled Receptor
Enzyme Linked Receptor
Receptors for acetylcholine, GABA, serotonin, and glycine have __ subunits, and receptors for glutamate have __ subunits
5;4
Nicotinic Acetylcholine Receptor = the first ion channel to:
1) Be purified
2) Have its sequence determined
3) Be reconstituted in a lipid bilayer
4) Provide a single channel recording
Nicotinic Acetylcholine Receptor Function
Consists of 5 subunits
Binding of ACh to the α subunits opens the channel causing influx of cations
Location of Nicotinic Acetylcholine Receptors
Neuromuscular Junction
Peripheral autonomic nervous system
Central Nervous system
GABA Receptor-Gated Cl- channels
Primary postsynaptic inhibitory transmitter in CNS
GABA binds receptor → CL- channels open → Influx of CL- into cell → Hyperpolarization and inhibitory response
G-protein coupled receptor passes through the plasma membrane ______ times
7
3 Parts of G-protein coupled receptor
Receptor provides specificity
Herterotrimeric G protein is the transducer (GTP-bound and GDP-bound states)
Effector provides the catalytic component to generate the second messenger
Heterotrimeric G Proteins
Three types of subunits (α,β, and γ) which function as a dimer
Gα: binds GTP and interacts with effectors (Hydrolyzes GTP→ GDP)
Gβγ: inhibits Gα; anchors to membrane
Different Gα subunits:
Gαs (stimulates adenylyl cyclase)
Gαi (inhibits adenylyl cyclase)
Gαq (activates phospholipase C)
Enzyme Linked Receptors
Tyrosine Kinase-Linked
Serine/threonine kinase-linked
Protein phosphatase-linked
Guanylyl cyclase-linked
Tyrosine Kinase-Linked Receptors
EGF (Epidermal growth factor) Receptor: Inactive is monomer; active is dimer
PDGF (Platelet-derived growth factor) Receptor: Inactive is monomer; active is dimer
Insuling Receptor: Inactive is dimer, active is tetramer
EGF Receptor: Ligand binding causes…
Dimerization
Activation of cross autophosphorylation
Binding of intracellular signaling molecules
4 classes of major receptors that are major drug targets
- Intracellular steroid receptors
- Ion Channel-linked receptors
- G Protein-coupled receptors
- Enzyme linked receptors
Transcription factors (CREB)
Proteins that bind DNA and regulate (either promote or inhibit) the transcription of genes
Activated transcription factors induce transcription by activating __________
RNA polymerase
Protein Kinases
Catalyze the addition of a phosphate group to the side chain of amino acids in proteins and peptides
Serine/threonine-specific protein kinases
PKA, PKC, Ca(2+)/calmodulin-dependent kinase, MAP kinase (such as ERK, JNK, or p38 kinase)
Tyrosine-specific protein kinases
Tyrosine kinase-linked receptors (e.g. EGF receptor) and cytoplasmic kinases such as Src and Abl
Dual specificity kinases
Can phosphorylate both threonine and tyrosine: Map kinases kinases (MKK1)
Protein phosphatases
Catalyze the cleavage of a phosphate group from the side chain of amino acids in proteins and peptides (less variety than protein kinases)
How are the activities of protein kinases and protein phosphatases regulated
Second messengers
Second messengers
Small, diffusable molecules that are generated in response to ligand-receptor interactions and activate downstream effectors
Cyclic adenosine monophosphate (cAMP)
Generation: when Gαs activates adenylyl cyclase (activated adenylyl cyclase converts ATP to cAMP)
Effector: cAMP activates Protein Kinase A
Diacylglycerol (DAG) and Inositol Triphosphate (IP3)
Generation: made when Gαq activates phospholipase C (PLC) - cleaves PIP2 to DAG and IP3
Effectors: DAG activates PKC; IP3 binds to IP3 receptors of ER - releases calcium
Ca(2+)
Generation: Generated by opening of ion channels
Effectors: Binds to proteins and activates PKC and other kinases and enzymes
cAMP pathway
1) Binding of ligand to GPCR activates Gαs
2) Active Gαs-GTP binds to adenylyl cyclase and activates it
3) Each activated adenylyl cyclase generates many cAMP molecules from ATP
4) cAMP molecules activate protein kinase A (PKA)
5) Each activated PKA can phophorylate and activate many copies of substrate X
6) Each copy of Substrate X may activate many copies of the next downstream substrate
How is PKA activated
cAMP binds to regulatory subunits and releases catalytic subunits
GPCRs coupled to Gαq regulate the ______-______ pathway
Inositol-lipid
Inositol-lipid pathway
1) Ligand activates a GPCR
2) Gαq is activated by binding GTP
3) PLC is activated by binding Gαq-GTP
4) Active PLC cleaves PIP2 into IP3 and DAG
5) IP3 diffuses into the cytoplasm and causes Ca (2+) release from ER
6) DAG remains in the membrane and activated PKC
7) Ca(2+) binds to calmodulin and activates Ca(2+)/calmodulin dependent kinase
Monomeric G proteins
Activated by interacting with proteins known as “guanine nucleotide exchange factors” (GEFs)
Inactivated by interacting with proteins known as “GTPase activating proteins” (GAPs)
Difference between heterotrimeric and monomeric G proteins
1) Heterotrimeric G proteins (Gα subunit) are activated by direct interaction with a GPCR (EGFR)
2) Monomeric G proteins (such as Ras) are activated by direct interaction with a GEF (SOS)
GEFs are activated when they are recruited to activated tyrosine kinase-linked receptors by _______ ________
Adaptor proteins (GRB2)
Activation Cycle of Ras (and other monomeric G Proteins)
1) Ras interacts with a GEF, which causes Ras-GDP to release its bound GDP and bind GTP, resulting in formation of active Ras-GTP
2) Active Ras-GTP binds and signals to other molecules
3) Ras has intrinsic GTPase activity and hydrolyzes bound GTP to GDP
4) Upon association with GAP, Ras-GTP hydrolyzes to inactive Ras-GDP
5) Ras-GDP is inactive until a new signal causes it to associated with a GEF
Ras-GTP activates the ____ ______ signaling cascade
MAP kinase
MAP kinase signaling cascade
Ras-GTP activates Raf (MKKK)
Raf phophorylates MKK1
MKK1 phosphoylates ERK (MAP kinase)
ERK phophorylates many different transcription factors
Gefitinib and Erlotinib
Drugs that target the kinase domain of the EGF receptor and inhibit signaling by the EGF receptor
Gefitinib and Erlotinib decrease lung tumor burden in ___ of patiens who have mutated EGF receptor
These drugs are effective in only ___ of patients w/o receptor mutations
80%; 10%
EGF receptor mutations occur more frequently in
Female patients of Asian descent who have never smoked
Attenutation of signaling is accomplished by:
Ligand inactivation or dissociation from the receptor
Receptors dissociate from other proteins
Adaptor protein complexes dissociate
G-proteins hydrolyze GTP to the GDP inactive state
Phophorylated proteins are de-phophorylated
Second messengers are metabolized or diffuse away
Ions are sequestered or pumped out
Adaptation
Receptor-mediated endocytosis
Occurs when signaling system responds to the intensity and frequency of stimulation
Coated-pits (formed by clathrin)
Specialized inward-curved membranes where receptor-mediated endocytosis occurs
CURL
Compartment of Uncoupling of Receptor and Ligand (these vesicles have low pH (4.5-5) to favor dissociation of ligand)
Retroendocytosis
Dissociated ligands incorporate into vesicles and are recycled
Homologous desensitization
Signaling by the receptor is attenuated - can occur due to covalent modification of the ligand-bound receptor
What two molecules work together to desensitize GPCR
phosphorylation by β-adreneregic kinase (βark) followed by binding of β-arrestin to phosphorylated receptor
How is GCPR desensitized by PKA
Stimulated receptor activates PKA, the activated PKA can phosphorylate the receptor and desensitize it
Heterologous desensitization
Signaling by both the stimulated receptor and by other different types of receptors is attenuated:
Both stimulated and unstimulated receptors are blocked
