signal transduction
is the relay of an extracellular signal to its final targets(s) through intracellular signaling molecules
-triggered by the binding of a signal to a receptor
-activated receptor relays the signal, initiating an intracellular cascade of molecular events
intracellular receptors
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Cortisol
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Cortisol function
adaptation to long term stress (raises blood glucose levels, blood pressure,mobilizes fat
binding of cortisol to its receptor is necessary for binding to DNA
-cortisol receptor is a TRANSCRIPTION FACTOR (but doesn't function as TF until it receives a signal)
-absence of cortisol, receptor remains inactive in the cytosol
-when the hormone binds: conformation change activates the receptor
-the activated hormone-receptor complex is transported to the nucleus and binds specific regulator (enhancer or repressor) sequences of target genes
activation of cortisol receptor
activated receptor-cortisol complex binds to regulatory region of target gene and activates transcription
**intracellular steroid hormone receptors have binging sites for a signaling molecule + DNA sequence. the same steroid hormone receptor which binds to specific DNA sequence can regulate different genes in different cell types?
TRUE
signaling cascade
extracellular messenger = primary
intracellular messenger = secondary
molecular switches
many signaling pathways require the activation of specific target enzymes that behave as molecular switches:
1)some target enzymes are ACTIVATED (turned on) by addition of a phosphate group (phosphorylation) by a PROTEIN KINASE
-inactivation (turned off) catalyzed by a PROTEIN PHOSPHATES
2)some target enzymes are activated by a GUANINE NUCLEOTIDE EXCHANGE FACTOR (GEF) which exchanges a found GDP for GTP
-inactivation catalyzed by a GTPase
G-protein coupled receptor
-largest family of cell surface receptors
-uses signaling by GTP biding protein
-crosses membrane 7 times
-activate gene protein (cytoplasmic domain)
-receptor = extracellular domain
-7 transmembrane domains
-activator of the G protein (cytoplasmic domain)
activation of the G protein: trimeric GTP binding protein
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G protein inactive form
all 3 subunits associated together
-attached to the membrane
G protein active form
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activation of alpha subunit requires
GEF (guanine nucleotide exchange factor) to exchange GDP to GTP
activated alpha subunit
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activated G-protein
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**which of the following happens when cell-surface receptors activates G protein?
a subunit exchanges its bound GDP to GTP
activated enzyme
2nd messenger that converts reactants to products
-signal amplification
acetycholine
causes reduction of heart rate
opening K+ channels
when opening channels, more positive ions on the outside (hyperpolarized) which is going to reduce the rate of heart beats
-alpha comes and removes beta so channel closes
** when ach binds to GPCR on heart muscle the activated receptor stimulates a G protein, which opens a K+ channel in the plasma membrane, which would enhance this effect of ach?
addition of high concentration of a non-hydrolyzable analogue of GTP
regulation of cAMP-dependant protein kinase A (PKA)
-PKA promotes glycogen metabolism
-inactive PKA is a tetramer: 2 regulatory subunits bound to 2 catalytic subunits
-cAMP ACTIVATES PKA by binding to the regulatory subunits, causing a conformational change that RELEASES and ACTIVATES the catalytic subunits
-kinase activity of the released catalytic subunits PHOSPHORYLATE multiple EFFECTOR PROTEINS
epinephrine (adrenaline) receptor
-derived from tyrosine
-a HORMONE produced/secreted by the adrenal medulla (interior)
-stimulates the breakdown of glycogen in anticipation of muscle activity (breakdown glycogen -> glucose increasing energy mode)
-inhibits the production of glycogen
-also used as a NEUROTRANSMITTER by the sympathetic nervous system ( the FIGHT-OR-FLIGHT RESPONSE to perceive threat
epinephrine effects
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epinephrine receptor
-G protein coupled receptor
-activated G protein activates enzyme ADENYLATE CYCLASE (at membrane( which catalyzes the conversion of ATP into cAMP
-this results int he elevation of cAMP levels and the activation of PROTEIN KINASE A (PKA)
cAMP
-2nd messenger
-small molecules
-carry away from membrane
-protein kinase a= very reactive to high cAMP levels
Fast response to PKA activation
-breakdown of glycogen (in SKELETAL muscle cells)
-PKA phosphorylates and activated PHOSPHORYLASE KINASE
-phosphorylase kinase phosphorylates and activates GLYCOGEN PHOSPHORYLASE
-glycogen phosphorylase breaks down glycogen into glucose subunits, converting them into glucose 1-P
-within second of binding to GTP receptor
-PKA phosphorylates an enzyme that activates and phosphorylates another enzyme = a process
-PKA breakdown -> glucose subunits are released
Phosphoglucomustase
another enzyme that ISOMERIZES glucose 1-P into glucose 6-P which enter glycolysis
-molecule that enters glycolysis that is used to generate ATP = needed for muscles to contract
regulation of glycogen metabolism by PKA
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epinephrine receptor cycle
-SLOW response due to activation of new gene expression to PKA activation
-brain/ hypothalamus
-adrenaline actives Gprotein subunit attached to kinase and activates PKA -> transported to nucleus
-same molecule with different function
-transcriptional regulator that is inactive until PKA is attached
-target gene= somatostatin (peptide hormone) thats released -> pancreas and stomach to inhibit enzyme and slow digest
**adrenaline stimulates glycogen breakdown in skeletal muscle cells by ultimately activating glycogen phosphorylase. consider skeletal muscle cells, which have a continuously active mutant form of PKA (remains on). whats false?
T: low unphosphorylated phosphokinase, not high affinity of adrenaline for receptor and high excess of glucose available
F:would lead to an excess in the amount of glycogen available
second messenger ca2+ and calmodulin
Calmodulin = small ca2+ binding PROTEIN that modulates man cellular processes by activating CALMODULIN-DEPENDANT PROTEIN KINASES and phoatases
-binds ca2+ ions
-in high concentration of ca2+, calmodulin will bind to target proteins (CAM) = protein kinase and activate kinases
Calmodulin
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calmodulin activation pathway
-beings with the activation of G PROTEIN
-activated alpha subunit activates PHOSPHOLIPASE (PLC)
-PLC catalyzes the hydrolysis of PHOSPHATIDYL INOSITOL BIPHOSPHATE (PIP2 - inositol phospholipid) into DIACYL GLYCEROL (DAG- which remains membrane bound) and INOSITOL TRIPHOSPHATE (IP3- released as a free cytoplasmic messenger)
-both DAG and IP3 can activate other cellular functions
IP3 is the ligand for
IP3 gated ca2+ channels on the smooth ER
-the released ca2+ bind calmodulin in cytosol which activated ca2+/ calmodulin-dependant (CAM) kinase
calmodulin pathway: fast or slow?
FAST; still everything already synthesized just has many intermediate steps (calmodulin is already floating)
Wnt signaling pathway
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Wnt pathway and cell proliferation: NO signal
degradation complex forms in the cytoplasm = default stage
-the APC-CONTAINING COMPLEX targets BETA-CATENIN for proteolytic degradation = break down no genes expressed.
Wnt pathway and cell proliferation: signal PRESENT
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Wnt pathway and cell proliferation: slow or fast?
SLOW response since there is gene expression in response to signal
dimerization and activation of receptor tyrosine kinase
RTK's are enzymatic receptors with single transmembrane domains that form dimers upon binding of the signal:
1) signal usually binds dimer. this causes
2) receptor subunits associate into a dimer
3) the cytoplasmic tyrosine kinase domains of the receptor subunits phosphorylate each other: ACTIVATION
4)the PHOSPHORYLATED TYROSINE'S can then bind and activate other targets: SH2 domains of intracellular signaling proteins
-signal always dimerizes to make 2 subunits: phosphorylated + makes SH2 domain proteins
**which mutation would reduce the amount of ca2+ released from the ER to the cytosol
mutation of PIP2 that cannot be hydrolyzed (so cannot send 2nd messenger)
receptor tyrosine kinase
-signal (growth factor = regulate growth, proliferation and cell survival = usually dimer)
-inactive RTK = 2 subunits that are not attached
-interaction= kinase domain activates and they phosphorylate each other -> docking sites for other receptors to downstream target in cells
-signaling molecules always have SH2 domains
-usually have 3 intracellular proteins
signaling dimer
can regulate pathway for phosphate = have phosphatase activity induced by another signal to move phosphatase and inactive receptor
the Ras-MAP pathway
-controls cell proliferation and differentiation in response to growth factors
-activated receptor tyrosine kinase is recognized by the ADAPTOR PROTEIN (SH2)
-adaptor protein recruits the RAS-ACTIVATING PROTEIN, a guanine nucleotide exchange factor (GEF)
-this GEF activates RAS (a monomeric GTP binding protein) by EXCHANGING its GDP -> GTP
-Ras-GTP activates a MAP KINASE-KINASE-KINASE triggering a MAP-kinase cascade of activation of protein kinases by phosphorylation
Ras can hydrolyze its GTP to
GDP to inactivate itself
Ras-MAP pathway cycle
-signal molecules induce growth and cell division -> dimerization which activated RTK, activating adaptor protein
-this recruits Ras activint protein (GEF)
-Goal = activate Ras protein
-Ras is activated by exhanging GDP -> GTP and will transmit signal to cascade of kinases
*MUST be able to regulate Ras protein (turn off and on) or else is would form tumors
Map kinase cascade
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**growth factor RGF stimulates and proliferation of cultured rat cells. it is bound by a receptor tyrosine kinase called RGFR. Which of the following types of alterations to RGF would be most likely to PREVENT receptor dimerization?
mutation that prevents RGFR from binding to RGF
Drosophila compound eye
-each of the 800 eye units consists of 8 photoreceptor cells and 12 lens cells
-all 20 cells differentiate from identical eye epithelial cells
-R8 differentiates into a photoreceptor cell, then induces 7 neighboring cells to become photoreceptors, ending with the R7 cell.
-the remaining 12 cells of the eye unit become lens cells by default
-1st photocenter = R8
-last = R7
-remaining cells in unit = lens
-ometidia = photo + lens cells
Roles of boss and sevenless in photoreceptor R7 differentiation
R8 induces the differentiation of R7 into a photoreceptor by direct cell-cell contact
1)Boss, 2)seveleness
-loss of function mutation in either gene results in absence of the R7 photoreceptor
Boss
(bride of sevenless)
-signal protein on the surgace of the R8 cell
Sevenless
a receptor tyrosine kinase, it the Boss receptor on the surface of R7 that activates a Ras pathyway