signal transduction

Question 1

intracellular steroid receptors

Answer 1

for small hydrophobic signaling molecules like steroid hormones, thyroid, retinoids and vit D, ligand activated transcription factors

2 unique features:
1. receptors are intracellular and bind ligands in cytoplasm or nucleus

2. receptors are ligand activated transcription factors

the receptors have a hormone binding site, a dna binding domain, and a transcription activating domain

Question 2

mechanism of signaling by intracellular steroid receptors

Answer 2

hydrophobic signaling molecules are solubalized by reversible binding carrier protein

After release from carrier protein hydrophobic molecule diffuses thru the PM

binding to receptor
Subcellular localization : either bound in cyto then move to nucleus r receptor already in the nucleus

hormone binding induces conformational change which releases an inhibitory protein and can bind to the hormone response element located near target gene

Regulation of gene espression: positive regulation by stabilizing binding of transcription factor
neg regulation by repression of transcription facts
only a few genes affected

Question 3

ion channel-linked receptor

Answer 3

multiple subunits, directly gates ion channel, major target for drugs, rapid signaling

nicotinic Ach receptor (5 subunits, cation selective, excitatory) located on NMJ, PNS, CNS converts Ach binding into electrical signal in the post synaptic cell

GABA receptor Cl- channels: main inhib post synapse (benzosm barb hypnotics, anesthesia)

Question 4

GPCR

Answer 4

multicomponent system, large family, mediate effects of NT, light, odorants, hormones,

passes thru the membrane 7 times

Question 5

heterotrimeric G protein

Answer 5

link the GPCR to an effector enzyme, provides specificity, (transducer), effector provides the catalytic component to the 2nd messenger

cycle between 2 states and act as a molecular switch, GDP-bound/inactive, and GTP bound/active

consist of 3 different proteins/subunits- Ga- binds GTP binds effectors, acts as GTPase)
GB and Gy- By dimer anchors to membrane and has itso own effectors

G protein is classified by the type of Ga protein
Gas- stimulates adenylyl cyclase
Gai- inhibits adenylyl cyclase
Gaq- stimulates phospholipase C

Question 6

Enzyme linked receptors

Answer 6

diverse group of receptors with different enzymatic activities (tyrosine kinase linked, serine threonine kinase)

Question 7

tyrosine kinase linked receptor

Answer 7

single protein with one transmembrane domaine which dimerizes upon ligand binding or a tetramer with 2 extracellular subunits and 2 transmembrane subunits

extracellular domains bind the ligand

cytoplasmic domain has tyrosine specific protein kinase activity but can also bind to tyrosine specific kinases

ligand binding leads to dimerization, activation by cross phosphorylation, binding of intracellular signaling molecules

regulate cell proliferation and differentiation in response to hormones, growth factors such as EGF

Question 8

Transcription factors

Answer 8

DNA-binding proteins that regulate transcription of specific genes
Signal transduction cascades regulate many properties of TFs (nuclear translocation and ability to bind DNA)

Activated TFs induce transcription by activating RNAP resulting in transcription of mRNA from the target gene

Some are activated by phosphorylation by protein kinases, and inactivated when dephosphorylated by protein phosphatases

Question 9

protein kinases

Answer 9

catalyze addition of phosphate group to side chain of AA of proteins and peptides

protein kinases are classified according to the type of AA which is modified

serine/threonine-specific protein kinases: PKA, PKC, Ca/calmodulin-dependent protein kinase, MAP kinases

tyrosine-specific protein kinases: EGF receptor

dual specificity protein kinases: phosphorylate both threonine and tyrosine: MAP kinases (like MKK1)

Question 10

protein phosphatases

Answer 10

catalyze cleavage of phosphate group from side chain, less variety than kinases

Question 11

second messengers

Answer 11

small diffusible signaling molecules that are generated in response to ligand-receptor binding and activate downstream signaling molecules:

cAMP: generated by adenyl cyclase which is activated by Gas protein, activates cAMP-dependent kinases (PKA, PKC)

diacylglycerol (DAG): generated when phospholipase C cleaves PIP2 to DAG and IP3, binds IP3 receptors on ER causing release of Ca from ER

Ca: generated by opening of ion channels, activates PKC and other protein kinases

Question 12

Cyclic nucleotide pathways

cAMP pathway regulated by GPCR coupled to Gas

Answer 12

ligand binds receptor, Gas binds GTP and dissociates from By, Gas-GTP activates adenylyl cyclase, Adenylyl cyclase converts ATP-> cAMP

cAMP activates PKA (a tetramer of 2 catalytic and 2 regulatory subunits, each R binds to 2 cAMP, R and C dissociate, C subunit becomes autophosphorylated

C subunit of PKA phosphorylates specific proteins on serine and threonine residues

The phosphorylation results in activation or inactivation of the phosphorylated protein

AMPLIFICATION of the original signal

PKA can phosphorylate many different substrates

Question 13

Cyclic nucleotide pathways

cAMP pathway regulated by GPCR coupled to Gai

Answer 13

activated Gai (Gai with GTP) inhibits adenylyl cyclase -> decreased production of cAMP and decreased activity of PKA

Question 14

phospholipid hydrolysis pathway

Answer 14

inosital lipid pathway

Ligand binds to and activates GPCR, Gaq binds GTP and dissociates from By, Gaq-GTP activates phospholipase C (PLC), PLC cleaves PIP2 to generate IP3 and DAG, IP# diffuses into cytoplasm and releasses Ca from ER, Ca binds to calmodulin and activates Ca/calmodulin-dependent protein kinase, DAG remains in PM and together with Ca activates PKC

Question 15

Monomeric G protein pathways

Answer 15

Ras etc, monomeric G proteins are recruited to receptor-linked tyrosine kinases and turned on by a GEF (nuanone nucleotided exchange factor) and turned off by a GAP (GTPase activateing protein)

Difference between a monomeric and heterotrimeric G protien (monomerics are activated by direct interaction with GEFs, as opposed to GPCRs)

GEF (like Sos) binds to monomeric G protein (like Ras), G protein releases GDP and binds GTP signals other molecules, when a GAP binds to ras it hydrolyzes GTP to GDP

Question 16

MAP kinase signaling cascade (Mitogen-activated protein)

Answer 16

MAP kinase kinase kinase (MAPKKK) phosphorylates a MAPKK which phosphorylates a MAP K

Phosphorylated MAPK usually phosphorylates a transcription factor (gene transcription)

Question 17

Ras and the MAP kinase signaling

Answer 17

ligand binds to tyrosine-linked receptor which dimerizes and cross autophosphorylated, phosphorylated receptor binds adaptor molecule called GRB2, GRB2 binds the GEF (Sos) to the Ras, Sos activates Ras by inducing ras to release GDP and binds GTP, Ras-GTP activates the MAPKKK (Raf), phosphorylates the MAPKK (MKK1), MKK1 phosphorylates MAPK (ERK) which phosphorylates transcirption factors –> increased gene transcription

Question 18

mutations of EGF/ Ras

Answer 18

tumors-> high activity of MAPK/ERK pathway

Agents that inhibit EGF receptor -> cancer therapeutics

Gefitinib, Erlotinib are tyrosine kinase inhibitors that target kinase domain of EGF receptor

Question 19

signal attenuation

Answer 19

signal needs to be turned off at every level:
Ligands dissociate from receptor
receptor complexes dissociate
adaptor proteins complex dissociate
Hydrolysis of GTP
Protein phosphatases dephosphorylate proteins (like protein kinases)
second messengers metabolized
ions are relocated

Question 20

Signal adaptation (Receptor mediated endocytosis)

Answer 20

regulates the amount of receptors

upon binding of ligands receptors migrate to coated pits (caltharin), vesicles fuse with tubular-reticular structures (CURL) uncoupling ligand and receptor low pH

Vesicles fuse with lysozymes

Question 21

Signal adaptation (Receptor desensitization)

Answer 21

homologous desensitization: only signal from the stimulated receptor is attenuated, may involve covalent modification (phosphorylation) B-arrestin or PKA

Heterologous desensitization: both stimuated and unstimulated receptors are blocked (phosphorylation or second messengers)