cell signaling test 3 Flashcards
GPCRs
activate heterotrimeric G proteins
mediate signaling in metabolism and are involved in sensory perceptions
contain 7 transmembrane alpha helices
NH3+ end in extracellular space; COO- end in cell cytosol
many are glycoproteins; carbohydrate functional groups in amino end in extracellular space
also called serpentine receptors
receptor agonist
activate receptor signaling by mimicking natural ligand
receptor antagonist
bind to receptors with high affinity and block the binding of physiological agonists without promoting structural changes needed for signal transduction
GPCR activation steps
- ligand-induced conformational change in the GPCR
- receptor mediated stimulation of guanine nucleotide exchange (GTP replaces GDP). This causes subunit dissociation. Subunits are modified and bound to plasma membrane
- regulation of downstream effector processes by GaGTP and Gby complexes
- termination of signal
shared pathway
2 ligands activate the same receptor
activation/deactivation of the same pathway
example: glucagon and epinephrine bind to different GPCRs which result in the same pathway for net glucose export
parallel pathway
1 ligand binds 2 receptors
example: epinephrine can bind B2 or A1 GPCR. Both result in different pathways & net glucose export
protein kinase A (PKA)
a downstream signaling protein
active when cAMP is high
Three distinct metabolic responses:
1. phosphorylation and inhibition of glycogen synthase and impedes glycogen synthesis
2. phosphorylation and activation of enzymes involved in glycogen degradation to produce glucose
3. phosphorylation and activation of enzymes involved in gluconeogenesis
PKA has two domains: regulatory & catalytic
cAMP binds regulatory domain and activates protein
PKAs role in epinephrine
B2-adrenegic receptor or glucagon receptor activated
activation of Gsa subunit convers ATP to cAMP
cAMP activates PKA
PKA turns off glycogen synthesis & turns on glycogen degradation and glucose synthesis resulting in net glucose export
or
a1-adrenegic receptor activated, Gqa subunit activated which activates PLC
PIP2 produces DAG + IP3
turn on glycogen degradation (IP3 from Ca+ release) and turns off glycogen synthesis (DAG turns on PKA) resulting in net glucose export
guanine nucleotide exchange factor (GEF)
promote GDP-GTP exchange
activate signaling
GTPase activating protein (GAP)
stimulate intrinsic GTP hydrolysis inhibiting signal transduction
3 subunits bind to GPCR
Termination of GPCR-Mediated Signaling by GRK
Receptor is desensitized after Gαβγ dissociation.
G protein coupled receptor kinases (GRK) phosphorylate the GPCR cytoplasmic domain on Ser and Thr residues.
Phosphorylation provides a docking site for β-arrestin, which prevents reformation of the Gαβγ complex.
β-Adrenergic Receptor Kinase (βARK) GRK cycle
GPCR activated
bark binds Gby subunit & recruits kinases to GPCR tail
phosphorylation of tail
b-arrestin binds to GPCR tail and transports protein to endocytic vesicle where GPCR is dephosphorylated and rebuilt
Receptor tyrosine kinase (RTK) signaling
transmits extracellular signals by ligand activation of an intrinsic tyrosine kinase function found in the cytoplasmic tail of receptor
inactive on cellular membrane as monomer; binding causes active dimer to form
phosphorylate downstream signaling proteins
Targets:
adaptor proteins and kinases
EGC: a serum growth factor
EGC binds EGRF1 and EGRF2 monomers> causes dimerization > leads to kinase activity on EGFR 1 and phosphorylation of Tyr on EGFR 2
EGFR2 changes conformation and phosphorylated Tyr on EGFR1
binding of SH2/SH3 domain adaptor proteins and initiation of downstream signaling
adapter protein > GEF protein > Ras + GTP activation
Ras proteins
G protein signaling molecule (Ga)
anchored to cytoplasmic side of plasma membrane w/ lipid
activated by GEFs
contain an intrinsic GTPase activity that is regulated by GAP
Ras GTP interacts with activated Raf
Interaction phosphorylates MEK which phosphorylates ERK which enters nucleus and increases rates of cell division
mutated Ras protein activated cell division without growth factors
insulin signaling pathway
insulin receptor stays in tact; held together with disulfide bond
insulin binds receptor and induces conformational change (only binds to one subunit)
autophosphorylation occurs, Ras activated downstream
MAP kinase signaling eventually leads to gene expression and cell division
insulin signaling pathway
insulin receptor stays in tact; held together with disulfide bond
insulin binds receptor and induces conformational change (only binds to one subunit)
autophosphorylation occurs, Ras activated downstream
MAP kinase signaling eventually leads to gene expression and cell division
and
PIP3 activation by IRS cause glucose uptake and glycogen synthesis
Tumor necrosis factor receptor signaling
regulates cell death and cell growth
signal through cytolytic adapter complexes
receptor is a homotrimer
A single receptor stimulates intracellular pathways with opposing cellular responses.
TNF adapter proteins
SODD: silencing of the death domain
TRADD: receptor associated with the death domain
Binding of TNF-alpha induces conformational change in the TNF death domain
This causes a release of silencing of the death domain (SODD) and recruitment of TNF receptor associated death domain (TRADD). (SODDs are exchanged for TRADDs)
apoptosis
proteases are not translated in active form
procaspase 8 is cleaved into caspase 8
Caspase 8 cleaves procaspase 3 and generates caspase 3, the “executioner” caspase.
Key regulatory molecules are degraded and the cell dies.
nuclear receptor signaling
intracellular receptors
not bound to membrane
serve as transcription factors that regulate gene expression
governed by three parameters:
cell-specific expression of nuclear receptors, localized bioavailability of ligands, differential accessibility of target gene DNA sequences in chromatic to nuclear receptor binding
steroid receptors
homodimer, head to head orientating, bind palindrome DNA, 3 nucleotides between binding sequence, ligands come from cholesterol
metabolite receptors
heterodimer, monomers oriented in the same direction, DNA biding sequence repeats, 1-5 nucleotides between DNA binding sequences, ligands derived from vitamins, unsaturated fatty acids, essential AA
Glucocorticoid signaling pathway (nuclear receptor example)
Glucocorticoid binds ligand in cytocol
Glucocorticoid signaling pathway (nuclear receptor example)
Glucocorticoid binds ligand in cytosol
activated monomer enters nucleus
monomer dimerizes and binds to gene
or
activated monomer binds other substrates and cases gene regulation by removal of transcription factors
This pathway decreases the expression of separate genes
