Mechanisms of Cell Signaling Flashcards
name 4 major classes of receptors
- ion channels
- steroid receptors (cytoplasm or nucleus)
- protein kinase (enzyme linked) receptors
- 7-transmembrane GCPR
what is wrong with the receptor in cystic fibrosis?
recessive disease, LOF mutation in CFTR chloride (Cl-) ion channel
normal ligand of CFTR is ATP
impaired channel function —> abnormal salt transport
—> thick mucus build-up in respiratory epithelial cells
how does tetrodotoxin (TTX) cause death?
(pufferfish toxin)
structurally similar to hydrated Na+ but larger -> binds sodium ion channels irreversibly
—> disrupts action potential propagation -> diaphragm paralysis and death
major families of steroid hormones (5)
- androgen/testosterone
- estrogen/estradiol
- progesterone
- glucocorticoid/cortisol
- mineralcorticoid/aldosterone
Bind receptor in cytosol or nucleus to regulate gene expression
derived from cholesterol
describe estrogen signaling
steroid hormone, derived from cholesterol
- ER attached to chaperone protein in cytosol in absence of ligand
- estrogen binding induces conformational change in ER - dissociates from chaperone to expose nuclear import signal
- ER-ligand complex homo-dimerizes
- dimerized receptors enter nucleus to bind ERE (DNA promoter sequence, estrogen response element)
tamoxifen
estrogen receptor antagonist, selective for breast tissue
represses estrogen over-expression in breast cancer
Addison’s disease, Cushing’s syndrome, and Kallman syndrome have this in common
steroid-hormone related diseases
Addison’s - autoimmune adrenal insufficiency
Cushing’s - secondary hyperaldosteronism, overproduction of ACTH
Kallman - delayed puberty, developmental disorder
describe structure of protein kinase receptors
and what are two subfamilies
extra cellular domain (binds ligand)
trans-membrane domain
cytoplasmic domain (kinase activity)
major types (defined by amino acid phosphorylation targets):
1. tyrosine kinases
2. serine/threonine kinases
phosphotases remove phosphate group by what mechanism (organic chemistry)
hydrolysis
in protein kinase receptor activation, ligand binding to extracellular domain induces monomer dimerization that brings cytosolic protein kinase domains into proximity with one another and phosphorylate each other.
Cytosolic proteins are then recruited to phosphorylated cytoplasmic domains, which induces signaling cascade via _____
Ras-MAPK pathway
(Ras - small G-protein)
ultimately activates TFs vis phosphorylation in nucleus.
[MAPK is only one downstream protein of activated ras-GTP]
describe each of these core elements of the Ras_MAPK signaling pathway (in protein kinase receptor signaling):
a. Grb
b. GEFS
c. SoS
d. Ras
e. G-proteins
f. GAPS
a. Grb (G protein receptor binding): contains SH2 domain that binds tyr-phosphorylated receptor cytoplasmic domain, recruits GEFS (such as SoS)
b. GEFS (guanine nucleotide exchange factors): activate G-proteins by promoting dissociation of GDP for GTP
c. SoS (son of sevenless): GEF that binds Grb and activates small G-proteins such as Ras via GTP/GDP regulation
d. Ras (rat sarcoma): monomeric small G-protein
e. G-proteins: binds GTP/GDP and act as molecular switches, have weak intrinsic GTPase activity
f. GAPS (GTPase activating proteins): inactive G-proteins by stimulating weak, intrinsic GTPase activity of G-proteins
describe the nucleotide exchange reaction that occurs in the Ras-MAPK pathway in protein kinase receptor signaling
- ras-GTP is active
- GAP binds ras-GTP to inactive
- ras-GDP is inactive
- SoS (ras-GEF) binds ras-GDP to activate
(it’s a cycle, Ras is G-protein molecular switch)
what do these families of small g-proteins generally do?
a. ras
b. rab
c. ran
d. rac
a. ras family: receptor signaling, cell division (s = signaling)
b. rab family: traffic of membrane vesicles (b = bus)
c. ran family: nucleus/ cytoplasm traffic (n = nucleus)
d. rac family: actin cytoskeleton (c = cytoskelton)
How does ras mutation play a role in cancer
ras: small G-protein in ras-MAPK pathway
oncogenic mutations in Ras common in tumors - makes it resistant to GAPs
mutated Ras is constitutively active
cause of neurofibromatosis type 1 (in cell signaling)
mutation in NF1, which encodes Ras-GAP (neurofirbomin-1)
mutation in GAP genes results in overactive Ras
cell signaling mutation that causes Noonan syndrome
mutation in PTPN11 gene, encodes protein tyrosine phosphatase (SHP2)
results in GOF, hyperactive Ras (defective Ras GAP activity)
[ras-MAPK is pathway downstream of protein kinase receptors]
most abundant receptor class in human genome
7-transmembrane receptors (seven alpha helices)
large extra cellular binding domains
most receptor-target drugs are directed at 7-TMRs
roles in vision (Opsins such as rhodopsin), smell (olfactory receptors), mood (serotonin, dopamine, GABA, glutamate), ANS
serotonin, dopamine, GABA, and glutamate all have this in common regarding their receptors
receptors are all 7-TMRs
what are the components of the 7-TMR trimeric complex?
7-TMRs coupled to large G-protein trimeric complex in cytoplasm (alpha, beta, gamma)
alpha subunit contains ras-like domain regulated by GTP/GDP
alpha and beta/gamma subunits dissociate form each other and receptor cytosolic domain following activation
7-TMR signaling mechanism steps
- ligand binding causes GDP->GTP by receptor cytoplasmic tail (acts as GEF)
- alpha and beta/gamma subunits dissociate from each other and cytosolic domain
- subtype of G-protein alpha subunit determines downstream affect :
a. Gs-alpha ACTIVATES adenylate cyclase —>PKA
b. Gi-alpha INHIBITS adenylate cyclase —>PKA
c. Gq-alpha ACTIVATES PLC —> PKC
what are the three subtypes of G-protein alpha subunit in 7-TMR signaling and what do they do
Gs-alpha: ACTIVATES adenylate cyclase, signaling target is PKA
Gi-alpha: INHIBITS adenylate cyclase, signaling target is PKA (inhibits formation)
Gq-alpha: ACTIVATES phospholipase-Cbeta (PLC), signaling target PKC
how does 7-TMR desensitization occur
signaling is terminated even in presence of ligand
- beta-adrenergic receptor (7-TMR) binds adrenal gland hormone ligands epinephrine and NT norepinephrine
- ligand bound receptor becomes substrate for beta-adrenergic receptor kinase (BARK)
- phosphorylated receptor is bound by beta-arrestin —> blocks re-association of cytosolic domain with G-alpha subunit
Gs-alpha of 7-TMR upregulates PKA downstream of ligand binding. What are the intermediary steps
Gs-alpha (subtype of G-alpha monomer) activates adenylate cyclase
AC changes ATP—> cAMP
cAMP activates PKA
PKA causes increase in intracelular calcium
cytosolic calcium is maintained at a very low concentration by calcium pumps in the ____
calcium pumps in the plasma membrane and SER
PKA induces receptor-mediated calcium influx, following upstream activation by Gs-alpha of 7-TMR
Explain steps of Inositol Triphosphate (IP3) and Calcium Signaling Pathway
- Gq-alpha-GTP activates PLC
- PLC hydrolyzes PiP2 into iP3 and DAG
- iP3 triggers calcium release from SER lumen
- DAG and Ca2+ together activate PKC
describe mechanism of PKC activation
- absence of signaling, C1 and C2 domains of PKC are arranged with psuedosubstrate occupying active site
- signaling produces DAG and PS (phosphatidylserine) and calcium release is triggered
- C1 domain binds DAG and exposes active site
- C2 domain binds PS when calcium is present —> linker region exposed
- PKC linker is cleaved to release constitutively active kinase subunit
how does calmodulin regulate CAMK
calmodulin is calcium-binding protein: when Ca2+ binds, tertiary structure is altered, and calmodulin activates proteins such as CAMK (Ca2+/calmodulin-activated protein kinase)
CAMKs are self-inhibiting proteins (activated by disinhibition) - serine/threonine kinase proteins
- absence of signaling - inhibitory subunits block kinase (catalytic) domain
- during signaling - Ca2+/calmodulin binds inhibitory subunit to expose active site
how does tamoxifen work to treat Estrogen-sensitive breast cancer (specifically)
antagonizes estrogen signaling by inhibiting, rather than activating, gene at ERE
does not affect localization to the nucleus (effect is after it gets to the nucleus)
