Cell Signaling Flashcards
3 roles of protein phosphorylation in signaling
- Imposes reversible changes in secondary structure that can cause conformational changes in the protein (stabilize active or inactive form)
- Sterically blocks interactions with another protein
- Recruits phospho-binding proteins
protein kinases
enzymes that are usually inactive due to an inhibitory loop that blocks the active site. Activation is associated with displacement of the inhibitory loop through phosphorylation which allows binding of substrate and transfer the γ-phosphate of ATP to a hydroxyl group of serine, threonine or tyrosine.
**Can be phosphorylated (activated) by other protein kinases
G protein-coupled receptor
characterized by 7 transmembrane domains, extracellular amino terminus and cytosolic carboxyl terminus; extracellular side binds ligands, intracellular side interact with G proteins to induce signaling
Unlike RTKs, can be desensitized and downregulated.
G proteins
heterotrimeric proteins (3 subunits, the smaller, tightly linked B and y, and larger alpha subunit); usually anchored to cytosolic side of plasma membrane; can bind GDP and GTP and have intrinsic GTPase activity; when in active, GTP-bound state, can regulate the activity of effectors
GEF (guanine nucleotide exchange factor)
interaction of a G protein with this leads to a conformational change that causes release of GDP from the G protein and replacement with GTP, thus activating the G protein
GAP (GTPase accelerating protein)
interaction of a G protein with this leads to conformational change that causes release of GTP and replacement with GDP, turning the G protein off. Example: RGS proteins (Regulators of G protein Signaling)
cAMP/PKA pathway
important GPCR pathway in which ligand binds GPCR, G protein alpha dissociates and interacts with adenylyl cyclase which activates cAMP, cAMP activates PKA (a Ser/Thr kinase) which has broad effector functions including activating CREB (transcription factor) in the nucleus
PKA
a ser/thr kinase, activation of PKA is the most important step in cAMP pathway, allows for broad effector functions and activation of CREB (cAMP-response element binding protein, a transcription factor)
IP3/PKC pathway
important GPCR pathway in which ligand binds, activates Gaq G protein which interacts with phospholipase C. Phospholipase C cleaves PIP2 into IP3 and DAG. IP3 binds to IP3-gated Ca2+ channel in the smooth ER which opens Ca2+ channels in the cell, influx of Ca2+ activates PKC. Activated PKC migrates to plasma membrane where DAG remains, DAG further activates PKC = cell response
calmodulin
small protein that binds Ca2+, when bound, induces conformational change that permits it to activate effector proteins –> important because this is a PKC-independent use for calcium.
Roles in neuronal plasticity, smooth muscle contraction, nitric oxide induced vasodilation
desensitization
the uncoupling of the receptor from G-protein; occurs when G-protein receptor kinases (GRK’s) phosphorylate the intracellular C-terminus of the receptor allowing for subsequent binding of arrestin. Rapid and reversible process
down-regulation
Continued stimulation of the receptor with ligand causes removal of receptors from the membrane; desensitization recruits arrestins which also serve as scaffolds for other proteins important in endocytosis of GPCR
toxin-mediated diseases
this type of disease uses our G protein signaling pathways against us, leading to overexpression of downstream effectors and associated side effects that can be severe; ex: pertussis, cholera
Mechanism: Cholera —> A1 toxin uses (NAD+) as substrate to add ADP-ribose to the alpha subunit of Gαs. ADP-ribosylation of Gαs inhibits its GTPase activity, and causes excessive stimulation of AC and increased cAMP. Cholera toxin activates the Gαs/AC/cAMP pathway in the intestinal epithelium and causes excessive fluid secretion leading to diarrhea.
VIP (vasoactive intestinal peptide)
peptide hormone that has diverse roles in many tissues but main function in intestines is to maintain motility by stimulating secretion of water and electrolytes. Signals through GPCR and Gas G protein and mediated by PKA and chloride channel activation. If a toxin like cholera takes over the signaling, can result in excess cAMP activation and diarrhea in the gut.
NO/cGMP pathway
pathway responsible for causing vasodilation and relaxation of smooth muscle. NOS synthesizes NO, NO diffuses thru plasma membrane and activates cytosolic guanylyl cyclate (sGC).
NO–>sGC–>cGMP–>PKG
PKG1
protein kinase that blocks calcium release when activated, and activates myosin light chain phosphatase (MLCP). (Both of those are necessary for contraction in the muscle.) Results in vasodilation and relaxation of smooth muscle
PDE5
protein that regulates cGMP activity by inactivating cGMP when in high concentrations. Drugs have been made to inhibit this protein, which reduces cGMP degradation and increases magnitude and duration of cGMP signaling. Ex: Viagra leads to erection by strengthening cGMP signaling which increases vasodilation
vision cGMP pathway
In the dark: rhodopsin is inactive, cGMP levels are elevated, and abundant cGMP gated Na+/Ca2+ channels (CNG) are open. This ion flow (called dark current) causes partial depolarization and steady release of glutamate that inhibits bipolar cell activity.
In the light: rhodopsin is active, cGMP levels decrease, and the CNG channels close. The membrane hyperpolarizes, and the calcium level in the photoreceptor cell drops. The amount of glutamate that is released by the cell also drops because Ca2+ is required for the glutamate-containing vesicles to fuse with cell membrane and release their contents.
receptor tyrosine kinases
large family of growth factor receptors, have a single membrane spanning domain, the cytosolic portion of the receptor contains enzyme activity. The extracellular structures bind ligand. Binding of ligand causes dimerization of the monomer, leading to autophosphorylation of adjacent tyrosine residues (scaffold). SH2 effector proteins bind the phosphotyrosine residues, activating the effector proteins
Ras/MAPK pathway
RTK pathway promoting cell proliferation; (SOS)–>Ras–>Raf–>ERK(MAPK)–>nucleus
PI3K/AKT pathway
RTK pathway promoting cell survival, inhibits apoptosis, increases protein synthesis.
p85 becomes phosphorylated–>PI3K puts PIP3 on membrane which recruits PDK1 and AKT, PDK1 activates AKT
AKT
Signaling protein for survival, when active- key functions include: (1) inhibiting cell death (apoptosis; by phosphorylating BIM, BAD and FoxO proteins), (2) promotes protein synthesis (by targeting mTOR), and (3) metabolism (e.g. by targeting FoxO and GSK1)
PI3K
phosphorylates PIP2 to PIP3 when activated, which allows PDK1 and AKT to bind the plasma membrane and initiate cell survival signaling
PTEN
phosphatase that regulates the AKT pathway by dephosphorylating PIP3 back into PIP2. Cancer can arise when there is a loss of function of this enzyme
