what is signal transduction
process by which cells receive & respond to cues from biological environment
essentialy for normal development, homeostatis
if wrong/aberrations, get disease like: cancer, ID, developmental genetic aberration, psychiatric disease, diabetes
target for most drugs = signal transduction
general categories of cell-cell signaling? how do they differ?
1) endocrine: cell produces hormone, it enters into circulation, travels long distance > target cell; i.e. pituitary-synthesized hormone
2) paracrine: signaling cell releases factor into local area; ddoesn't enter bloodstream; i.e. SHH
3) neuronal: specialized paracrine signaling w/ neutrotransmitters
4) contact-dependent: action limited to cell adjacent, cells must be in contact to hit target cell
5) autocrine: cell both makes signaling factor & responds to it; i.e. cancer cells
types of signaling factors?
amino acids/their derivatives
drugs of abuse
what is the basic principle of cell-surface receptors?
high affinity btwn hydrophilic signal molecule & cell-surface receptor makes them bind
causes signal trasnduction event w/in cell; just diffusion wouldn't work
describe process of FAST signaling
what does it cause?
extracellular signal molecule > cell-surface receptor protein > intracellular signaling pathway > altered protein function > altered cytoplasmic machinery > altered cell behavior
< sec to mins
occurs in CYTOPLASM only
describe process of SLOW signaling
what does it cause?
extracellular signal molecule > cell-surface receptor protein > nucleus > altered protein synthesis by changing transcriptional program > altered cytoplasmic machinery > altered cell behavior
SLOW: mins to hrs
occurs in both CYTOPLASM and NUCLEUS
major classes of cell surface receptors?
1) G-protein coupled receptors (GPCRs)
2) Receptor tyrosine kinases (RTKs)
3) Cytokine receptors
what transmits through GPCRs (what are the LIGANDS)?
neurotransmitters (epinephrine, serotonin, dopamine)
sensory stimuli (light, odorants)
many prescription drugs
what is the structure of the GPCR?
how does it basically work?
protein w/ 7 transmembrane domains
ligand binds the receptor extracellularly, causing conformational change in the receptor
change allows intracellular domain to interact w/ heterotrimeric G protein
what is the structure of the heterotrimeric G protein?
what is each subunit able to do?
has 3 subunits: alpha, beta, gamma
beta-gamma are always found together
alpha is able to bind GT & to hydrolyze bound GTP to GDP
alpha and gamma have post-translational fatty acid residue connected - can insert into inner surface of plasma membrane
describe activation pathway of the G protein
1) heterotrimeric G protein is inactive when bound to GDP, all 3 subunits are interacting then
2) ligand binds GPCR > receptor changes conformation to allow interaction w/ alpha subunit of G protein
3) alpha subunit releases GDP, binds GTP
4) alpha subunit w/ GTP dissociates from beta-gamma subunit
5) alpha-GTP and beta-gamma subunits now interact w/ downstream receptors
what causes alpha subunit of G protein to switch GDP for GTP when activating?
[GTP] > [GDP] in cytoplasm
so, if GDP is expelled, GTP replaces it
describe the pathway for inactivation of the G protein
1) after activating its target protein, alpha subunit hydrolyzes GTP to GDP, with aid of an RGS, regulatory of G-protein signaling
2) GDP-bound alpha subunit is inactive
3) inactive alpha subunit re-associates w/ beta-gamma subunit b/c GDP-alpha has affinity for beta-gamma subunit
what is RGS? what is its function?
membrane-bound regulatory of G protein signaling protein
increases GTP-ase activity of alpha subunit of heterotrimeric G-protein, allows it to inactivate and "reset" much faster
aka helps alpha subunit hydrolyze GTP to GDP
how many GPCRs are there, and how many different alpha/beta/gamma subunits?
1000 GPCRs, 25 alpha/beta/gamma subunits
so many different GPCRs will work thru similar pathways to the subunits
what are small or secondary messengers?
a non-protein/enzyme intermediate molecule that acts as an intermediate to propagate a signal from a receptor
i.e. adenylyl cyclase, cAMP, IP3, DAG, Ca
what are the 3 classes of G-protein's alpha subunits, what do they directly do?
Gs: activates adenylyl cyclase
GI: inactivates adenylyl cyclase
Gq: activates phospholipase C
describe the pathway of GS on its downstream targets
1) activated G-alpha-S activates adenylyl cyclase
2) adenylyl cyclase converts ATP to cAMP
3) cAMP activates targets, like PKA
4) cAMP binding to PKA activates it by dissociating its regulatory subunits
5) PKA phosphorylates targets, i.e. glycogen phosphorylase & glycogen synthase
what is a kinase? what is a protein kinase?
kinase: enzyme that phosphorylates, usually from ATP to a substrate
protein kinase: enzyme in which substrate is a protein
what are the 2 major types of protein kinases?
1) kinases that phosphorylate Ser/Thr residues
2) kinases that phosthorylate Tyr residues
what is the structure of PKA?
how is it activated?
PKA has 4 subunits: 2 regulatory, 2 catalytic
when cAMP binds to the 2 regulatory subunits, they no longer have affinity for catalytic subunits
catalytic subunits go forward & phosphorylate substrates, propagate signal
describe pathway involving adrenaline and GS
what is the result? and what kind of response is this?
1) situation of fight/flight, adrenaline binds GPCR, activates GS
2) GS binds & activates adenylyl cyclase
3) adenylyl cyclase converts ATP to cAMP
4) cAMP activates PKA
5) PKA phosphorylates & activates phosphorylase kinase
6) phosphorylase kinase phosphorylates phosphorylate glycogen by hydrolyzing ATP to ADP
7) phosphorylated phosphorylate glycogen causes glycogen breakdown
this gives your muscles energy to run!
what steps in the GS/adrenaline pathway are sites of amplification? which are not sites of amplification?
Amplification sites (>1:1 ratio of activator>activated):
adenylyl cyclase activates cAMP
PKA phosphorylates substrates
Not amplification sites (1:1 ratio of activator>activated)
GS activates adenylyl cyclase
cAMP activates PKA
what is meant by a site of amplification?
signaling such that limited amount of information input causes explosive burst of information output
what is signal branching? example?
when 1 receptor protein can activate more than 1 kind of G protein
i.e. PKA has numerous substrates
why are signal transduction pathways reversible? what are examples of this?
because if signaling goes awry, that can result in disease
1) alpha subunit + RGS cause GTP hydrolysis to GDP
2) cAMP phosphodiesterase digests cAMP to AMP, which is a biologically inert signaling molecule
3) protein phosphotases inactivate substrates that require phosphate for activation
describe the phospholipase C / Gq pathway
1) signal molecule activates its GPCR
2) Gq subunit of G protein binds & activates phospholipase C-beta
3) phospholipase C breaks down the membrane phospholipid PIP2 into IP3 and DAG
4) IP3 can activate release of Ca2+ inside the cell, which activates PKC, which causes other signaling functions
DAG directly activates PKC also
what are IP3 and DAG examples of?
from the phospholipase C pathway
what is the function of Gi?
inhibits adenylyl cyclase
describe activation and pathway of Gt
1) rhodopsin receptor activates Gt
2) Gt activates cGMP phosphodiesterase
3) cGMP phosphodiesterase breaks down cGMP