Signal transduction Flashcards
what are types of plasma membrane receptors?
- some activate G-protein switch proteins that trigger downstream pathways or generation of intracellular second messengers
- tyrosine kinases that trigger signal transduction pathways involving sequential activation of downstream kinases
Der and the glucocorticoid receptor pathway?
- Dex activates the glucocorticoid receptor
- without Dex the receptor is in the cytoplasm
- with Dex the receptor-hormone complex moves to the nucleus
- the control b-galactosidase is cytoplasmic regardless of Dex
what is the glucocorticoid receptor pathway
- hormone diffuses into the cell and binds to ligand binding domain
- hormone binding displaces chaperon proteins allowing the LBD-hormone complex to move to the nucleus and DNA binding domains can associate with response elements and transcription can happen
how do hydrophilic signalling molecules act?
- binding of the signal molecule to a specific cell surface receptor leads to a conformational change
- activated receptor initiates downstream signal transduction proteins/secondary messengers
- signal transduction leads to the activation of effector proteins, which usually turns off the active receptor
what are the 4 types of extracellular signalling
- endocrine: affects target cells some distance from the site of synthesis, hormone travels through blood
- paracrine: affects adjacent target cells
- autocrine: cells respond to signals that they produce
- signalling by plasma membrane-attached proteins
How does signal transduction by protein phosphorylation work
- active target proteins are dephosphorylated by protein phosphatases to make them inactive
- inactive target proteins are phosphorylated by a proteins kinase to make them active
what are characteristics of secondary messenger molecules
- small short-lived molecules
- diffuse rapidly
- allow enzymatic amplification
what is the function of G-proteins and GPCRs
- hydrophilic hormones that function as on-off switches for intracellular signalling by binding to membrane receptors to form GPCRs
- G-proteins exchange GDP to GTP leading to a conformational change and activation
- active G-proteins send a signal
- once the signal is sent GTP is converted back to GDP and the G-protein inactivates itself
what are the activator and inactivator proteins in G-proteins
GEF = activator protein: exchangers GDP for GTP, G-protein is now capable of interacting with downstream effector proteins - produces secondary messenger molecules
GAP = inactivator protein: exchanges GTP for GDP, G-protein is capable of interacting with upstream activators
What is the signal transduction pathway for epinephrine
- epinephrine hormone binds to the membrane receptor
- G-protein associates with receptor and exchanges GDP for GTP
- causes G-alpha subunit to dissociate and bind to adenylyl cyclase
- adenylyl cyclase can now produce secondary messenger molecules (cAMP) which go on to activate protein kinase A
- protein kinase A is now able to phosphorylate target proteins
subunits of G-proteins
3 subunits: Ga, Gb, Gy
- Ga and Gy are covalently bound to the membrane
- Gy and Gb are tightly associated
- ligand binding alters receptor confirmation allowing binding of Ga subunit causing GDP to be replaced with GTP
- Ga subunit disassociates and moves to an inactive effector to make it active
mechanisms of GPCR signaling
- hormone binds to induce conformational change of receptor
- activated receptor binds to Ga subunit
- activated receptor causes conformational change in Ga and triggers dissociation of GDP
- binding of GTP to Ga triggers dissociation of Ga both from the receptor and Gby
- hormone dissociates from receptor and Ga binds effector to activate it
- hydrolysis of GTP to GDP causes Ga to dissociate from effector and reassociate with Gby
what is the GEF and GAP In the GPCR pathway
GEF = receptor - exchanges GDP for GTP
GAP = effector - increases the rate of GTP hydrolysis and deactivation of the G-protein
what are receptor tyrosine kinases - IMPORTANT
- when activated they phosphorylate downstream targets and initiate a signal transduction cascade
- usually, the hormone or peptide leads to dimerization of 2 kinase polypeptides
- dimerization of RTKs leads to cytoplasmic domain autophospholylation
how are RTKs activated
- ligand binds to the ligand binding region
- causes dimerization and phosphorylation of activation loop tyrosines on cytosolic side
- active protein tyrosine kinase phosphorylates additional tyrosine residues
RTK example: EGF receptor
- EGF binds to its receptor
- dimerization alters the conformation of the intracellular domain leading to kinase activation
- reciprocal phosphorylation of tyrosine residues on the donor and acceptor domains of the activated receptor
what is the Ras/MAP kinase pathway
- activated by RTKs
- active Ras stimulates the formation of a signal transduction complex which contains 3 sequentially acting protein kinases: Raf, MEK and ERK
- culminates in phosphorylation and activation of transcription factors controlling gene expression
Ras/MAP kinase pathway in 6 steps - IMPORTANT
- hormone binds to LBD causing receptor dimerization and autophosphorylation of the RTK
- binding of adaptor proteins (GRB2 and Sos) bring inactive Ras. the SH3 domains of GRB2 bind to Sos and the SH2 domain interacts with phosphorylated tyrosine residues
- Sos acts as the GEF and promotes exchange of GDP for GTP to activate Ras, active Ras then dissociates from Sos
- active Ras recruits Raf and activates first kinase (Raf aka MAPKKK). 14-3-3 protein is not longer inhibiting Raf
- Raf (MAPKKK) phosphorylates MEK (MAPKK). Ras hydrolyzes its GTP in response and becomes inactive
- Phosphorylated MEK phosphorylates ERK (MAPK). ERK can move to the nucleus to activate various TFs via phosphorylation
what are 2 ways gene expression is induced at the end of the MAP kinase pathway
- ERK (MAPK) translocates to the nucleus once activated and is able to transcribe the SRE gene
- ERK (MAPK) can activate p90RTK via phosphorylation which can translocate to the nucleus and phosphorylate other TFs