Signal Transduction Flashcards
What are the two main types of kinases?
Tyrosine Kinase and serine/threonine kinase
What are the two main types of GTP-binding proteins?
Trimeric G proteins and Monomeric GTPase
How do G-protein-coupled receptors (GPCRs) work?
Ligand binds to receptor which activates a G-protein which in turn activates or inhibits another protein. Often this protein is an enzyme that generates a specific second messenger
Describe the structure of a G-protein
They have 7 membrane spanning regions with their amino termini on the extracellular face and their carboxy termini on the cytoplasmic face.
Why is signal transduction needed?
To mediate cell activity
Most cell-surface receptors are coupled to what kind of G-protein and what does this mean?
To a trimeric G-protein, meaning it is compsed of three different subunits (alpha, beta and gamma)
Describe the mechanism of GPCRs
The binding of the ligand causes a conformational change to the receptor causing it to bind to the G,alpha protein in a way that displaces GDP and GTP binds. This triggers G,beta,gamma dissociation activating downstream pathways. However this activation is short lived as GTP hydrolyses to GDP in seconds leading to the re-association of G,alpha with G,beta,gamma and the inactivation of adenylate cyclase
The overall system of GPCRs require what?
A receptor, a transducer (G-protein) and an amplified (adenylate cyclase) which generates lots of second messenger
Name three G,alpha subunits (different G proteins) and what they do
G(alpha)q or Gq - stimulates phospholipase C
Gs - Stimulates adenylate cyclase, increasing cAMP
Gi - Inhibits adenylate cyclase and therefore decreases cAMP
What is the role of Phospholiapse C
To cleave PIP(2) into IP(3) and DAG. The IP(3) can open calcium channels in the ER releasing calcium. This causes PKC to translocate to the membrane where it is activated by DAG.
How is PLC activated?
By Gq protein (G,alpha,q)
What is calcium involved in?
Exocytosis, contraction, metabolism, gene transcription, fertilisation (so lots of cellular events)
What causes the initial spark of fertilisation? and then what occurs after this?
PLC-zeta by triggering the opening of surface calcium channels. The wave of calcium then triggers the start of embryonic development and prevents other sperm from entering the cell.
Where are the substrates for PKC?
Some in the cytoplasm but some isoforms can translocate to the nucleus and phosphorylate nuclear proteins. It can also act indirectly to alter gene transcription
What is the receptor for adrenaline?
Beta (2) adrenergic receptor
What happens when adrenaline binds to its receptor?
Receptor - Beta2 adrenergic receptor.
It causes the increase of intracellular concentrations of cAMP (Cyclic AMP) as the receptor couples to G(alpha)s.
What is cAMP synthesised by and from?
From ATP by Adenylate cyclase
What is cAMP degraded by?
cAMP phosphodiesterase
How does cAMP activate the cAMP-dependant protein kinase (PKA)
cAMP binds to regulatory subunits of the PKA which causes the release of the catalytic subunits
What does signal transduction result in?
The amplification of a signal meaning a small change at the top of the chain can have a large change at the bottom.
What can PKA phosphorylate?
Substrates on serine or threonine residues
What does adrenaline cuase?
The release of glucose and fatty acids from liver/fat and the increased contraction of cardiac muscle
What can PKA do?
Phosphorylate a protein called CREB which can then bind to cAMP response element regions (CRE’s) and therefore switching on gene transcription
Describe what cholera toxins do
It becomes cleaved and active and then enters the intestinal epithelium cells to activate Galpha(s). The overstimulation of cAMP production results in release of water and ions into the lumen of the small intestine leading to rapid fluid loss and dehydration.
How does pertussis toxins work?
It inhibits Galpha(i) to increase cAMP production in lung epithelium.
How do are receptor tyrosine kinases (RTKs) activated?
A signalling molecule in a form a dimer binds to the receptors causing two receptors to come together. They then phosphorylate each other. Intracellular proteins can then bind to these phosphates and mediate signal transduction
What is an example of a small GTPase protein and how is it activated?
Ras and it is activated by receptor tyrosine kinase (RTK)
Describe how Ras is activated?
Adaptor protein attached to RTK interacts with the Ras-activating protein. This protein then causes the GDP on the inactive Ras protein to switch to GTP, activating it.
What small GTPases are direct targets for clostridial cytotoxins?
Rac/Rho subfamily
Mutations in what small GTPase is found in aprox. 20% of cancers?
Ras
What does Ras activate?
The MAPK (mitogen-activated protein kinase) pathway which results in changes in protein activity or changes in gene expression
What is the EGFR (epidermal growth factor receptor) activated by and what odes it do?
It is a RTK which is activated by TGFἁ (transforming growth factor alpha), it then goes onto activate Ras via Grb2/SOS.
Mutations in EGFR, Ras and Raf are associated with what?
Tumorigenesis
