Theme 3 - intracellular signalling Flashcards
give four examples of intracellular signals
ions, proteins, gases and second messengers
name three molecules that need receptors to get through the membrane
proteins, peptides and charged molecules
name two ways ligands can cause downstream effects
by directly interacting with the cell surface receptor or binding co-receptors or accessory molecules on the cell surface
what type of proteins are most signalling molecules and where are they found?
hydrophilic proteins in the cytoplasm
where are hydrophobic signalling molecules most likely to be found?
embedded in the membrane
name three cellular processes that may be changed due to a signalling cascade activating an effector protein
altered metabolism, altered cell shape or altered gene expression
give an example of where signal amplification occurs
in the eye - rhodopsin detects low levels of light
what is distribution in signal amplification?
activation of one molecule that then goes on to activate many others
name three ways in which signalling molecules are controlled
by post translational modifications (phosphorylation), binding GTP/GDP or interaction with activators such as Ca++ and cAMP
how does a kinase phosphorylate a protein?
removes a phosphate from ATP in the cytoplasm which is then added to another protein, causing a shape change
how do phosphatases turn a signal off?
hydrolyse and take the phosphate group off the protein
does phosphorylation always activate proteins?
usually it does but it can also inactivate them
what three amino acids can a phosphate group be added on to?
tyrosine, threonine and serine
why can phosphate groups only be added onto specific amino acids?
they have a free hydroxyl group where a kinase can attach a phosphate
give three examples of serine threonine kinases
CaM kinase (calcium calmodulin dependent kinase), PKA (produces glucose), PKC (leaning and memory), MAPK (cytokine production)
what are the two subclasses of tyrosine kinases?
receptor and non receptor tyrosine kinases
give an example of a non receptor tyrosine kinase
Src
give an example of an RTK
EGFR (the receptor itself in a kinase)
what are small GTPases and where are they usually found?
GTP binding proteins that are usually found in the cytoplasm
what is an inactive small GTPase bound to?
GDP
what needs to happen for a small GTPase to bind GTP?
GDP needs to be released so it can preferentially bind GTP in the cytoplasm
how does a small GTPase switch the signal off?
proteins have their own intrinsic GTPase activity which is a motif in its protein sequence that hydrolyses GTP back to GDP
give an example of a GTPase
Ras
how long does it take for Ras to activate and inactivate?
activates in 10 seconds and turns itself off in 10 mins
what mutations can lead to signalling errors?
ras - mutation from glycine to valine at amino acid 12
what does mutations in ras cause the protein to be?
constitutively active - no longer hydrolyses GTP to GDP so can’t switch itself off
name two provisional activators of proteins
Calcium and cAMP
how does calcium regulate proteins?
binds calmodulin which induces a conformational change so it can engage with target protein Cam Kinase which then phosphorylates myosin
what type off folding predominates in GPCRs?
alpha helices
what is a G protein bound to in the inactive state
alpha subunit is bound to GDP and alpha, beta, gamma are attached
what happens when GTP binds the alpha subunit of a G protein?
the alpha subunit dissociates from the beta gamma subunit
does the alpha subunit have GTPase activity?
yes - it can switch itself off
which G proteins are involved in adenylate cyclase and cAMP signalling?
Gs and Gi
which ligand binds which receptor to activate the Gs signalling cascade?
adrenaline binds an adrenergic receptor
what is the role of adenylate cyclase?
sits in the membrane and converts ATP to cAMP when contacted by a GTP bound Gs protein
what is PKA activated by and what does it do?
activates when cAMP binds and it goes to the nucleus to phosphorylate transcription factors to drive glucose production
what receptors are involved in Gq activation?
M1 muscarinic acetylcholine receptors
what does an activated Gq bind?
PLC - causes cleavage of PIP2 to DAG and IP3
what is the effector molecule in the Gq cascade?
PKC - which is activated by DAG and calcium and cause phosphorylation of other molecules
which receptor slows down heart rate when activated?
M2 muscarinic Ach receptors
how does binding of Ach to an M2 receptor lead to decrease heart rate?
Ach binds the M2 receptor, beta gamma unit says in the membrane and binds to K+ channels causing them to ope, this causes K+ to move out the cell and hyperpolarise the cell therefore heart rate slows down and decreased FOC
Name two conditions that are implicate G protein signalling
cholera and whooping cough
what type of G protein does cholera toxin inhibit and what are the consequences of this?
inhibits GTPase activity Gs therefore it cant switch itself off - prolonged activity causes water and chloride ions to move out of intestinal cells - dehydration, diarrhoea and sometimes death
what toxin is produced by whooping cough?
bordetella pertussis - pertussis toxin
what G protein does bordetella pertussis inhibit and what are the consequences?
pertussis toxin stops Gi associating when its GPCR and also releases an active form of adenylate cyclase - double wham of not being able to switch off adenylate cyclase and introducing an extra active version - prolonged signalling which stimulates coughing
where can kinase activity occur on an enzyme coupled receptor?
built into the receptor or association with a molecule that has kinase activity
how many TM domains do enzyme coupled receptors usually have?
one - type I transmembrane receptor
what happens to an enzyme coupled receptor when a ligand binds?
dimerisation
Initial signalling cascade of an RTK
ligand binds and two receptors are brought together to form a dimer, then they can cross phos/phos themselves - this activates the receptorand phos sites allow proteins to dock and have further downstream effects
what type of enzyme activity do RTKs have?
intrinsic (built in)
