cell signalling Flashcards
what are the four types of cell signalling?
direct cell-cell signalling
paracrine signalling
endocrine signalling
autocrine signalling
where are the two places cell receptors can be located?
intracellularly
on cell surface
what is glucocorticoid?
steroid hormone produced by the adrenal glands in response to stress
explain glucocorticoid receptor binding and activation.
ligand binding causes dissociation from Hsp 90
the ligand is then translocated to the nucleus where it binds to glucocorticoid response elements on target genes
where is the glucocorticoid receptor located?
in the cytosol in inactive form, bound to Hsp 90
where is the thyroid hormone receptor located?
always bound to thyroid response elements found on DNA
what is the thyroid hormone receptor bound to in the absence of the ligand?
HDAC
a corepressor
what is the thyroid hormone receptor bound to in the presence of the ligand?
histone acetyletransferase
a coactivator
activates target genes
what type of molecules do intracellular receptors respond to?
small hydrophobic molecules that can diffuse across the plasma membrane
what type of molecules do cell surface receptors respond to?
hydrophilic molecules
what are the main types of cell surface receptors?
ion channel-linked receptors
G protein-linked receptors
enzyme-linked receptors
what can enzyme-linked receptors activate?
a catalytic domain or an enzyme
what are the different classes of GPCR’s?
classes A-F
unclassified
what are physiological roles of G protein receptor signalling?
visual sense (rhodopsin)
gustatory sense (gustducin)
sense of smell
regulation of immune system and inflammation
both sympathetic and parasympathetic systems are regulated by GPCR’s
what is the structure of GPCR’s?
7 membrane-spanning alpha helices
3 subunits designated alpha, beta and gamma
how are GPCR’s structurally different to enzyme-linked receptors?
enzyme-linked usually only have one transmembrane alpha helices
GPCR’s have seven
explain the activation of adenylyl cyclase.
hormone binds to receptor and activates G protein
GDP is converted to GTP
G protein binds to adenylyl cyclase
this synthesises cAMP through the use of ATP
how are G proteins regulated?
G protein is activated through conversion of GDP to GTP
G protein translocated to target once activated
they quickly become inactive again
regulators of G protein signalling hydrolyse GTP forming GDP and causing the G protein to enter its inactive state again
what is PKA?
protein kinase A
aka cAMP-dependant protein kinase
how many regulatory and catalytic subunits does inactive PKA have?
2 regulatory and 2 enzymatic
where does cAMP bind to on PKA and what happens as a result?
binds to regulatory subunits which dissociate from catalytic subunits
free catalytic subunits then phosphorylate serine on target proteins
which two enzymes, involved in glycogen metabolism regulation by adrenaline, does PKA phosphorylate?
phosphorylase kinase (activates it) glycogen synthase (inactivates it)
explain the regulation of glycogen metabolism by adrenaline.
adrenaline binds to receptor which activates G proteins through the conversion of GDP to GTP
this then activates adenylyl cyclase which drives the synthesis of cAMP which binds to PKA which activates the catalytic units which phosphorylate different enzymes
what is the function of phosphorylase kinase?
activates glycogen phosphorylase which catalyses glycogen breakdown
what is the function of glycogen synthase?
when activated it drives glycogen synthesis
what is the function of a rod photoreceptor cell?
turns light signal into electrical signal
what are examples of other second messengers?
cGMP
phospholipids and Ca2+
explain a rod photoreceptor cell becomes hyperpolarised.
light strikes rhodopsin and the 11-cis-retinal is isomerised to 11-trans-retinal
this causes conformational changes in transducin ( a G protein) activating it
phosphodiesterase is then activated and hydrolyses cGMP into GMP
cGMP levels fall closing cationic channels causing the cell to become hyperpolarised
what is the largest family of enzyme-coupled receptors
tyrosine-kinase receptors
how many receptor tyrosine kinases does the human genome encode?
58
what are key features of receptor tyrosine kinases?
N-terminal extracellular ligand binding domain usually have cysteine rich domains
single transmembrane alpha helix
C-terminal tyrosine kinase domain in cytosol
what is dimerisation and auto-phosphorylation in relation to tyrosine-kinase receptors?
ligand binding leads to dimer formation and activation of the cytosolic kinase domains
this results in receptor autophosphorylation which is when two polypeptide chains on each receptor cross-phosphorylate each other
what are the two roles of autophosphorylation?
to ensure the phosphorylation of the catalytic domain
to ensure the signal gets transmitted
how is phospholipase C activated?
phospholipase C-gamma binds to receptor tyrosine kinases via the SH2 domain
tyrosine phosphorylation increases PLC-gamma activity, stimulating the hydrolysis of PIP2
the second messengers diacylglycerol (DAG) and inositol 1,4,5 triphosphate (IP3) stimulate downstream pathways
ultimately allows influx of calcium 2+ ions
what is phospholipase C?
an enzyme involved in glycogen breakdown and muscle contraction
what are downstream effects of increased calcium 2+ ions?
calmodulin is activated when two or more ions bind
it is a major calcium binding protein
this then binds to target proteins including protein kinases
what is the function of CaM kinases?
they phosphorylate metabolic enzymes, ion channels and transcription factors
one form regulates synthesis and release of neurotransmitters
what are CaM kinases?
protein kinases that are activated by calcium ions/calmodulin
explain insulin mediated glucose transport
insulin binds to the alpha subunit of the insulin receptor dimer
causes conformational changes in the receptor leading to auto phosphorylation
insulin receptor substrate 1 forms a complex with Pi3 kinase at the SH2 domain
Akt is then phosphorylated sending the glucose transporter Glut 4 to the plasma membrane which allows glucose transport into the cell
which type of receptor is the insulin receptor?
tyrosine kinase receptor
what does MAP kinase stand for?
mitogen activated protein kinase
how are Ras proteins regulated?
activated when guanine nucleotide exchange factors stimulate exchange of GDP to GTP
inactivated when GTP is hydrolysed back into GDP
explain the MAP kinase activation pathway
tyrosine kinase receptors are activated by ligand binding
SH2 domain and guanine exchange factor are phosphorylated
GEF activates Raf protein through conversion of GDP to GTP
Raf then activates MEK through the conversion of ATP to ADP
MEK then phosphorylates extracellular-regulated kinase (ERK) through conversion of ATP to ADP
which then goes on to activate downstream targets such as transcription factors
what are examples of receptors that are coupled to transcription factors?
transforming growth factor beta
NF-kB
Wnt
TGF-beta receptor structure
dimers of type I and type II polypeptides that associate following ligand binding
type II phosphorylates type I which then phosphorylates smad proteins
how many different members of TGF-beta family are found in humans?
30
what are NF-kB family?
transcription factors
responsible for both innate and adaptive immune responses
what do TGF-beta receptors play a role in?
inflammation
cancer
autoimmune diseases
where are NF-kB proteins found in unstimulated cells?
bound to inhibitory IkB proteins and are inactive
what are the effects of TNF receptor activation?
activates IkB kinase
this phosphorylates the IkB protein bound to NF-kB, labelling it with ubiquitin which tells the cell IkB needs to be degraded
this frees NF-kB which translocates to the nucleus and induces the expression of target genes
activation is very short lived
what is the NF-kB negative feedback loop?
NF-kB is activated by the phosphorylation and the degradation of IkB
but a gene that is activated by NF-kB encodes for IkB
this generates a negative feedback loop
