Lecture 5- Regulation of protein activity (ligand-gated ion channels)

Question 1

Cytoplasmic effects

Answer 1

Cytoplasmic effects have to do with activating protein kinases which can change the activity of proteins that are already in the cytoplasm
Protein kinase is an enzyme that adds a phosphate on to proteins
Very important as they effectively regulate everything in the cell and because they are so important you cannot just have them on all the time, they need to be able to be turned off, they get turned on and off by other molecules, one if probably cyclic AMP and other other one could represent calcium

Question 2

Where does cyclic AMP and calcium come from?

Answer 2

Receptor gets activated and then can go on to form these molecules

Question 3

Ligand gated ion channels also called

Answer 3

ionotropic receptors

Question 4

Ligand gated ion channels

Answer 4

A multisubunit protein that crosses through the plasma membrane
A channel may be closed or open
When open ions move into, or out of, the cell depending on their electrochemical gradient
The type of ion(s) moving though the channel is determined by the channel’s structure

Channels may be opened by different stimuli
Changes in voltage across the membrane
Deformation of the membrane
Ligands at the cell surface
Note = some channels are always open
A further class of ion channel are opened in response to internal signals
Often ‘downstream’ of GPCRs
Include GIRK (G-protein gated inwardly-rectifying potassium channels) which allow K+ to move out of a cell

Ligand gated ion channels provide rapid and normally short lasting signals to cells
Particularly well suited to signalling in the nervous system
Channels may be modulated by additional factors e.g. ethanol acts on the GABA receptor which is why it has the effect that it does

Question 5

Nicotinic acetylcholine receptor subunits and transmembrane domains

Answer 5

Channels are composed of 4-5 subunits, the number depending on the channel type, channel is in the middle
Each subunit makes 4 passes across the membrane called transmembrane domains
Transmembrane domains - each one is made up of protein that goes through the membrane backwards and forwards (makes 4 passes through the membrane)

Question 6

Nicotinic receptor located at the

Answer 6

NMJ

Question 7

Nicotinic receptor

Answer 7

One of the best studied ligand-gated ion channels is the nicotinic receptor at the neuromuscular junction
Made up of 5 subunits
Influx of Na+ causes the muscle to depolarise
Opens voltage-gated channels to release Ca2+ from internal stores which leads to muscle contraction
Nicotinic receptors also present elsewhere including the brain
These have different subunit structures and thus different properties
Vary with brain region and development
Like most cellular proteins the nAChR functions as part of a complex with other proteins not like the first picture - more like the colourful one
Complex involved in correctly maintaining the positioning of the receptor for most effective functioning
rapsyn concentrates the nAChR at the correct location opposite the synapse
Lost of dystrophin results in Duchenne muscular dystrophy, an X-linked muscle wasting disorder
Similar protein complexes support the functioning of most receptor-types including ion channels, GPCR and cytokine receptors

Question 8

To describe how Ca2+ influx through a ligand-gated ion channel can regulate protein activity and protein expression in a target cell

Answer 8

The relationship between ion channel receptor and protein kinases (cartoon above). Examine synaptic transmission as an example
Neurons communicate across a synapse
But the strength of that communication can be modified
Increased strength of communication may provide a mechanism for memory formation
Called long term potentiation (LTP).
Involves two types of glutamate receptor
AMPA receptors are ion channels that allow Na+ into the post synaptic target cell and depolarise it
NMDA receptors are ion channels that allow Ca2+ into the post synaptic target cell, depolarises it and other things
Ca2+ concentrations inside a cell are very low
Therefore a rise in Ca2+ provides a dramatic signal to a cell
AN increase in the intracellular Ca2+ concentration can stimulate multiple cellular events, many of which are mediated by Ca2+ - dependent protein kinases
Including Ca2+/calmodulin-dependent protein kinase II (CaMKII)

Activated CaMKII can phosphorylate multiple protein targets
Can increase Ca2+ influx through Ca2+ channels (opens more of the channels)
Can cause AMPA receptors to keep them open longer
Can create mechanism to put more AMPA receptors in the membrane
Can activate transcription factors to turn genes on (or off) - through phosphorylation
ALl of these changes can strengthen communication across this synapse - increases the ability of the postsynaptic side of the nerve terminal to detect the signal more quickly or strongly
And thus change neuron activity
Mechanism to form memorise?