Week 3 Na+ and K+ channels Flashcards
what is the importance of Na+ channels
major excitory current in cells
rising phase of AP
generates pacemaker currents
what is the structure of a VGSC
single protein termed alpha subunit
9 different in humans
also has auiliary beta subunits whcih are imporat for locataion and fuction
what family of proteins do VGSC belong to?
6-transmembrane helices
how many domains are there in VGSC?
4 transmembrane domains with 6 transmembrane helies each
how do VGSC sense voltage?
S4 of each domain is a voltage sensor - rich in positively charged amino acids
when cell depolarises positve charge at surface increases
elcteostactic repultion causes domain shift upwards towards extracellular
shift pulls intracellular linkers causing conformational change and opening of pore
How do VGSC fast inactivate?
Fast and slow present in most
intracellular loop between domains 3 and 4 is the inactivation gate - contians a IFM sequence isoleucine-phenalalainine methionine
the binding site for this is on the inner face of the pore which is occulded when pore is closed
when pore open this is exposed
gate moves to binding site closing the pore
How is slow inactivation mediated in VGSC?
not well understood
why are VGSC ion selective?
depends on speicif amino acids on the P loop - exists between S5 and S6
and form the start of the conducting pore
what is the Alpha Alpha motif responsible for Na+ selectivity?
the DEKA
D = apartic acid
E = glutamic acid
K = lysine
A = alanine
How many Beta subunits are there
5
beta1, beta1B, Beta2, beta3, beta4
what is the structure of beta subunits, what is the exception?
all single transmembrane spenning segment with intercellula c and ectracellular N terminal - apart forn beta1B
the extracellular domain contains immunoglobumilin loop
Beta1B is soluable as no transmembrane doamin
What roles do beta subunits have in modulating alpha subunits?
localisation
biophysics - peak current voltage dependence and inactivation
will modulate some voltage gated K+ channels
act as cell adhesion molecules - imoortant during pathfinding and axonal fasciculation
Why can beta subunits act as cell adhesion molecules?
struculalry similar to cell adhestion molecuels
have IG domains that bind extracellular proteins
independednt of A units
but the role is to maintain alpha subits at the cell surface
how are beta subunits involved in pathfinding
cell adhestion molceules interact with neuroens duering axon outgrowth and pathfinding
Beta subunits are expressed on tip of extending nurites and interact with extracellular proteins
KO prevents axonalk extention eg defasciculate and migrate in random direction
how are Beta1B subunits involved in cell adheastion
not membrane bound - soluble and secreated
assosiatie with alpha subunts to modulate properties
also evidence for soluble cell adheastion molecule
What are VGCC involved in?
transmitter relases, hormone secreation
excitation contraction coupling
excitation transciption couplujng - sysnaptic assembly
What is the structre of VGCC
single channel forming subinut Alpha1 sububit
are membres of 6 transmembrane helixes family
contsind
where is the voltage sensor in VGCC
S4 of each domain
How can VGCC sense voltage
S4 has rich positively charged amino acids
depolaration of the cell leads to incerase in postative charges at the membrane
when a threshold is reached positively charged domias is forces upwards
this pulls on intracelllular linkers connceted to S4 resulting in a conformational change in the perotin leadinto central pore opening
what can inactivate VGCC?
calcium
voltage - fast and slow
how to test for voltage inactivation ?
use barium as the charge carrier , this elimates Ca2+ dependent inactivation
what is the mechism for fast VGCC inactivation
uniline VGSC not to do with loop between domains 3 and 4
S6 domains seem to be important - comprises part of the pore forming region
inadition intracellular loops bwtween 1 and 2 domains seems to be imporant - as these components are distrubuted - pore collapse?
how is CDI achieved in VGCC
ca2+ calmodulin binding
c terminal of alpha1and CaM
intracellular loop betweem domians 1 and 2
when channel is closed CaM binds to peptide A regoin of C terminal of alpha subunit
when channel oppen ca2+ binds to CaM and the EF motif prevents binding of 1-2 domain intracellular loop form binding to the channel pore
as ca2+ increases CaM conformational change and cuases EF domain to move and allows for the binding of 1-2 loop to trhe pore
what is the amino acid motif for selctivity filter of VGCC
EEEE - all glutamic acid
what is glutamate?
an amino acid
what is gultamates mojor role in th CNS?
major excitatory neurotransmitter
70% of synapses are glutamergic
what types of glutamate receptors do we have?
4
ionotropic - AMPA Kainate and NMDA
Metabotropic
mGluR1-7
What does Mg+ do to NMDAR
blocks current flow at negative potentials
how can we block current flow theough AMPARs?
inactivation
unbinding
