Week 3 Na+ and K+ channels

Question 1

what is the importance of Na+ channels

Answer 1

major excitory current in cells
rising phase of AP
generates pacemaker currents

Question 2

what is the structure of a VGSC

Answer 2

single protein termed alpha subunit
9 different in humans
also has auiliary beta subunits whcih are imporat for locataion and fuction

Question 3

what family of proteins do VGSC belong to?

Answer 3

6-transmembrane helices

Question 4

how many domains are there in VGSC?

Answer 4

4 transmembrane domains with 6 transmembrane helies each

Question 5

how do VGSC sense voltage?

Answer 5

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

Question 6

How do VGSC fast inactivate?

Answer 6

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

Question 7

How is slow inactivation mediated in VGSC?

Answer 7

not well understood

Question 8

why are VGSC ion selective?

Answer 8

depends on speicif amino acids on the P loop - exists between S5 and S6
and form the start of the conducting pore

Question 9

what is the Alpha Alpha motif responsible for Na+ selectivity?

Answer 9

the DEKA
D = apartic acid
E = glutamic acid
K = lysine
A = alanine

Question 10

How many Beta subunits are there

Answer 10

5
beta1, beta1B, Beta2, beta3, beta4

Question 11

what is the structure of beta subunits, what is the exception?

Answer 11

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

Question 12

What roles do beta subunits have in modulating alpha subunits?

Answer 12

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

Question 13

Why can beta subunits act as cell adhesion molecules?

Answer 13

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

Question 14

how are beta subunits involved in pathfinding

Answer 14

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

Question 15

how are Beta1B subunits involved in cell adheastion

Answer 15

not membrane bound - soluble and secreated
assosiatie with alpha subunts to modulate properties
also evidence for soluble cell adheastion molecule

Question 16

What are VGCC involved in?

Answer 16

transmitter relases, hormone secreation
excitation contraction coupling
excitation transciption couplujng - sysnaptic assembly

Question 17

What is the structre of VGCC

Answer 17

single channel forming subinut Alpha1 sububit
are membres of 6 transmembrane helixes family
contsind

Question 18

where is the voltage sensor in VGCC

Answer 18

S4 of each domain

Question 19

How can VGCC sense voltage

Answer 19

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

Question 20

what can inactivate VGCC?

Answer 20

calcium
voltage - fast and slow

Question 21

how to test for voltage inactivation ?

Answer 21

use barium as the charge carrier , this elimates Ca2+ dependent inactivation

Question 22

what is the mechism for fast VGCC inactivation

Answer 22

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?

Question 23

how is CDI achieved in VGCC

Answer 23

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

Question 24

what is the amino acid motif for selctivity filter of VGCC

Answer 24

EEEE - all glutamic acid

Question 25

what is glutamate?

Answer 25

an amino acid

Question 26

what is gultamates mojor role in th CNS?

Answer 26

major excitatory neurotransmitter
70% of synapses are glutamergic

Question 27

what types of glutamate receptors do we have?

Answer 27

4
ionotropic - AMPA Kainate and NMDA
Metabotropic
mGluR1-7

Question 28

What does Mg+ do to NMDAR

Answer 28

blocks current flow at negative potentials

Question 29

how can we block current flow theough AMPARs?

Answer 29

inactivation
unbinding