Module 2 Lecture 3 Flashcards
what does the slope of the IV curve represent
conductance
where do beta subunits dock on rat voltage-gated potassium channel (Kv 1.2)
T1 subunits, formed by alpha subunits
what is the alpha subunit responsible for in rat voltage-gated potassium channel (Kv 1.2)
channel activation, selectivity, TEA binding
what are the 6 transmembrane sequences formed by on Kv 1.2
alpha helical subunits
Kv 1.2 structure
4 alpha subunits, each with 6 transmembrane sequences
S4 function in Kv 1.2
voltage sensor
S4 structure in Kv 1.2
evenly spaced positively charged residues (every 3rd residue)
S5 and S6 function in Kv 1.2
form the pore
what is Kv 1.2
rat voltage-gated potassium channel
where is the T1-tetramerization domain in Kv 1.2
NH2 tail
which side does TEA work from
the intracellular side
which mutations of channel types have the least effect on TEA concentration required to block the channel
V437T, T439S
which mutations of channel types have the most effect on TEA concentrations required to block the channel
M440I, and T441S
main characteristic of the Shaker channel
it is a voltage-gated potassium that inactivates
- found in Drosophila
what did Hartmann do to test Kv channel function
took a portion of Kv 3.1 aa sequence and stuck it in Kv 2.1 aa sequence –> produced chimera that looks mostly like Kv 2.1, but has the sequence of the SS1 and SS2 domains of the linker
results of Hartmann’s experiment
the chimera caused a transfer of conductance properties; the chimera conducted ions like Kv 3.1
what did Hartmann conclude from his experiment
the conducting properties of the channel are due to the sequence that was tranferred to the chimera
how is the Kv channel highly selective for the smaller K+ ion?
the surface of the channel is lined with carbonyl oxygen atoms - creating 4 binding sites and hydration cages for K+ ions
how do hydrated K+ ions interact with the Kv channel
enter pore and exchange water cage for a carbonyl cage, enter channel
why can’t Na+ enter the Kv channel
Na+ too large with hydration cage/ too small without
- very happy when hydrated, not happy when partially bound by carbonyl
- significant energy cost to go from hydrated –> carbonyl bound
how is energy spent when K+ enters Kv channel
energy lost from losing hydration cage is gained back by getting hydrated by backbone
characteristics of A-C bacterial K+ channel selectivity filters
only selects nonhydrated K+ ions (Na+ too small and can’t be stabilized)
- 4 subunits, 2 transmembrane domains each
characteristics of D-F human Nav 1.7 selectivity filter
only selects partially hydrated Na+ ions
- 4 repeated motifs of 6 transmembrane regions (24 TM)
- voltage sensors
how are positive charges selected for in the channel
negative charges inside pore
what does the presence of multiple ions in the pore cause
pushes each other through and speeds up diffusion
how does S4 move
positive charges build up in the cell and push S4 up and out
why are gating currents hard to measure
whenever there is a gating current, there’s often a much bigger ionic current
what experiment did the Benzanilla lab do
- gating currents and ionic currents were recorded from squid giant axon
- compared calculated conductance of gating current and ionic current at different voltages
what does gating current signify
charge movement
what does ionic current signify
channel opening
what was the conclusion of the Bezanilla lab experiment
the two curves are different, which suggests multiple closed conformational states
what do we know about potassium flow if the second voltage step is Ek
no K flow; if there’s any current left in the channel, it must be due to another ion
why is it hard to measure conductance during a depolarization
during a depolarization, the driving force changes and the number of open channels changes, so it’s hard to estimate the conductance
- tail currents help
how do you increase K+ tail currents
remove any Na+ current
relationship between Na tail current and ionic current
usually in the same direction
why do Na tail currents need shorter times of voltage pulses?
inactivation of the channel
when does gate 4 open
only when gate 3 is also open
when does inactivating gate close
after 4th gate opens
when is Na current inward and strongest
right after depolarization phase
- continuously gets weaker until it’s almost not flowing on average
main characteristic of Kv channel current
non-inactivating outward K+ current that lasts the depolarization phase
main characteristic of Nav channel current
inactivating inward Na+ current
what subunits does the voltage-gated sodium channel (NavAb) have
alpha and ancillary beta subunits
what is the alpha subunit responsible for in the NavAb channel
channel activation, selectivity, TTX binding, pore, gatingwhat
what is the alpha subunit composed of in the NavAb channel
four repeat domains (I - IV) covalently linked together
- not tetrameric protein; it’s all 1 protein
how many transmembrane sequences does each domain have in the NavAb channel
6
- S4 = voltage sensory
- S5 and S6 = pore, along with S5/S6 linker
- III/IV domain linker is especially important (IFM/IFMT)
importance of glutamate residue in Nav conductance pore
found in the pore loop of the 1st domain; controls sensitivity to TTX
- mutating glutamate –> glutamine = abolishes TTX sensitivity
two major types of inactivation
fast = N-type
slow = C-type
how was inactivation affected by holding the membrane at more negative voltages
less inactivation
how was inactivation affected by holding the membrane at more positive voltages
ton of inactivation
what did Hodgkin and Huxley look at to study inactivation
- studied inactivation of various holding voltages on inactivation
- studied impact of the length of time held at various holding voltages on inactivation
results of Hodgkin and Huxley’s experiment on inactivation
inactivation was the strongest after a long period of time at a positive potential
- least inactivation after a shorter time at a negative potential
what did H&H propose to explain the dynamics of the K+ and Na+ channels
- K current can be conceived as ocurring after 4 independent voltage sensors move (n)
- Na current can be conceived as occurring after 3 independent voltage sensors moving (m); but can be inactivated by an independent voltage sensor (h)
what happens if you add pronase intracellularly
abolished inactivation
what are the different conformations for a channel
active, inactivated, or closed
what is fast inactivation of Na+ channels associated with
the intracellular linker between domains III and IV acting as an inactivation particle
proteolysis function
removes N-type inactivation
what does shortening the N-terminus do
gets rid of Shaker N-type inactivation
what is revealed after removing N-type inactivation
slower C-type
what causes C-type inactivation
conformational changes at the extracellular pore
what promotes “closing” the C-type inactivation gate
local increases (followed by decreases) in extracellular K+ near the pore
