propagation of the action potential-Wagner

Question 1

Mutations Within the _-Subunit of the Skeletal Muscle, Voltage-Gated Na+ Channel

Answer 1

(SCN4A on chromosome 17)
~20 mutations can cause three distinct yet related disorders:
hyperkalemic periodic paralysis
potassium-aggravated myotonia
paramyotonia congenita
Mutations primarily localized to three regions of the α-subunit:
the cytoplasmic side of the S5 & S6 segments (i.e., the inner mouth of the ion pore)
the inactivation loop the S4 segment

lose subunit=lose function

Question 2

Hyperkalemic periodic paralysis

Answer 2

caused by 4 mutations known to confer a susceptibility to exercise-, stress- or diet-induced hyperkalemia
symptoms include muscular weakness and paralysis , which may be preceded by myotonia (increased muscle tension) or fasciculations (rippling contractions)

due largely to mutations at residues 704, 1360 and 1592 that compromise the interaction between the S5/S6 segments (on cytoplasmic side) and the inactivation loop and latch.
results in incomplete inactivation → sustained Na+ current and muscle cell depolarization → widespread Na+ inactivation → paralysis

alpha subunit less interaction with inactivating group–> residual Na+ flowing through these channels. –> incomplete inactivation

raises Vm so more difficult to depolarize (can’t get to closed state)

Question 3

Potassium-aggravated myotonia

Answer 3

caused by 6 mutations known to elicit muscular hyperexcitability in response to mild elevations in extracellular potassium levels

due primarily to three known mutations at residue 1306 lysine is substituted for one of these (rank order of severity: glutamate > valine > alanine).

mutated channels show a marked slowing in their inactivation kinetics → persistent fractional inward Na+ current → near-threshold muscle cell depolarization –> incomplete inactivation

long lasting firing

duration of inward Na+ is longer==> open longer because slowed inactivation

fractional current not as bad as hyperkalemic periodic paralysis, so we don’t see the period of paralysis.

Question 4

Paramyotonia congenita

Answer 4

caused by 9 mutations known to evoke muscular hyperexcitability induced by COLD and worsened by exercise.
symptoms include muscular rigidity and stiffness that, in some instances, may be followed by paralysis
can look like hyperkalemic periodic paralysis. difference is in the exposure to cold.

due principally to mutations within the S4 segment that interfere with transition from activation to inactivation–> slow inactivation kinetics and interfere with the cycling between activation and inactivation.

exception: T1313M causes latent paralysis following cold-induced myotonia
results in a small Na+ current and muscle cell depolarization toward threshold

results in a small Na+ current and muscle cell depolarization toward threshold

fractional inward current not as bad as Hyperkalemic periodic paralysis

Question 5

voltage gated Na+ channel characteristics (make up and residues that allow activation and inactivation)

Answer 5

1 alpha subunit has 4 homologous repeats (transmembrane domains)
each transmembrane domain has 5 membrane spanning regions)
with S5 and S6 and intervening loop forming the pore (having a selectivity filter)
intra and extracellular loops
intracellular N and C termini

beta subunit is a glycoprotein
modulates the amplitude of the current–> modulates the kinetics

Critical residues include lysine at the 1422nd position of repeat III and alanine at the 1712th position of repeat IV
Constitute the selectivity filter of the channel for Na+

S4=the voltage sensor (senses depolarization)
basic residues (arginine and lysine) at every 3rd position of S4 enables S4 to be amphipathic and to respond to depolarization with a clockwise rotational conformation change that externalizes 2 positive charges--> gives rise to the gating current.--> activation of the channel 

inactivation of the channel is due to: isoleucine, phenylalanine & methionine residues at positions 1488-1490 of the cytoplasmic loop connecting repeats III & IV (forming a hydrophobic latch)
Serve as inactivation gate–> regulate flux

Question 6

TTX sensitivity

Answer 6

due to tyrosine and glutamate residues at the Y385 and E387 positions in the loop between S5 and S6 of repeat 1 (extracellular loop)

Currently eight known subtypes of α subunits
all subtypes have this GLUTAMATE RESIDUE (critical for the sensitivity).
Those in the CNS (subtypes 1-3) show 200X greater TTX sensitivity and have an additional tyrosine at the 385th position, whereas the others have a cysteine–> increase sensitivity

Question 7

characteristic of voltage gated K+ channels

Answer 7

Four subfamilies of α subunits (each with 6 membrane spanning domains)
Each family containing up to nine subtypes of α subunits
Arranged as a tetramer, but not linked as repeats
Homotetramers & heterotetramers (comprising different subtypes of α subunits) both exist
T domain: sequence of 114 N-terminal amino acids critical for tetramer formation
Can assemble with β subunits 1:1

β subunits on the cytoplasmic side: influence current amplitude and kinetics, and stabilize the alpha subunit through interaction with the T domain

pore: H5 loop lines the pore
S6 and cytoplasmic loop linking S4 and S5 form the mouth pore.

Voltage sensing S4 segment: contains regularly spaced basic residues that help form the voltage sensing amphipathic helix

inactivation: a lot of diversity. some channels do not inactivate at all (remain open as long as the stimulus is present–> “the delayed rectifier”)
Fast (N-type) inactivation
N-terminus or β subunit contains an “inactivation ball” interacts with cytoplasmic loop between S4 & S5 to form “inactivation gate” (e.g., A-type K+ channel–> slows the rate of rise of depolarization to increase the time it takes for the plasma membrane to get to threshold. regulates the interspike/firing rate)

C-type inactivation
Due to conformational changes in the C terminal domain

Question 8

TEA sensitivity

Answer 8

Due to consensus sequence within H5 loop involved in K+ channel selectivity (T/S)xxtxGYG)
Region also contains several residues involved in TEA binding

TEA=tetraethylammonium=residues recognize TEA and K+ and TEA can bind to and plug the pore so K+ cannot enter.

Question 9

characteristics of voltage gated Ca2+ channels

Answer 9

Six currently known subtypes: T, L, N, P, Q & R
Subtypes differ in their: Depolarization threshold for activation, Rates of inactivation, and Sensitivity to channel blockers & toxins

T-type=transient=low threshold Ca2+ to activate but the most rapidly inactivated
L-type=slowest inactivation

L-type channels blocked by dihydropyridine drugs
N-type channels blocked by shellfish toxins (ω-conotoxins)
P- & Q-type channels blocked by spider toxins (ω-agatoxins–>secreted by spunnelweb spider)

Comprised of five subunits (α1, α2, β, γ & δ)
The α1 subunit (pore forming) contains: Four covalently linked repeats (pore), Depolarization sensor for activation in the S4 segment (clockwise rotation—> open pore by pulling on loops between S4 and S5–> Ca2+ enter), An extracellular loop connecting S5 & S6 that lines the pore and forms the extracellular mouth of the pore (Glutamate residues have a negative charge and provide high-affinity Ca2+ binding and serve as the selectivity filter), Numerous modulatory sites

Beta subunit=intracellular=influence amplitude and kinetics

inactivation: 
S6 of repeat 1
Cytoplasmic loop linking repeats I & II 
interacts with βγ subunits of G proteins
C terminus 
involved in Ca2+ dependent inactivation
Other important structural determinants
Cytoplasmic loop connecting repeats II & III
Involved in exctation-contraction coupling and stimulus secretion coupling