Lecture 10 Flashcards
Nav and Kv basics of Pore loop channels
Nav channels are pore loops that conduct outward Na currents that are responsible for depolarizing the Vm during the rising phase of the action potential
Kv channels are pore loops that are activated after Nav channels, they conduct outward K currents that are responsible for strongly repolarizing the Vm during the falling phase of the action potential
All Nav, Kv, basically all P-loop channels have their evolutionary origins in bacteria
Describe KIR channels
P-loop channels emerged billions of years ago in bacteria
- The initial prototype was a KIR channel
- These channels have 2 TMH’s separated by an extracellular pore-loop that dips back into the membrane
- They are tetrameric, meaning that 4 subunits come together to make up a channel
- The 4 pore-loops project key amino acids into the pore that help define K+ ion selectivity (they make up the ion selectivity filter/motif)
- KIR homologs are found in all cellular life forms including humans
What do you get when you duplicate a KIR gene?
Evolutionary changes in the KIR channel gave rise to other types of p-loop channels
-Tandem duplication of the KIR channel gene gave rise to the 2-pore K leak channel
-It is dimeric (only need to proteins to make a channel)
-Major contributor to potassium leak currents
-Thus critical for setting up the RMP (increase in Gk)
-K2P channels are only found in eukaryotes
What happens when inversion plus addition happens to the KIR channel
Inversion across the membrane along with the addition of a transmembrane helix gave rise to ionotropic glutamate receptors
-Ex. AMPA, NMDA, and Kainite receptors of vertebrates
-Crucial roles in excitatory synaptic transmission
-Ex. Glutamatergic synapses
-Like all P-loop channels, these are tetrameric
-Ancient proteins may have originated in bacteria
What happens when KIR was fused?
Fusion with a separately evolved protein in bacteria, the S1-S4 voltage sensors, gave rise to the voltage gated channels
-Kv channels and Ca2+ sensitive SK channels
-Some lost selectivity for K+ to conduct Na+ and/or Ca2+ (only in eukaryotes)
-Ex.1 HCN channels, these channels are activated by hyperpolarization, cAMP, and cGMP
-Ex.2 TRP channels, these channels have the most lost voltage sensitivity, they are activated by various chemicals, mechanical stimuli (touch) and temperature, they also have important roles in sensory biology
Digression point #1
The voltage sensor domain may have originated as a separate ion channel. However, humans have 2 genes for voltage sensor proteins (i.e. S1-S4 helices) without P-loops
-Transmembrane phosphatase with tensin homology (TPTE and TPTE2)
-These conduct outward pH and voltage sensitive H+ currents
Digression point #2
Mutations in the voltage sensor of the P-loops can create leak cation currents which are toxic to cells. An example of this is the arginine to histidine mutation in the domain II s4 helix of NAV1.4 creates a leak proton current. Deviation in the voltage sensor currents can also conduct Ca2+ which we know is toxic to cells.
What happened when a TMH was added to the N-terminus?
Addition of a transmembrane helix to the N-terminus of a Kv/SK/TRP channel structure gave rise to BK channels. Like SK channels, BK channel gating is regulated by rises in cytoplasmic Ca2+
What happens when a Kv channel is duplicated?
Tandem duplication of a Kv like channel gave rise to the Two Pore channels, or TPC. These channels are found in plants and animals, they are dimeric, they are cationic (Ca2+ and Na+), and lastly, the release Ca2+ into the cell from acidic organelles known as endo-lysosomes
What happens when a TPC like channel is duplicated?
Tandem duplication of the TPC like channel gave rise to 4-domain channels
-1 protein makes a channel
-Nav channels (responsible for depolarization during an action potential)
-Cav channels (responsible for muscle contraction, gene expression, and synaptic transmission)
-NALCN (sodium leak channels, Na+ current responsible for setting the RMP)
-Yeast calcium channel 1 (CCH1), these are homologous to NALCN, found in fungi
4 domain channels have only been documented in eukaryotes. There are vast lineages of 4-domain channels that remain undescribed in eukaryotes
What revealed the mechanism for K+ selectivity?
The X-ray crystal structure of the KIR like channel revealed the mechanism for K+ ion selectivity. The arrangement of carbonyl oxygen atoms in the selectivity filter mimics the hydration shell of K+ ions. This makes it energetically favorable for K+ to strip its hydration shell before entering the pore. It is unfavorable for ions like Na+ and Ca2+.
What happens to K+ ions as they move through the selectivity filter?
K+ ions are selectively and temporarily stripped of their hydration shells as they pass through the K+ channel selectivity filter
Who got the Nobel prize for the mechanisms in K+ ion channel selectivity?
Rodrick MacKinnon
How does Na+ ion selectivity work
Although we know what Na+ ion channels look like we are still unsure about the selectivity. We know that unlike Kv channels, Na enters the pore in a semi-hydrated state. Another thing that we know is that the Na pore is lined with amino acids that select for Na+ ions. Altogether, the pore dimensions and spatial positioning of the selectivity filter amino acids help optimize the Na+ ions interactions.
What is the Nav channel selectivity filter that is found in humans and most invertebrates?
The selectivity filter in humans and most vertebrates is called DEKA. The glutamate and aspartate have negatively charged side groups that attract cations. Lysine has a positively charged side group which seems to be the key in Na+ selectivity. Altogether these channels are highly selective for Na+.
