Calcium-activated potassium channel subunit alpha-1

Function

BK channels are activated (opened) by changes in membrane electrical potential and/or by increases in concentration of intracellular calcium ion (Ca2+). Opening of BK channels allows K+ to passively flow through the channel, down the electrochemical gradient. Under typical physiological conditions, this results in an efflux of K+ from the cell, which leads to cell membrane hyperpolarization (a decrease in the electrical potential across the cell membrane) and a decrease in cell excitability (a decrease in the probability that the cell will transmit an action potential).

BK channels are essential for the regulation of several key physiological processes including smooth muscle tone and neuronal excitability. They control the contraction of smooth muscle and are involved with the electrical tuning of hair cells in the cochlea. BK channels also contribute to the behavioral effects of ethanol in the worm C. elegans under high concentrations ( 100 mM, or approximately 0.50% BAC). It remains to be determined if BK channels contribute to intoxication in humans.

Structure

BK channels have a tetrameric structure. Each monomer of the channel-forming alpha subunit is the product of the KCNMA1 gene. Modulatory beta subunits (encoded by KCNMB1, KCNMB2, KCNMB3, or KCNMB4) can associate with the tetrameric channel. Alternatively spliced transcript variants encoding different isoforms have been identified.

Each BK channel alpha subunit consists of (from N- to C-terminal):

1. A unique transmembrane domain (S0) that precedes the 6 transmembrane domains (S1-S6) conserved in all voltage-dependent K+ channels.
2. A voltage sensing domain (S1-S4).
3. A K+ channel pore domain (S5, selectivity filter, and S6).
4. A cytoplasmic C-terminal domain (CTD) consisting of a pair of RCK domains that assemble into an octameric gating ring on the intracellular side of the tetrameric channel. The CTD contains four primary binding sites for Ca2+, called "calcium bowls", encoded within the second RCK domain of each monomer.

Available X-ray structures include:

• – Open structure of the BK channel gating ring
• – Crystal structure of the human BK gating apparatus
• – Structure of the intracellular gating ring from the human high-conductance Ca2+ gated K+ channel (BK Channel)

Pharmacology

BK channels are pharmacological targets for the treatment of stroke. Various pharmaceutical companies developed synthetic molecules activating these channels in order to prevent excessive neurotoxic calcium entry in neurons. But BMS-204352 (MaxiPost) a molecule developed by Bristol-Myers Squibb failed to improve clinical outcome in stroke patients compared to placebo. BK channels have also been found to be activated by exogenous pollutants and endogenous gasotransmitters carbon monoxide and hydrogen sulphide.

BK channels are blocked by tetraethylammonium (TEA), paxilline and iberiotoxin.

Related conditions

Researchers have identified a rare disease in humans caused by mutations in the gene.  KCNMA1-linked channelopathy can cause neurological conditions like seizures and movement disorders. An episode of the Diagnosis TV show, based on a column in the New York Times, was about a young girl with a KCNMA1 disorder that caused transient episodes of muscle weakness.

References

References

1. "HomoloGene - NCBI".
2. "Entrez Gene: KCNMA1 potassium large conductance calcium-activated channel, subfamily M, alpha member 1".
3. (2000). "An overview of the potassium channel family". Genome Biology.
4. (July 2010). "Structure of the human BK channel Ca2+-activation apparatus at 3.0 A resolution". Science.
5. (December 2003). "A central role of the BK potassium channel in behavioral responses to ethanol in C. elegans". Cell.
6. (December 1996). "Determinant for beta-subunit regulation in high-conductance voltage-activated and Ca(2+)-sensitive K+ channels: an additional transmembrane region at the N terminus". Proceedings of the National Academy of Sciences of the United States of America.
7. (July 2010). "Structure of the gating ring from the human large-conductance Ca(2+)-gated K(+) channel". Nature.
8. (March 2001). "Structure of the RCK domain from the E. coli K+ channel and demonstration of its presence in the human BK channel". Neuron.
9. Pico A. 2003. RCK domain model of calcium activation in BK channels. PhD thesis. The Rockfeller University, New York.
10. (January 2008). "The RCK2 domain of the human BKCa channel is a calcium sensor". Proceedings of the National Academy of Sciences of the United States of America.
11. (September 1997). "A novel calcium-sensing domain in the BK channel". Biophysical Journal.
12. (December 2011). "Open structure of the Ca2+ gating ring in the high-conductance Ca2+-activated K+ channel". Nature.
13. (May 2005). "Voltage-gated cation channel modulators for the treatment of stroke". Expert Opinion on Investigational Drugs.
14. (April 2001). "Maxi-K potassium channels: form, function, and modulation of a class of endogenous regulators of intracellular calcium". The Neuroscientist.
15. (2002). "BMS-204352: a potassium channel opener developed for the treatment of stroke". CNS Drug Reviews.
16. (February 2005). "Continuous inhalation of carbon monoxide attenuates hypoxic pulmonary hypertension development presumably through activation of BKCa channels". Cardiovascular Research.
17. (March 2008). "The RCK1 high-affinity Ca2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels". Proceedings of the National Academy of Sciences of the United States of America.
18. (February 2010). "Hydrogen sulfide increases calcium-activated potassium (BK) channel activity of rat pituitary tumor cells". Pflügers Archiv.
19. (January 2005). "Paxilline, from Fermentek".
20. (August 1992). "Mode of action of iberiotoxin, a potent blocker of the large conductance Ca(2+)-activated K+ channel". Biophysical Journal.
21. (October 2019). "KCNMA1-linked channelopathy". The Journal of General Physiology.
22. Sanders, Lisa. (2018-09-11). "A Diagnosis Update: New Information on a Young Girl's Rare Genetic Condition". The New York Times.