BK channels (Big Potassium), also called Maxi-K or slo1, are potassium channels characterized by their large conductance of potassium ions (K+) through cell membranes. These 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 (an increase 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 exogenous doses (> 100 mM)  that have been shown to correspond to biologically relevant internal ethanol concentrations. It remains to be determined if BK channels contribute to intoxication in humans.
As with most other voltage-gated potassium channels, 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 tetrametic channel.
BK channels are a prime example of modular protein evolution. Each BK channel alpha subunit consists of (from N- to C-terminal):
- A unique transmembrane domain (S0) that precedes the 6 transmembrane domains (S1-S6) conserved in all voltage-dependent K+ channels.
- A voltage sensing domain (S1-S4).
- A K+ channel pore domain (S5, selectivity filter, and S6).
- A cytoplasmic C-terminal domain (CTD) consisting of a pair of RCK (Regulator of Conductance of K+) 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:
- Template:PDB2 - Crystal Structure of the Human BK Gating Apparatus
- Template:PDB2 - Structure of the Intracellular Gating Ring from the Human High-conductance Ca2+ gated K+ Channel (BK Channel)
- Template:PDB2 - Open Structure of the BK channel Gating Ring
BK channels are pharmacological targets for the treatment of several medical disorders including stroke and overactive bladder. Although pharmaceutical companies have attempted to develop synthetic molecules targeting BK channels, their efforts have proved largely ineffective. For instance, 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 gazotransmitters carbon monoxide and hydrogen sulphide.
BK channels can be readily inhibited by a range of compounds including tetraethylammonium (TEA), paxilline and iberiotoxin.
- ^ Miller C (2000). "An overview of the potassium channel family". Genome Biol. 1 (4): reviews0004.1–reviews0004.5. PMC 138870. PMID 11178249. doi:10.1186/gb-2000-1-4-reviews0004.
- ^ a b c Yuan P, Leonetti MD, Pico AR, Hsiung Y, MacKinnon R (July 2010). "Structure of the human BK channel Ca2+-activation apparatus at 3.0 A resolution". Science 329 (5988): 182–6. PMC 3022345. PMID 20508092. doi:10.1126/science.1190414.
- ^ EntrezGene 3778
- ^ Davies AG, Pierce-Shimomura JT, Kim H, VanHoven MK, Thiele TR, Bonci A, Bargmann CI, McIntire SL (December 2003). "A central role of the BK potassium channel in behavioral responses to ethanol in C. elegans". Cell 115 (6): 655–66. PMID 14675531. doi:10.1016/S0092-8674(03)00979-6.
- ^ Alaimo JT, Davis SJ, Song SS, Burnette CR, Grotewiel M, Shelton KL, Pierce-Shimomura JT, Davies AG, Bettinger JC. (April 2012). "Ethanol metabolism and osmolarity modify behavioral responses to ethanol in C. elegans". Alcohol Clin Exp Res 36 (11): 1840–50. PMC 3396773. PMID 22486589. doi:10.1111/j.1530-0277.2012.01799.x.
- ^ Wallner M, Meera P, Toro L (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". Proc. Natl. Acad. Sci. U.S.A. 93 (25): 14922–7. PMC 26238. PMID 8962157. doi:10.1073/pnas.93.25.14922.
- ^ a b c Wu Y, Yang Y, Ye S, Jiang Y (July 2010). "Structure of the gating ring from the human large-conductance Ca(2+)-gated K(+) channel". Nature 466 (7304): 393–7. PMC 2910425. PMID 20574420. doi:10.1038/nature09252.
- ^ Jiang Y, Pico A, Cadene M, Chait BT, MacKinnon R (March 2001). "Structure of the RCK domain from the E. coli K+ channel and demonstration of its presence in the human BK channel". Neuron 29 (3): 593–601. PMID 11301020. doi:10.1016/S0896-6273(01)00236-7.
- ^ Pico AR (2003). RCK domain model of calcium activation in BK channels (PhD thesis). New York: The Rockfeller University. hdl:10209/211.
- ^ Yusifov T, Savalli N, Gandhi CS, Ottolia M, Olcese R (January 2008). "The RCK2 domain of the human BKCa channel is a calcium sensor". Proc. Natl. Acad. Sci. U.S.A. 105 (1): 376–81. PMC 2224220. PMID 18162557. doi:10.1073/pnas.0705261105.
- ^ a b Schreiber M, Salkoff L (September 1997). "A novel calcium-sensing domain in the BK channel". Biophys. J. 73 (3): 1355–63. PMC 1181035. PMID 9284303. doi:10.1016/S0006-3495(97)78168-2.
- ^ a b Yuan P, Leonetti MD, Hsiung Y, MacKinnon R (January 2012). "Open structure of the Ca2+ gating ring in the high-conductance Ca2+-activated K+ channel". Nature 481 (7379): 94–7. PMC 3319005. PMID 22139424. doi:10.1038/nature10670.
- ^ Gribkoff VK, Starrett JE Jr, Dworetzky SI. (2001). "Maxi-K potassium channels: form, function, and modulation of a class of endogenous regulators of intracellular calcium.". Neuroscientist 2 (2): 166–77. PMID 11496927. doi:10.1177/107385840100700211.
- ^ Jeffrey J. Layne , Bernhard Nausch , Søren-Peter Olesen , Mark T. Nelso (2009). "BK channel activation by NS11021 decreases excitability and contractility of urinary bladder smooth muscle". American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 298 (R378-R384): R378–84. PMC 2828174. PMID 19923353. doi:10.1152/ajpregu.00458.2009.
- ^ Gribkoff VK, Winquist RJ (May 2005). "Voltage-gated cation channel modulators for the treatment of stroke". Expert Opin Investig Drugs 14 (5): 579–92. PMID 15926865. doi:10.1517/135437126.96.36.1999.
- ^ Jensen BS (2002). "BMS-204352: a potassium channel opener developed for the treatment of stroke". CNS Drug Rev 8 (4): 353–60. PMID 12481191. doi:10.1111/j.1527-3458.2002.tb00233.x.
- ^ Dubuis E, Potier M, Wang R, Vandier C (February 2005). "Continuous inhalation of carbon monoxide attenuates hypoxic pulmonary hypertension development presumably through activation of BKCa channels". Cardiovasc. Res. 65 (3): 751–61. PMID 15664403. doi:10.1016/j.cardiores.2004.11.007.
- ^ Hou S, Xu R, Heinemann SH, Hoshi T (March 2008). "The RCK1 high-affinity Ca2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels". Proc. Natl. Acad. Sci. U.S.A. 105 (10): 4039–43. PMC 2268785. PMID 18316727. doi:10.1073/pnas.0800304105.
- ^ Sitdikova GF, Weiger TM, Hermann A (February 2010). "Hydrogen sulfide increases calcium-activated potassium (BK) channel activity of rat pituitary tumor cells". Pflugers Arch. 459 (3): 389–97. PMID 19802723. doi:10.1007/s00424-009-0737-0.
- ^ "Paxilline, from Fermentek".
- ^ Candia S, Garcia ML, Latorre R (August 1992). "Mode of action of iberiotoxin, a potent blocker of the large conductance Ca(2+)-activated K+ channel". Biophys. J. 63 (2): 583–90. PMC 1262182. PMID 1384740. doi:10.1016/S0006-3495(92)81630-2.